Review of Apparent Vertical Movement Rates in the
Great Lakes Region
Jacob Bruxer
Chuck Southam
I. Table of Contents
II. List of Figures ........................................................................................................... iv
III. List of Tables ............................................................................................................ vi
1.
2.
3.
Introduction................................................................................................................ 1
Past Studies ................................................................................................................ 2
Method ....................................................................................................................... 2
3.1. Water Level Difference Plots and Linear Regression.......................................... 2
3.2. Variability ............................................................................................................ 3
3.3. Outliers................................................................................................................. 5
3.4. Running Average Plots ........................................................................................ 6
3.5. Breakpoint Analysis............................................................................................. 7
4.
Lake Superior............................................................................................................. 8
4.1. Overview.............................................................................................................. 8
4.2. Point Iroquois, Michigan ..................................................................................... 9
4.3. Marquette, Michigan.......................................................................................... 10
4.4. Ontonagon, Michigan ........................................................................................ 10
4.5. Duluth, Minnesota.............................................................................................. 11
4.6. Two Harbors, Minnesota ................................................................................... 11
4.7. Grand Marais, Minnesota................................................................................... 12
4.8. Thunder Bay, Ontario ........................................................................................ 13
4.9. Rossport, Ontario ............................................................................................... 14
4.10. Michipicoten, Ontario ........................................................................................ 15
4.11. Gros Cap, Ontario .............................................................................................. 16
4.12. Summary ............................................................................................................ 17
5.
Lakes Michigan-Huron ............................................................................................ 18
5.1. Overview............................................................................................................ 18
5.2. Harbor Beach, Michigan.................................................................................... 20
5.3. Ludington, Michigan.......................................................................................... 21
5.4. Holland, Michigan ............................................................................................. 22
5.5. Calumet Harbor, Illinois .................................................................................... 23
5.6. Milwaukee, Wisconsin....................................................................................... 25
5.7. Kewaunee, Wisconsin........................................................................................ 27
5.8. Sturgeon Bay Canal, Wisconsin ........................................................................ 28
5.9. Green Bay, Wisconsin ....................................................................................... 29
5.10. Port Inland, Michigan ........................................................................................ 29
5.11. Lakeport, Michigan............................................................................................ 30
5.12. Essexville, Michigan.......................................................................................... 31
5.13. Harrisville, Michigan ......................................................................................... 32
5.14. Mackinaw City, Michigan ................................................................................. 33
5.15. De Tour, Michigan............................................................................................. 35
5.16. Thessalon, Ontario ............................................................................................. 35
5.17. Little Current, Ontario ....................................................................................... 37
5.18. Parry Sound, Ontario ......................................................................................... 38
ii
5.19. Collingwood, Ontario ........................................................................................ 39
5.20. Tobermory, Ontario ........................................................................................... 41
5.21. Goderich, Ontario .............................................................................................. 42
5.22. Summary ............................................................................................................ 46
5.23. Outlet at Lakeport .............................................................................................. 47
6.
Lake Erie.................................................................................................................. 50
6.1. Overview............................................................................................................ 50
6.2. Buffalo, New York............................................................................................. 51
6.3. Sturgeon Point, New York................................................................................. 51
6.4. Barcelona, New York......................................................................................... 52
6.5. Erie, Pennsylvania.............................................................................................. 52
6.6. Fairport, Ohio..................................................................................................... 53
6.7. Cleveland, Ohio ................................................................................................. 54
6.8. Marblehead, Ohio .............................................................................................. 56
6.9. Toledo, Ohio ...................................................................................................... 57
6.10. Monroe, Michigan.............................................................................................. 60
6.11. Fermi Power Plant, Michigan ............................................................................ 61
6.12. Bar Point, Ontario .............................................................................................. 62
6.13. Kingsville, Ontario............................................................................................. 63
6.14. Erieau, Ontario................................................................................................... 64
6.15. Port Stanley, Ontario.......................................................................................... 65
6.16. Port Dover, Ontario............................................................................................ 66
6.17. Port Colborne, Ontario....................................................................................... 67
6.18. Summary ............................................................................................................ 70
7.
Lake Ontario ............................................................................................................ 71
7.1. Overview............................................................................................................ 71
7.2. Cape Vincent, New York................................................................................... 72
7.3. Oswego, New York............................................................................................ 73
7.4. Rochester, New York......................................................................................... 74
7.5. Olcott, New York............................................................................................... 74
7.6. Port Weller, Ontario........................................................................................... 75
7.7. Port Dalhousie, Ontario ..................................................................................... 76
7.8. Burlington, Ontario ............................................................................................ 77
7.9. Toronto, Ontario ................................................................................................ 78
7.10. Cobourg, Ontario ............................................................................................... 80
7.11. Kingston, Ontario............................................................................................... 81
7.12. Summary ............................................................................................................ 83
8.
Conclusions.............................................................................................................. 84
9.
Recommendations.................................................................................................... 86
10. References................................................................................................................ 88
iii
II. List of Figures
Figure 3-1: Water level difference plot of Point Iroquois minus Duluth (Lake Superior). ............ 3
Figure 3-2: Harbor Beach minus Calumet Harbor water level difference plot with 1907 outlier
included. .......................................................................................................................................... 6
Figure 3-3: Water level difference plot of Harbor Beach minus Milwaukee for the period 1904 to
2006 with 5-year centred runing average. ....................................................................................... 7
Figure 3-4: Harbor Beach minus Goderich water level difference plot, with piecewise regression
and the estimated breakpoint indicated. .......................................................................................... 8
Figure 4-1: Lake Superior gauge locations...................................................................................... 9
Figure 4-2: Point Iroquois minus Marquette water level difference plot. .................................... 10
Figure 4-3: Point Iroquois minus Ontonagon water level difference plot. ................................... 11
Figure 4-4: Point Iroquois minus Two Harbors water level difference plot.................................. 12
Figure 4-5: Point Iroquois minus Grand Marais water level difference plot................................. 13
Figure 4-6: Point Iroquois minus Thunder Bay water level difference plot.................................. 14
Figure 4-7: Point Iroquois minus Rossport water level difference plot......................................... 15
Figure 4-8: Point Iroquois minus Michipicoten water level difference plot. ................................ 16
Figure 4-9: Point Iroquois minus Gros Cap water level difference plot........................................ 17
Figure 5-1: Lakes Michigan-Huron gauge locations..................................................................... 19
Figure 5-2: Harbor Beach minus Ludington water level difference plot....................................... 21
Figure 5-3: Milwaukee minus Ludington water level difference plot........................................... 22
Figure 5-4: Harbor Beach minus Holland water level difference plot. ......................................... 23
Figure 5-5: Harbor Beach minus Calumet Harbor water level difference plot with 1907 outlier
removed. ........................................................................................................................................ 24
Figure 5-6: Harbor Beach minus Calumet Harbor water level difference plot with 5-year centred
running average. ............................................................................................................................ 24
Figure 5-7: Harbor Beach minus Milwaukee water level difference plot. .................................... 25
Figure 5-8: Harbor Beach minus various Lake Michigan gauges 5-year centred running averages
of water level difference plots. ...................................................................................................... 26
Figure 5-9: Harbor Beach minus Kewaunee water level difference plot. ..................................... 27
Figure 5-10: Harbor Beach minus Sturgeon Bay Canal water level difference plot. .................... 28
Figure 5-11: Harbor Beach minus Green Bay water level difference plot. ................................... 29
Figure 5-12: Harbor Beach minus Port Inland water level difference plot. .................................. 30
Figure 5-13: Harbor Beach minus Lakeport water level difference plot....................................... 31
Figure 5-14: Harbor Beach minus Essexville water level difference plot..................................... 32
Figure 5-15: Harbor Beach minus Harrisville water level difference plot. ................................... 33
Figure 5-16: Harbor Beach minus Mackinaw City water level difference plot............................. 34
Figure 5-17: Harbor Beach minus Mackinaw City water level difference plot with 5-year centred
running average. ............................................................................................................................ 34
Figure 5-18: Harbor Beach minus De Tour water level difference plot........................................ 35
Figure 5-19: Harbor Beach minus Thessalon water level difference plot. .................................... 36
Figure 5-20: Harbor Beach minus Thessalon water level difference plot with 5-year centred
running average. ............................................................................................................................ 37
Figure 5-21: Harbor Beach minus Little Current water level difference plot. .............................. 38
Figure 5-22: Harbor Beach minus Parry Sound water level difference plot. ................................ 39
Figure 5-23: Harbor Beach minus Collingwood water level difference plot. ............................... 40
Figure 5-24: Harbor Beach minus Collingwood water level difference plot with 5-year centred
running average. ............................................................................................................................ 40
Figure 5-25: Harbor Beach minus Collingwood breakpoint estimation........................................ 41
iv
Figure 5-26: Harbor Beach minus Tobermory water level difference plot. .................................. 42
Figure 5-27: Harbor Beach minus Goderich water level difference plot. ..................................... 43
Figure 5-28: Goderich minus Thessalon water level difference plot. ........................................... 44
Figure 5-29: Goderich minus Collingwood water level difference plot........................................ 45
Figure 5-30: Goderich minus Milwaukee water level difference plot........................................... 46
Figure 6-1: Lake Erie gauge locations........................................................................................... 50
Figure 6-2: Buffalo minus Sturgeon Point water level difference plot. ........................................ 51
Figure 6-3: Buffalo minus Barcelona water level difference plot. ................................................ 52
Figure 6-4: Buffalo minus Erie, PA, water level difference plot................................................... 53
Figure 6-5: Buffalo minus Fairport water level difference plot. ................................................... 54
Figure 6-6: Buffalo minus Cleveland water level difference plot. ................................................ 55
Figure 6-7: Buffalo minus Cleveland water level difference plot with 5-year centred running
average........................................................................................................................................... 56
Figure 6-8: Buffalo minus Marblehead water level difference plot. ............................................. 57
Figure 6-9: Buffalo minus Toledo water level difference plot. ..................................................... 58
Figure 6-10: Comparison of Toledo minus Fermi Power Plant and Toledo minus Erie, PA water
level difference plots. .................................................................................................................... 59
Figure 6-11: Buffalo minus Toledo breakpoint estimation for recording gauge period................ 60
Figure 6-12: Buffalo minus Monroe water level difference plot................................................... 61
Figure 6-13: Buffalo minus Fermi Power Plant water level difference plot. ................................ 62
Figure 6-14: Buffalo minus Bar Point water level difference plot. ............................................... 63
Figure 6-15: Buffalo minus Kingsville water level difference plot............................................... 64
Figure 6-16: Buffalo minus Erieau water level difference plot..................................................... 65
Figure 6-17: Buffalo minus Port Stanley water level difference plot............................................ 66
Figure 6-18: Buffalo minus Port Dover water level difference plot.............................................. 67
Figure 6-19: Buffalo minus Port Colborne water level difference plot......................................... 68
Figure 6-20: Buffalo minus Port Colborne water level difference plot with 5-year centred running
average........................................................................................................................................... 68
Figure 6-21: Buffalo minus various Lake Erie gauges 5-year centred running averages of water
level difference plots. .................................................................................................................... 69
Figure 6-22: Comparison of Buffalo minus Port Dover and Buffalo minus Port Colborne water
level difference plots. .................................................................................................................... 70
Figure 7-1: Lake Ontario gauge locations. .................................................................................... 72
Figure 7-2: Cape Vincent minus Oswego water level difference plot........................................... 73
Figure 7-3: Cape Vincent minus Rochester water level difference plot........................................ 74
Figure 7-4: Cape Vincent minus Olcott water level difference plot.............................................. 75
Figure 7-5: Cape Vincent minus Port Weller water level difference plot. .................................... 76
Figure 7-6: Cape Vincent minus Port Dalhousie water level difference plot................................ 77
Figure 7-7: Cape Vincent minus Burlington water level difference plot. ..................................... 78
Figure 7-8: Cape Vincent minus Toronto water level difference plot........................................... 79
Figure 7-9: Cape Vincent minus Toronto water level difference plot with 5-year centred running
average........................................................................................................................................... 79
Figure 7-10: Cape Vincent minus Cobourg water level difference plot (June to Sep. average). .. 80
Figure 7-11: Cape Vincent minus Cobourg water level difference plot (July to Oct. average). ... 81
Figure 7-12: Cape Vincent minus Kingston water level difference plot. ...................................... 82
Figure 7-13: Comparison of Kingston water level difference plots. ............................................. 83
v
III. List of Tables
Table 4-1: Lake Superior gauge stations and periods of record. ..................................................... 9
Table 4-2: Summary of Lake Superior rates of apparent vertical movement and uncertainty
relative to Point Iroquois. .............................................................................................................. 18
Table 5-1: Lake Michigan gauge stations and periods of record................................................... 19
Table 5-2: Lake Huron gauge stations and periods of record........................................................ 20
Table 5-3: Summary of Lakes Michigan-Huron rates of apparent vertical movement and
uncertainty relative to Harbor Beach............................................................................................. 47
Table 5-4: Comparison of rates of movement on Lakes Michigan-Huron relative to Harbor Beach
and Lakeport.................................................................................................................................. 48
Table 5-5: Summary of Lakes Michigan-Huron rates of apparent vertical movement and
uncertainty relative to Lakeport..................................................................................................... 49
Table 6-1: Lake Erie gauge stations and periods of record. .......................................................... 50
Table 6-2: Summary of Lake Erie rates of apparent vertical movement and uncertainty relative to
Buffalo........................................................................................................................................... 71
Table 7-1: Lake Ontario gauge stations and periods of record...................................................... 72
Table 7-2: Summary of Lake Ontario rates of apparent vertical movement and uncertainty relative
to Cape Vincent. ............................................................................................................................ 84
vi
vii
1. Introduction
During the last glacial era, glaciers covered much of North America, including the
Great Lakes region. Under the great weight of these glaciers the Earth’s crust was
deformed. When the ice in the Great Lakes region melted some 10,000 years ago, the
crust began to rebound, and it is believed that this rebounding has continued ever since.
As the glaciers receded, they generally tended to do so in a north-easterly direction.
For this reason, the northern and eastern areas of the Great Lakes regions tended to be
covered with a greater amount of ice for a longer period of time than areas to the south
and west. As such, rebound rates have been observed to occur at a faster rate as one
travels to the north and to the east. In general, the result is that observed water levels on
a given lake appear to be falling over time on the northern and eastern shores, while
water levels appear to be rising over time on the southern and western shores. By
comparing the water level differences over time between pairs of gauges on a given lake,
the relative rate of apparent vertical movement between these gauges can be estimated.
The phrase “apparent vertical movement” has been used intentionally here in place of
“glacial isostatic adjustment”, “post-glacial rebound”, “crustal movement”, or any other
phrase that would suggest movement of the Earth’s crust specifically. While often the
apparent vertical movement between gauges is assumed to be exclusively the result of
glacial isostatic adjustment, in some cases the movement may be the result of other
known or unknown factors. An example would be at a gauge station where in the
immediate local vicinity the land, including both the gauge and the local benchmark, is
subsiding as a result of something other than glacial isostatic adjustment. In fact such an
occurrence has been observed on Lake Erie at the Fairport, Ohio gauge. Other such
events that could be responsible for inconsistencies in the water level difference plots
may include the relocation of a gauge or a change in the data collection method from staff
gauge readings to continuously recording water level measurements.
