Nearshore Bathymetric Mapping along a 7-mile Reach of Lake Sharpe Shoreline near Lower Brule, South Dakota, 2013

By

Ryan F. Thompson

99°36' 99°35' 99°34' 99°33'

44°08'

44°07'

44°06'

44°05'

44°04'

EXPLANATION

Lower Brule

Lake Sharpe

7-Mile reach

Lagoons

0 0.5 1.0 MILE

0 0.5 1.0 KILOMETER

Study area

SOUTH DAKOTA

NEBRASKA

IOWA

MINNESOTA

NORTH DAKOTA

MONTANA WYOMING

Oahe Dam

Big Bend Dam

Base from U.S. Department of Agriculture, National Agriculture 

Imagery Program (NAIP), 2012

Universal Transverse Mercator projection, zone 14

Figure 1. Location of 7-mile reach of Lake Sharpe shoreline surveyed in study area near Lower Brule, South Dakota, in 2013.

ABSTRACT 

Shoreline erosion rates along Lake Sharpe, a Missouri River 

reservoir, near the community of Lower Brule, South Dakota, were 

studied previously during 2011‒12 by the U.S. Geological Survey, the Lower Brule Sioux Tribe, and Oglala Lakota College. The 

rapid shoreline retreat has caused many detrimental effects along 

the shoreline of Lake Sharpe, including losses of cultural sites, 

recreation access points, wildlife habitat, irrigated cropland, and 

landmass. The Lower Brule Sioux Tribe is considering options to 

reduce or stop erosion. One such option for consideration is the 

placement of discontinuous rock breakwater structures in shallow 

water to reduce wave action at shore. Information on the depth of 

water and stability characteristics of bottom material in nearshore 

areas of Lake Sharpe is needed by the Lower Brule Sioux Tribe to 

develop structural mitigation alternatives. To help address this need, 

a bathymetric survey of nearshore areas of Lake Sharpe near Lower 

Brule, South Dakota, was completed in 2013 by the U.S. Geological 

Survey in cooperation with the Lower Brule Sioux Tribe.

HYPACK® hydrographic survey software was used to plan data 

collection transects for a 7-mile reach of Lake Sharpe shoreline near 

Lower Brule, South Dakota. Regular data collection transects and 

oblique transects were planned to allow for quality-assurance/quality-control comparisons.

Two methods of data collection were used in the bathymetric 

survey: (1) measurement from a boat using bathymetric instrumentation where water was more than 2 feet deep, and (2) wading using 

Real-Time Kinematic Global Navigation Satellite System equipment on shore and where water was shallower than 2 feet deep. A 

dual frequency, 24- or 200-kilohertz narrow beam, depth transducer 

was used in conjunction with a Teledyne Odom CV100 dual frequency echosounder for boat-based data collection. In water too 

shallow for boat navigation, the elevation and nature of the reservoir 

bottom were mapped using Real-Time Kinematic Global Navigation 

Satellite System equipment.

Once the data collection effort was completed, data editing was 

performed in HYPACK® to remove erroneous data points and to 

apply water-surface elevations. Maps were developed separately 

for water depth and bottom elevation for the study area. Lines of 

equal water depth for 2, 3, 3.5, 4, and 5 feet from the water surface 

to the lake bottom were mapped in nearshore areas of Lake Sharpe. 

Overall, water depths stay shallow for quite a distance from shore. In 

the 288 transects that crossed a 2 foot depth line, this depth occurred 

an average of 88 feet from shore. Similarly, in the 317 transects that 

crossed a 3 foot depth line, this did not occur until an average of 

343 feet from shore. Elevation contours of the lake bottom were 

mapped primarily for elevations ranging from 1,419 to 1,416 feet 

above North American Vertical Datum of 1988.

Horizontal errors of the Real-Time Kinematic Global Navigation Satellite System equipment for the study area are essentially 

inconsequential because water depth and bottom elevation were 

determined to change relatively slowly. The estimated vertical error 

associated with the Real-Time Kinematic Global Navigation Satellite System equipment for the study area ranges from 0.6 to 0.9 inch. 

