Mississippi Valley Division
Engineer Research and Development Center
Measuring Connectivity of
Floodplain Waterbodies to
the Lower Mississippi River
by Amanda J. M. Oliver, Catherine E. Murphy, Charles D.
Little, Jr., and K. Jack Killgore
MRG&P Tech Note No. 1 • October 2016
Approved for public release; distribution is unlimited.
MRG&P Tech Note No. 1
INTRODUCTION: Prior to levee construction, Mississippi River floodwater spread across a 30to 124-mile-wide floodplain, exchanging nutrients, sediment, and organisms between the main
channel and floodplain waterbodies. Levees now constrict the floodplain to an average 5-mile
width and thus also reduce the number of waterbodies that the river can connect to (Biedenharn et
al. 2000). Additionally, Lower Mississippi River (LMR) bendway cutoffs have increased the
river’s slope and stream power (Biedenharn et al. 2000), potentially changing connectivity.
Maintaining a gradient of connectivity—from waterbodies that are always connected to those that
are rarely connected—is essential for ecosystem health (Ward and Tockner 2001). Rarely
connected waterbodies support unique species assemblages (Tockner and Stanford 2002) and
ecosystem diversity (Lubinski et al. 2008; Thomaz et al. 2007) while more frequently connected
waterbodies can sequester and exchange nutrients from the main channel and provide areas for
spawning, rearing, and refuge for riverine organisms (Baker et al. 1991; Stoffels et al. 2014). The
U.S. Army Corps of Engineers has the ability to change connectivity by notching dikes, dredging
to remove sand plugs and to maintain tie channels, and strategically placing revetment and dikes.
To quantify connectivity and evaluate habitat restoration potential, a study was initiated
encompassing the river and floodplain downstream of Helena, Arkansas, extending from
approximately River Mile (RM) 620 to 642. This area was selected because of available longterm gage data, elevation data, accessibility, and the presence of numerous waterbodies that ideally
were discrete and arrayed along a gradient of connectivity.
The study attempted to answer
several questions, one of which was
how the biological community and
biogeochemical processes differ
between waterbodies with different
degrees of connection. Elevation
data and gage data were used to
locate connecting channels, to
identify connection thresholds and
locations, and to determine Figure 1. Illustration of the location of a connection threshold
between two discrete waterbodies.
connection frequency. Ward et al.
(1999) define connectivity as the
transfer of water and material between the river’s main channel and the floodplain. A connection
threshold is “the elevation river water must reach to enter” or, conversely, the elevation below
which “water cannot gravity drain from” a discrete waterbody (Cobb et al. 1984) (Figure 1). This
technical note documents a portion of the study: the data and methods to develop an elevation
model and to determine connection thresholds and connection frequency.
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METHODOLOGY: Bare-earth elevation data were gathered from aerial and bathymetric surveys
and used to create a continuous computer model of the ground in the Island 63 project area. Within
the model, the channels connecting floodplain waterbodies to the Mississippi River main channel
were investigated to locate the highest elevation, or connection threshold, within the channel. The
connection threshold was then compared to observed river gage water surface elevations to
determine the frequency of connection between each floodplain waterbody and the river.
Elevation Data. Light Detection and Ranging (LiDAR) and bathymetric elevation data were
gathered to create a continuous bare-earth model of the project area, including river and lake beds
(Table 1). The most current and a second older, more complete set of aerial LiDAR data were
compiled for the project area. The LiDAR technique determines ground elevation by transmitting
a laser beam towards the earth, measuring the strength of the reflected beam, converting reflected
strength to distance (range), and subtracting the distance from the known elevation of the receiver.
The laser beam cannot penetrate turbid water, thus bathymetric data were acquired for submerged
areas (Figure 2).
Table 1.
Data
*
The source and native horizontal and vertical reference of elevation data used in this study.