On the other hand, jumps, shifts, changes in rate, step-like patterns or other
discontinuities in water level difference plots might actually be the result of a poorly
understood glacial isostatic adjustment process. For example, the assumption of a linear
trend over the short-term between gauge pairs may be incorrect. Linear trend lines
calculated from water level difference plots are used to determine the rate of apparent
vertical movement between gauges; however, in reality the processes responsible for
observed glacial isostatic adjustment may be non-linear, but may instead follow some
different pattern or no pattern at all. The use of long-term linear regression of water level
differences may be useful for some purposes, but not all.
For these reasons, this report provides a revision of the rates of glacial isostatic
adjustment as calculated from water level difference plots using linear regression, and
also illustrates the difficulties and anomalies that occur in this type of analysis.
Significant discontinuities in the water level difference plots are identified, and where
possible, potential causes for the discontinuities are discussed.
1
2. Past Studies
Glacial isostatic adjustment has been studied for some time in the Great Lakes region.
A report by the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic
Data (Coordinating Committee, 1977) summarizes a number of earlier studies. The
Coordinating Committee compared differences between the four-month mean June to
September monthly average water levels for gauge pairs on each of the Great Lakes.
Linear regression lines were fit to the water level difference versus time plots, with the
slope of the line indicating the relative rate of movement between the two gauges in
question.
Other researchers (e.g. Tushingham, 1992) have used similar methods that are
referenced in the Coordinating Committee’s 2001 report which provides one of the most
recent updates. In their study, the Coordinating Committee used monthly mean water
level data from all twelve months with outliers removed, as opposed to the traditional
June to September average used in earlier studies. The vertical velocity rates and
standard errors were calculated for all gauges from this data. The findings were also
reported in Mainville and Craymer (2005).
It should be noted that these past studies did not deal directly with non-homogeneous
data, trend shifts, step-like patterns, or other inconsistencies, whereas this current study
does.
3. Method
3.1.
Water Level Difference Plots and Linear Regression
Similar to past studies, plots of water level differences versus time between gauge
pairs on each lake were constructed, and a linear least-squares regression line was fit to
the data. The slope of this line indicates the approximate rate of apparent vertical
movement between the two gauges. Published monthly mean water level data at gauges
across the Great Lakes region for the period of record up until 2006 was obtained for use
in this study. Data for Canadian gauges was obtained from the Canadian Hydrographic
Service, while data from U.S. gauges was obtained from the U.S. National Ocean Service.
As an example, Figure 3-1 shows a water level difference plot for Point Iroquois and
Duluth. Point Iroquois is located at the eastern end and approximate outlet of Lake
Superior, while Duluth is located at the western end of the lake. Rates of apparent
vertical movement are normally reported relative to the outlet of the lake, but can also be
determined for any gauge pair on a given lake. The four-month (June to September)
mean monthly water level as recorded at Duluth is subtracted from the same four-month
mean level as recorded at Point Iroquois for all years of record and plotted as shown. The
2
downward slope of the linear regression line indicates that over time the water level
observed at Duluth appears to be increasing with respect to the water level recorded at
Point Iroquois at a rate of approximately 0.0027 metres/year (m/year) or
27 centrimetres/century (cm/century). This apparent vertical movement is normally
assumed to be the result of the earth’s crust at Duluth falling with respect to the crust at
Point Iroquois. Please note that in absolute terms (i.e. relative to the earth’s geo-centre)
the crust could be falling or rising at either or both gauge locations, but that it is not
possible to determine what the case is from these diagrams. Rather it is the relative rate
between the two gauges that is illustrated by the water level difference plots.
In addition to the water level difference plots and calculated rates of vertical
movement, the uncertainty as given by the standard deviation of the slope value was also
calculated.
Point Iroquois - Duluth
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1920
1940
1960
1980
2000
2020
Year
Figure 3-1: Water level difference plot of Point Iroquois minus Duluth (Lake Superior). Water
levels were averaged from June to September in this case.
3.2.
Variability
Figure 3-1 also illustrates the fact that plots of water level differences versus time
show noise or variability in addition to the assumed linear trend. This is believed to be
primarily the result of meteorological impacts, such as storm and wind events, differences
in barometric pressure, ice effects at gauge locations, etc. To help reduce some of this
3
variability, in past studies monthly mean water level differences between gauge pairs
have normally been averaged. A four-month average from June to September was
commonly used as these four months were believed to be those least susceptible to the
meteorological impacts outlined.
One notable exception was the recent Coordinating Committee (2001) report, which
used monthly values from all twelve months as opposed to the four-month June to
September average approach. Outliers were removed in this study where appropriate,
and while the plots using data from all months tend to show greater variability than fourmonth average plots, the linear trend in the data can still be identified. However, one of
the problems that results from using this method is that the density of the data and the
greater variability can mask important features in the water level differences plots, such
as non-linear patterns, shifts, steps and other discontinuities.
Taking this into consideration, initial investigations conducted for this study tried to
identify which months, if any, showed the least amount of noise and variability with
regards to water level differences between gauge pairs. It was believed that these months
would be the best choices for use in the average water level difference plots used to infer
apparent vertical movement of water level gauges over time.
Each lake was investigated individually in this analysis, since the lakes vary
significantly in their geographic and climatic attributes, which could cause certain months
of data to be most appropriate for use on a certain lake but not on others. For example,
water levels on Lake Erie, due to such factors as the lake’s size, depth, geographic
orientation and climate, tend to be more affected by wind setup and seiche effects than
some of the other lakes. Therefore, the best months used on Lake Erie (i.e. those least
affected by meteorological impacts) may not be the most ideal months for use on the
other lakes.
A number of tests were done on water level difference plots for each lake and for
each month. These included determining the mean and standard deviations of the
monthly differences, determining the standard deviation of the year-to-year lag of the
monthly differences, and calculating the R-squared value of the monthly difference plots
over time.
The six traditionally ice-free months of May to October generally performed better
than the other months in these tests, which was as expected. Between these six months,
however, the results did not show conclusive evidence that any months in particular
would be significantly better for use in the water level difference analysis than others.
The latter months (July to October) showed the least variability in terms of mean
differences, while earlier months (May to August) showed the lowest variability when
standard deviations about the mean year-to-year differences were plotted. These months
also had the best fit according to the R-squared statistic. July and August tended to be the
best regardless of test chosen, but again, these did not perform significantly better than
the other ice-free months investigated. Somewhat surprisingly, this was the case
regardless of the lake chosen.
4
For a number of reasons, it was decided that the traditional four-month, June to
September average be used primarily in this study. First, as noted, none of the months
performed especially better than any of the other months, but the months of June to
September were four of the best according to the variability investigation. Second, the
uncertainty is reduced in linear regression calculations as more data is available, which
includes both the number of years of available data, as well as the completeness of these
available years. Water level records are sometimes missing in the period of record for a
given gauge due to equipment malfunctions, gauge maintenance, relocation or any
number of other reasons. Since all months of data for a given average calculation must
be available for the average to be computed and used in the regression analysis, the more
months required for a calculation, the more likely it is that one or more of these months
are missing. If one or more months are missing, the average cannot be calculated and this
year will have to be omitted. For example, if only the month of February is missing in a
given year, then the annual average for that year cannot be calculated and the year cannot
be used in the regression analysis. On the other hand, the June to September average for
this same year can be calculated and used. As a result, averages that include a smaller
number of months (such as the four-month June to September average) can be calculated
more often than averages that require more months (such as the 12-month annual average
or even a six-month May to October average). An alternative option would be to use less
than four months, or even choose a single month of data as opposed to an average
calculation; however, the use of an average helps to reduce some of the variability and
noise in the water level difference plots. The choice of a four-month average (in this case
June to September) was done in part to balance the need for available data and the need
to reduce variability in the water level difference relationship.
Lastly, using the June to September average is consistent with many previous studies,
thus allowing for direct comparison. In addition to those studies already mentioned, June
to September averages have also been used for International Great Lakes Datum (IGLD)
adjustments. Furthermore, rates of apparent vertical movement were calculated during
this study for other combinations of months, and in general the results normally tended to
be in close agreement. Additional calculated rates are reported in this document where
deemed necessary. However, unless otherwise stated, the rates of apparent vertical
movement determined from the four-month June to September average water level
differences are reported.
3.3.
Outliers
Outliers were subjectively removed only if they were obviously the result of some
issue beyond those known to cause normal variability observed in the water level
difference plots, or if they had a significantly disproportional impact on the calculated
rate of apparent vertical movement. Only a small number of outliers were removed in
this way, and those removed are documented in this report. It should be noted that
normally even these most extreme outliers did not make a significant difference in the
calculated rates of vertical movement. As an example, Figure 3-2 shows one of the most
5
significant outliers removed, occurring in 1907 in a plot of Harbor Beach minus Calumet
Harbor. When this outlier was removed the difference in calculated rate of apparent
vertical movement was only 0.80 cm/century.
Harbor Beach - Calumet Harbor
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 3-2: Harbor Beach minus Calumet Harbor water level difference plot with 1907 outlier
included.
3.4.
Running Average Plots
In order to smooth out some of the noise in the water level difference plots, running
averages of the water level differences were calculated for certain gauge pairs. Three- to
nine-year centred averages were calculated about the central year and plotted against the
water level differences. By smoothing out some of the noise in the water level difference
plots, the running average plots are able to better illustrate potential shifts, changes in
slope, stepwise patterns, or other features that may not be as apparent in the original
water level difference plots.
An example of a running average plot is shown in Figure 3-3. The June to September
average differences are plotted along with the five-year centred running averages for June
to September. While also evident in the original water level difference plot, the running
average plot more clearly indicates what appears to be a step-like pattern in the calculated
differences.
6
Harbor Beach - Milwaukee
0.3
June-Sep. Avg.
June-Sep. 5-Year Centred Avg.
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 3-3: Water level difference plot of Harbor Beach minus Milwaukee for the period 1904 to
2006. The June to September average differences are plotted along with the June to September
five-year centred running average plot showing a step-like pattern.
3.5.
Breakpoint Analysis
Breakpoint analysis was performed on certain water level difference plots if evidence
suggested a distinct change in rate of apparent vertical movement has occurred. For
example, Figure 3-4 shows a plot of Harbor Beach minus Goderich June to September
average water level differences. Since visual inspection of the original plot suggested a
change in slope at some point around the 1950’s, piecewise regression was performed
and the estimated breakpoint identified. Linear regression is performed before and after
the breakpoint, which is assumed to exist. The slope and intercept of lines before and
after the breakpoint are calculated, and the breakpoint itself is identified as the year in
which the overall R-squared value of the fitted lines is a maximum. This was found
using the Solver function in Microsoft Excel. In the case of Harbor Beach minus
Goderich, the breakpoint was identified at 1955.
Due to the uncertainty and variability in the water level difference plots, the
breakpoint found in this way can only be accepted as an estimate. Nonetheless, this
method gives an unbiased means of estimating the breakpoint, and also allows subjective
comparisons to be made between different plots.
7
Harbor Beach - Goderich
0.3
June-Sep. Avg.
Piecewise Regression
Estimated Breakpoint
Water Level Difference (m)
0.2
0.1
1955
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 3-4: Harbor Beach minus Goderich water level difference plot, with piecewise regression and
the estimated breakpoint indicated.
4. Lake Superior
4.1.
Overview
Water level data from a total of ten gauge stations were analyzed on Lake Superior
(Figure 4-1). Four of these gauge stations are located in Canada, while the remaining six
gauge stations are located in the United States. Table 4-1 gives a list of the gauge
stations used in this analysis, as well as a summary of the periods of record for each
station. Data at some locations were divided between gauges due to gauge relocations.
These datasets were combined and treated as one homogeneous dataset in this analysis
unless otherwise noted, and are identified in the Station ID with a backslash indicating
the two final digits of the two gauges (for example, Marquette data comes from two
individual gauges, 9099016 and 9099018, which cover different time periods at
essentially the same location). Furthermore, if a full year at a given station contained no
data (i.e. no monthly data available from January to December for a given year) this year
was identified. Only full years of missing data are identified, but note also that additional
years within the dataset not identified may still be missing individual or many months.
This applies to gauges on all of the Great Lakes.
8
Figure 4-1: Lake Superior gauge locations.
Start End
Missing (Full)
Location
Year Year Years
Point Iroquois, MI,
USA
9099004
1930 2006
1945-49
9099016/8 Marquette, MI, USA
1860 2006
-9099044
Ontonagon, MI, USA
1959 2006
-9099064
Duluth, MN, USA
1860 2006
1874-79
Two Harbors, MN,
1888-98; 1901-29;
USA
1932-34; 1936-40
9099070
1887 1988
Grand Marais, MN,
9099090
1966 2006
-USA
Thunder Bay, ON,
10050
1907 2006
1912-1913
Canada
Rossport, ON,
Canada
10220
1967 2006
-Michipicoten, ON,
10750
1915 2006
-Canada
Gros Cap, ON,
Canada
10920
1926 2006
1930-1960
Table 4-1: Lake Superior gauge stations and periods of record.
Station ID
4.2.
Number of Full
Years Missing
Total Years
of Data
5
0
0
6
72
147
48
141
48
54
0
41
2
98
0
40
0
92
31
50
Point Iroquois, Michigan
The Point Iroquois gauge was selected as the Lake Superior outlet. Gros Cap is also
located close to the outlet of Lake Superior; however, Point Iroquois has traditionally
been the gauge used to represent the outlet, likely owing to the fact that it has a longer
period of record than Gros Cap. Data at Point Iroquois begins in 1930 and continues
through to 2006, with the exception of a period of missing data from October 1944
through October 1950. The gauge was not moved during this period of missing data, but
it was moved in 1970. All data at Point Iroquois was obtained from a continuous
recording gauge.
9
4.3.
Marquette, Michigan
The water level record at Marquette is a combination of data from two gauge stations.
The data begins in 1860 and was collected at gauge 909-9016 (“Marquette”) until 1980.
The data collected from 1980 until 2006 comes from gauge 909-9018 (“Marquette C.G.”).
The data were merged, with data from station 909-9018 being used for the overlapping
year of 1980, and it was assumed that this represented a homogeneous dataset.
The water level difference plot for Point Iroquois minus Marquette is shown in Figure
4-2. The rate of vertical movement as determined from the slope of the linear regression
line is -10.50 cm/century, indicating that Marquette is falling relative to the lake’s outlet
at Point Iroquois. Visual inspection of the plot does not reveal any significant
discontinuities in the slope of the linear relationship.
Point Iroquois - Marquette
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0.0
-0.1
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
Year
Figure 4-2: Point Iroquois minus Marquette water level difference plot.
4.4.
Ontonagon, Michigan
The water level record at Ontonagon begins during 1959, with the first June to
September average being calculated in 1960. The water level difference plot for Point
Iroquois minus Ontonagon is shown in Figure 4-3. The rate of vertical movement as
determined from the slope of the linear regression line is -17.32 cm/century, indicating
10
that Ontonagon is falling relative to the lake’s outlet. Visual inspection of the plot does
not reveal any significant discontinuities.
Point Iroquois - Ontonogan
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 4-3: Point Iroquois minus Ontonagon water level difference plot.
4.5.
Duluth, Minnesota
The water level record begins in 1860 at Duluth. The water level difference plot for
Point Iroquois minus Duluth was shown previously in Figure 3-1. The rate of vertical
movement as determined from the slope of the linear regression line is -26.85 cm/century,
indicating that Duluth is falling relative to the lake’s outlet. Data at Duluth prior to 1950
was collected from staff gauges read tri-daily (Coordinating Committee, 1978).
Although the variability of this earlier data does not appear to be significantly different
than the recording gauge data collected post-1950, the slope of the linear relationship in
the earlier data does appear to be slightly different than the slope of the latter data. If the
earlier data is omitted the downward slope of the regression line increases
to -29.00 cm/century.