This vertical error is small relative to the accuracy of the bathymetric data.

Accuracy assessments of the data collected for this study were 

computed according to the National Standard for Spatial Data Accuracy. The maps showing the lines of equal water depth and elevation 

contours of the lake bottom are able to support a 1-foot contour 

interval at National Standards for Spatial Data Accuracy vertical 

accuracy standards, which require a vertical root mean squared error

of 0.30 foot or better and a fundamental vertical accuracy calculated 

at the 95-percent confidence level of 0.60 foot or better.

INTRODUCTION

Shoreline erosion along a 7-mile reach of Lake Sharpe, near the 

community of Lower Brule, South Dakota, was studied previously 

during 2011‒12 by the U.S. Geological Survey, the Lower Brule 

Sioux Tribe, and Oglala Lakota College (Neitzert and others, 2012). 

Lake Sharpe is a reservoir on the Missouri River created by the 

construction of Big Bend Dam and operated today by the U.S. Army 

Corps of Engineers. The loss of land area associated with this erosion has caused many detrimental effects in the nearshore areas of 

Lake Sharpe, including the losses of cultural sites, recreation access 

points, wildlife habitat, and irrigated cropland and landmass. Water 

treatment costs at the Lower Brule Rural Water System have risen 

due to increased sediment found in water from Lake Sharpe, and 

sedimentation around the intake structure has necessitated extending the intake farther into Lake Sharpe (Jim MacCauley, Program 

99°36' 99°35' 99°34' 99°33'

44°08'

44°07'

44°06'

44°05'

44°04'

Lake Sharpe

Lower Brule

EXPLANATION

Lagoons

Transects

 Regular

 Oblique

U.S. Geological Survey 

reference point used for base 

station—Punch point in top of 

rebar driven into ground along 

the edge of concrete pad. 

Elevation of reference point is 

1,428.86 feet above North 

American Vertical Datum of 

1988

0 0.5 1.0 MILE

0 0.5 1.0 KILOMETER

Base from U.S. Department of Agriculture, National Agriculture 

Imagery Program (NAIP), 2012

Universal Transverse Mercator projection, zone 14

Figure 2. Locations of planned regular and oblique transects in nearshore areas of Lake Sharpe in the study area, 2013.

Manager, Lower Brule Rural Water System, oral commun., 2014). 

Of special concern is the rapid advance of the shoreline toward the 

municipal wastewater lagoons (fig. 1) located southeast of Lower 

Brule, S. Dak.

The Lower Brule Sioux Tribe is considering options that would 

reduce or stop erosion and perhaps mitigate some of the habitat that 

has already been lost. One option being considered is the placement 

of discontinuous rock breakwater structures in shallow water to 

reduce wave action at the shore and potentially allow a plant margin 

to establish. The calmer water between these structures and shore 

also may provide backwater habitat. Information on the depth of 

water and stability characteristics of bottom material in nearshore 

areas of Lake Sharpe is needed by the Lower Brule Sioux Tribe to 

determine the proper placement of breakwater structures and estimate the quantity of material required for construction of breakwater 

structures. To help address this need, a bathymetric survey of nearshore areas of Lake Sharpe near Lower Brule, South Dakota, was 

completed in 2013 by the U.S. Geological Survey in cooperation 

with the Lower Brule Sioux Tribe.