Source
Horizontal
Vertical
2005
LiDAR
St. Louis
District
(MVS)
WGS 84, UTM Z15N,
U.S. Survey ft*
NAVD88 (Geoid 03) ,
U.S. Survey ft
2009
LiDAR
Earth
Explorer
NAD83, State Plane
MS W, U.S. Survey ft
NAVD88 (Geoid 03) ,
U.S. Survey ft
2013
Bathymetry
Memphis
District
(MVM)
WGS 84, UTM Z15N,
U.S. Survey ft
NAVD88 (Geoid 03) ,
U.S. Survey ft
2014
Bathymetry
MVM
WGS 84, UTM Z15N,
U.S. Survey ft
NAVD88 (Geoid 03) ,
U.S. Survey ft
2015
Bathymetry
ERDCCHL§
NAD83, UTM Z15N,
meters
NAVD88 (Geoid 12A||),
meters
Vertical
Accuracy
Horizontal
Accuracy
<+0.5 sFT†
<+1.65 sFT †
+0.3 sFT ‡
+0.5 sFT ‡
One U.S. survey foot (sFT) is equal to 0.30480061 while an international survey foot is equal to exactly 0.3048.
†
Accuracy information represents equipment error after post processing (Aeroquest 2011).
‡
MVM provided accuracy information that represents equipment error after post-processing.
§
U.S. Army Engineer Research and Development Center Coastal and Hydraulics Laboratory
||
As per the National Geodetic Survey (NGS) Interactive Computations page, elevation in the Island 63 area differs by an average of 0.36 ft between
Geoid 03 and Geoid 12A (NGS 2015).
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Figure 2.
(A) Extent of 2009 LiDAR and areas where 2005 LiDAR were added. White space
represents submerged areas with no LiDAR data. (B) Elevation model with study
sites labeled: MBP = Modoc Borrow Pits, I63 = Island 63 Secondary Channel, GrB =
Graveyard Blue Hole, ORC = Old River Chute, McW = McWilliams Lake, JmS = Jim
Samples Lake, GlH = Glory Hole Lake, I64 = Island 64 Secondary Channel, DsL =
Desoto Lake, Mlw = Mellwood Lake, and SfD = Sunflower Dikes Secondary Channel.
One set of LiDAR data was downloaded from the U.S. Geological Survey (USGS) Earth Explorer
as laser (LAS) files (USGS 2015). These data were originally collected from 19 February 2009 to
2 August 2010 (Table 1). Data extended across the majority of the project area and were collected
as points with a 1 m2 average spacing (Figure 2) (Aeroquest 2011). LAS file versions 1.1 onward
contain a classification scheme (0 = never classified, 1 = unassigned, 2 = bare earth, etc.). To
investigate the classification scheme and check data quality, a LAS dataset was created in ArcGIS,
and the downloaded LAS files were loaded into it. Statistics were generated to determine point
spacing, Z minimum, Z maximum, classifications, and total number of points (see ESRI
[Environmental Systems Research Institute] 2008 for techniques and an in-depth discussion).
Afterwards, the LAS files underwent two processing steps in RiSCAN Pro (RIEGL 2015). The
first step generated a bare-earth dataset from the LAS files by removing all vegetation, building,
and water classifications. Next, to improve processing speeds, the Octree filter was used to
decrease point density in flat areas based on user-designated thresholds for horizontal distance and
elevation change. The end products were ArcGIS multipoint files. The second set of LiDAR data
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MRG&P Tech Note No. 1
was acquired in 2005 at a lower water surface elevation than the 2009 data. These data were
provided as multipoint files by the St. Louis District, U.S. Army Corps of Engineers (USACE),
and were used to fill in areas of missing data within the project area (Figure 2).
For study waterbodies where there were no LiDAR data, bathymetric data were gathered. In 2013,
bathymetric transect data were gathered in 1000 ft intervals by side-scan sonar. Each transect was
perpendicular to flow, and the survey encompassed the Mississippi River main channel and
connected secondary channels. The Memphis District gathered multibeam bathymetry in 2014 for
the Island 63 secondary channel, and ERDC-CHL gathered additional data on 7–8 April 2015 for
the five small waterbodies southwest of Island 63.