4.6.
Two Harbors, Minnesota
The water level record begins in 1887 at Two Harbors, but very little data is available
until 1941. In addition, the water level record ends in 1988. The water level difference
11
plot for Point Iroquois minus Two Harbors is shown in Figure 4-4. The rate of vertical
movement as determined from the slope of the linear regression line is -19.90 cm/century,
indicating that Two Harbors is falling relative to the lake’s outlet. Visual inspection of
the plot does not reveal any significant discontinuities.
Point Iroquois - Two Harbors
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1920
1940
1960
1980
2000
Year
Figure 4-4: Point Iroquois minus Two Harbors water level difference plot.
4.7.
Grand Marais, Minnesota
The water level record begins in 1966 at Grand Marais. The water level difference
plot for Point Iroquois minus Grand Marais is shown in Figure 4-5. The rate of vertical
movement as determined from the slope of the linear regression line is -10.26 cm/century,
indicating that Grand Marais is falling relative to the lake’s outlet. Visual inspection of
the plot does not reveal any significant discontinuities.
12
Point Iroquois - Grand Marais
0.3
Jun-Sep Avg
Linear (Jun-Sep
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1980
2000
2020
Year
Figure 4-5: Point Iroquois minus Grand Marais water level difference plot.
4.8.
Thunder Bay, Ontario
The water level record begins in 1907 at Thunder Bay. The water level difference
plot for Point Iroquois minus Thunder Bay is shown in Figure 4-6. The rate of vertical
movement as determined from the slope of the linear regression line is 3.84 cm/century,
indicating that Thunder Bay is rising relative to the lake’s outlet. However, visual
inspection of Figure 4-6 shows that though it is not entirely misleading, a linear
relationship in this case may not be an accurate representation of the apparent vertical
movement, since the plot shows some interesting discontinuities. Two alternative
possibilities were investigated. The first was that the data prior to 1951 is inconsistent
with the data recorded from 1951 on. The rate of vertical movement calculated with only
data collected from 1951 on is 1.29 cm/century. The second possibility investigated was
that a change in rate of vertical movement has occurred at some point during the period
of record. A breakpoint analysis was performed, which identified a possible breakpoint
in 1985 if the pre-1951 data is included, or 1989 if only data collected from 1951
onwards is used.
However, there is little evidence suggesting reasons for either of these alternative
possibilities. All data were collected from continuously recording gauges. A number of
gauge relocations have occurred, but only one, at Thunder Bay in 1953, has occurred
close to any of the dates of the discontinuities suggested by the additional analysis. This
relocation was a short distance, and it seems unlikely that this was the cause of the
13
discontinuity. No other gauges on Lake Superior show similar issues with pre-1951 data,
so it seems unlikely that the data at Point Iroquois is the problem. Comparisons of
Thunder Bay versus other long-term gauges on Lake Superior (i.e. Duluth, Michipicoten,
and Marquette) appear to show similar shifts in the pre-1951 data, though these are not as
discernible due to the greater slope of the data in these plots. The mild slope of the Point
Iroquois minus Thunder Bay plot makes the shift more noticeable. Lastly, the period of
record after the possible breakpoints in the mid- to late-1980s is too short to adequately
define a change in rate.
Point Iroquois - Thunder Bay
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1951+)
Water Level Difference (m)
0.2
0.1
0
-0.1
1920
1940
1960
1980
2000
2020
Year
Figure 4-6: Point Iroquois minus Thunder Bay water level difference plot.
4.9.
Rossport, Ontario
The water level record begins in 1967 at Rossport. The water level difference plot for
Point Iroquois minus Rossport is shown in Figure 4-7. The rate of vertical movement as
determined from the slope of the linear regression line is 24.55 cm/century; however,
visual inspection of the plot shows a significant jump occurring in 1973. Tushingham
(1992) reports that the Rossport gauge has been tied to an unstable benchmark in the past.
While this could not be confirmed, it seems a likely explanation for the observed jump,
especially since the observed rate of apparent vertical movement is fairly constant after
this point in time. The rate of apparent vertical movement as calculated from 1973 data
onwards is 15.41 cm/century, which seems to be a more realistic estimate.
14
Point Iroquois - Rossport
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1973+)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
2020
Year
Figure 4-7: Point Iroquois minus Rossport water level difference plot.
4.10. Michipicoten, Ontario
The water level record begins in 1915 at Michipicoten. The water level difference
plot for Point Iroquois minus Michipicoten is shown in Figure 4-8. The rate of vertical
movement as determined from the slope of the linear regression line is 23.39 cm/century.
Visual inspection of the plot does not reveal any significant discontinuities. A breakpoint
analysis was performed, which suggested a potential change in rate around 1965.
However, since it is not possible to confirm a rate change, and since the linear regression
line for the entire period of record does provide what appears to be a reasonably good fit
(R-squared = 0.968), the rate of 23.39 cm/century is acceptable.
15
Point Iroquois - Michipicoten
0.3
Jun-Sep Avg
Linear (Jun-Sep
A )
Water Level Difference (m)
0.2
0.1
0
-0.1
-0.2
1920
1940
1960
1980
2000
2020
Year
Figure 4-8: Point Iroquois minus Michipicoten water level difference plot.
4.11. Gros Cap, Ontario
The water level record begins in 1961 at Gros Cap. The water level difference plot
for Point Iroquois minus Gros Cap is shown in Figure 4-9. The rate of vertical movement
as determined from the slope of the linear regression line is -2.91 cm/century. However,
visual inspection of the plot reveals what appear to be significant discontinuities or
outliers in the period of record, especially in the early years. In more recent years (post1986), the water level difference relationship appears more stable. The rate of vertical
movement calculated for this period is -1.11 cm/century. Though it represents a very
short period of record, this rate is also more in line with what one would expect, since
Gros Cap and Point Iroquois are located very close together. In fact, for many purposes,
due to their close proximity, the rate calculated at Gros Cap with respect to Point Iroquois
is likely redundant.
16
Point Iroquois - Gros Cap
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1986+)
Water Level Difference (m)
0.2
0.1
0
-0.1
1940
1960
1980
2000
2020
Year
Figure 4-9: Point Iroquois minus Gros Cap water level difference plot.
4.12. Summary
The calculated rates of apparent vertical movement and their standard deviations for
Lake Superior are given in Table 4-2. The water level gauge, period of record, and its
calculated rate of apparent vertical movement and associated standard error with respect
to the outlet of Lake Superior at Point Iroquois are given. Rates and standard error values
calculated for additional scenarios are also given in italics. These scenarios were
computed for the reasons outlined in the text above (i.e. where gauge issues were
observed or suspected). Note that rates were also calculated with 1951 data onwards for
all long-term gauges due to the observed issue with the pre-1951 data in the Thunder Bay
minus Point Iroquois water level difference plot. Though this potential issue, if it exists,
might be related to Thunder Bay only, the calculated rates with 1951 data onwards also
provides an estimate of vertical movement with a consistent period of record between all
gauges that does not include staff gauge data or any other potential pre-1951 data issues.
In general, water level difference plots on Lake Superior are fairly uncomplicated.
No single outliers were removed from the Lake Superior plots, although consecutive
years of questionable data were removed for the analysis at both Rossport and Gros Cap.
With the possible exception of Thunder Bay, water level difference plots on Lake
Superior do not show any significant discontinuities or anomalies. Most of the water
level difference plots on Lake Superior appear to be well-represented by a linear equation,
17
and the variability in the plots does not mask or lead one to question the observed trend in
most cases.
Gauge (Period of Record)
Rate
Standard Error
*Additional Scenarios
(cm/century)
(cm/century)
Marquette (1931-2006)
-10.50
0.32
-9.78
0.45
*Marquette (1951-2006)
Ontonagon (1960-2006)
-17.32
1.27
Duluth (1931-2006)
-26.85
0.53
-29.00
0.73
*Duluth (1951-2006)
Two Harbors (1931-1987)
-19.90
1.08
Grand Marais (1966-2006)
-10.26
1.01
Thunder Bay (1931-2006)
3.84
0.70
1.29
1.06
*Thunder Bay (1951-2006)
Rossport (1967-2006)
24.55
2.21
15.41
1.64
*Rossport (1973-2006)
Michipicoten (1931-2006)
23.39
0.52
20.63
0.71
*Michipicoten (1951-2006)
Gros Cap (1961-2006)
-2.91
2.52
-1.11
1.33
*Gros Cap (1986-2006)
Table 4-2: Summary of Lake Superior rates of apparent
vertical movement and uncertainty relative to Point Iroquois.
5. Lakes Michigan-Huron
5.1.
Overview
Lake Michigan and Lake Huron are connected by the wide and deep Straits of
Mackinac, which causes the two lakes to behave as one in terms of hydraulics. For this
reason they are considered together in this analysis. Water level data from a total of eight
gauge stations on Lake Michigan and twelve gauge stations on Lake Huron were
analyzed, for a total of 20 gauges on the two lakes combined. On Lake Michigan all
eight gauges are located within the United States, as are six of the gauges on Lake Huron.
The remaining six gauge stations on Lake Huron are located within Canada. Table 5-1
and Table 5-2 give a list of the gauge stations used in this analysis, as well as a summary
of the periods of record for each station, including any full years of data that were not
available.
18
Figure 5-1: Lakes Michigan-Huron gauge locations.
Station ID
Location
Start
Year
End
Year
Missing (Full) Years
Ludington, MI,
1898-99; 1901; 1905; 1907;
USA
1895 2006 1909-34; 1938; 1940-43; 1948-49
Holland, MI,
1898; 1901-02; 1904; 1909-34;
9087031
1894 2006
USA
1936-40; 1943-55; 1957-58
Calumet
9087044
-Harbor, IL, USA 1903 2006
Milwaukee, WI,
9087057/8 USA
1860 2006
-Kewaunee, WI,
9087068
1974 2006
-USA
Sturgeon Bay
9087072
1920-21; 1923-24; 1926
Canal, WI, USA 1905 2006
Green Bay, WI,
9087078/9 USA
1953 2006
-Port Inland, MI,
9087096
1964 2006
-USA
Table 5-1: Lake Michigan gauge stations and periods of record.
9087023
Number of Full
Years Missing
Total Years
of Data
38
74
50
63
0
104
0
147
0
33
5
97
0
54
0
43
19
Station ID
Location
9075002
9075014
9075034/5
9075059
9075080
Lakeport, MI, USA
Harbor Beach, MI,
USA
Essexville, MI, USA
Harrisville, MI, USA
Mackinaw City, MI,
USA
9075098/9
Start
Year
1955
End
Year
2006
1860
1953
1961
1899
Missing (Full)
Years
--
Number of Full
Years Missing
0
Total Years
of Data
52
2006
2006
2006
----
0
0
0
147
54
46
2006
-1897-98; 1900;
1904-33; 1937-43
0
108
40
71
0
81
0
48
0
47
2
99
0
45
0
97
De Tour, MI, USA
1896 2006
Thessalon, ON,
Canada
11070
1926 2006
-Little Current, ON,
11195
1959 2006
-Canada
Parry Sound, ON,
11375
1960 2006
-Canada
Collingwood, ON,
Canada
11500
1906 2006
1912-1913
Tobermory, ON,
11690
1962 2006
-Canada
Goderich, ON,
11860
1910 2006
-Canada
Table 5-2: Lake Huron gauge stations and periods of record.
5.2.
Harbor Beach, Michigan
The Harbor Beach gauge on Lake Huron has traditionally been selected as the outlet
of the two lakes, and was originally selected in this analysis as well. The Lakeport gauge
was also considered as it is located closer to the actual outlet at the head of the St. Clair
River. The period of record for Lakeport is much shorter than that at Harbor Beach. For
this reason Harbor Beach has traditionally been chosen as the outlet of Lakes MichiganHuron. However, the period of record at Lakeport now contains greater than 50 years of
water level data, and, as will be shown in Section 5.11, there appears to be some
movement between Lakeport and Harbor Beach. For these reasons and the close
proximity of Lakeport to the head of the St. Clair River, Lakeport was also considered
separately as the outlet of the two lakes in Section 5.23. A third gauge station, Goderich,
is also located fairly close to the actual outlet, but potential gauge issues at this location
as outlined below prevented it from being chosen.
Data at Harbor Beach begins in 1860 and continues through to 2006; however, it has
been noted by Quinn and Southam (2008) that water level data from 1860 to August 1874
is actually Milwaukee data transferred to Harbor Beach using an estimated rate of vertical
movement of 14 cm/century. It should also be noted that data from September 1874 until
March 1901 was collected from staff gauge readings, while data after this point in time
20
until present has been collected with a continually recording gauge. Both of these issues
are discussed in greater detail below.
5.3.
Ludington, Michigan
The water level record begins in 1895 at Ludington; however, until 1950 monthly
data is limited to only a handful of months, if any, having records in a given year.
Furthermore, data from this period was collected using staff gauges. Only beginning in
August 1950, after the first continuously recording gauge was installed in July of that
year, does monthly data become continually available.
The water level difference plot for Harbor Beach minus Ludington is shown in Figure
5-2. The rate of vertical movement as determined from the slope of the linear regression
line is -12.77 cm/century. However, if one omits the pre-1951 data, the rate
becomes -17.73 cm/century. For the reasons outlined above, it is suggested that this rate
is the more representative estimate.
Harbor Beach - Ludington
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1951+)
Water Level Difference (m)
0.2
0.1
0
-0.1
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-2: Harbor Beach minus Ludington water level difference plot.
Visual inspection of the plot followed by breakpoint analysis indicated a potential
breakpoint at approximately 1971. The rate of movement calculated after 1971
is -13.81 cm/century. However, the period of record in this case is likely too short, and it
21
may simply be that the variability in the plot gives the impression of a breakpoint.
Another alternative is that the breakpoint is actually a sudden drop. Evidence of a drop is
also apparent when Ludington is plotted versus other long-term Lakes Michigan-Huron
gauges, including Milwaukee (Figure 5-3), Mackinaw City and Calumet Harbor.
However, no evidence has been found to support a possible reason for either a change in
rate or a sudden jump at the Ludington station.
Milwaukee - Ludington
0.3
Jun-Sep Avg
Water Level Difference (m)
0.2
0.1
0
-0.1
1940
1950
1960
1970
1980
1990
2000
2010
Year
Figure 5-3: Milwaukee minus Ludington water level difference plot.
5.4.
Holland, Michigan
The water level record begins in 1894 at Holland; however, similar to Ludington,
monthly data is limited in the early period, and was collected using staff gauges. Only
beginning in May 1959, after the first continuously recording gauge was installed at this
location, does monthly data become continually available.
The water level difference plot for Harbor Beach minus Holland is shown in Figure
5-4. The rate of vertical movement as determined from the slope of the linear regression
line is -9.16 cm/century. If one omits the pre-1959 data, however, the rate
becomes -11.70 cm/century. For the reasons outlined above, it is suggested that this rate
is the more representative estimate. Visual inspection does not show any significant
discontinuities.
22
Harbor Beach - Holland
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1959+)
Water Level Difference (m)
0.2
0.1
0
-0.1
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-4: Harbor Beach minus Holland water level difference plot.
5.5.
