EXPLANATION

Elevation, in feet above North American 

Vertical Datum of 1988

6,124

6,115

6,106

6,096

6,087

0 1 2 3

0 1 2 3

MILES

KILOMETERS

43°N

111°40'W

111°35'W

42°55'N

42°50'N

Split

Island

Benchmark

Benchmarks

Long Island

111°40'W

42°55'N

111°35'W

42°50'N

0 1 2 3 4 MILES

0 1 2 3 4 KILOMETERS

2000–2001 LiDAR collection area

2011 multibeam echosounder (MBES) transect lines

EXPLANATION

Bathymetric Survey

Ground Survey

MBES and 2000–2001 LiDAR area of overlap

2011 ground-survey data collection point

43°N

111°35'W

42°55'N

42°50'N

111°40'W

0 1 2 3

0 1 2 3

MILES

KILOMETERS

EXPLANATION

Topography

6,130

6,140

6,150

6,160

6,170

6,180

6,190

6,200

6,210

6,220

6,230

6,090

6,095

6,100

6,105

6,110

6,115

6,120

Bathymetry

Reservoir full pool 

at 6,124

Elevation, in feet above North American 

Vertical Datum of 1988 

6,240

6,250

6,260

6,270

6,280

6,290

6,300

6,310

6,320

6,330

6,340

")

34

Henry

111°50'W

111°40'W

111°30'W

111°20'W

111°10'W

43°N

42°50'N

42°40'N

IDAHO

IDAHO FALLS

SODA SPRINGS

Blackfoot Subbasin

Blackfoot River

Blackfoot 

Reservoir

Meadow Creek

Corral Creek

Blackfoot River

Brush Creek

Slug Creek

Diamond Creek

GOVERNMENT

DAM #

Northern

Eastern 

#

#

Southern

0 3 6 9 12 MILES

0 3 6 9 12 KILOMETERS

Blackfoot subbasincontinues northwest

EXPLANATION

Stage monitoring station 

 and name

Wind monitoring station 

 and name

#

Southern

Northern

Figure 2. Data-collection points used to 

calculate a revised stage-capacity table for 

Blackfoot Reservoir, Idaho, from a 2000–2001 

LiDAR survey and 2011 MBES and ground 

surveys.

Figure 6. Bathymetric and topographic 

contours of Blackfoot Reservoir, Idaho, 2011.

Figure 5. Bathymetric surface elevations in Blackfoot Reservoir, 

Idaho, 2011. A historical river channel visible in the northern half 

of the reservoir and its absence in the southern and eastern parts of 

the reservoir indicates some sedimentation.

tac12-0762_fig02

tac12-0762_fig04ab

0

2

4

6

8

10

12

14

16

18

20

22

24

26

6,122.7

6,122.8

6,122.9

6,123.0

6,123.1

6,123.2

6,123.3

6,123.4

6,123.5

6,123.6

6,123.7

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

320

340

360

6,122.7

6,122.8

6,122.9

6,123.0

6,123.1

6,123.2

6,123.3

6,123.4

6,123.5

6,123.6

6,123.7

07-19-11

12:00 

07-19-11

15:00 

07-19-11

18:00 

07-19-11

21:00 

07-20-11

0:00 

Stage, in feet above the North American Vertical Datum of 1988 

Wind direction, in degrees 

Wind speed, in miles per hour

A.

B.

Wind speed

Northern station

Stage

Northern station

Eastern station

Southern station

EXPLANATION

Wind direction

Northern station

Stage

Northern station

Eastern station

Southern station

EXPLANATION

An example of a wind event

on July 19, 2011, when wind 

speed and direction did affect 

reservoir surface slope.

(North) (South) (North) (West) (East) 

90 ̊

180 ̊

270 ̊

0/360 ̊

Frequency of occurrence,

in percent (%)

Wind speed, in 

miles per hour

EXPLANATION

≥ 25

20–25

13–20

8–13

5–8

 1–5 

Calms: 2.87 percent

tac12-0762_fig01

Abstract

The U.S. Geological Survey (USGS), in cooperation with the Shoshone-Bannock 

Tribes, surveyed the bathymetry and selected above-water sections of Blackfoot Reservoir, 

Caribou County, Idaho, in 2011. Reservoir operators manage releases from Government 

Dam on Blackfoot Reservoir based on a stage-capacity relation developed about the time 

of dam construction in the early 1900s. Reservoir operation directly affects the amount of 

water that is available for irrigation of agricultural land on the Fort Hall Indian Reservation 

and surrounding areas. The USGS surveyed the below-water sections of the reservoir using 

a multibeam echosounder and real-time kinematic global positioning system (RTK-GPS) 

equipment at full reservoir pool in June 2011, covering elevations from 6,090 to 6,119 feet 

(ft) above the North American Vertical Datum of 1988 (NAVD 88). The USGS used data 

from a light detection and ranging (LiDAR) survey performed in 2000–2001 to map 

reservoir bathymetry from 6,116 to 6,124 ft NAVD 88, which was mostly in depths too 

shallow to measure with the multibeam echosounder, and most of the above-water section 

of the reservoir (above 6,124 ft NAVD 88). Selected points and bank erosional features 

were surveyed by the USGS using RTK-GPS and a total station at low reservoir pool in 

September 2011 to supplement and verify the LiDAR data. The stage-capacity relation 

was revised and presented in a tabular format. The data sets show a 2.0-percent decrease in 

capacity from the original survey, due to sedimentation or differences in accuracy between 

surveys. A 1.3-percent error also was detected in the previously used capacity table and 

measured water-level elevation because of a questionable reference elevation at monitoring 

stations near Government Dam. Reservoir capacity in 2011 at design maximum pool of 

6,124 ft above NAVD 88 was 333,500 acre-ft.