Bathymetric data were gathered in 2014 using a Reson SeaBat® 7101-ER multibeam echosounder
and in 2015 by an Odom Echotrac® CV300 echosounder (Teledyne Thousand Oaks, CA) outfitted
with a 240 kHz and 200 kHz transducer, respectively. The echosounder emits a sound pulse and
records the time it takes for the sound pulse to bounce off the floor of the waterbody and be
received by the echosounder’s microphone. By calibrating the equipment to the speed that sound
travels through the location’s water, time can be converted into distance, which is subtracted from
the water surface elevation to determine the bed elevation. Horizontal position for each bed
elevation point (latitude and longitude) were received by a Trimble® R8 RTK GPS receiver
(Trimble Navigation Limited, Sunnyvale, CA) from two Trimble® R8 base stations (survey-grade
GPS units) centered on NGS control points (benchmarks). The base stations transmitted position
correction information at random intervals to the receiver. This information along with gage
readings was used to create water-level files that were edited and paired with echosounder readings
by HYPACK® software (Xylem Brand, Middletown, CT) to yield a X, Y, Z file (ASCII). These
files were then converted into ArcGIS multipoint files by using the ASCII 3D to Feature Class tool
(ESRI 10.2, Redlands, CA).
Elevation Modeling. To display the elevation data and locate connection thresholds, a bareearth model of the project area was created in the form of an ArcGIS terrain model (Figure 2). To
create the model, all bathymetry and LiDAR data were converted into a common horizontal
projection (NAD1983 UTM Zone15N). Vertical projection and Geoid were also investigated to
understand the vertical variation introduced and to control for this where possible. Data were then
combined within an ArcGIS feature dataset, and the terrain model was created from this dataset.
Terrain parameters were 5 ft point spacing, window size pyramid type, point selection method of
Z minimum with no secondary thinning, and five pyramid levels. A terrain model is the newer
version of an ArcGIS Triangulated Irregular Network (TIN) model. Pyramids (lower resolution
versions) allow terrain models to draw and process faster than TIN models, but terrains still contain
and use the full data for calculations. As with TINs, terrain models generate a surface by creating
a network of triangles following the Delaunay triangulation method. A terrain model was chosen
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over a raster (digital elevation model, DEM) because the triangulation method allows unique
elevation values across the entire surface. For a full discussion of terrain design, see ESRI (2016).
Connection Thresholds. Once
the model was created, it was visually
studied to identify and digitize all
channels that connect to the study
sites. Any high areas within these
channels were identified as potential
connection thresholds (Figure 3).
Connection-threshold identification
was aided by creating profile graphs
of the digitized channels (3D Analyst,
ESRI 10.2, 2013). Thresholds with
the appearance of steep, tall berms
(Figure 3) are frequently, if not
always, sites of water control
structures and should be field
verified. The bottom of the control
structure could represent the
connection threshold; however, other
higher-elevation locations may exist
within the channel (Figure 3). Once
connection
thresholds
were
identified, polygons were drawn
around each location (Figure 3).
Figure 3. Detail of connecting channel, berm with culvert,
connection threshold and its elevation, and
Because the LiDAR and bathymetric
LiDAR points clipped from the LiDAR multipoint
point data were contained within
file for Graveyard Blue Hole.
multipoint files, individual point
elevations could not be displayed. Thus, LiDAR and bathymetric point files were clipped by the
connection-threshold polygons, returning only the points within each polygon. The X, Y, Z data
were added to the subsequent point file. The points were then symbolized by their Z values to
identify the elevation water must exceed to pass through the area (connection threshold)
(Figure 3).
For sites with only bathymetric transect data, a series of aerial imagery collected at different river
stages was used to identify connecting-channel locations. The elevations of the bathymetric
transect points falling within these channels were studied to determine the approximate
connection-threshold elevation. For some connecting channels with no associated bed elevation
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data, field observations of connectivity at specific gage heights were used to establish the
connection threshold.
Connection Frequency. Water surface elevation at the Helena, AR, gage; the average river
slope between Helena at RM 663.1 and Friar Point at RM 652.5; and connection-threshold
elevations were used to determine timing and frequency of surface-water connections between
waterbodies and the Mississippi River main channel. Timing and frequency are approximations
because they rely on a comparison of a onetime measurement of the connection-threshold
elevation; in reality, the connection-threshold elevation may change over time. Connectivity is
ecologically relevant in conservation biology. Infrequent connection promotes unique species
assemblages, increasing overall ecosystem diversity. A more frequent single surface connection
allows aquatic nutrients and organisms to move between the river’s main channel and floodplain
waterbodies. Two or more connections provide flow-through conditions, flushing accumulated
suspended organic matter and equalizing water quality. Surface connectivity was quantified by
the following metrics:
1.
2.
3.
4.