Calumet Harbor, Illinois
The water level record begins in 1903 at Calumet Harbor. The water level difference
plot for Harbor Beach minus Calumet Harbor was shown previously in Figure 3-2. A
visual inspection of this plot shows what appears to be an outlier occurring in 1907. The
June to September monthly average difference in 1907 appears to disagree significantly
with the other data, and was therefore not used in this analysis. The rate of vertical
movement as determined from the slope of the linear regression line with the 1907 outlier
removed (Figure 5-5) is -12.12 cm/century, which is only slightly smaller than the rate
calculated with the outlier included (-12.92 cm/century). For the reasons outlined above,
it is suggested that the rate of -12.12 cm/century is the more reasonable estimate.
A visual inspection of the data and an additional plot of the five-year centred running
average (Figure 5-6) also indicate what appears to be a stepwise pattern to the data. As
will be shown below, this pattern is similar to other patterns observed on Lake Michigan,
particularly at Milwaukee.
23
Harbor Beach - Calumet Harbor
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-5: Harbor Beach minus Calumet Harbor water level difference plot with 1907 outlier
removed.
Harbor Beach - Calumet Harbor
0.3
June-Sep. Avg.
June-Sep. 5-Year Centred Avg.
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-6: Harbor Beach minus Calumet Harbor water level difference plot with 5-year centred
running average.
24
5.6.
Milwaukee, Wisconsin
The water level record begins in 1860 at Milwaukee. The water level difference plot
for Harbor Beach minus Milwaukee is shown in Figure 5-7. Quinn and Southam (2008)
have outlined a number of issues with this plot, which can also be seen here. First, as
was discussed in Section 5.2, the water level data at Harbor Beach for the period from
1960 to August 1874 is actually Milwaukee data transferred to Harbor Beach using an
estimated rate of vertical movement of 14 cm/century (i.e. the Harbor Beach data for this
period was not measured at Harbor Beach). This can be seen clearly by the reduced
variability during this period in Figure 5-7. Furthermore, data collected at Milwaukee
after this period until September 1903 was collected from staff gauge readings. From this
point on, except for a short period from March 1946 until July 1951, recording gauges
have been in use. The water level differences appear to show greater variability during
the staff gauge period than during the recording gauge period, at least prior to 1903. This
increase in variability is not as evident in the shorter staff gauge period that began in
March 1946.
Harbor Beach - Milwaukee
0.3
Jun-Sep Avg
1875
Linear (1904+)
1904
Water Level Difference (m)
0.2
Linear (1875+)
0.1
0
-0.1
1840
1860
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-7: Harbor Beach minus Milwaukee water level difference plot.
The early data (up until 1874) transferred from Milwaukee was discarded from this
analysis, and the average rate of apparent vertical movement was calculated for both
1875 data onwards and 1904 data onwards. The rate calculated from 1875 on, which
includes the staff gauge data, is -14.77 cm/century. The rate calculated from 1904 on,
which does not include the staff gauge data, is -16.14 cm/century.
25
A plot of the five-year centred running average for Harbor Beach minus Milwaukee
was previously shown in Figure 3-3. It indicates what appears to be a stepwise pattern to
the water level differences. Interestingly, the stepwise pattern observed at Milwaukee is
similar to the patterns seen at other gauges on Lake Michigan (Figure 5-8). The pattern
at Calumet Harbor in particular appears to be similar to that observed at Milwaukee. In
addition, Sturgeon Bay Canal and Mackinaw City, though not quite as apparent, also
appear to have steps at similar places, such as the one seen around 1960.
Harbor Beach - Lake Michigan Gauges
5-year Centred Running Average
0.3
HB-MIL
HB-CAL
HB-StB
Water Level Difference (m)
0.2
HB-MAC
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-8: Harbor Beach minus various Lake Michigan gauges 5-year centred running averages of
water level difference plots: HB = Harbor Beach; MIL = Milwaukee; CAL = Calumet Harbor; StB =
Sturgeon Bay Canal; MAC = Mackinaw City.
One possible explanation is that the Harbor Beach data is the cause of the pattern
observed; however, the pattern is not seen at gauges on Lake Huron when compared to
Harbor Beach, as would be expected if this was the case (although this might be the result
of some other issue on Lake Huron, which is discussed below). Another explanation for
the similar patterns is that the gauges, in particular Milwaukee and Calumet Harbor, are
susceptible to the same forces, whether gauge-related, meteorological or geological in
nature. For example, perhaps both gauges tend to be affected by the same storm events,
or both locations are actually falling with respect to Harbor Beach in a stepwise pattern,
as opposed to linearly, due to glacial isostatic adjustment. The proximity of the two
gauges lends credence to these possible explanations. While a detailed investigation into
the root causes of the stepwise effects is beyond the scope of this document, the pattern
highlighted should be taken into consideration in future studies, including measurement
26
programs involving Global Positioning Systems (GPS) where short-term movement rates
are important.
5.7.
Kewaunee, Wisconsin
The water level record begins in 1974 at Kewaunee. With only 33 years of data, this
is a relatively short period of record. The water level difference plot for Harbor Beach
minus Kewaunee is shown in Figure 5-9. The rate of vertical movement as determined
from the slope of the linear regression line is -11.99 cm/century; however, rates
calculated from two other four month averages (i.e. May to August and July to October)
showed significantly different results (-13.65 and -9.75 cm/century, respectively). The
regression line in Figure 5-9 is heavily influenced by the higher data occurring before
1977, and the apparent outlier occurring in 2002. With this data excluded the rate
becomes only -4.19 cm/century; however, there is no evidence to suggest any reason for
the exclusion of this data, especially given the short period of record. The short period of
record does not appear to be sufficient for estimating the rate of vertical movement in this
case, and in future years, as more data is added to the Kewaunee period of record, a better
understanding of the vertical movement occurring at this gauge may become apparent.
Harbor Beach - Kewaunee
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1970
1975
1980
1985
1990
1995
2000
2005
2010
Year
Figure 5-9: Harbor Beach minus Kewaunee water level difference plot.
27
5.8.
Sturgeon Bay Canal, Wisconsin
The water level record begins in 1905 at Sturgeon Bay Canal, but there is a gap in the
record from April 1919 to June 1927, and the gauge location was moved during this time.
Furthermore, data collected before June 1945 is staff gauge data, while data from this
month onwards is collected from a continuously recording gauge.
The water level difference plot for Harbor Beach minus Sturgeon Bay Canal is shown
in Figure 5-10. The rate of vertical movement as determined from the slope of the linear
regression line is -4.36 cm/century, indicating Sturgeon Bay Canal is falling relative to
Harbor Beach.
Harbor Beach - Sturgeon Bay Canal
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1927+)
Water Level Difference (m)
0.2
Linear (1945+)
0.1
0
-0.1
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-10: Harbor Beach minus Sturgeon Bay Canal water level difference plot.
Visual inspection of the plot, however, shows at least two additional features. First,
the early data collected from 1905 until the missing period that begins in 1919 does not
appear to be well represented by a linear regression line fitted over the entire period of
record. The rate of vertical movement with this early data excluded is -6.73 cm/century.
Second, whether this early data is included or not, there appears to be a breakpoint, which
according to the breakpoint analysis occurs in the mid to late 1940s. Since the data
collection method was changed to a recording gauge in 1945, this year was selected as
the breakpoint. The rate of vertical movement calculated with recording gauge data only
28
(1945 onwards) is -8.26 cm/century. This regression line most closely represents the
apparent vertical movement occurring at this location in recent time.
5.9.
Green Bay, Wisconsin
The water level record begins in 1953 at Green Bay. The water level difference plot
for Harbor Beach minus Green Bay is shown in Figure 5-11. The rate of vertical
movement as determined from the slope of the linear regression line is -9.37 cm/century,
indicating Green Bay is falling relative to Harbor Beach. Visual inspection of the plot
does not show any significant discontinuities.
Harbor Beach - Green Bay
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 5-11: Harbor Beach minus Green Bay water level difference plot.
5.10. Port Inland, Michigan
The water level record begins in 1964 at Port Inland. The water level difference plot
for Harbor Beach minus Port Inland is shown in Figure 5-12. The rate of vertical
movement as determined from the slope of the linear regression line is 6.03 cm/century,
indicating Green Bay is rising relative to Harbor Beach. Visual inspection of the plot
does not show any significant discontinuities.
29
Harbor Beach - Port Inland
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
Year
Figure 5-12: Harbor Beach minus Port Inland water level difference plot.
5.11. Lakeport, Michigan
The water level record begins in 1955 at Lakeport. As stated in Section 5.2, Lakeport
is located closer to the outlet of Lakes Michigan-Huron at the head of the St. Clair River
than Harbor Beach is. Harbor Beach has traditionally been chosen as the outlet of the
two lakes due to its location and longer period of record. However, since Lakeport now
has greater than 50 years of data, it was also considered as the outlet in this analysis
(Section 5.23). As more data is added to the period of record in coming years, Lakeport
may become the preferred choice to represent the Lakes Michigan-Huron outlet.
The water level difference plot for Harbor Beach minus Lakeport is shown in Figure
5-13. The data for 1976 was removed as an outlier, as there was an obvious problem
with the water level gauge at Lakeport during June and July of this year, which resulted
in unrealistic recorded water levels that were between 0.6 and 0.7 metres less than those
recorded in the months of May and August. With this year removed, the rate of vertical
movement as determined from the slope of the linear regression line is -3.19 cm/century,
indicating that Lakeport is falling relative to Harbor Beach. Visual inspection of the plot
does not show any significant discontinuities.
While the rate calculated is not large, it does indicate that Lakeport is falling relative
to Harbor Beach, and that Harbor Beach may not be as good a representation of the outlet
30
as has been previously believed. Further discussion on Lakeport as the outlet is given in
Section 5.23.
Harbor Beach - Lakeport
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 5-13: Harbor Beach minus Lakeport water level difference plot.
5.12. Essexville, Michigan
The water level record begins in 1953 at Essexville. The water level difference plot
for Harbor Beach minus Essexville is shown in Figure 5-14. The rate of vertical
movement as determined from the slope of the linear regression line is -5.58 cm/century,
indicating that Essexville is falling relative to Harbor Beach. Visual inspection of the
plot does not show any significant discontinuities.
31
Harbor Beach - Essexville
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 5-14: Harbor Beach minus Essexville water level difference plot.
5.13. Harrisville, Michigan
The water level record begins in 1961 at Harrisville. The water level difference plot
for Harbor Beach minus Harrisville is shown in Figure 5-15. The rate of vertical
movement as determined from the slope of the linear regression line is 5.48 cm/century,
indicating that Harrisville is rising relative to Harbor Beach. Visual inspection of the plot
does not show any significant discontinuities. In fact, Harbor Beach minus Harrisville
provides an excellent example of how gauges located in close proximity can show little
variance in terms of water level differences, as the plot shows that the linear regression
line appears to fit the data well.
32
Harbor Beach - Harrisville
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
Year
Figure 5-15: Harbor Beach minus Harrisville water level difference plot.
5.14. Mackinaw City, Michigan
The water level record begins in 1899 at Mackinaw City. The water level difference
plot for Harbor Beach minus Mackinaw City is shown in Figure 5-16. The rate of
vertical movement as determined from the slope of the linear regression line is
9.22 cm/century, indicating that Mackinaw City is rising relative to Harbor Beach.
Visual inspection of the plot does not show any significant discontinuities. A plot of
the 5-year centred running average does appear to indicate a stepped pattern (Figure
5-17), though not a significant one, and this observation may only be the result of
variability in the water level difference relationship.
33
Harbor Beach - Mackinaw City
0.1
Water Level Difference (m)
0
-0.1
-0.2
Jun-Sep Avg
Linear (Jun-Sep Avg)
-0.3
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-16: Harbor Beach minus Mackinaw City water level difference plot.
Harbor Beach - Mackinaw City
0.1
Water Level Difference (m)
0
-0.1
-0.2
June-Sep. Avg.
June-Sep. 5-Year Centred Avg.
-0.3
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-17: Harbor Beach minus Mackinaw City water level difference plot with 5-year centred
running average.
34
5.15. De Tour, Michigan
The water level record begins in 1896 at De Tour; however, monthly data is limited to
only a handful of months, if any, having records in a given year until 1951. Only
beginning in 1951 does monthly data become continually available. Furthermore, data
was collected using staff gauges until June 1954.
The water level difference plot for Harbor Beach minus De Tour is shown in Figure
5-18. The rate of vertical movement as determined from the slope of the linear regression
line is 14.63 cm/century. In this case omitting the pre-1954 staff gauge data made little
difference to the calculated rate (14.15 cm/century with recording gauge data only).
There do not appear to be any significant discontinuities in the plot.
Harbor Beach - De Tour
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-18: Harbor Beach minus De Tour water level difference plot.
5.16. Thessalon, Ontario
The water level record begins in 1926 at Thessalon. The water level difference plot
for Harbor Beach minus Thessalon is shown in Figure 5-19. The rate of vertical
movement as determined from the slope of the linear regression line is 18.60 cm/century,
indicating that Thessalon is rising relative to Harbor Beach.
35
Harbor Beach - Thessalon
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1961-)
Water Level Difference (m)
0.2
Linear (1980+)
0.1
0
-0.1
-0.2
1920
1940
1960
1980
2000
2020
Year
Figure 5-19: Harbor Beach minus Thessalon water level difference plot.
Visual inspection of the plot does not show any significant discontinuities; however, a
plot of the 5-year centred running average does appear to indicate a stepped pattern, or
plateau, occurring approximately during the 1960s and 1970s (Figure 5-20). Of note is
that if two regression lines are drawn both before and after the apparent plateau, the rate
of apparent vertical movement is greater than that calculated for the full data set, and
approximately equal (21.73 and 20.85 cm/century, respectively), suggesting two
approximately parallel periods of movement. However, it must also be noted that the
variability in the plot appears to be greatest during the plateau period, and that this may in
fact be the cause for any semblance of a plateau in the first place. This period should be
investigated further.
36
Harbor Beach - Thessalon
0.1
Water Level Difference (m)
0
-0.1
-0.2
June-Sep. Avg.
June-Sep. 5-Year Centred Avg.
-0.3
1920
1930
1940
1950
1960
1970
1980
1990
2000
2010
2020
Year
Figure 5-20: Harbor Beach minus Thessalon water level difference plot with 5-year centred running
average.
5.17. Little Current, Ontario
The water level record begins in 1959 at Little Current. The water level difference
plot for Harbor Beach minus Little Current is shown in Figure 5-21. The rate of vertical
movement as determined from the slope of the linear regression line is 25.93 cm/century.
However, visual inspection of the plot shows a significant jump occurring in 1963.
Tushingham (1992) reports that the Little Current gauge has been tied to an unstable
benchmark in the past. The same issue was reported by Tushingham for the Rossport
gauge on Lake Superior (Section 4.9), and the plots of both Rossport and Little Current
show a similar looking jump. While it could not be confirmed, it seems likely that the
unstable benchmark explains the observed jump at both these gauges. Like Rossport, the
observed rate of apparent vertical movement is fairly constant at Little Current after this
point in time. The rate as calculated from post-1963 data is 21.33 cm/century, which is
likely a more realistic estimate of the apparent vertical movement at this location.
37
Harbor Beach - Little Current
0.1
Water Level Difference (m)
0
-0.1
-0.2
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1963+)
-0.3
1950
1960
1970
1980
1990
2000
2010
Year
Figure 5-21: Harbor Beach minus Little Current water level difference plot.
5.18. Parry Sound, Ontario
The water level record begins in 1960 at Parry Sound. The water level difference plot
for Harbor Beach minus Parry Sound is shown in Figure 5-22. The rate of vertical
movement as determined from the slope of the linear regression line is 20.80 cm/century,
indicating that Parry Sound is rising relative to Harbor Beach. Visual inspection of the
plot does not show any significant discontinuities.