Introduction

Blackfoot Reservoir is an 18,000-acre lake located about 15 mi north of Soda Springs, 

Caribou County, in southeastern Idaho (fig. 1). The reservoir was formed in 1909–10, after 

the completion of Government Dam on the Blackfoot River, and primarily is used now 

(2012) for irrigation supply and recreation. Reservoir releases are managed by the Bureau 

of Indian Affairs (BIA) based on a stage-capacity relation developed in 1933 using watersurface elevations (stages) referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29). The reservoir supplies irrigation water for about 50,000 acres of agricultural 

land near Pocatello and on the Fort Hall Indian Reservation. Elevation of the reservoir 

bottom ranges from 6,090 to 6,124 ft above the North American Vertical Datum of 1988 

(NAVD 88), and the reservoir holds 333,500 acre-ft of water at full pool elevation (6,124 ft 

above NAVD 88). Drainage area to the reservoir is 581 mi2 (Harenberg and others, 1989). 

The U.S. Geological Survey (USGS) operated a non-recording stage-monitoring station on 

the northern end of the reservoir near the dam from 1912 to 1925 and from 1929 to 1989 

(USGS station 13065000). The BIA and Idaho Power Company also have operated stagemonitoring stations in the reservoir near the dam over various time periods.

The USGS surveyed the bathymetry of Blackfoot Reservoir in 2011 to determine 

temporal changes due to sedimentation and other factors, and to improve the overall 

accuracy of the stage-capacity table. Bathymetric surveys of high accuracy and resolution 

are possible through advances in instrumentation, notably multibeam echosounders 

(MBESs). An MBES coupled with real-time kinematic global positioning systems 

(RTK-GPS) has advantages over traditional techniques for surveying reservoir bathymetry, 

such as single-beam echosounders, acoustic Doppler profilers, or soundings, because 

they can provide more coverage of the reservoir bed and real-time data corrections. The 

topography (above-water) of the reservoir, which could not be measured with the MBES, 

was surveyed by Horizons, Inc., in 2000 using terrestrial light detection and ranging 

(LiDAR), and verified in selected areas by USGS in 2011 using RTK-GPS surveying 

equipment. 

Wind is a factor that is not accounted for in a typical bathymetric or terrestrial survey, 

but it can influence an observer’s perception of reservoir capacity and operations. Wind 

moving across a reservoir can cause the water surface to be sloped, resulting in increased 

stage on one end of the reservoir. For example, if wind causes high stage near the dam 

at Blackfoot Reservoir, reservoir managers may release water under the assumption that 

the stage at the dam is constant across the reservoir. In this case, the current storage in 

the reservoir would be overestimated, and too much water may be released. The USGS 

evaluated this phenomenon through wind and stage monitoring in May–November 2011.

Previous Investigations

The original stage-capacity table for Blackfoot Reservoir was developed in 1933 

using data from topographic maps that were made about the time of construction of 

Government Dam (David Bollinger, Bureau of Indian Affairs, written commun., 2012). 

Selected elevations and capacities from the 1933 stage-capacity table are published in U.S. 

Geological Survey (1956) and in various USGS Annual Water Data Reports from 1912 

through 1989 (Harenberg and others, 1989). Horizons, Inc., surveyed the above-water 

section of the reservoir in autumn 2000, when stages were about 6,116 ft above NAVD 88, 

using terrestrial LiDAR equipment resulting in a point spacing of 14 × 7 ft. Horizons, Inc., 

provided the LiDAR data to Dowl HKM in March 2001, who used the data to prepare a 

map of the exposed reservoir pool with 2-ft contour intervals from 6,116 to 6,124 ft above 

NAVD 88 (Gary Elwell, Dowl HKM, written commun., 2004). No parts of the reservoir 

below 6,116 ft were surveyed during this study. Dowl HKM noted a 2-percent decrease 

in reservoir capacity from 1933 to 2000 in the range of surveyed elevations (Gary Elwell, 

Dowl HKM, written commun., 2004).

The U.S. Army Corps of Engineers rebuilt the spillway at Government Dam in 1984, 

and the spillway crest elevation was raised 2 ft from 6,122.5 to 6,124.5 ft NAVD 88 

(reported as 6,118.5–6,120.5 ft NGVD 29) (Gary Elwell, Dowl HKM, written commun., 

2004). Dowl HKM noted a discrepancy in the reported spillway crest elevation during 

their 2000–2001 study. Project personnel observed water spilling over the spillway crest 

when the reservoir pool elevation was measured at 6,124.41 ft NAVD 88 (reported as 

6,120.41 ft NGVD 29), and determined that the true spillway elevation must be closer to 

6,124.0 ft NAVD 88 (reported as 6,120.0 ft NGVD 29). The USGS verified the spillway 

elevation as near 6,124.0 ft NAVD 88 (6,120.0 ft NGVD 29) during the 2011 study using 

RTK-GPS surveying equipment. The spillway is slightly sloped with highest elevations 

on the northern side (6,124.2 ft NAVD 88), and lowest elevations on the southern side 

(6,124.0 ft NAVD 88). 

Methods

Field data were collected at various times from May 29 to November 30, 2011. The 

USGS collected stage data at three locations and wind data at one location. Reservoir 

bathymetry was measured using MBES equipment on June 6–24, 2011, at maximum 

pool (average 6,124.0 ft NAVD 88), and selected ground elevations were measured using 

RTK-GPS and a total station on September 26–30, 2011, at minimum pool for the year 

(average 6,119.3 ft NAVD 88). 