The approximate percent of time the site was connected from 2000 to 2015
The study site’s connection status on the sampling day
The number of days prior to the sampling day since the site had been connected
The number of days connected during this connection event
A measure of slope was needed to account for north to south elevation change; the LMR at Cairo
is approximately 315 ft above sea level (asl) and 0 ft asl at its mouth. The slope between Helena
and Friar Point and Helena and Fair Landing was calculated at four discrete discharges and the
average slope chosen. This method does not account for daily variation in water surface slope. A
more accurate method that does account for daily changes is to use daily readings from two
different gages in close proximity or that bracket the project area. This method was not used
because there was no second suitable gage. Nearby gages had long periods with no readings or
were downstream of tributaries whose inputs change the water surface slope of the river. The
analysis period of 2000–2015 was chosen because it encompasses a full range of hydrologic
conditions (drought to flood years) and stage discharge comparisons indicate a relatively stable
river bed at the study site during this time.
Helena gage water surface elevations were downloaded from rivergages.com and converted from
NGVD29 to NAVD88 with the VERTCON (2015) conversion of −0.151 ft. Water surface
elevations for days with no data were determined by averaging the previous and subsequent day’s
data. The slope, river miles, and the Helena gage were used to convert the connection-threshold
elevation to an elevation at the Helena gage. For example, the Helena gage is located at RM 663.1,
and river water enters Mellwood Lake at RM 625.8. Thus Mellwood’s connection threshold can
be converted to a Helena stage by converting 37.3 miles to feet, multiplying by the slope, and
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MRG&P Tech Note No. 1
adding the value to the connection threshold. This value is then compared to Helena gage daily
water surface elevations by using tables and functions within Microsoft Excel to calculate the
connectivity metrics. Connectivity metrics were then validated by comparing predicted
connection dates with observed connection dates recorded during field sampling and by HOBO®
U20L-01 data loggers (Onset Computer Corporation, Bourne, MA) deployed at a subset of sites.
SUMMARY: Connectivity conservation is a global initiative to create or rehabilitate corridors
between areas of high biodiversity (Crooks and Sanjayan 2006). The approach outlined in this
document is the first step in understanding connectivity thresholds and frequency between the
main channel of the Mississippi River and its floodplain. LiDAR and bathymetric data were used
to create a nearly continuous bare-earth model of a 20-mile reach around Island 63. This model
was used to find channels that connected floodplain waterbodies to the main channel of the
Mississippi River. Connection thresholds were then located, a river mile assigned, and the
frequency of connection determined using gage data. Study results will provide quantifiable
metrics to rank planning projects, to evaluate current conditions, and to forecast future conditions,
aiding in the management and enhancement of riverine and floodplain biodiversity. Monitoring
and improving connectivity will also satisfy certain terms and conditions of USACE’s
Conservation Plan in the LMR (Killgore et al. 2014), inform reasonable and prudent measures in
the U.S. Fish and Wildlife Service’s (USFWS) Biological Opinion (USFWS 2013), and support
preparation of environmental assessments. Future studies of the location, elevation, and frequency
of connection on the remaining connected Mississippi floodplain waterbodies at a district- or
division-wide scale could document whether an array of connectivity is being maintained and
prioritize areas to improve conservation corridors in the LMR.
ADDITIONAL INFORMATION: This Mississippi River Geomorphology and Potamology
Program (MRG&P) Technical Note was prepared by Amanda J. M. Oliver, OSE Jaya Corporation,
and Catherine E. Murphy, Charles D. Little, Jr., and K. Jack Killgore, U.S. Army Engineer
Research and Development Center, Vicksburg, MS. The study was funded by MRG&P. Additional
information pertaining to MRG&P may be obtained from Dr. Barb Kleiss, Mississippi Valley
Division. This Technical Note should be cited as follows:
Oliver, A. J. M., C. E. Murphy, C. D. Little, Jr., and K. J. Killgore. 2016. Measuring
Connectivity of Floodplain Waterbodies to the Lower Mississippi River. MRG&P Tech
Note No. 1. Vicksburg, MS: U.S. Army Engineer Research and Development Center.
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MRG&P Tech Note No. 1
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NOTE: The contents of this technical note are not to be used for advertising, publication,
or promotional purposes. Citation of trade names does not constitute an official
endorsement or approval of the use of such products.
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