38
Harbor Beach - Parry Sound
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 5-22: Harbor Beach minus Parry Sound water level difference plot.
5.19. Collingwood, Ontario
The water level record begins in 1906 at Collingwood. The water level difference
plot for Harbor Beach minus Collingwood is shown in Figure 5-23. The rate of vertical
movement as determined from the slope of the linear regression line is 14.21 cm/century,
indicating that Collingwood is rising relative to Harbor Beach.
However, visual inspection of the plot shows some interesting features. These are
especially evident in the 5-year centred running average plot (Figure 5-24). First, the pre1916 data does not appear to fit the rest of the data plot. This may be the result of the
gauge being moved in 1915 (Coordinating Committee, 1978). Second, the assumption of
a linear trend in this case appears to be invalid. Rather, the slope of the plot appears to
shift at some point, or is possibly even curvilinear. Breakpoint analysis estimated a
breakpoint to occur in 1967 (Figure 5-25). The slope calculated prior to the breakpoint is
19.14 cm/century, while the slope calculated after the breakpoint is 7.65 cm/century.
One possible explanation is that the water level gauge at Collingwood was moved in
1965 (Coordinating Committee, 1978), although this is just one possibility, and since
similar issues are seen at Goderich (Section 5.21), additional forces may be the cause.
39
Harbor Beach - Collingwood
0.1
Water Level Difference (m)
0
-0.1
1900
1920
1940
1960
1980
2000
2020
-0.2
Jun-Sep Avg
Linear (Jun-Sep Avg)
-0.3
Year
Figure 5-23: Harbor Beach minus Collingwood water level difference plot.
Harbor Beach - Collingwood
0.1
Water Level Difference (m)
0
-0.1
-0.2
June-Sep. Avg.
June-Sep. 5-Year Centred Avg.
-0.3
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-24: Harbor Beach minus Collingwood water level difference plot with 5-year running
average.
40
Harbor Beach - Collingwood
0.1
Water Level Difference (m)
0
1967
-0.1
-0.2
June to Sep Avg.
Piecewise Regression
Estimated BreakPoint
-0.3
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-25: Harbor Beach minus Collingwood breakpoint estimation.
5.20. Tobermory, Ontario
The water level record begins in 1962 at Tobermory. The water level difference plot
for Harbor Beach minus Tobermory is shown in Figure 5-26. The rate of vertical
movement as determined from the slope of the linear regression line is 15.74 cm/century,
indicating that Tobermory is rising relative to Harbor Beach. Visual inspection of the
plot does not show any significant discontinuities, with the possible exception of a
breakpoint occurring around 1969. This point coincides approximately with the
breakpoint observed at Collingwood, although the direction is reversed. It could simply
be the result of variability in the water level difference plot, however, and due to the short
period of record it would not be prudent to make any more significant statements in this
regard.
41
Harbor Beach - Tobermory
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 5-26: Harbor Beach minus Tobermory water level difference plot.
5.21. Goderich, Ontario
The water level record begins in 1910 at Goderich; however, water level data appears
only to have been collected during the summer and fall months until 1920, although twice
during this period monthly records were kept until December. Furthermore, the gauge at
Goderich was relocated in 1919, and has been moved a number of additional times since
then (Coordinating Committee, 1978). The water level difference plot for Harbor Beach
minus Goderich is shown in Figure 5-27. The rate of vertical movement as determined
from the slope of the linear regression line is -2.97 cm/century, indicating that Goderich
is falling relative to Harbor Beach.
However, visual inspection of the plot shows some significant features. First, the pre1920 data does not appear to fit the rest of the data plot very well, which may be the
result of the gauge relocation in 1919 (although, as noted, there have been several other
gauge relocations at this site that do not show similar effects).
42
Harbor Beach - Goderich
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0.0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-27: Harbor Beach minus Goderich water level difference plot.
Second, from approximately 1966 to 1978, the variability in the water level difference
relationship appears to be significantly greater than the variability throughout the
remainder of the plot. Interestingly, this period of increased variability seems to coincide
with a similar period of interest in the Harbor Beach minus Thessalon water level
difference relationship, during which there appeared to be either a pause in movement or
the impression of a pause due to increased variability in the plot (Section 5.16). A plot of
Goderich minus Thessalon is shown in Figure 5-28. This plot shows the same increase in
variability occurring approximately during the 1970s as was seen in the plots of both
Goderich and Thessalon versus Harbor Beach. Also, while the regression line in Figure
5-28 appears to be a relatively good fit, breakpoint analysis indicated that a change in
slope of approximately 6 cm/century may have occurred sometime during the 1960s. The
breakpoint, however, is not as obvious in this case due to the greater magnitude of the
slope values calculated before and after the point in question. In fact, other plots can
show similar traits, in which breakpoint analysis estimates a point indicating what would
seem to be a significant change in slope numerically, but visual inspection of the plot
does not readily reveal the same estimated breakpoint. This is a result of the variability
in these plots, and it shows that while both visual inspection and the breakpoint analysis
can be useful in indicating a potential breakpoint, the decision as to whether a break has
actually occurred is somewhat subjective.
43
Goderich - Thessalon
0.1
Water Level Difference (m)
0
-0.1
-0.2
Jun-Sep Avg
Linear (Jun-Sep Avg)
-0.3
1920
1940
1960
1980
2000
2020
Year
Figure 5-28: Goderich minus Thessalon water level difference plot.
Finally, similar to observations made at the Collingwood gauge, the assumption of a
linear trend in the case of Goderich appears to be invalid. Rather, the slope of the plot
appears to shift at some point, or is possibly even curvilinear. As shown earlier in
Section 3.5, breakpoint analysis estimated a breakpoint to occur in 1955. Considering the
variability in the plots and the inexact nature of identifying the breakpoint, this value for
Goderich is not far removed from the breakpoint of 1967 identified at Collingwood
(Figure 5-25). The slope calculated in the Harbor Beach minus Goderich plot prior to the
breakpoint is 1.22 cm/century, while the slope calculated after the breakpoint
is -6.51 cm/century, indicating that Goderich went from rising relative to Harbor Beach,
to falling relative to the lake’s outlet.
One possible explanation is that salt mining, which began in the late 1950s at
Goderich, may be causing the land in the area to subside. However, this explanation
would apply to Goderich only, and since similar issues are seen at Collingwood, this may
indicate that additional forces are the cause. A plot of Goderich minus Collingwood
water levels tends to support this contention (Figure 5-29). The slope of the linear
regression line is 17.21 cm/century, and as can be seen from the plot, a breakpoint is not
evident. In fact, the slopes of linear regression lines calculated before and after a
breakpoint of 1955 are within 1 cm/century of each other. This implies that not only do
both Collingwood and Goderich show a breakpoint occurring at approximately the same
time in the linear regression plot when compared to Harbor Beach, but also the change in
44
the slope value after the breakpoint is approximately equal (i.e. the slope of the plot
decreases by approximately the same amount in both cases). A plot of Goderich minus
Sturgeon Bay Canal (not shown) showed a similar result.
Goderich - Collingwood
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
-0.2
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-29: Goderich minus Collingwood water level difference plot.
Unfortunately the periods of records at other nearby gauges, such as Tobermory and
Parry Sound for example, are not long enough to lend any further evidence to support a
shift in the rate of apparent vertical movement. Nevertheless, as one last example to
further illustrate the complexity inherent in this analysis, Figure 5-30 shows a plot of
Milwaukee minus Goderich water levels. As can be seen, there does not appear to be a
noticeable breakpoint, and in fact breakpoint analysis did not indicate any significant
changes in slope. A stepwise pattern, however, similar to those seen in previous
Milwaukee plots, is somewhat evident, as is larger year-to-year variability.
45
Goderich - Milwaukee
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 5-30: Goderich minus Milwaukee water level difference plot.
5.22. Summary
Water level difference plots between gauge pairs on Lakes Michigan-Huron tended to
be more complex than those plots observed on Lake Superior. While water level
differences at some gauge pairs follow a mostly linear relationship, others, including
Milwaukee and Calumet Harbor, appear to follow a more stepwise pattern. Others, such
as Goderich and Collingwood, appear to show a shift in the rate of apparent vertical
movement. Possible explanations include climate factors, mining activities, and actual
geologic forces among others. Yet even when an explanation seems plausible (e.g. salt
mining at Goderich as the cause of a change in rate of apparent vertical movement
between Harbor Beach and Goderich), evidence from another gauge pair can be
contradictory (e.g. Goderich minus Thessalon and Goderich minus Collingwood do not
show the same change). While a full investigation of these issues is beyond the scope of
this document, what this demonstrates is that our understanding of the underlying forces
behind these observations is still incomplete.
A summary of the calculated rates of apparent vertical movement for Lakes
Michigan-Huron relative to Harbor Beach are shown in Table 5-3. The uncertainty
(standard error of the slope value) is also shown, as are additional scenarios as described
in previous sections.
46
Gauge (Period of Record)
Rate
Standard Error
*Additional Scenarios
(cm/century)
(cm/century)
Ludington (1897-2006)
-12.77
0.77
-17.73
0.83
*Ludington (1951-2006)
-13.81
1.55
*Ludington (1971-2006)
Holland (1906-2006)
-9.16
0.58
-11.70
0.92
*Holland (1959-2006)
Calumet Harbor (1903-2006)
-12.92
0.81
-12.12
0.51
*Calumet Harbor (w/o 1907 outlier)
Milwaukee (1860-2006)
-14.33
0.30
-14.77
0.36
*Milwaukee (1875-2006)
-16.14
0.41
*Milwaukee (1904-2006)
Kewaunee (1974-2006)
-11.99
2.82
-4.19
1.54
*Kewaunee (1977-2006 & w/o 2002 outlier)
Sturgeon Bay Canal (1905-2006)
-4.36
0.44
-6.73
0.48
*Sturgeon Bay Canal (1927-2006)
-8.26
0.57
*Sturgeon Bay Canal (1945-2006)
Green Bay (1955-2006)
-9.37
0.93
Port Inland (1965-2006)
6.03
1.21
Lakeport (1956-2006)
-3.19
0.67
Essexville (1953-2006)
-5.58
0.71
Harrisville (1962-2006)
5.48
0.52
Mackinaw City (1899-2006)
9.22
0.32
De Tour (1934-2006)
14.63
0.53
Thessalon (1927-2006)
18.60
0.41
21.73
0.90
*Thessalon (1927-1961)
20.85
1.60
*Thessalon (1980-2006)
Little Current (1960-2006)
25.93
1.73
21.33
1.03
*Little Current (1963-2006)
Parry Sound (1960-2006)
20.80
0.94
Collingwood (1906-2006)
14.21
0.36
19.14
0.63
*Collingwood (1916-1967)
7.65
1.10
*Collingwood (1967-2006)
Tobermory (1962-2006)
15.74
1.08
Goderich (1910-2006)
-2.97
0.41
1.22
0.88
*Goderich (1910-1955)
-6.51
1.10
*Goderich (1955-2006)
Table 5-3: Summary of Lakes Michigan-Huron rates of apparent vertical
movement and uncertainty relative to Harbor Beach.
5.23. Outlet at Lakeport
As discussed in Sections 5.2 and 5.11, Lakeport may be a better choice of outlet for
Lakes Michigan-Huron than Harbor Beach, which has traditionally been chosen as the
outlet. Lakeport is located closer to the true outlet at the head of the St. Clair River and
47
according to the water level difference plots Lakeport appears to be falling relative to
Harbor Beach at a rate of -3.19 cm/century. In addition, water levels have now been
recorded at Lakeport for more than 50 years, which is likely a sufficient period of record
for calculating rates of apparent vertical movement, at least for the most recent time
period.
For these reasons, the rates of apparent vertical movement were also calculated with
Lakeport chosen as the outlet of Lakes Michigan-Huron. Relative rates of movement
with respect to both Harbor Beach and Lakeport are shown in Table 5-4 for comparison
purposes. The rates shown for Harbor Beach were calculated for the same period of
record as those for Lakeport. The period of record begins in 1955 at Lakeport, but the
June to September averages only begin in 1956.
Rate of Movement
Difference
(cm/century)
Net Sum
Gauge (Period of Record)
in Rate
(Diff. – (HB-LP rate))
Harbor
(cm/cent.)
Lakeport
Beach
-16.30
-13.11
-3.19
0.00
Ludington (1956-2006)
-11.25
-7.54
-3.71
-0.52
Holland (1956-2006)
-12.91
-9.86
-3.05
0.14
Calumet Harbor (1956-2006)
-17.92
-14.59
-3.33
-0.14
Milwaukee (1956-2006)
-11.99
-6.55
-5.44
-2.25
Kewaunee (1974-2006)
-7.99
-4.37
-3.62
-0.43
Sturgeon Bay Canal (1956-2006)
-9.68
-6.24
-3.44
-0.25
Green Bay (1956-2006)
6.03
9.99
-3.96
-0.77
Port Inland (1965-2006)
-5.78
-2.55
-3.23
-0.04
Essexville (1956-2006)
5.48
8.20
-2.72
0.47
Harrisville (1962-2006)
10.21
13.72
-3.51
-0.32
Mackinaw City (1956-2006)
14.57
17.81
-3.24
-0.05
De Tour (1956-2006)
19.14
22.50
-3.36
-0.17
Thessalon (1956-2006)
25.93
28.91
-2.98
0.21
Little Current (1960-2006)
20.80
23.57
-2.77
0.42
Parry Sound (1960-2006)
9.04
12.11
-3.07
0.12
Collingwood (1956-2006)
15.74
18.20
-2.46
0.73
Tobermory (1962-2006)
-6.27
-2.77
-3.50
-0.31
Goderich (1956-2006)
Average
-3.37
-0.18
Table 5-4: Comparison of rates of movement on Lakes Michigan-Huron relative to Harbor Beach
and Lakeport.
The key point to note from this table is the differences between Harbor Beach and
Lakeport and the net sum of the differences. Note that if one triangulates between any
three gauges on a given lake, the net sum of the differences moving around the triangle
should be zero. For example, the difference between the rates of movement between
Ludington and Harbor Beach and Ludington and Lakeport should theoretically be equal
to the rate of movement between Harbor Beach and Lakeport, which was calculated
as -3.19 cm/century in Section 5.11. In the case of Ludington the net sum is in fact zero.
However, due to uncertainty in the calculated rates of apparent vertical movement, the
48
net sum does not end up being exactly zero at all gauges. Nevertheless, the differences
between the rates calculated relative to Harbor Beach and those calculated relative to
Lakeport are all fairly close to each other, and the average difference in rates is equal
to -3.37 cm/century, which is quite close to the rate of movement of -3.19 cm/century
calculated between Harbor Beach and Lakeport directly.
These results provide further evidence that Lakeport is falling relative to Harbor
Beach at a rate of approximately -3 cm/century or so. Again, since the period of record at
Lakeport is now greater than 50 years, and since Lakeport is located much closer to the
actual lake outlet, it should be given consideration for use as the outlet in future studies.
Many of the issues and scenarios described in previous sections for Harbor Beach apply
to Lakeport as well, so a full treatment of these is not given again here, but the rates of
movement calculated for Lakeport and their uncertainty is provided in Table 5-5 for each
gauge and a selection of similar scenarios to those outlined for Harbor Beach. Note that
some scenarios investigated for Harbor Beach are not included since they occur prior to
the start of the Lakeport period of record, or only shortly thereafter.