Stage and Wind Monitoring

The USGS installed three stage-monitoring stations and one wind monitoring station 

on Blackfoot Reservoir in May and June 2011 (fig. 1). The stage and wind monitoring 

station (USGS station 13065000) on the northern end of the reservoir near Government 

Dam was installed on May 27, 2011. Station equipment included a Sutron ACCUBAR® 

bubbler pressure sensor to measure reservoir stage, a Vaisala WINDCAP® sonic 

anemometer to measure wind speed and direction, and a Sutron SatLink2 datalogger to 

record and transmit data. Data were recorded every 15 minutes and transmitted through 

satellite telemetry every hour. The anemometer stored instantaneous and 12-minute 

average readings of wind speed and direction. Average wind readings were used for the 

analysis unless an anomaly was detected, such as an erroneous reading resulting from a 

leaf or bird obstructing one of the sensors. In this case, the instantaneous readings were 

used in place of the average readings if they were reasonable and did not appear to be 

outliers. Stage data from the northern station from May 27 to November 30, 2011, are 

published in U.S. Geological Survey (2011). 

The other two stage-monitoring stations were installed on the eastern and southern 

sides of the reservoir on June 21, 2011. The two additional stations were compared with 

the stage and wind monitoring data at the northern station to determine whether wind 

causes enough slope in the reservoir’s water surface to warrant consideration of wind in 

reservoir operation. The stations were equipped with two In-Situ Level TROLL® 500 

vented pressure transducers mounted on a weighted platform on the reservoir bed. The 

vented cables extended to the water surface to a buoy, and the cable ends were housed in 

a locked case attached to the top of the buoy, enabling USGS personnel to periodically 

download data from the transducers. Stage at all three monitoring stations was referenced 

to a staff plate or other stable reference point installed near each station. Reference points 

on each staff plate were surveyed during the ground RTK-GPS survey to tie sensor 

readings into a common datum for comparison. Stage sensors were reset to the staff plate 

reading at each visit. Corrections for sensor drift or malfunction were applied as needed 

by pro-rating over time depending on the cause and magnitude of the correction. 

Bathymetric Survey 

USGS personnel determined that a full-coverage MBES bathymetric survey for the 

reservoir was not required to create a stage-capacity table that would accurately represent 

the true capacity of the reservoir. Wilson and Richards (2006) stated that a planned 

line interval of approximately 1 percent of the longitudinal length of the reservoir is 

suitable for defining a bathymetric surface. The spacing of MBES survey lines used for 

the Blackfoot Reservoir survey was 500 ft, which is approximately 0.6 percent of the 

longitudinal length of the reservoir (fig. 2). This smaller spacing better defined some of 

the more complex bathymetric features around the islands and shorelines. A 2-ft interval 

contour map created from the LiDAR data was used to create the boundary extent for 

the bathymetric survey. The LiDAR survey (2000) was conducted at a stage of 6,116 ft 

NAVD 88, which is 8 ft below full pool. This data set provided spatial data in parts of the 

reservoir that would normally have been below water (6,116–6,124 ft NAVD 88). Due to 

depth constraints, most parts of the reservoir in this elevation range were unmeasurable 

by the MBES system. The MBES survey was planned to include all areas less than 

around 6,116 ft and include overlap of the MBES and LiDAR data above 6,116 ft 

NAVD 88 wherever possible.

Horizontal and Vertical Control

Horizontal and vertical control for the bathymetric survey was established by setting 

three temporary benchmarks on two islands located within the reservoir. A National 

Geodetic Survey-designated benchmark “Gray GPS”, located about 7 mi east of the 

center of the reservoir, was used to survey and verify the temporary benchmarks. Two 

benchmarks were established on the reservoir to be used as daily set-up locations for the 

bathymetric survey, and a third was used as a daily coordinate check benchmark. Two 

benchmarks were established on Long Island and one benchmark was established on 

Split Island (fig. 2).

Data Collection and Processing

The equipment used to conduct the bathymetric survey included an Odom 

Hydrographic Systems ES-3 multibeam echosounder; an International IndustriesTM 

Figure 1. Location of Blackfoot Reservoir and watershed in Caribou County, Idaho.

Figure 3. Distribution and frequency of wind measured at the northern monitoring 

station on Blackfoot Reservoir, Idaho, June 22–November 30, 2011.

Figure 4. Graphs showing wind speed (A) and direction (B) measured at the 

northern monitoring station, and impacts on stage at the eastern, northern, and 

southern monitoring stations on Blackfoot Reservoir, Idaho, July 19–20, 2011. 

Oval indicates a wind event on July 19, 2011, when wind speed and direction 

affected reservoir slope.

capacity of 1.0–1.5 percent, respectively. Reservoir capacity calculated from 

the average elevation of the three stations compared within 0.9 percent on 

average of reservoir capacity calculated from only the stage at the northern 

station.

As a whole, most differences in stage due to wind and gradients within 

the reservoir were relatively small and probably are within the measurement 

error of the stage-monitoring instruments and of the uncertainty of the 

MBES survey (0.21 ft). Therefore, reservoir operations based on stage at the 

northern station are considered sufficient to represent true capacity of the 

reservoir during the period of study. 