SURVEY PREPARATION

HYPACK® hydrographic survey software was used (HYPACK, 

Inc., 2013) to plan data collection activities. Data collection transects 

were planned for nearshore areas in a 7-mile reach of Lake Sharpe 

shoreline in the study area near Lower Brule, S. Dak. (fig. 2). Regular data collection transects were laid out approximately perpendicular to shore and 100 feet apart. Oblique transects were laid out 

approximately 1,000 feet apart and oriented at roughly 45 degrees 

from regular transects to allow for quality-assurance/quality-control 

comparisons. This resulted in a total of 318 regular transects and 

31 oblique transects. Because nearshore depths were unknown before 

the bathymetric survey, the required length of the regular transects 

also was unknown to achieve water depths of at least 5 feet. Regular 

transects typically were planned with a 400-foot length, but a second 

segment was added to most transects to extend them to approximately 800 feet. This allowed some regular transects to be angled to 

avoid crossing adjacent transects and achieve better data coverage 

(fig. 2). The intent was to collect data on regular transects from the 

shore outward into the lake a minimum of 400 feet, or farther if 

required until water was at least 5 feet deep. One additional transect 

was planned, which ran approximately parallel to shore in water 

about 3 to 4 feet deep. It is hoped that data from this parallel transect 

can be used in conjunction with future geophysical studies of the 

area.

Vertical and horizontal control for data collection was from a 

reference point established for use in a previous study (Neitzert and 

others, 2012). A 4.75-hour static Global Navigation Satellite System (GNSS) occupation on a nearby Bureau of Land Management 

cadastral disk (with no known published coordinates, but determined to be located at latitude 44°07'49.64693" North, longitude 

99°34'45.65437" West) was processed using the Online Positioning 

User Service (OPUS; National Geodetic Survey, 2014a). Using the 

cadastral disk as a base, a 5-minute Real-Time Kinematic (RTK) 

occupation was completed to establish the coordinates of a punch 

point in the end of a length of rebar driven into the ground near a 

concrete pad. At the next site visit, the cadastral disk was found to 

have been destroyed, so the rebar was used as control for the bathymetric survey for this study and is referred to as the base station 

(fig. 2). Additional OPUS occupations were made during data collection to ensure that the rebar had not moved.

DATA COLLECTION METHODS

Two methods of data collection were used in the bathymetric 

survey. From July 29 to August 7, 2013, boat-based data were collected using bathymetry instrumentation where water was more 

than 2 feet deep. From August 12 to 15, wading-based data were 

collected using Real-Time Kinematic Global Navigation Satellite 

System (RTK GNSS) equipment on shore and in water about 2 feet 

deep or shallower. Boat-based data collection also used RTK GNSS 

for navigation and determination of the water-surface elevation. 

The RTK GNSS is a global positioning method where a stationary 

receiver (known as a base station) uses a radio to send out realtime differential correction signals to one or more mobile receivers 

(known as rovers). The GNSS surveying methods used to support 

this study met the criteria for a Level IV GNSS survey (Rydlund and 

Densmore, 2012). The data collection procedures and equipment 

used are described in the following subsections.

BOAT-BASED DATA COLLECTION

A 171⁄2-foot, flat-bottom aluminum boat was used for boat-based 

data collection activities. This hull shape has a shallow draft that 

allows the vessel to travel in shallow water. Because boat-based data 

collection is substantially faster than wading-based data collection, it 

was preferable to use the boat on as much of any given data transect 

as was practical. A mount was attached near the front of the boat and 

used to deploy a Teledyne ODOM OTSBB 200/24-4-20 (over-theside, broadband, dual-frequency [200-kilohertz and 4-degree beam 

width or 24-kilohertz and 20-degree beam width]) depth transducer 

in an Airmar M-42 housing and the RTK GNSS receiver used for 

guidance. The 4-degree beam width provides a smaller “footprint” 

on the reservoir bottom than the 20-degree beam, and can provide 

more accurate depths, especially in steep areas. The 200-kilohertz 

and 4-degree beam width were used for data collection on the regular and oblique transects. Data streams from the 200-kilohertz, 