Standard
Error
(cm/century)
Ludington (1956-2006)
-13.11
1.10
*Ludington (1971-2006)
-7.81
1.39
Holland (1956-2006)
-7.54
1.07
*Holland (1959-2006)
-8.07
1.08
Calumet Harbor (1956-2006)
-9.86
1.13
Milwaukee (1956-2006)
-14.59
1.05
Kewaunee (1974-2006)
-6.55
2.81
*Kewaunee (1977-2006 & w/o 2002 outlier)
-1.16
1.95
Sturgeon Bay Canal (1956-2006)
-4.37
1.01
Green Bay (1956-2006)
-6.24
1.09
Port Inland (1965-2006)
9.99
1.40
Essexville (1956-2006)
-2.55
0.85
Harrisville (1962-2006)
8.20
1.12
Mackinaw City (1956-2006)
13.72
0.99
De Tour (1956-2006)
17.81
1.08
Thessalon (1956-2006)
22.50
1.29
*Thessalon (1980-2006)
24.46
2.57
Little Current (1960-2006)
28.91
2.02
*Little Current (1963-2006)
24.27
1.50
Parry Sound (1960-2006)
23.57
1.29
Collingwood (1956-2006)
12.11
1.02
*Collingwood (1967-2006)
12.18
1.45
Tobermory (1962-2006)
18.20
1.60
Goderich (1956-2006)
-2.77
1.32
Table 5-5: Summary of Lakes Michigan-Huron rates of apparent vertical
movement and uncertainty relative to Lakeport.
Gauge (Period of Record)
*Additional Scenarios
Rate
(cm/century)
49
6. Lake Erie
6.1.
Overview
Water level data from a total of 16 gauge stations were analyzed on Lake Erie (Figure
6-1). Six of these gauge stations are located in Canada, while the remaining ten gauge
stations are located in the United States. Table 6-1 gives a list of the gauge stations used
in this analysis, as well as a summary of the periods of record for each station.
Figure 6-1: Lake Erie gauge locations.
Station
ID
9063020
9063028
9063032
9063038
9063053
9063063
9063079
9063085
9063087
Location
Buffalo, NY, USA
Sturgeon Point, NY, USA
Barcelona, NY, USA
Erie, PA, USA
Fairport, OH, USA
Cleveland, OH, USA
Marblehead, OH, USA
Start
Year
1860
1869
1960
1958
1975
1860
1959
End
Year
2006
2006
1987
2006
2006
2006
2006
Missing (Full)
Years
1870-1886
------1878-1903;
1909-10
--
Toledo, OH, USA
1877 2006
Monroe, MI, USA
1975 1988
Fermi Power Plant, MI,
9063090 USA
1963 2006
-12005
Bar Point, ON, Canada
1966 2006
-12065
Kingsville, ON, Canada
1962 2006
-12250
Erieau, ON, Canada
1957 2006
-12400
Port Stanley, ON, Canada
1908 2006
1912-1925
12710
Port Dover, ON, Canada
1958 2006
-Port Colborne, ON,
12865
1911 2006
-Canada
Table 6-1: Lake Erie gauge stations and periods of record.
Number of Full
Years Missing
17
0
0
0
0
0
0
Total Years
of Data
130
138
28
49
32
147
48
28
0
102
14
0
0
0
0
14
0
44
41
45
50
85
49
0
96
50
6.2.
Buffalo, New York
The Buffalo gauge was selected as the outlet of Lake Erie. The Buffalo gauge is
located close to the head of the Niagara River, has traditionally been used as the outlet
and has a relatively long period of record. Buffalo data begins in 1860, but early data is
incomplete and only available during the months of April to December until 1870, and
from 1871 until 1887 there is a gap in which no data is available. Data only becomes
continuously available from 1887 onwards. Moreover, data up until 1899 is staff gauge
data, while continuously recording gauges were only used from this point onwards.
6.3.
Sturgeon Point, New York
The water level record begins in 1969 at Sturgeon Point. With only 38 years of data,
this is a relatively short period of record. The water level difference plot for Buffalo
minus Sturgeon Point is shown in Figure 6-2. The rate of vertical movement as
determined from the slope of the linear regression line is 1.14 cm/century, indicating that
Sturgeon Point is rising slightly relative to Buffalo. However, the short period of record
certainly affects the accuracy of the estimate, and in future years, as more data is added to
the Sturgeon Point period of record, a better understanding of the vertical movement
occurring at this gauge may become apparent.
Buffalo - Sturgeon Point
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
Year
Figure 6-2: Buffalo minus Sturgeon Point water level difference plot.
51
6.4.
Barcelona, New York
The water level record begins in 1960 at Barcelona, but ends in 1987. As was the
case at Sturgeon Point, with only 28 years of data, this is a relatively short period of
record. The water level difference plot for Buffalo minus Barcelona is shown in Figure
6-3. The rate of vertical movement as determined from the slope of the linear regression
line is -6.43 cm/century, indicating that Barcelona is falling relative to Buffalo. However,
rates calculated for other combinations of months were significantly different (for
example, the rate calculated taking the annual average of all monthly data
was -3.49 cm/century). The short period of record certainly affects the accuracy of the
estimate, and unfortunately, the Barcelona gauge has been discontinued, so it is unlikely
that additional water level data will be added to the period of record. Since the calculated
rate of apparent vertical movement appears questionable for these reasons, the inclusion
of Barcelona should be questioned, if not omitted, in any additional studies on glacial
isostatic adjustment.
Buffalo - Barcelona
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
Year
Figure 6-3: Buffalo minus Barcelona water level difference plot.
6.5.
Erie, Pennsylvania
The available water level record begins in 1958 at Erie, Pennsylvania. The water
level difference plot for Buffalo minus Erie is shown in Figure 6-4. The rate of vertical
52
movement as determined from the slope of the linear regression line is -7.69 cm/century,
indicating that Erie is falling relative to Buffalo. Visual inspection of the plot shows a
break in the available data in 1984, which also gives an illusion of a possible step or drop
at this point. A plot of the July to October averages, however, does not show evidence of
this same issue.
Buffalo - Erie, PA
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 6-4: Buffalo minus Erie, PA, water level difference plot.
6.6.
Fairport, Ohio
The available water level record begins in 1975 at Fairport. The water level
difference plot for Buffalo minus Fairport is shown in Figure 6-5. The rate of vertical
movement as determined from the slope of the linear regression line is -21.81 cm/century,
indicating that Fairport is falling relative to Buffalo.
However, it has been brought to the attention of the Coordinating Committee that
both the water level gauge and the benchmark at Fairport are subsiding. It has been
suggested that this may be the result of salt mining in the area. Regardless of the
underlying cause, the rate of movement at Fairport is far greater than what would be
expected when compared to other Lake Erie water level gauges. For these reasons, it is
suggested that Fairport be omitted from further analysis of glacial isostatic adjustment
rates.
53
Buffalo - Fairport
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1970
1980
1990
2000
2010
Year
Figure 6-5: Buffalo minus Fairport water level difference plot.
6.7.
Cleveland, Ohio
The available water level record begins in 1860 at Cleveland; however, data collected
up until November 1903 is staff gauge data (Coordinating Committee, 1987). It is
evident in the water level difference plot for Buffalo minus Cleveland that the variability
in the staff gauge data collected prior to 1903 is greater than the variability in the data
collected with a recording gauge after this year (Figure 6-6). The rate of vertical
movement as determined from the slope of the linear regression line for the full period of
record is -9.01 cm/century, indicating that Cleveland is falling relative to Buffalo.
Alternatively, if the staff gauge data is excluded and a linear regression line is fit to the
post-1903 data only, the rate of apparent vertical movement is only -7.09 cm/century.
Moreover, removal of what appears to be an outlier in 1905 causes the rate to decline
slightly more to -6.69 cm/century. Since the Buffalo gauge was moved in 1911
(Coordinating Committee, 1987), removal of at least this outlier seems reasonable.
54
Buffalo - Cleveland
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (Post-1903)
Water Level Difference (m)
0.2
0.1
0
-0.1
1840
1860
1880
1900
1920
1940
1960
1980
2000
2020
Year
Figure 6-6: Buffalo minus Cleveland water level difference plot.
In addition, note that the Cleveland gauge was moved several times during the past
century of data collection. Though it can not be said with any certainty, this may be
partly the cause of what appears to be a stepwise pattern to the water level difference plot
as shown in Figure 6-7. There appear to be a number of sudden drops, interspersed with
longer periods of relative stability between the two gauges. While this impression may
simply be the result of variability in the plot, it also lends further evidence to the
suggestion that, despite the assumptions of this and past studies, the apparent vertical
movement between gauge pairs may not be linear.
55
Buffalo - Cleveland
0.3
June to Sep. Avg.
5-year Centred
Running Avg.
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 6-7: Buffalo minus Cleveland water level difference plot with 5-year centred running average.
6.8.
Marblehead, Ohio
The available water level record begins in 1959 at Marblehead. The water level
difference plot for Buffalo minus Marblehead is shown in Figure 6-8. The rate of vertical
movement as determined from the slope of the linear regression line is -6.84 cm/century,
indicating that Marblehead is falling relative to Buffalo. There do not appear to be any
significant discontinuities in the water level difference plot.
56
Buffalo - Marblehead
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 6-8: Buffalo minus Marblehead water level difference plot.
6.9.
Toledo, Ohio
The water level record at Toledo begins in 1877, but up until 1911 only a handful of
measurements are available at this location. Only after 1911 does the water level record
at Toledo become more or less continuously available. In addition, recording gauges
were installed at Toledo beginning in 1940, with staff gauges used prior to this time
(Coordinating Committee, 1987). Furthermore, until 1939, the gauge was located on the
Maumee River approximately 6 kilometres upstream of the current location, which is still
on the river, but near the mouth and Maumee Bay. Shorter relocations have also taken
place at both of these main gauge locations.
The water level difference plot for Buffalo minus Toledo is shown in Figure 6-9. The
rate of vertical movement as determined from the slope of the linear regression line
is -5.93 cm/century, indicating that Toledo is falling relative to Buffalo. If the pre-1940
staff gauge data is omitted, which is also the data collected prior to moving the gauge
closer to Maumee Bay, the calculated rate is -6.76 cm/century.
57
Buffalo - Toledo
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1964+)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 6-9: Buffalo minus Toledo water level difference plot.
A significant amount of variability can be observed in the water level difference plot
of Buffalo minus Toledo. In addition, although the recording gauge data collected from
1940 on is more complete than the staff gauge data collected prior to this date, unlike
some other gauges noted previously (e.g. Milwaukee vs. Harbor Beach), it is difficult to
tell whether there is significantly more noise in the staff gauge data than in the recording
gauge data. This could be the result the gauge being located on the Maumee River prior
to 1940. Furthermore, Lake Erie, due primarily to its geographic orientation, size and
depth, is known to be the Great Lake whose recorded water levels are most affected by
climate-related events. For example, it is not uncommon for sustained heavy winds to
cause water levels measured at one side of Lake Erie to be different from water levels
measured at the opposite side by a metre or more. Water levels at Toledo and Buffalo,
which are located on opposite ends of the lake, would therefore be highly affected by
such climatic events. An example is shown in Figure 6-10, which compares plots of
water level differences between Toledo and Fermi Power Plant with those of Toledo and
Erie, PA. As can be seen, because of their close proximity, the Toledo minus Fermi
Power Plant plot of water level differences shows less variability and noise than the plot
of Toledo minus Erie, PA, which are located further apart. Though the months of June
through September were chosen to try and minimize these issues, they cannot be avoided
entirely, and this would partly explain the higher degree of variability seen in these water
level difference plots than in those of other gauge pairs.
58
Toledo - Fermi Power Plant vs. Toledo - Erie, PA
June to September Average
0.3
Toledo - Fermi P.P.
Toledo - Erie, PA
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 6-10: Comparison of Toledo minus Fermi Power Plant and Toledo minus Erie, PA water level
difference plots.
The high degree of variability in the plot makes it difficult to be confident in the
estimated rate of apparent vertical movement. The variability may also be responsible for
what appears to be a change in slope, which according to breakpoint analysis, occurred
around 1977 (Figure 6-11). Although only recording gauge data is shown in Figure 6-11,
the breakpoint was also estimated at 1977 when staff gauge data was included.
However, the gauge was relocated in 1964, with the intake located downstream
approximately 50 feet from the previous location (Coordinating Committee, 1987). This
is one possible cause for the apparent change in slope. The rate of apparent vertical
movement calculated with only post-1964 data is -12.64 cm/century.
In any case, the rate of apparent vertical movement between Buffalo and Toledo
remains difficult to estimate due to noise in the data, coupled with a number of gauge
relocations, and the change of data collection method from staff gauge to recording gauge
readings.
59
Buffalo - Toledo
0.3
June to Sep. Avg.
Piecewise Regression
Estimated Breakpoint
Water Level Difference (m)
0.2
0.1
1977
0
-0.1
1930
1940
1950
1960
1970
1980
1990
2000
2010
Year
Figure 6-11: Buffalo minus Toledo breakpoint estimation for recording gauge period.
6.10. Monroe, Michigan
According to the Coordinating Committee (1987), water levels started being collected
in 1859 at Monroe; however, water levels were only collected in fragmented intervals,
with large gaps in between. More importantly, published water levels are now only
available from September 1975 until November 1988. With only 13 years of complete
data available, this is a very short period of record and likely far too short for estimating
rates of apparent vertical movement at this location. Nevertheless, the water level
difference plot for Buffalo minus Monroe is shown in Figure 6-12. The rate of vertical
movement as determined from the slope of the linear regression line is -25.11 cm/century,
indicating that Monroe is falling relative to Buffalo. However, rates calculated for other
combinations of months were significantly different (for example, the rate calculated
taking the annual average of all monthly data was -75.33 cm/century). Furthermore, both
of these calculated rates far exceed those calculated for other gauges near Monroe, or
anywhere else on Lake Erie for that matter, and these rates are well beyond those which
can reasonably be expected due to glacial isostatic adjustment. The short period of
record certainly affects the accuracy of the estimate, and unfortunately, the Monroe gauge
has been discontinued, so it is unlikely that additional water level data will be added to
the period of record. Since the calculated rate of apparent vertical movement appears
questionable for these reasons, Monroe should be omitted from further analysis.
60
Buffalo - Monroe
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1970
1975
1980
1985
1990
Year
Figure 6-12: Buffalo minus Monroe water level difference plot.
6.11. Fermi Power Plant, Michigan
Water level data has been collected since 1963 at Fermi Power Plant. The water level
difference plot for Buffalo minus Fermi Power Plant is shown in Figure 6-13. The rate of
vertical movement as determined from the slope of the linear regression line
is -10.10 cm/century, indicating that Fermi Power Plant is falling relative to Buffalo.
This rate seems reasonable based on the location of this gauge, the calculated rates at
other gauges on Lake Erie, and the adequate length of the period of record. Visual
inspection of the plot does not reveal any significant discontinuities.
61
Buffalo - Fermi Power Plant
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
Year
Figure 6-13: Buffalo minus Fermi Power Plant water level difference plot.
6.12. Bar Point, Ontario
The water level record begins in 1966 at Bar Point. The water level difference plot
for Buffalo minus Bar Points is shown in Figure 6-14. The rate of vertical movement as
determined from the slope of the linear regression line is -18.64 cm/century, indicating
that Bar Point is falling relative to Buffalo. However, this rate was greater than the rates
calculated using other combinations of months, including that calculated taking the
annual average of all monthly data, which had the slowest calculated rate
of -15.01 cm/century. Regardless, these rates are all greater than that which would
reasonably be expected due to glacial isostatic adjustment alone.