DSM-10 TSS dynamic motion reference unit (roll, pitch, yaw); and an Odom DIGIBARPro profiling sound velocimeter to provide continuous near-surface sound velocity data. A 

HemisphereTM VS110 heading and position receiver using two GPS antennas mounted over 

the echo sounder transducer provided a precise heading. A more detailed description of the 

equipment used for the bathymetric survey is described in Fosness (in press). Horizontal and 

vertical positioning was accomplished using an RTK-GPS receiver mounted directly above the 

MBES and radio linked to a static base-station receiver using a GPS antenna positioned over 

one of the three temporary benchmarks. The elevation for the reservoir-bottom was calculated 

in real-time, so a water-surface measurement was not necessary for the bathymetric survey.

Bathymetry data were collected using HYPACK® software’s HYSWEEP® software 

(HYPACK, Inc., 2011). The raw data were post-processed to remove erroneous data caused 

by macrophyte growth and false echo returns. An algorithm called the Combined Uncertainty 

Bathymetric Estimator (CUBE) was built into the HYSWEEP® software suite. The CUBE 

method is a robust statistical filtering approach that incorporated both data points and the total 

propagated uncertainty of each component of the MBES to determine the estimate for position and 

elevation within a gridded surface. The CUBE results from the Blackfoot Reservoir bathymetry 

were output to a 3×3 ft gridded surface that defined the Easting (X), Northing (Y), and elevation 

(Z) in ASCII format. In some areas, a CUBE estimate could not be resolved in the software. In 

those areas, the data were reduced to the median value and output to a 3×3 ft gridded surface.

Ground Survey

Elevations of major erosional bank surfaces and reference points near monitoring stations 

were measured on September 26–30, 2011 (fig. 2). The ground survey was used to verify 

that the Dowl HKM LiDAR data set from 2000 to 2001 properly represented major erosional 

banks and other features that could affect the capacity-table calculation. USGS personnel used 

a survey-grade RTK-GPS, in conjunction with a Trimble VX total spatial station to collect 

ground-survey data. The RTK-GPS and Trimble VX total spatial station system provide 

a combined accuracy of 0.039 ft horizontally and 0.072 ft vertically (Trimble Navigation 

Limited, 2003, 2010), and has built-in constraints to exclude any data collected outside of 

the desired accuracy limits. The RTK-GPS data were collected using the 2009 Geoid model 

(Roman and others, 2010) to derive NAVD 88 elevations, and post-processed in Trimble 

Business Center software (Trimble Navigation Limited, 2011). The ground-survey data were 

referenced to the National Geodetic Survey’s Continuously Operating Reference Station 

(CORS) network (Snay and Soler, 2008) by establishing accurate base-station locations using 

the Static Online Positioning User Service (OPUS-S) (Weston and others, 2007). The survey 

data used the same spatial reference as the bathymetric and LiDAR data: State Plane Idaho 

East Zone referenced by the North American Datum of 1983 and NAVD 88. 

Quality Control and Quality Assurance

Static observations were collected daily at the RTK-GPS base station to ensure accuracy 

within the survey and provide quality assurance to the horizontal and vertical positioning 

established for each of the temporary benchmarks. The National Geodetic Survey’s OPUS-S 

was used to post-process the static data for each day of surveying. Daily benchmark checks 

were made to ensure the RTK-GPS was properly set up and that the coordinates matched those 

established at the temporary benchmarks.

Physical offsets were measured for each of the MBES components and entered into the 

software to ensure proper translation for the MBES data solutions. A patch test was conducted 

for the multibeam system prior to the start of the survey to ensure that the physical offsets 

were correctly aligned for latency, pitch, roll, and yaw according to the National Oceanic and 

Atmospheric Administration (2012). The physical offset measurements, the results of the patch 

test, and uncertainty estimates for each component of the MBES were entered into the total 

propagated uncertainty (TPU) editor module built into HYPACK®. The TPU module quantifies 

all sources of error in the survey by accounting for the settings, field conditions, physical 

offsets, and patch test results. 

The CUBE module also was used to output an uncertainty estimate for each 3×3 ft gridded 

cell. The CUBE module utilized the TPU editor to account for the uncertainty associated with 

each component used for the MBES survey. The estimated mean uncertainty for the bathymetric 

survey was approximately 0.21 ft.

The MBES and 2000–2001 LiDAR data overlapped between elevations 6,116–6,119 ft, 

allowing a comparison between methods and detection of changes in parts of the reservoir 

(areas with black dots in fig. 2). More than 48,000 points were available for a comparison 

between data sets made using a 6-ft grid. The reported vertical accuracy of the LiDAR survey 

was ±0.7 ft (Horizons, Inc., written commun., 2001). On average, the MBES elevations were 

0.95 ft lower than the LiDAR elevations in areas where data sets overlapped. Thirty-four 

percent of the comparison points matched within the combined uncertainty, calculated as the 

square root of the sum of square errors, of the MBES survey (0.21 ft) and LiDAR survey 

(0.7 ft), which was 0.73 ft. 

The 2011 ground survey data confirm that the LiDAR data were collected in the presence 

of the major erosional banks, and the LiDAR data does represent the vertical nature of the 

erosional banks within the limits of the LiDAR data density. The RTK-GPS ground survey 

elevations were on average 0.8 ft lower than the LiDAR elevations in areas where comparisons 

were possible. This difference is just slightly higher than the combined uncertainty (0.704 ft) 

of the RTK-GPS ground survey (0.072 ft) and LiDAR survey (0.7 ft). Some of the differences 

may have been due to varying benchmarks or control points used to establish vertical and 

horizontal control and true differences in location and elevation of comparison points. 

