Thunder Lake
11-0167-00 CASS COUNTY
Summary
Thunder Lake is located eight miles south of Remer, MN in Cass County. It
covers 1,347 acres, which places it in the upper 10% of lakes in Minnesota
in terms of size. Thunder Lake occurs just south of the Chippewa National
Forest boundary and near the Land O’Lakes State Forest.
Thunder Lake has two inlets and one outlet, which classifies it as a
drainage lake. It sits in the western corner of the Mississippi River - Grand
Rapids Watershed and receives flow from Laura Lake to the west and from
Kidney Lake to the south. Water leaves the Thunder Lake outlet on the
east side and flows into Pughole Lake, north into Big Rice Lake and then to
the Willow River which flows east to the Mississippi River.
Water quality data have been collected on Thunder Lake since 1973. These data show that the
lake is at the oligotrophic/mesotrophic border (TSI 39-41), which is characterized by clear water
throughout the summer and excellent recreational opportunities (page 9).
The Thunder Lake Association’s mission is to improve or maintain water quality through the
education and implementation of sound, proven management practices and processes, while
balancing the wide range of water surface and recreational uses this beautiful Lake has to offer.
The Association has been involved in numerous activities including water quality monitoring,
education, Healthy Lakes Initiative, and is a member of the Association of Cass County Lakes
(ACCL).
Vitals
MN Lake ID:
County:
Ecoregion:
Major Drainage
Basin:
Latitude/Longitude:
Water Body Type:
Monitored Sites
(Primary):
Monitored Sites
(Secondary):
Physical Characteristics
11-0062-00
Cass
Northern Lakes and Forest
Upper Mississippi River
46.95416667/-93.97111111
Public
203
201, 202, 204, 205, 206, 207,
208, 209, 210, 211
Surface area (acres):
Littoral area (acres):
% Littoral area:
Max depth (ft):
Mean depth (ft):
Lakeshed size (acres):
Lakeshed : lake area ratio
Inlets
Outlets
Accesses
1,347
226
17%
95
(m): 22.5
24
(m): 7.3
9,189
6.8:1
2
1
1 public
Invasive species present: none documented
Data Availability
Transparency data
Numerous yearly secchi readings from 1973-2008
through the MPCA CLMP program.
Chemical data
Total Phosphorus and Chlorophyll a data have been
collected in 2005-2008.
Inlet/Outlet data
No inlet/outlet data exist for Thunder Lake.
Recommendations
For recommendations refer to page 12.
Thunder Lake
Report generation: 2008, RMB Environmental Laboratories
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Figure 1. Map of Thunder Lake illustrating bathymetry, lake sample site locations, stream inlets and outlets
and aerial land use. The pink shaded areas in the lake illustrate the littoral zone, where the sunlight can
usually reach the lake bottom allowing aquatic plants to grow.
Lake Site
Depth (ft)
Monitoring Programs
201
202
203* Primary Site
204
205
206
50
20
70
60
65
70
CLMP: 1973-1975
CLMP: 1974-1975
CLMP: 1982-1984; Outdoor Corps: 2005-2006; RMB Lab: 2008
CLMP: 1988-1994
CLMP: 1988-1993;
CLMP: 1995-2007; RMB Lab: 2007
Sites 207-211 only have one year of data each and are not included in this table.
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Report generation: 2008, RMB Environmental Laboratories
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The information below describes available chemical data for both sites of Thunder Lake through
2008. The data set is limited, and all parameters with the exception of total phosphorus,
chlorophyll a and secchi depth, are means from 1980, 1991, 2000, 2007 DNR Fisheries Surveys.
Minnesota is divided into seven ecoregions based on land use, vegetation, precipitation and
geology. The MPCA has developed a way to determine the "average range" of water quality
expected for lakes in each ecoregion. For more information on ecoregions and expected water
quality ranges, see page 11.