These disparities are in part due to the differences in available data in any given year.
Specifically, if data for a certain month is missing in a given year, then an average
including that missing month cannot be calculated for that year, while an average that
does not include the missing month can be calculated and is used in the regression
analysis. For example, if for some reason only the month of June is missing in a certain
year, then the four-month May to August and June to September averages can not be
calculated for this year. On the other hand, the July to October four-month average could
be calculated. A water level difference value for this year would therefore be included
for July to October but not for the other two four-month averages, and if for some reason
this year were to have a significant influence on the slope of the regression line, it would
62
cause a significant discrepancy between the calculated rates of vertical movement. To
account for this, only concurrent data (i.e. only years where all combinations of monthly
averages can be calculated) can be used. When this was done for Bar Point, the
calculated rates of apparent vertical movement showed somewhat better agreement.
Differences in the calculated rates, however, do still remain, and these remaining
disparities are a reflection of the short period of record and the high degree of variability
in the water level differences between Bar Point and Buffalo, which result at least in part
due to meteorological-related factors similar to those described for Toledo above.
Buffalo - Bar Point
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
Year
Figure 6-14: Buffalo minus Bar Point water level difference plot.
6.13. Kingsville, Ontario
The water level record begins in 1962 at Kingsville. The water level difference plot
for Buffalo minus Kingsville is shown in Figure 6-15. The rate of vertical movement as
determined from the slope of the linear regression line is -9.80 cm/century, indicating that
Kingsville is falling relative to Buffalo. Despite the variability in the plot, this rate is
similar to rates calculated for other combinations of months. Furthermore, there do not
appear to be any significant discontinuities in the water level difference plot.
63
Buffalo - Kingsville
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
Year
Figure 6-15: Buffalo minus Kingsville water level difference plot.
6.14. Erieau, Ontario
The water level record begins in 1957 at Erieau. The water level difference plot for
Buffalo minus Erieau is shown in Figure 6-16. The rate of vertical movement as
determined from the slope of the linear regression line is -6.31 cm/century, indicating that
Erieau is falling relative to Buffalo. The rate calculated for other combinations of months
varies somewhat (for example, the rate calculated for the May to August average
is -7.18 cm/century), again the result of variability in the plot. Otherwise, there do not
appear to be any significant discontinuities in the water level difference plot.
64
Buffalo - Erieau
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 6-16: Buffalo minus Erieau water level difference plot.
6.15. Port Stanley, Ontario
The water level record begins in 1908 at Port Stanley; however, data was collected
only in the summer months until 1911, and no data was collected from November 1911
until collection was resumed in June 1926. The gauge was also relocated during this
period.
The water level difference plot for Buffalo minus Port Stanley is shown in Figure
6-17. The pre-1926 period clearly does not fit well with the remaining data. The rate of
vertical movement as determined from the slope of the linear regression line
is -2.14 cm/century if pre-1926 data is included, and -3.47 cm/century if only data from
1926 onwards is used. The latter rate seems to be the more acceptable of the two,
especially for the current period. Either way, these rates indicate that Port Stanley,
similar to nearly all other stations on Lake Erie, is falling relative to Buffalo.
Also of note is that the Port Stanley gauge is susceptible to silting issues, and is
scheduled for relocation in the near future; however, these issues are mainly a concern
with instantaneous readings (Carol Robinson, personal communication, 2008). Silting
issues should not, therefore, have a significant impact on the monthly mean data used in
this study.
65
Buffalo - Port Stanley
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1926+)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 6-17: Buffalo minus Port Stanley water level difference plot.
6.16. Port Dover, Ontario
The water level record begins in 1958 at Port Dover. The water level difference plot
for Buffalo minus Port Dover is shown in Figure 6-18. The rate of vertical movement as
determined from the slope of the linear regression line is -1.82 cm/century, indicating that
Port Dover is falling relative to Buffalo. Other than a few periods of missing data, there
do not appear to be any significant discontinuities in the water level difference plot.
66
Buffalo - Port Dover
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 6-18: Buffalo minus Port Dover water level difference plot.
6.17. Port Colborne, Ontario
The water level record begins in 1911 at Port Colborne. The water level difference
plot for Buffalo minus Port Colborne is shown in Figure 6-19. The rate of vertical
movement as determined from the slope of the linear regression line is -5.11 cm/century,
indicating that Port Colborne is falling relative to Buffalo.
The 5-year centred average plot indicates what appears to be a stepped pattern to the
water level differences (Figure 6-20). In addition to the stepped pattern, it appears that
any apparent vertical movement occurring in the past has been nearly stopped or
suspended since approximately 1979 (the rate calculated from 1979 onwards is
0.34 cm/century). With Buffalo and Port Colborne being located only approximately
30 kilometres apart, one would expect such a slow rate of apparent vertical movement
between these two locations. In addition, their close proximity would suggest there
should be less variability in the plot. While this does appear to be true after
approximately 1979, it does not appear to be evident prior to this time.
The observed stepped pattern is similar to that seen at Cleveland, and less so to a
pattern seen at Port Stanley (Figure 6-21). The only other long-term water level gauge
on Lake Erie is Toledo, but this gauge does not provide much additional information or
an accurate comparison due to the issues at Toledo already discussed in Section 6.9.
67
Buffalo - Port Colborne
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 6-19: Buffalo minus Port Colborne water level difference plot.
Buffalo - Port Colborne
0.3
June to Sep. Avg.
5-year Running Avg.
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 6-20: Buffalo minus Port Colborne water level difference plot with 5-year centred running
average.
68
Buffalo - Lake Erie Gauges
5-year Centred Running Average
0.3
Buf.-Cleveland
Buf.-Pt. Colborne
Buf.-Pt. Stanley
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 6-21: Buffalo minus various Lake Erie gauges 5-year centred running averages of water level
difference plots.
There is little evidence suggesting why such a pattern exists. The Buffalo gauge has
been located at the same location since 1911, while the Port Colborne gauge has been
moved twice (Coordinating Committee, 1987); the first move occurred in 1925, when the
gauge was moved approximately one kilometre from within the Old Welland Canal to
closer to the lake itself; the second relocation occurred in 1964, but was only a short
distance from the previous location. While the first relocation potentially explains what
appears to be a step occurring around the same time, neither relocation explains what
appears to be a more significant step occurring sometime between 1940 and 1960, nor do
they explain why Port Colborne has gone from appearing to be falling relative to Buffalo
to appearing to be relatively stable.
Lastly, Figure 6-22 shows a comparison of Buffalo minus Port Dover and Buffalo
minus Port Colborne water level difference plots. Port Colborne has a longer period of
record than Port Dover, and the rate of movement calculated for the entire period of
record at Port Colborne is much different than the rate calculated for the entire period of
record at Port Dover, which only begins in 1958. After 1958, however, during which
data was collected at both locations, the water level differences appear to be quite similar.
Due to the relatively close proximity of the two gauges, this is expected, and it again
illustrates the importance of the period of record length on the calculated rates of
apparent vertical movement between gauge pairs.
69
Buffalo - Port Dover
vs.
Buffalo - Port Colborne
0.3
Port Dover (1958+)
Port Colborne (1958+)
Port Colborne (p-o-r)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 6-22: Comparison of Buffalo minus Port Dover and Buffalo minus Port Colborne water level
difference plots.
6.18. Summary
Similar to Lakes Michigan-Huron, water level difference plots between gauge pairs
on Lake Erie are often complicated. While water level differences at some gauge pairs
follow a mostly linear relationship, others, such as Cleveland and Port Colborne, appear
to follow a more stepwise pattern. In addition, the meteorological impacts on Lake Erie
lead to significant variability and noise in the water level difference plots. This
variability is mostly the result of meteorological factors, such as wind events, which
cause significant short-term differences in water levels between different areas of the lake,
in particular between the eastern and western ends. This variability increases the
uncertainty of the slope estimates, and can also cause inconsistencies to be observed in
the average water level difference plots. The variability can also potentially either hide
or exaggerate inconsistencies caused by other possible factors, further complicating the
analysis.
A summary of the calculated rates of apparent vertical movement for Lake Erie are
shown in Table 6-2. The uncertainty as given by the standard error of the slope value is
also shown, as are the additional scenarios as described in previous sections.
70
Gauge (Period of Record)
Rate
Standard Error
*Additional Scenarios
(cm/century)
(cm/century)
Sturgeon Point (1969-2006)
1.14
0.79
Barcelona (1961-1987)
-6.43
3.09
Erie, PA (1958-2006)
-7.69
1.02
Fairport (1975-2006)
-21.81
1.63
Cleveland (1860-2006)
-9.01
0.36
-7.09
0.44
*Cleveland (1903-2006)
-6.69
0.37
*Cleveland (1903-2006 & w/o 1905 outlier)
Marblehead (1959-2006)
-6.84
1.35
Toledo (1906-2006)
-5.93
0.74
-6.76
1.19
*Toledo (1940-2006)
-12.64
1.84
*Toledo (1964-2006)
Monroe (1976-1988)
-25.11
9.87
Fermi Power Plant (1964-2006)
-10.10
1.72
Bar Point (1966-2006)
-18.64
2.30
Kingsville (1962-2006)
-9.80
1.74
Erieau (1958-2006)
-6.31
1.34
Port Stanley (1909-2006)
-2.14
0.65
-3.47
0.62
*Port Stanley (1926-2006)
Port Dover (1958-2006)
-1.82
0.91
Port Colborne (1912-2006)
-5.11
0.38
0.34
1.18
*Port Colborne (1979-2006)
Table 6-2: Summary of Lake Erie rates of apparent vertical movement and
uncertainty relative to Buffalo.
7. Lake Ontario
7.1.
Overview
Water level data from a total of ten gauge stations were analyzed on Lake Ontario
(Figure 7-1). Six of these gauge stations are located in Canada, while the remaining four
gauge stations are located in the United States. Table 7-1 gives a list of the gauge
stations used in this analysis, as well as a summary of the periods of record for each
station.
71
Figure 7-1: Lake Ontario gauge locations.
Station ID
Location
Start
Year
End
Year
Missing
(Full) Years
1899-1913;
1915
-1908-34;
1936-52
--
Cape Vincent, NY,
USA
1898 2006
Oswego, NY, USA
1860 2006
Rochester, NY,
9052058
1860 2006
USA
9052076
Olcott, NY, USA
1967 2006
Port Weller, ON,
Canada
13030
1929 2006
1932-1955
Port Dalhousie,
13040
1910 1956
-ON, Canada
Burlington, ON,
Canada
13150
1970 2006
-Toronto, ON,
13320
1906 2006
-Canada
Cobourg, ON,
13590
1956 2006
-Canada
Kingston, ON,
Canada
13988
1909 2006
-Table 7-1: Lake Ontario gauge stations and periods of record.
9052000
9052030
7.2.
Number of Full
Years Missing
Total Years
of Data
16
0
93
147
44
0
103
40
24
54
0
47
0
37
0
101
0
51
0
98
Cape Vincent, New York
The Cape Vincent, NY, gauge was selected as the outlet of Lake Ontario. The Cape
Vincent gauge is located closest to the outlet of Lake Ontario, it has traditionally been
used as the outlet in past studies and the gauge has a relatively long period of record.
Alternatively, the Kingston, ON, gauge also has a long period of record and is located
close to both the outlet and the Cape Vincent gauge. The Kingston gauge was also
investigated as a potential outlet in order to confirm or provide an alternative to results
72
obtained from Cape Vincent, although unfortunately there are a number of potential
issues with the Kingston gauge that impact the results (Section 7.11).
The Cape Vincent period of record begins in 1898; however, only four months of data
are on record until 1916, after which point data is for the most part continuous. Also of
note is that water level data collected between 1916 and 1935 is recording gauge data,
data from 1936 until April 1955 is primarily staff gauge data (although May to November
of 1954 were collected with a recording gauge), and following April 1955 all data is
recording gauge data. Since 1916 the gauge has been relocated twice (1936 and 1939).
7.3.
Oswego, New York
The water level record begins in 1860 at Oswego. The water level difference plot for
Cape Vincent minus Oswego is shown in Figure 7-2. The rate of vertical movement as
determined from the slope of the linear regression line is -3.59 cm/century, indicating that
Oswego is falling relative to Cape Vincent. There do not appear to be any significant
discontinuities in the water level difference plot.
Cape Vincent - Oswego
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 7-2: Cape Vincent minus Oswego water level difference plot.
73
7.4.
Rochester, New York
The water level record begins in 1860 at Rochester; however, from 1907 to 1953 no
data is available other than five months worth in 1935. Furthermore, the data collected
prior to 1907 is almost entirely staff gauge data, whereas the data collected after 1953 is
from recording gauges (Coordinating Committee, 1987). The water level difference plot
for Cape Vincent minus Rochester is shown in Figure 7-3. The pre-1953 data has been
omitted from the analysis. The rate of vertical movement as determined from the slope of
the linear regression line is -7.99 cm/century, indicating that Rochester is falling relative
to Cape Vincent (the rate is -7.91 cm/century when the 1935 data is included). There do
not appear to be any significant discontinuities in the water level difference plot.
Cape Vincent - Rochester
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 7-3: Cape Vincent minus Rochester water level difference plot.
7.5.
Olcott, New York
The published water level record begins in 1967 at Olcott. The water level difference
plot for Cape Vincent minus Olcott is shown in Figure 7-4. The rate of vertical
movement as determined from the slope of the linear regression line is -12.52 cm/century,
indicating that Oswego is falling relative to Cape Vincent. However, the rates calculated
for other combinations of months differed fairly significantly. For example, the rates
calculated for the annual, May to August and July to October averages were -8.80, -10.52
and -11.56 cm/century, respectively. This is at least partially the result of the relatively
74
short period of record at Olcott, as well as additional incomplete data within years. The
annual average rate in particular was calculated with only 26 full years of data, whereas
the June to September average was calculated with 37 values. The calculated rate of
apparent vertical movement should become more certain in future years as more data is
added to the period of record. Lastly, there do not appear to be any significant
discontinuities in the water level difference plot.
Cape Vincent - Olcott
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
Year
Figure 7-4: Cape Vincent minus Olcott water level difference plot.
7.6.
Port Weller, Ontario
The water level record begins in 1929 at Port Weller; however, data is not available
from November 1931 until January 1956, and the gauge was relocated during this time
(Coordinating Committee, 1987). The water level difference plot for Cape Vincent
minus Port Weller is shown in Figure 7-5. The rate of vertical movement as determined
from the slope of the linear regression line is -14.32 cm/century if the pre-1956 data is
included and -17.13 cm/century when only data from 1956 onwards is used. Either way,
Port Weller appears to be falling relative to Cape Vincent. Since the gauge was relocated
prior to 1956, and since the pre-1956 data significantly affects the calculated rate of
apparent vertical movement, the rate calculated post-1956 is the more representative of
the actual relationship between these two gauges. Lastly, there do not appear to be any
significant discontinuities in the water level difference plot.
75
Cape Vincent - Port Weller
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Linear (1956+)
Water Level Difference (m)
0.2
0.1
0
-0.1
1920
1940
1960
1980
2000
2020
Year
Figure 7-5: Cape Vincent minus Port Weller water level difference plot.
7.7.
Port Dalhousie, Ontario
The water level record begins in 1910 but ends in 1956 at Port Dalhousie. The water
level difference plot for Cape Vincent minus Port Dalhousie is shown in Figure 7-6. The
rate of vertical movement as determined from the slope of the linear regression line
is -15.06 cm/century, indicating that Port Dalhousie is falling relative to Cape Vincent.