Overall, however, the LiDAR data are considered to be good quality and were used along with 

USGS-collected data sets to create a revised stage-capacity table.

Effect of Wind on Reservoir Surface Slope

The principal wind directions during the study were from the west (48 percent of observations), and 

from the south (32 percent of observations), with an overall average wind vector from the southwest 

(fig. 3). USGS personnel found that wind speed and direction generally do not substantially affect 

reservoir surface slope except during episodic events of a few hours in duration. Seven distinct events 

were identified during the study during days when wind speed and direction did affect reservoir surface 

slope. During those events, increased wind speeds from the west decreased stage at the northern station 

and increased stage at the eastern and southern stations. An example of such a wind event on July 19, 

2011, is shown within the oval in figure 4, when a spike in wind of 24 mph from 290 degrees (west) 

caused stage at the northern station to decrease about 0.3 ft, and stage at the eastern and southern 

stations to increase 0.1 and 0.2 ft, respectively. Degree of response at the eastern and southern stations 

differed among events. Increased wind speeds from the south increased stage at the northern station and 

decreased stage at the eastern and southern stations, similar to but opposite the pattern shown in figure 

4. Fluctuations occurred over several hours throughout the day, but did not result in a sustained change in 

reservoir surface slope. Change in stage among the stations ranged from 0.1 to 0.3 ft over the seven events. 

During the study period, the average difference in stage was –0.2 ft between the northern and 

eastern stations, and –0.3 ft between the northern and southern stations. Some of the differences may be 

attributed to differences in the accuracy of the instruments used to measure stage at the eastern, southern, 

and northern stations. The instruments installed at the eastern and southern stations were intended 

to be temporary and were less accurate (accuracy ± 0.1 percent of depth; In-Situ Inc., 2010) than the 

instrument installed at the northern station (accuracy ± 0.02 percent of depth; Sutron Corp., 2011). At 

full pool elevation, a difference of 0.2–0.3 ft in reservoir surface elevation results in a difference in 

Revised Stage-Capacity Relation

Geographic information system (GIS) software was used to produce a threedimensional surface of the reservoir-bottom elevations from the bathymetric, 

LiDAR, and ground-survey data (fig. 5). The surface was created by combining 

the point measurements from the LiDAR, bathymetric, and ground surveys, and 

using the Natural Neighbor interpolation tool in ArcGISTM (Environmental Systems 

Research Institute, Inc., 2011) to create a raster surface with a 3-ft cell size. The 

USGS then used ArcGISTM to create an area and capacity table based on the raster 

surface showing surface area and capacity (volume) at specified elevations in 0.2-ft 

increments (table 1). Interpolation to 0.2 ft was desired for comparison with the 

stage-capacity table previously in use, although capacities at 0.8-ft intervals are 

more reliable because the combined uncertainty of the MBES and LiDAR surveys 

was 0.73 ft. The bathymetry and near-shore surface (derived from LiDAR and 

ground-survey data only) were contoured using ArcGISTM, and the contours were 

cartographically edited to create a topographic map with 5-ft contours below and 

10‐ft contours above 6,124 ft NAVD 88 (fig. 6). Reservoir capacity at full pool 

(6,124 ft NAVD 88) is 333,500 acre-ft based on 2011 bathymetric and ground survey 

data, and 2000–2001 LiDAR data. Reservoir capacity at minimum pool measured 

during the study period (6,119 ft NAVD 88) is 248,500 acre-ft (table 1). 

Comparisons in capacity from the original 1933 survey, the 2000–2001 Dowl 

HKM study, and 2011 USGS study are shown in table 2. The comparison is limited 

to elevations between 6,116 and 6,124 ft NAVD 88 because reservoir elevations 

below 6,116 ft were not surveyed during the Dowl HKM study. Reservoir capacity 

in this range of elevations has decreased on average by 2.0 percent since the original 

survey, which supports results from the Dowl HKM study. The differences could 

be caused by changes in the accuracy of survey methods, and minor sedimentation 

in the reservoir. The bathymetric survey showed an absence of a historical river 

channel in the southern and eastern parts of the reservoir, which indicates some 

sedimentation (fig. 5). A well-defined thalweg was evident in the northern half of the 

reservoir indicating that little, if any, sedimentation has occurred over time in that 

region.

The USGS noted discrepancies in stages at monitoring stations near Government 

Dam operated by the USGS and other agencies. The Idaho Power Company 

monitoring station is referenced to the U.S. Army Corps of Engineers benchmark 

called “BDR-5” located on a hill north of Government Dam above the spillway, 

which was assumed to have an elevation of 6,161.79 ft NAVD 88 (reported as 

6,157.79 ft NGVD 29 in a 1989 USGS station description) (Mark Kolowith, Idaho 

Power Company, written commun., 2012). The BIA monitoring station is referenced 

to a finished floor elevation at the BIA dam pump house. The USGS station was 

temporarily referenced upon installation to the same finished floor elevation as the 

BIA station. When the USGS surveyed reference marks in September 2011 using 

RTK-GPS, USGS station elevations were then referenced to benchmarks established 

during the MBES survey. The stage at the USGS station based on the 2011 RTKGPS survey decreased 0.40 ft after the survey correction. After the correction 

was applied to the USGS station, the BIA and Idaho Power monitoring stations 

measured an average 0.23 ft lower and 0.52 ft higher, respectively, than the USGS 

station during the 2011 study period. The USGS surveyed the U.S. Army Corps of 

Engineers “BDR-5” benchmark in May 2012 and found that the previous elevation 

used was about 0.48 ft too high, which explains most of the discrepancy between 

the stages measured at the Idaho Power and USGS monitoring stations. The BIA 

station has been used as the principal data source for managing reservoir operation 

from 1989 to 2011. An error of –0.23 ft in stage results in an average 1.3 percent 

underestimation of reservoir capacity in both the old and revised stage-capacity table 

over the elevation range of 6,116–6,124 ft above NAVD 88. The stages and stagecapacity table described herein are based on the elevations measured at the 2011 

USGS monitoring station with applied corrections.