Impaired
Waters
Standard2 Interpretation
Parameter
Mean
Ecoregion
Range1
Total phosphorus (ug/L)
12.8
14 - 27
> 35
Chlorophyll a (ug/L) 3
3.8
4 - 10
> 12
Chlorophyll a max (ug/L)
7
<15
Secchi depth (ft)
13.1
7.5 - 15
Dissolved oxygen
see page 8
Total Kieldahl Nitrogen
1.10
0.40 - 0.75
Indicates insufficient nitrogen to
support summer nitrogen-induced
algae blooms.
Alkalinity (mg/L)
123
40 - 140
Indicates a low sensitivity to acid
rain and a good buffering
capacity.
Color (Pt-Co Units)
9
10 - 35
Indicates very clear water with
little to no tannins (brown stain).
pH
8.4
7.2 - 8.3
Indicates a hard water lake. Lake
water pH less than 6.5 can affect
fish spawning and the solubility of
metals in the water.
Chloride (mg/L)
--
0.6 - 1.2
Data nonexistent.
Total Suspended Solids
2.4
<1 - 2
Indicates low suspended solids
and clear water.
Conductivity (umhos/cm)
261
50 - 250
Slightly above the ecoregion
average. High conductivity
indicates high dissolved solids,
which can come from watershed
or human sources.
Total Nitrogen :Total
Phosphorus
--
25:1 – 35:1
Thunder Lake is most likely
phosphorus limited, which means
that algae growth is limited by the
amount of phosphorus in the lake.
Results are better than the
expected range for the ecoregion.
For more information about
Impaired Waters Assessment,
see page 12.
< 4.5
Dissolved oxygen depth profiles
show that the deep areas of the
lake are anoxic in late summer.
(mg/L)
(mg/L)
Data Source: 1980, 1991, 2000, 2007 DNR Fisheries Surveys
1
th
th
The ecoregion range is the 25 -75 percentile of summer means from ecoregion reference lakes
For further information regarding the Impaired Waters Assessment program, refer to http://www.pca.state.mn.us/water/tmdl/index.html
3
Chlorophyll a measurements have been corrected for pheophytin
Units: 1 mg/L (ppm) = 1,000 ug/L (ppb)
2
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Report generation: 2008, RMB Environmental Laboratories
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Water Quality Characteristics - Historical Means
Years monitored: 1973-1975, 1982-2008
Parameters
Total Phosphorus Mean (ug/L):
Total Phosphorus Min:
Total Phosphorus Max:
Number of Observations:
Chlorophyll a Mean (ug/L):
Chlorophyll-a Min:
Chlorophyll-a Max:
Number of Observations:
Secchi Depth Mean (ft):
Secchi Depth Min:
Secchi Depth Max:
Number of Observations:
Primary
Site
203
Site
201
Site
202
Site
204
Site
205
Site
206
12.8
8
24
14
10
7
13
3
3.8
1
7
14
3.5
2
5
2
13.1
5.5
19
47
13.4
8
19
49
11.7
9
16
25
14.5
6.5
20
73
13.0
11.5
15.5
62
17.4
8.5
30
174
Figure 2. Thunder Lake total phosphorus, chlorophyll a and transparency historical ranges. The arrow
represents the range and the black dot represents the historical mean (Primary Site 203). Figure adapted
after Moore and Thornton, [Ed.]. 1988. Lake and Reservoir Restoration Guidance Manual. (Doc. No. EPA 440/5-88-002)
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Report generation: 2008, RMB Environmental Laboratories
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Transparency (Secchi Depth)
Transparency is how easily light can pass through a substance. In lakes it is how deep sunlight
penetrates through the water. Plants and algae need sunlight to grow, so they are only able to
grow in areas of lakes where the sun penetrates. Water transparency depends on the amount of
particles in the water. An increase in particulates results in a decrease in transparency.
The transparency varies year-to-year due to changes in weather, precipitation, lake use, flooding,
temperature, lake levels, etc. The annual means for Thunder Lake range from 9-21 ft.
Transparency was highest in 1995 and 2004. Figure 3 shows that the transparency at site 206 is
better than at site 203; however the data from site 203 is limited and disjointed. Transparency
monitoring should be continued at both sites 203 and 206 to see if they are similar in the future.
Since site 203 is in the middle of the long portion of the lake, it may get mixed up more than the
western bay (site 206), causing the transparency to be lower.