There appears to be a possible shift or step down in the plot around 1933, but since the
period of record is not very long and ends in 1956, this was not investigated further.
76
Cape Vincent - Port Dalhousie
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1910
1920
1930
1940
1950
1960
Year
Figure 7-6: Cape Vincent minus Port Dalhousie water level difference plot.
7.8.
Burlington, Ontario
The water level record begins in 1970 at Burlington. The water level difference plot
for Cape Vincent minus Burlington is shown in Figure 7-7. The rate of vertical
movement as determined from the slope of the linear regression line is -24.05 cm/century,
indicating that Burlington is falling relative to Cape Vincent. Calculations of rates for
other combinations of months yielded significantly different results, including a rate
of -16.61 cm/century for the annual average (though only 19 years of full annual data are
available) and -22.62 and -20.95 cm/century for the May to August and July to October
averages, respectively. These discrepancies are a result of the short period of record at
Burlington combined with missing data (a maximum of 33 years of data were available
for the calculations for any monthly combination). In addition, the Burlington gauge was
moved in 1981, and is now located on a concrete pier close to the sand bar and filled land
that divides Lake Ontario and Hamilton Harbour. Tushingham (1992) noted that this
gauge is potentially unstable for these reasons. There do not appear to be any significant
discontinuities in the water level difference plot, but due to the reasons outlined the rate
of apparent vertical movement calculated at Burlington should be treated with caution.
77
Cape Vincent - Burlington
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1960
1970
1980
1990
2000
2010
Year
Figure 7-7: Cape Vincent minus Burlington water level difference plot.
7.9.
Toronto, Ontario
The water level record begins in 1906 at Toronto. The water level difference plot for
Cape Vincent minus Toronto is shown in Figure 7-8. The rate of vertical movement as
determined from the slope of the linear regression line is -10.93 cm/century, indicating
that Toronto is falling relative to Cape Vincent.
The water level difference plot appears to show a somewhat stepwise pattern, which
is particularly evident in the 5-year centred running average plot (Figure 7-9). This is
similar to some of the patterns seen on other lakes, such as at Calumet Harbor and at
Milwaukee on Lake Michigan, for example (Sections 5.5 and 5.6, respectively). On Lake
Ontario, however, a similar pattern is not as obvious at other long-term gauges.
78
Cape Vincent - Toronto
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 7-8: Cape Vincent minus Toronto water level difference plot.
Cape Vincent - Toronto
0.3
June-Sep. Avg.
June-Sep. 5-year Centred
A
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 7-9: Cape Vincent minus Toronto water level difference plot with 5-year centred running
average.
79
7.10. Cobourg, Ontario
The water level record begins in 1956 at Cobourg. The water level difference plot for
Cape Vincent minus Cobourg is shown in Figure 7-10. The rate of vertical movement as
determined from the slope of the linear regression line is -8.51 cm/century, indicating that
Cobourg is falling relative to Cape Vincent.
There appears to be a downward shift or jump in the water level difference plot
occurring in 1977, which is also concurrent with a gap in the data. However, while this
apparent shift appears obvious in this plot, the same shift is not evident in the July to
October plot, in which there is no data gap (Figure 7-11). The rate of vertical movement
as calculated from the July to October average is -8.30 cm/century, which is close to that
calculated from the June to September averages. Therefore, it seems likely that what
appears to be a shift in the June to September plot is actually just an illusion caused by
variability and the gap in the data in this case. This helps illustrate the difficulty in
making assumptions of this sort based on a single plot of water level differences.
Cape Vincent - Cobourg
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 7-10: Cape Vincent minus Cobourg water level difference plot (June to September average).
80
Cape Vincent - Cobourg
0.3
Jul-Oct Avg
Linear (Jul-Oct Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1950
1960
1970
1980
1990
2000
2010
Year
Figure 7-11: Cape Vincent minus Cobourg water level difference plot (July to October average).
7.11. Kingston, Ontario
The water level record begins in 1909 at Kingston. The water level difference plot
for Cape Vincent minus Kingston is shown in Figure 7-12. The rate of vertical
movement as determined from the slope of the linear regression line is 3.03 cm/century,
indicating that Kingston is rising relative to Cape Vincent.
However, there appear to be a number of discontinuities in the water level difference
plot. First, there appears to be a shift or plateau running from 1929 until 1941, at which
point the water level differences shift down again. There is then a short rising period
which continues until approximately 1954, after which the plot appears to level out or
even descend slightly. Alternatively, the plot can instead be seen to rise until sometime
in the late-1970s, after which it begins to descend slightly.
The reasons for the odd shape of the plot are unknown. The apparent shift upwards
occurring between 1929 and 1941 is also evident in a plot of Toronto versus Kingston,
and somewhat less evident in a plot of Oswego versus Kingston, which would imply that
Kingston data is the cause (Figure 7-13). Furthermore, though not major, a change in
slope sometime in the 1970s or 1980s is also evident in these plots. A number of gauge
relocations have taken place at Kingston, including in 1962, 1974 and 1977. The 1962
relocation in particular was a fairly significant one, as the gauge was moved more than
81
3 kilometres. The rate of apparent vertical movement calculated with data collected after
1962 is -2.85 cm/century. However, neither this date nor the other relocation dates
appears to coincide well with any of the shifts or discontinuities occurring in the water
level difference plot.
Cape Vincent - Kingston
0.3
Jun-Sep Avg
Linear (Jun-Sep Avg)
Water Level Difference (m)
0.2
0.1
0
-0.1
1900
1920
1940
1960
1980
2000
2020
Year
Figure 7-12: Cape Vincent minus Kingston water level difference plot.
Since the Cape Vincent and Kingston gauges are only located approximately
16 kilometres apart, the expected rate of apparent vertical movement between the two
locations should be small. The rate of 3.03 cm/century calculated seems high, especially
when compared to other locations on Lake Ontario, and the differences between the rates
calculated for them. For example, the difference in rate calculated for Oswego and
Rochester is only 4.32 cm/century, despite being separated by approximately 90
kilometres, while the difference between Toronto and Olcott is only 1.59 cm/century,
despite being separated by approximately 68 kilometres.
82
Kingston Water Level Difference Comparisons
0.1
Water Level Difference (m)
0
-0.1
Cape Vincent-Kingston
-0.2
Toronto-Kingston
Oswego-Kingston
-0.3
1900
1920
1940
1960
1980
2000
2020
Year
Figure 7-13: Comparison of Kingston water level difference plots.
7.12. Summary
Water level difference plots on Lake Ontario are somewhat less complicated than
those on Lakes Michigan-Huron and Lake Erie, but some issues are still evident, such as
at Kingston, for example. Water level differences at most gauge pairs appear to follow a
mostly linear relationship, though in addition to the complications at Kingston, Toronto
appears to show a somewhat stepwise pattern similar to that seen on other lakes.
A summary of the calculated rates of apparent vertical movement for Lake Ontario
are shown in Table 7-2. The uncertainty (standard deviation of the slope value) is also
shown, as are additional scenarios as described in previous sections.
83
Gauge (Period of Record)
Rate
Standard Error
*Additional Scenarios
(cm/century)
(cm/century)
Oswego (1916-2006)
-3.59
0.28
Rochester (1935-2006)
-7.91
0.58
-7.99
0.63
*Rochester (1953-2006)
Olcott (1967-2006)
-12.52
1.16
Port Weller (1930-2006)
-14.32
1.05
-17.13
1.06
*Port Weller (1956-2006)
Port Dalhousie (1916-1956)
-15.06
1.33
Burlington (1970-2006)
-24.05
1.70
Toronto (1916-2006)
-10.93
0.42
Cobourg (1957-2006)
-8.51
0.89
Kingston (1916-2006)
3.03
0.36
-2.85
0.74
*Kingston (1962-2006)
Table 7-2: Summary of Lake Ontario rates of apparent vertical movement
and uncertainty relative to Cape Vincent.
8. Conclusions
This analysis has first and foremost provided an update to the rates of apparent
vertical movement (believed to be caused primarily by glacial isostatic adjustment) as
calculated from the slope of the linear regression line determined from water level
differences between gauge pairs. The rates of movement and their associated uncertainty
have been calculated between all gauges on a given lake and that lake’s outlet with all
published data collected up to and including 2006.
In addition to the revised rates, the key findings of this study are as follows:
•
The four-month June to September average water level differences should be
used in this type of analysis.
•
The length of the period of record is important. Shorter periods of record
(usually less than 40 years, approximately) are inadequate and should be
omitted.
•
Glacial isostatic adjustment may not be linear in nature. Discontinuities,
shifts, stepwise patterns, changes in rate and other interesting features can be
seen in some water level difference plots. In some cases an explanation for
these features can be suggested, whereas in other cases no explanation is
immediately evident. Furthermore, in some instances a feature seen in one
plot can be contradicted by its absence in another plot.
•
Water level difference plots can be helpful in identifying issues in the water
level data itself.
84
•
Meteorological effects can have an impact.
•
The rate of movement calculated for any given period of time may not be
representative of the rate of movement over the entire period of record, or
during any other time.
•
In future glacial isostatic adjustment studies, Lakeport, Michigan, should be
considered for use as the outlet of Lakes Michigan-Huron as opposed to
Harbor Beach, since Lakeport is located closer to the outlet, there appears to
be movement between Lakeport and Harbor Beach, and since the period of
record at Lakeport is now greater than 50 years.
Water level difference plots can show significant variability. This has normally been
attributed mostly to meteorological related factors. While this may be valid, it has also
been shown that the data collection method (i.e. staff gauge versus recording gauge data)
can influence the amount of observed variability. This variability increases the
uncertainty with which the rates of vertical movement can be calculated. By using the
four-month June to September average water level differences, the variability is reduced
to some degree without substantially increasing the possibility of having missing data. In
addition, the results are comparable to those of many past studies, which also used the
June to September average.
The length of the period of record used is important. Shorter periods of record tended
to show rates of movement that were inconsistent with the rates calculated at other
gauges on a given lake, and sometimes these rates were beyond the rates of movement
that might be reasonably expected due to glacial isostatic adjustment.
It has also been shown in a number of cases that sometimes the assumption of a linear
relationship is invalid. Instead, water level difference plots can show a stepped pattern
(e.g. Harbor Beach minus Milwaukee), a change in the rate of movement (e.g. Harbor
Beach minus Goderich), or other inconsistencies that do not follow any obvious pattern
(e.g. Cape Vincent minus Kingston). This is not the first study to report observed data
issues. The Coordinating Committee (2001) report, for example, stated that systematic
trends in outliers at a number of gauges should be investigated further. These trends
could be the result of any number of issues outlined here.
Determining the exact cause of these observed inconsistencies is beyond the scope of
this document, and may actually be impossible to determine for certain in most cases.
The observed non-linear patterns could be the result of any number of factors or a
combination of them. Possible explanations include data collection method changes,
meteorological impacts, gauge relocations or even actual geologic movements. For the
most part the assumption of a linear relationship may be valid, but in reality glacial
isostatic adjustment may not always be linear in nature. Perhaps in some areas the land
remains relatively stable for periods of time, followed by short periods of faster
movement, which would help explain the stepped patterns observed between some gauge
pairs. On the other hand, the stepped pattern could instead simply be the result of a
85
cyclical meteorological impact. For example, perhaps the average wind conditions on a
given lake vary in a multi-year, cyclical pattern (i.e. periods of stronger winds on average
lasting for several years followed by periods of weaker winds). Since high winds can
cause wind setup such that water levels at one end of the lake differ significantly from
water levels at the other end, a cyclical difference in wind patterns would not have to be
significant to cause a noticeable stepped pattern in the average water level difference
plots, especially when one considers that the average water level differences themselves
are measured in centimetres, whereas wind setup can cause short-term water level
differences of a metre or more.
Furthermore, perhaps the lakes themselves differ in how they are recovering from the
weight of the glaciers. For the most part, water level differences on Lake Superior appear
to follow a linear relationship, whereas discrepancies were observed more often on the
other lakes. This could be the result of much of Lake Superior being located within the
Canadian Shield, whereas the other lakes are not, or perhaps similar observations are only
hidden on Lake Superior because it is generally recovering at a faster rate than the other
lakes. Regardless, there is obviously still a lot we do not understand about glacial
isostatic adjustment in the Great Lakes region.
9. Recommendations
It should be noted that the importance of these issues is likely to vary depending on
the problem being investigated. These issues may be less important in a study requiring
analysis of long-term impacts of apparent vertical movement than in a study requiring
analysis of short-term impacts.
For example, to explain the land-to-water relationship around a lake and how the
observed water levels at each location have changed over the past century, the average
rate of movement calculated for the gauges using linear regression is likely adequate,
regardless of whether the relationship is truly linear or not. Similarly, the volumes of
each lake are believed to be changing with time, such that each lake is either storing or
decanting water due to glacial isostatic adjustment. The magnitude of these changes,
however, likely requires that long-term average rates of change be used in most cases.
However, these issues can have more serious consequences if shorter periods are
being investigated. For example, water level transfers are sometimes used to replace
missing data at a certain gauge station. The measured water levels from one gauge are
transferred to the other gauge to replace the missing data by using an estimated rate of
vertical movement between the two gauges. In such cases, a long-term linear relationship
between these gauges may not accurately describe the real relationship occurring during
the period of missing data. For example, if the relationship is actually stepwise, as
opposed to linear, the rate of vertical movement during the period in question may be
significantly different than the long-term linearly calculated rate. The transferred data
might be sufficient for some if not most purposes, but may be entirely inadequate for
86
other uses (such as determining the rate of apparent vertical movement between gauge
pairs, for example).
As another example, rates of apparent vertical movement are also being investigated
using Global Positioning Systems (GPS). In these studies, GPS are used to measure rates
of apparent vertical movement, and this is often done over far shorter periods of time than
those studies that use water level differences. Again, the rate of apparent vertical
movement measured over a shorter period may not be representative of the rate occurring
over a longer period. On the other hand, GPS studies might be able to help support or
refute some of the issues and discrepancies documented in this report.
All things considered, it is advised that the rates of apparent vertical movement as
calculated from linear regression of the water level difference relationships be used with
caution depending on the analysis being undertaken.
87
10. References
Coordinating Committee (1977). Apparent vertical movement over the Great Lakes.
Report by the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic
Data. July 1977.
Coordinating Committee (1978). History of water level gauges: Upper Great Lakes and
the St. Clair – Detroit Rivers. Report by the Coordinating Committee on Great Lakes
Basic Hydraulic and Hydrologic Data. January 1978.
Coordinating Committee (1987). History of water level gauges: Lower Great Lakes and
the International Section of the St. Lawrence River. Report by the Coordinating
Committee on Great Lakes Basic Hydraulic and Hydrologic Data. March 1987.
Coordinating Committee (2001). Apparent vertical movement over the Great Lakes Revisited. Report by the Coordinating Committee on Great Lakes Basic Hydraulic and
Hydrologic Data. November 2001.
Coordinating Committee (2006). 81st meeting minutes of the Coordinating Committee on
Great Lakes Basic Hydraulic and Hydrologic Data. November 14-15, 2006.
Mainville, A. and Craymer, M. R. (2005). Present-day tilting of the Great Lakes region
based on water level gauges. Geologic Society of America (GSA) Bulletin. 117(7/8),
July/August 2005, p. 1070-1080.
Quinn, F. H. and Southam, C. (2008). Lake Huron water level gage analysis. Report for
the International Upper Great Lakes Study.
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