Acknowledgments

The USGS wishes to thank Elese Teton with the Shoshone-Bannock Tribes, Brett Bovee 

with Natural Resources Consulting Engineers, and Dale Cunnington and Barbara Turcotte 

with Dowl HKM for providing information and various data sets. Special thanks also go to 

USGS field personnel who assisted with data collection, including Marshall Williams, Jay 

Bateman, Jake Jacobson, Alex Etheridge, and Dorene MacCoy.

References Cited

Environmental Systems Research Institute, Inc., 2011, ArcGIS Desktop Help 10–Natural Neighbor 

(Spatial Analyst): Environmental Systems Research Institute, Inc. website accessed December 1, 

2011, at http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//009z0000006p000000.htm.

Fosness, R.L., in press, Bathymetric surveys of the Kootenai River near Bonners Ferry, Idaho, water 

year 2011: U.S. Geological Survey Data Series 694.

Harenberg, W.A., Jones, M.L., O’Dell, I., and Cordes, S.C., 1990, Water resources data, Idaho, water 

year 1989: U.S. Geological Survey Water Data Report ID-89-1, 695 p. 

HYPACK, Inc., 2011, Hypack Hydrographic Survey Software User Manual: Middletown, Conn., 

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manual: Silver Spring, Md., National Oceanic and Atmospheric Administration, accessed May 1, 

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United States, development of GEOID09: National Geodetic Survey, accessed March 26, 2012, 

at http://www.ngs.noaa.gov/PC_PROD/GEOID09/.

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gauge/recorder 56-0134—Operations and maintenance manual, rev. 1.50: Sterling, Va., Sutron 

Corp., accessed April 30, 2011, at http://www.sutron.com/downloads/manuals.htm. 

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Sunnyvale, Calif., Trimble Navigation Limited, 216 p.

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Navigation Limited.

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September 1950, Part 13, Snake River Basin: U.S. Geological Survey Water-Supply Paper 1317, 572 p. 

(Also available at http://pubs.er.usgs.gov/publication/wsp1317.) 

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search.jsp.

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International, v. 21, no. 4, p. 23–25.

Wilson, G.L., and Richards, J.M., 2006, Procedural documentation and accuracy assessment of bathymetric 

maps and area/capacity tables for small reservoirs: U.S. Geological Survey Scientific Investigations Report 

2006–5208, 24 p. plus oversize figs. (Also available at http://pubs.usgs.gov/sir/2006/5208/.)

Stage or reservoir 

elevation 

(feet above NGVD 29)

Reservoir 

elevation 

(feet above NAVD 88)

Area 

(acres)

Capacity 

(acre-feet)

 6,086.0 6,090.0 0.12 0.05 

 6,086.2 6,090.2 0.15 0.08 

 6,086.4 6,090.4 0.18 0.11 

 6,086.6 6,090.6 0.21 0.15 

 6,086.8 6,090.8 0.26 0.19 

 6,087.0 6,091.0 0.32 0.25 

 6,087.2 6,091.2 0.38 0.32 

 6,087.4 6,091.4 0.50 0.41 

 6,087.6 6,091.6 0.88 0.53 

 6,087.8 6,091.8 1.68 0.78 

 6,088.0 6,092.0 2.66 1.21 

 6,088.2 6,092.2 3.83 1.85 

 6,088.4 6,092.4 5.25 2.75 

 6,088.6 6,092.6 7.23 3.98 

 6,088.8 6,092.8 10.7 5.73 

 6,089.0 6,093.0 17.0 8.42 

 6,089.2 6,093.2 28.2 12.8 

 6,089.4 6,093.4 48.5 20.3 

 6,089.6 6,093.6 79.7 32.9 

 6,089.8 6,093.8 118 52.7 

 6,090.0 6,094.0 155 80.0 

 6,090.2 6,094.2 185 114 

 6,090.4 6,094.4 213 154 

 6,090.6 6,094.6 240 199 

 6,090.8 6,094.8 266 250 

 6,091.0 6,095.0 294 306 

 6,091.2 6,095.2 322 367 

 6,091.4 6,095.4 351 435 

 6,091.6 6,095.6 379 508 

 6,091.8 6,095.8 411 587 

 6,092.0 6,096.0 443 672 

 6,092.2 6,096.2 478 764 

 6,092.4 6,096.4 519 863 

 6,092.6 6,096.6 572 972 

 6,092.8 6,096.8 636 1,093 

 6,093.0 6,097.0 728 1,229 

Stage or reservoir 

elevation 

(feet above NGVD 29)

Reservoir 

elevation 

(feet above NAVD 88)

Area 

(acres)

Capacity 

(acre-feet)