Transparency: Annual Means
25
Secchi Depth (ft)
20
15
10
Site 203
Site 206
5
Site 206, Long-term mean
08
20
06
20
04
20
02
20
00
20
98
19
96
19
94
19
92
19
90
19
88
19
86
19
84
19
19
82
0
Figure 3. Annual mean transparency compared to long-term mean transparency, sites 203 and 206.
Thunder Lake transparency ranges from 8.5 to 30 ft at site 206. This range is indicative of a
dynamic lake. Figure 4 shows the seasonal transparency dynamics. Thunder Lake transparency
does not appear to follow a pattern. Some lakes vary throughout the summer (dynamic lakes)
while some lakes stay constant. Transparency dynamics have to do with algae population
dynamics and lake turnover.
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Figure 4. Seasonal transparency dynamics and year-to-year comparison (Site 206). Lines connect the data
for the past five years (2002-2006).
User Perceptions
When volunteers collect secchi depth readings, they record their perceptions of the water based on
the physical appearance and the recreational suitability. These perceptions can be compared to
water quality parameters to see how the lake "user" would experience the lake at that time.
Looking at transparency data, as the secchi depth decreases the perception of the lake's physical
appearance rating decreases. Thunder Lake was rated as being "not quite crystal clear" 90% of
the time between 1988-2007 (Figure 5).
1%
9%
Physical Appearance Rating
9%
Crystal clear water
90%
Not quite crystal clear – a little algae visible
1%
Definite algae – green, yellow, or brown color
apparent
0%
High algae levels with limited clarity and/or mild
odor apparent
0%
Severely high algae levels
90%
Figure 5. Physical appearance rating, as rated by the volunteer monitor (1988-2007).
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As the secchi depth decreases, the perception of recreational suitability of the lake decreases.
Thunder Lake was rated as being "beautiful" and “minor aesthetic problems” 99% of the time from
1988-2007 (Figure 6).
Recreational Suitability Rating
1%
49%
50%
49%
Beautiful, could not be better
50%
Very minor aesthetic problems; excellent for
swimming, boating
1%
Swimming and aesthetic enjoyment of the lake
slightly impaired because of algae levels
0%
Desire to swim and level of enjoyment of the lake
substantially reduced because of algae levels
0%
Swimming and aesthetic enjoyment of the lake
nearly impossible because of algae levels
Figure 6. Recreational suitability rating, as rated by the volunteer monitor (1988-2007).
Total Phosphorus
Total Phosphorus (ug/L)
Site 203, 2005
Site 203, 2006
25
Site 203, 2008
Site 206, 2007
20
15
10
5
ct
13
-O
p
29
-S
e
p
-S
e
15
Se
p
1-
g
-A
u
18
Au
g
4-
l
-J
u
21
Ju
l
7-
23
-J
u
n
0
Ju
n
Total phosphorus was
evaluated in Thunder
Lake in 2005-2008
(Figure 7). The data
indicate an increase in
phosphorus toward the
end of the summer.
Both sites have similar
concentrations in each
year of monitoring.
Total Phosphorus
30
9-
Thunder Lake is most
likely phosphorus
limited, which means
that algae and aquatic
plant growth is
dependent upon
available phosphorus.
Figure 7. Historical total phosphorus concentrations (ug/L) for Thunder Lake (data
sets from 2005-2008).
Phosphorus should
continue to be
monitored to track any future changes in water quality.
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Chlorophyll a
Site 203, 2005
20
Site 203, 2006
18
16
Site 203, 2008
14
Site 206, 2007
12
10
8
6
4
2
ct
-O
13
-S
ep
29
-S
ep
15
Se
p
1-
-A
ug
18
Au
g
4-
ul
21
-J
l
Ju
7-
-J
23
Ju
n
un
0
9-
Chlorophyll a
concentrations greater
than 10 ug/L are
perceived as a mild
algae bloom, while
concentrations greater
than 20 ug/L are
perceived as a nuisance.
Chlorophyll a
Chlorophyll a (ug/L
Chlorophyll a is the
pigment that makes
plants and algae green.