4-degree beam and the 24-kilohertz, 20-degree beam were collected 

on the parallel transect. The transducer was used in conjunction 

with a Teledyne Odom CV100 dual-frequency echosounder, which 

was connected to a laptop running HYPACK® software. The RTK 

GNSS receiver provided a very accurate horizontal location (within 

an inch) for each depth data point, and allows HYPACK® to display 

the position of the vessel relative to data transects. This allows the 

boat operator to adjust travel direction and maintain position on the 

data transects. During data collection, an effort was made to keep 

the transducer within 10 feet on either side of data transects; however, this was not always possible. In some locations, the boat was 

navigated around weedbeds, whereas in other locations, data needed 

to be collected far beyond the end of the data transect to reach water 

depths of 5 feet (fig. 3). In addition to horizontal location, the RTK 

GNSS connected to the echosounder also recorded water-surface 

elevation for each depth data point. These water-surface elevation 

data were averaged by transect and used with depth and antenna 

offset (distance between the GNSS antenna and the water surface) to 

determine the elevation of the lake bottom.

WADING-BASED DATA COLLECTION

In water too shallow for boat navigation (less than 2 feet), the 

elevation and nature of the reservoir bottom were mapped using 

RTK GNSS. Wading-based data collection took place after boatbased data collection was completed. In this way, portions of 

transects where data had already been collected by boat could be 

identified, and wading-based data collection only needed to extend 

to (or slightly overlap) the boat-based data (fig. 3). Because the 

wading-based data were not collected simultaneously with boatbased data, changes in water surface elevation would mean that 

water depths between the two data collection methods would not 

be directly comparable. Windy conditions and resulting waves 

during wading-based data collection would have made it difficult to 

estimate depth accurately. To avoid these problems, depth was not 

recorded with bottom elevation and firmness. Wading-based data 

points were collected from the toe of the bank out to the approximate location where the boat data began. Once wading was completed for all transects, an additional point was collected at the top 

of bank for those transects that had a visible bank. Data points along 

the transect were variably spaced, depending on changes in slope. 

Points were as close as 1 foot apart in steep areas, but generally 

were several feet apart in flatter areas. An average of 3 seconds 

worth of data was stored as the elevation of the lake bottom. The 

bottom of the rove pole was not allowed to sink into the lake bottom, but was held at the surface of the lake bottom material. Similar 

to the collection of the boat-based data, an effort was made to collect 

wading-based data within 10 feet on either side of the data transects. 

However, many locations had fallen timber that made it difficult to 

stay near the transect, and a few areas had overhanging tree canopy 

that interfered with satellite signal. 

At each wading-based data point, a qualitative assessment of the 

firmness of the bottom material was made by the surveyor, based 

on how much the surveyor’s feet sank in the bottom material. A 

label indicating the degree of firmness was assigned to the point and 

stored with the elevation data. Labels used to describe the bottom 

firmness are summarized in table 1; of the 3,609 wading-based data 

99°36' 99°35' 99°34' 99°33'

44°08'

44°07'

44°06'

44°05'

44°04'

Lake Sharpe

Lower Brule

EXPLANATION

Data collection method

 Wading oblique

 Wading regular

 Boat oblique

 Boat regular

U.S. Geological Survey reference 

point used for base station— 

Punch point in top of rebar driven 

into ground along the edge of 

concrete pad. Elevation of 

reference point is 1,428.86 feet 

above North American Vertical 

Datum of 1988

0 0.5 1.0 MILE

0 0.5 1.0 KILOMETER

Base from U.S. Department of Agriculture, National Agriculture 

Imagery Program (NAIP), 2012

Universal Transverse Mercator projection, zone 14

Figure 3. Extent of boat- and wading-based data collected in nearshore areas of Lake Sharpe in the study area, 2013.

Table 1. Labels used to describe bottom firmness at wading-based 

data points.

Label Description

F Firm bottom. The feet of the surveyor did not sink detectably.

S1 Soft bottom, with the feet of the surveyor sinking approximately 1 inch.

S2 Soft bottom, with the feet of the surveyor sinking approximately 2 inches.

S3 Soft bottom, with the feet of the surveyor sinking approximately 3 inches.