 6,093.2 6,097.2 894 1,389 

 6,093.4 6,097.4 1,269 1,600 

 6,093.6 6,097.6 1,894 1,914 

 6,093.8 6,097.8 2,541 2,358 

 6,094.0 6,098.0 3,135 2,927 

 6,094.2 6,098.2 3,626 3,605 

 6,094.4 6,098.4 4,059 4,374 

 6,094.6 6,098.6 4,435 5,224 

 6,094.8 6,098.8 4,790 6,148 

 6,095.0 6,099.0 5,080 7,135 

 6,095.2 6,099.2 5,368 8,180 

 6,095.4 6,099.4 5,629 9,280 

 6,095.6 6,099.6 5,882 10,430 

 6,095.8 6,099.8 6,132 11,630 

 6,096.0 6,100.0 6,382 12,880 

 6,096.2 6,100.2 6,627 14,190 

 6,096.4 6,100.4 6,875 15,540 

 6,096.6 6,100.6 7,119 16,930 

 6,096.8 6,100.8 7,350 18,380 

 6,097.0 6,101.0 7,586 19,870 

 6,097.2 6,101.2 7,810 21,420 

 6,097.4 6,101.4 8,015 23,000 

 6,097.6 6,101.6 8,211 24,620 

 6,097.8 6,101.8 8,395 26,280 

 6,098.0 6,102.0 8,567 27,980 

 6,098.2 6,102.2 8,731 29,710 

 6,098.4 6,102.4 8,888 31,470 

 6,098.6 6,102.6 9,034 33,260 

 6,098.8 6,102.8 9,172 35,080 

 6,099.0 6,103.0 9,305 36,930 

 6,099.2 6,103.2 9,443 38,800 

 6,099.4 6,103.4 9,585 40,710 

 6,099.6 6,103.6 9,721 42,640 

 6,099.8 6,103.8 9,847 44,600 

 6,100.0 6,104.0 9,972 46,580 

 6,100.2 6,104.2 10,090 48,580 

Table 1. Reservoir area and capacity (volume) at specified elevations for Blackfoot Reservoir, Idaho, 2011.

[Elevations were converted from National Geodetic Vertical Datum of 1929 (NGVD 29) to North American Vertical Datum of 1988 (NAVD 88) using the 

National Oceanic and Atmospheric Administration VERTCON website (http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl) based on the latitude and 

longitude of the northern monitoring station. Conversion was 3.995 feet]

Stage or reservoir 

elevation 

(feet above NGVD 29)

Reservoir 

elevation 

(feet above NAVD 88)

Area 

(acres)

Capacity 

(acre-feet)

 6,100.4 6,104.4 10,210 50,610 

 6,100.6 6,104.6 10,320 52,670 

 6,100.8 6,104.8 10,440 54,740 

 6,101.0 6,105.0 10,550 56,840 

 6,101.2 6,105.2 10,660 58,960 

 6,101.4 6,105.4 10,760 61,100 

 6,101.6 6,105.6 10,870 63,270 

 6,101.8 6,105.8 10,970 65,450 

 6,102.0 6,106.0 11,080 67,660 

 6,102.2 6,106.2 11,180 69,880 

 6,102.4 6,106.4 11,280 72,130 

 6,102.6 6,106.6 11,380 74,390 

 6,102.8 6,106.8 11,480 76,680 

 6,103.0 6,107.0 11,590 78,990 

 6,103.2 6,107.2 11,690 81,320 

 6,103.4 6,107.4 11,790 83,660 

 6,103.6 6,107.6 11,900 86,030 

 6,103.8 6,107.8 11,990 88,420 

 6,104.0 6,108.0 12,090 90,830 

 6,104.2 6,108.2 12,180 93,260 

 6,104.4 6,108.4 12,280 95,700 

 6,104.6 6,108.6 12,370 98,170 

 6,104.8 6,108.8 12,470 100,700 

 6,105.0 6,109.0 12,570 103,200 

 6,105.2 6,109.2 12,660 105,700 

 6,105.4 6,109.4 12,750 108,200 

 6,105.6 6,109.6 12,850 110,800 

 6,105.8 6,109.8 12,940 113,400 

 6,106.0 6,110.0 13,030 116,000 

 6,106.2 6,110.2 13,120 118,600 

 6,106.4 6,110.4 13,210 121,200 

 6,106.6 6,110.6 13,290 123,900 

 6,106.8 6,110.8 13,370 126,500 

 6,107.0 6,111.0 13,460 129,200 

 6,107.2 6,111.2 13,560 131,900 

 6,107.4 6,111.4 13,640 134,600 

Stage or reservoir 

elevation 

(feet above NGVD 29)

Reservoir 

elevation 

(feet above NAVD 88)

Area 

(acres)

Capacity 

(acre-feet)