Chlorophyll a is tested in
lakes to determine the
algae concentration or
how "green" the water is.
Figure 8. Chlorophyll a concentrations (ug/L) for Thunder Lake (data
Chlorophyll a was
sets from 2005-2008).
evaluated in Thunder
Lake in 2005-2008 (Figure 8). Chlorophyll a concentrations for all dates at all sites remained
below 10 ug/L, indicating clear water most of the summer. There was not much variation between
sites over the years monitored. Chlorophyll a concentrations increased slightly over the course of
the summer.
Dissolved Oxygen
Dissolved Oxygen (DO) is the amount of oxygen dissolved in lake water. Oxygen is necessary for
all living organisms to survive, except for some bacteria. Living organisms breathe in oxygen that
is dissolved in the water. Dissolved oxygen levels of <5 mg/L are typically avoided by game fish.
There are no dissolved oxygen data for Thunder Lake. Monitoring dissolved oxygen in future
years would describe dynamics occurring in the water column and the habitat quality for game fish.
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Trophic State Index
Phosphorus (nutrients), chlorophyll a (algae
concentration) and Secchi depth
(transparency) are related. As phosphorus
increases, there is more food available for
algae, resulting in increased algal
concentrations. When algal concentrations
increase, the water becomes less
transparent and the Secchi depth
decreases.
Trophic State Index
Site 203
Site 206
TSI Total Phosphorus 40
Insufficient data
TSI Chlorophyll-a
42
Insufficient data
TSI Secchi
41
36
TSI Mean
41
Insufficient data
Oligotrophic
Mesotrophic
Trophic State:
Numbers represent the mean TSI for each parameter.
The results from these three measurements cover
different units and ranges and thus cannot be directly
compared to each other or averaged. In order to
standardize these three measurements to make them
directly comparable, we convert them to a trophic state
index (TSI).
100
Hypereutrophic
70
The mean TSI for Thunder Lake falls in the oligotrophicmesotrophic range (Figure 9). There is
good agreement between the TSI for
Thunder Lake
phosphorus, chlorophyll a and
transparency, indicating that these
variables are strongly related. The TSI
for site 206 is slightly better than site 203.
Eutrophic
50
Mesotrophic
40
Oligotrophic
Lakes on the oligotrophic/mesotrophic border (TSI 3941) are characterized by clear water throughout the
summer and are excellent for recreation.
0
Figure 9. Trophic state index chart with
corresponding trophic status.
TSI
<30
30-40
40-50
50-60
60-70
70-80
>80
Attributes
Oligotrophy: Clear water, oxygen throughout
the year at the bottom of the lake, very deep
cold water.
Bottom of shallower lakes may become anoxic
(no oxygen).
Mesotrophy: Water moderately clear most of
the summer. May be "greener" in late summer.
Eutrophy: Algae and aquatic plant problems
possible. "Green" water most of the year.
Blue-green algae dominate, algal scums and
aquatic plant problems.
Hypereutrophy: Dense algae and aquatic
plants.
Algal scums, few aquatic plants.
Fisheries & Recreation
Trout fisheries dominate.
Trout fisheries in deep lakes only. Walleye,
Tullibee present.
No oxygen at the bottom of the lake results in
loss of trout. Walleye may predominate.
Warm-water fisheries only. Bass may
dominate.
Dense algae and aquatic plants. Low water
clarity may discourage swimming and boating.
Water is not suitable for recreation.
Rough fish (carp) dominate; summer fish kills
possible.
Source: Carlson, R.E. 1997. A trophic state index for lakes. Limnology and Oceanography. 22:361-369.
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Trend Analysis
For detecting trends, a minimum of 8-10 years of data with 4 or more readings per season are
recommended. Minimum confidence accepted by the MPCA is 90%. This means that there is a
90% chance that the data are showing a true trend and a 10% chance that the trend is a random
result of the data. Only short-term trends can be determined with just a few years of data, because
there can be different wet years and dry years, water levels, weather, etc., that affect the water
quality naturally.