S4 Soft bottom, with the feet of the surveyor sinking approximately 4 inches.

points, most (2,704) had a firm bottom. The spatial distribution of 

the wading-based data points labeled with a soft bottom is shown on 

figure 4. Only 126 points (about 3.5 percent) had labels indicating 

the feet of the surveyor sank 3 or 4 inches.

QUALITY-ASSURANCE/QUALITY-CONTROL 

MEASURES

Quality-assurance/quality-control measures were made to ensure 

that the data collected were accurate, meaningful, and allowed an 

analysis of uncertainty. The quality-assurance/quality-control measures used in data collection are described in this section.

Following good surveying practices (Rydlund and Densmore, 

2012) can help ensure accuracy of the GNSS data. A 6.562-foot 

(2-meter) fixed-height tripod was used at the base station to avoid 

antenna measurement errors. Birds-eye levels were used on the base 

tripod and rove poles to ensure the antennae were held plumb and 

vertical while collecting data. An average of three measurements 

was used for each wading-based data point. Data were only collected when integer ambiguities were fixed (an indicator of solution 

stability), and with a minimum of four common satellites between 

the base and rover GNSS receivers.

The accuracy of depth data output from the echosounder and 

transducer was verified at the beginning and end of each day of 

boat-based data collection by using bar checks. During a bar check, 

an underwater object (in this study, a perforated metal plate) is 

passed below the transducer at multiple known depths, which are 

compared to the depths output by the echosounder. When differences exist between the known and output depths, data corrections 

can be calculated and applied to the data. Care must be taken during 

bar checks to ensure that the plate stays parallel to the water surface, 

and its depth below the water surface can be accurately determined. 

Using a heavy weight, tethering the corners of the metal plate to the 

weight, and avoiding areas of waves or high-water velocity during 

bar checks were techniques used to ensure that the plate stayed parallel to the water surface. For this study, the barcheck apparatus was 

suspended from the boat using a brake winch with a depth gage. For 

each bar check, the depth gage was set to zero when the plate was 

held at the water surface. The depths selected for bar checks must 

bracket the range of depths expected during data collection. Because 

the water in the study area was relatively shallow and is mixed by 

currents and waves, wide temperature variations within the water 

column are not expected. Water temperature was used to select an 

appropriate value for the speed of sound in water (www.engineeringtoolbox.com/sound-speed-water-d_598.html, accessed July 29, 

2013).

Another way to assure quality data is to assess the accuracy 

of an instrument through independent repeated measurements. A 

description of data quality based on comparison of repeat measurements is presented in the “Accuracy Assessment of Data and Maps” 

section.

99°36' 99°35' 99°34' 99°33'

44°08'

44°07'

44°06'

44°05'

44°04'

Lake Sharpe

Lower Brule

EXPLANATION

Bottom firmness

 Sunk 1 inch

 Sunk 2 inches

 Sunk 3 inches

 Sunk 4 inches

0 0.5 1.0 MILE

0 0.5 1.0 KILOMETER

Base from U.S. Department of Agriculture, National Agriculture 

Imagery Program (NAIP), 2012

Universal Transverse Mercator projection, zone 14

Figure 4. Spatial distribution of wading-based data points where a soft bottom was found.

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not 

imply endorsement by the U.S. Government.

This map was printed on an electronic plotter directly from digital files. Dimensional calibration may vary 

between electronic plotters and between X and Y directions on the same plotter, and paper may change 

size due to atmospheric conditions; therefore, scale and proportions may not be true on plots of this map.

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment—visit http://www.usgs.gov or call 1–888–ASK–USGS 

This report is available at http://dx.doi.org/10.3133/sim3307.

Suggested Citation: 

Thompson, R.F., 2014, Nearshore bathymetric mapping of a 7-mile reach of Lake Sharpe shoreline near 

Lower Brule, South Dakota, 2013: U.S. Geological Survey Scientific Investigations Map 3307, 2 sheets,

http://dx.doi.org/10.3133/sim3307.

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

Lower Brule Sioux Tribe

Scientific Investigations Map 3307

Sheet 1 of 2

ISSN 2329-132X (online)