 6,107.6 6,111.6 13,720 137,400 

 6,107.8 6,111.8 13,800 140,100 

 6,108.0 6,112.0 13,880 142,900 

 6,108.2 6,112.2 13,960 145,700 

 6,108.4 6,112.4 14,030 148,500 

 6,108.6 6,112.6 14,100 151,300 

 6,108.8 6,112.8 14,170 154,100 

 6,109.0 6,113.0 14,240 156,900 

 6,109.2 6,113.2 14,310 159,800 

 6,109.4 6,113.4 14,370 162,700 

 6,109.6 6,113.6 14,440 165,600 

 6,109.8 6,113.8 14,510 168,400 

 6,110.0 6,114.0 14,570 171,400 

 6,110.2 6,114.2 14,630 174,300 

 6,110.4 6,114.4 14,700 177,200 

 6,110.6 6,114.6 14,760 180,200 

 6,110.8 6,114.8 14,820 183,100 

 6,111.0 6,115.0 14,900 186,100 

 6,111.2 6,115.2 14,980 189,100 

 6,111.4 6,115.4 15,060 192,100 

 6,111.6 6,115.6 15,130 195,100 

 6,111.8 6,115.8 15,200 198,100 

 6,112.0 6,116.0 15,270 201,200 

 6,112.2 6,116.2 15,330 204,200 

 6,112.4 6,116.4 15,400 207,300 

 6,112.6 6,116.6 15,470 210,400 

 6,112.8 6,116.8 15,540 213,500 

 6,113.0 6,117.0 15,610 216,600 

 6,113.2 6,117.2 15,680 219,700 

 6,113.4 6,117.4 15,750 222,900 

 6,113.6 6,117.6 15,820 226,000 

 6,113.8 6,117.8 15,900 229,200 

 6,114.0 6,118.0 15,970 232,400 

 6,114.2 6,118.2 16,040 235,600 

 6,114.4 6,118.4 16,110 238,800 

 6,114.6 6,118.6 16,180 242,000 

Table 1. Reservoir area and capacity (volume) at specified elevations for Blackfoot Reservoir, Idaho, 2011.—Continued

[Elevations were converted from National Geodetic Vertical Datum of 1929 (NGVD 29) to North American Vertical Datum of 1988 (NAVD 88) using the 

National Oceanic and Atmospheric Administration VERTCON website (http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl) based on the latitude and 

longitude of the northern monitoring station. Conversion was 3.995 feet]

Stage or reservoir 

elevation 

(feet above NGVD 29)

Reservoir 

elevation 

(feet above NAVD 88)

Area 

(acres)

Capacity 

(acre-feet)

 6,114.8 6,118.8 16,250 245,300 

 6,115.0 6,119.0 16,330 248,500 

 6,115.2 6,119.2 16,400 251,800 

 6,115.4 6,119.4 16,470 255,100 

 6,115.6 6,119.6 16,540 258,400 

 6,115.8 6,119.8 16,600 261,700 

 6,116.0 6,120.0 16,660 265,000 

 6,116.2 6,120.2 16,720 268,400 

 6,116.4 6,120.4 16,770 271,700 

 6,116.6 6,120.6 16,820 275,100 

 6,116.8 6,120.8 16,870 278,500 

 6,117.0 6,121.0 16,920 281,800 

 6,117.2 6,121.2 16,960 285,200 

 6,117.4 6,121.4 17,000 288,600 

Stage or reservoir 

elevation 

(feet above NGVD 29)

Reservoir 

elevation 

(feet above NAVD 88)

Area 

(acres)

Capacity 

(acre-feet)

 6,117.6 6,121.6 17,040 292,000 

 6,117.8 6,121.8 17,080 295,400 

 6,118.0 6,122.0 17,120 298,900 

 6,118.2 6,122.2 17,160 302,300 

 6,118.4 6,122.4 17,200 305,700 

 6,118.6 6,122.6 17,240 309,200 

 6,118.8 6,122.8 17,280 312,600 

 6,119.0 6,123.0 17,310 316,100 

 6,119.2 6,123.2 17,350 319,500 

 6,119.4 6,123.4 17,380 323,000 

 6,119.6 6,123.6 17,420 326,500 

 6,119.8 6,123.8 17,460 330,000 

 6,120.0 6,124.0 17,490 333,500

 6,120.2 1

6,124.2 17,530 337,000 

Table 1. Reservoir area and capacity (volume) at specified elevations for Blackfoot Reservoir, Idaho, 2011.—Continued

[Elevations were converted from National Geodetic Vertical Datum of 1929 (NGVD 29) to North American Vertical Datum of 1988 (NAVD 88) using the 

National Oceanic and Atmospheric Administration VERTCON website (http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl) based on the latitude and 

longitude of the northern monitoring station. Conversion was 3.995 feet]

1

 Full pool elevation is 6,124 feet NAVD 88, but stages up to 6,124.1 feet were measured at the northern station during the 2011 study. Reservoir area and 

capacity for a stage of 6,124.2 feet is included to bracket measured values.

Table 2. Comparison of reservoir capacity (volume) at selected 

elevations for 1933, 2000–2001, and 2011 surveys for Blackfoot 

Reservoir, Idaho.

[Reservoir elevations below 6,116 feet North American Vertical Datum of 

1988 (NAVD 88) were not surveyed in the 2000–2001 HKM study. Reservoir 

capacity at 6,116 feet was assumed to be the same as the 1933 Bureau of 

Indian Affairs (BIA) table. HKM, now Dowl HKM; USGS, U.S. Geological 

Survey]

Stage or 

reservoir 

elevation 

(feet above 

NAVD 88)

Capacity (acre-ft) Percentage of 

change in 

capacity, 

1933–2011

1933 BIA 

table

2000–01 

HKM 

Study

2011 

USGS

Study

6,116.0 205,400 205,400 201,200 -2.0

6,118.0 237,500 234,800 232,400 -2.1

6,120.0 270,600 267,100 265,000 -2.1

6,122.0 304,700 300,500 298,900 -1.9

6,124.0 339,800 334,800 333,500 -1.9

Average change -2.0

Bathymetry and Capacity of Blackfoot Reservoir, Caribou County, Idaho, 2011

By 

Molly S. Wood, Kenneth D. Skinner, and Ryan L. Fosness

2012

Scientific Investigations Map 3229

Bathymetry and Capacity—Sheet 1

By Wood, M.S., Skinner, K.D., and Fosness, R.L., 2012, Bathymetry and 

Prepared in cooperation with the Shoshone-Bannock Tribes capacity of Blackfoot Reservoir, Caribou County, Idaho, 2011

U.S. Department of the Interior

U.S. Geological Survey