There is not enough historical data to perform trend analysis for total phosphorus or chlorophyll a
on Thunder Lake. Site 206 has over 8 years of transparency data, which is enough data to
perform a short-term and long-term trend analysis (Figure 10). The data was analyzed using the
Mann Kendall Trend Analysis.
Lake Site
Parameter
Date Range
Trend
Probability
206
Transparency
1995-2007
Improving
95%
Figure 10. Transparency trend (ft) for site 206 from 1995-2007.
Site 206 shows a statistically significant improving trend in transparency from 1995-2007. The
transparency has improved an average of approximately one foot since 1995. Transparency
monitoring should continue at both sites so that this trend can be tracked in future years.
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Ecoregion Comparisons
Minnesota is divided into seven ecoregions based on
land use, vegetation, precipitation and geology. The
MPCA has developed a way to determine the
"average range" of water quality expected for lakes in
each ecoregion. From 1985-1988, the MPCA
evaluated the lake water quality for reference lakes.
These reference lakes are not considered pristine, but
are considered to have little human impact and
therefore are representative of the typical lakes within
the ecoregion. The "average range" refers to the 25th
- 75th percentile range for data within each ecoregion.
For the purpose of this graphical representation, the
means of the reference lake data sets were used.
60
30
50
25
30
20
20
Secchi depth (ft)
40
15
10
NLF
Ecoregion
Thunder
b
10
15
crystal
clear
0
0
increased
algae
20
5
10
a
0
5
Chlorophyll-a (ug/L, ppb)
Total Phosphorus (ug/L, ppb)
Thunder Lake is in the
Northern Lakes and Forests
Ecoregion. The mean total
phosphorus, chlorophyll a and
transparency (secchi depth) for
Thunder Lake are all within the
expected ecoregion ranges
(Figures 11a-c.
25
NLF
Ecoregion
Thunder
c
NLF
Ecoregion
Thunder
Figures 11a-c. Thunder Lake ranges compared to Northern Lakes and Forest Ecoregion ranges. The
Thunder Lake total phosphorus and chlorophyll a ranges are from 14 data points collected in MaySeptember of 2005-2008. The Thunder Lake secchi depth range is from 47 data points collected in MaySeptember from 1982-1984, 2005-2006 and 2008.
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Inlet/Outlet Data Assessment
No inlet/outlet data exist for Thunder Lake.
Assessment/Findings Recommendations
Transparency
Transparency monitoring at sites 203 and 206 should be continued annually. It is important to
continue transparency monitoring weekly or at least bimonthly every year to enable year-to-year
comparisons and trend analyses.
Impaired Waters Assessment 303(d) List
There are two main types of Impaired Waters Assessment for lakes: eutrophication (excess
phosphorus) for aquatic recreation and mercury in fish tissue for aquatic consumption. Thunder
Lake is not listed as impaired for mercury in fish tissue or eutrophication; however, it is due to
insufficient data. The 2008 lake data will count towards the 2010 assessment.
Aquatic Recreational Use Assessment 305(b)
In the 2008 MPCA Aquatic Use Assessment (305(b)), Thunder Lake was classified as having
insufficient data for this assessment.
Inlet/Outlet Assessment
Because of the lack of inlet/outlet data, a mass balance project should be considered. This study
answers questions about nutrient loading into the lake and nutrient budget within the lake.
Organizational contacts and reference sites
Thunder Lake Association
http://www.mnwaters.org/index.php?uberKey=1340
Cass County Environmental
Services Department
303 Minnesota Avenue W, P.O. Box 3000, Walker, MN 56484-3000
(218) 547-7241
http://www.co.cass.mn.us/esd/home_esd.html
DNR Fisheries Office
1601 Minnesota Drive
Brainerd, MN 56401
(218) 828-2550
http://www.dnr.state.mn.us/lakefind/index.html
Regional Minnesota Pollution
Control Agency Office
7678 College Road, Suite 105, Baxter, MN 56425
(218) 828-2492
http://www.pca.state.mn.us
Regional Board of Soil and Water
Resources Office
1601 Minnesota Drive, Brainerd, MN 56401
(218) 828-2383
http://www.bwsr.state.mn.us
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