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Suggested Citation: Kohn, M.S., 2012, Bathymetry of Groundhog

Reservoir, Dolores County, Colorado, 2011: U.S. Geological

Survey Scientific Investigations Map 3202, scale 1:8,000

[http://pubs.usgs.gov/sim/3202/].

Bathymetry of Groundhog Reservoir, Dolores County, Colorado, 2011

By

Michael S. Kohn

2012

station also continuously records its position. The rover communicates with the base station and is used to 

collect and record individual data points throughout the study area based on the position of the base station.

The daily water-surface elevation was determined from the RTK GPS survey-data points. The RTK GPS 

dataset not only included more than 700 water-surface elevation points over the course of the study, but almost 

6,000 data points were obtained on the shoreline and in the shallow areas of the lake that were not navigable 

by boat. The shoreline was surveyed from a lake depth of 3 ft to 2–3 ft above the top of the dam. 

All survey data were collected with a common coordinate system, geoid, ellipsoid, and datum. The 

coordinate system used was Universal Transverse Mercator (UTM), zone 12 north, the horizontal datum was 

NAD 83, and the vertical datum was North American Vertical Datum 1988 (NAVD 88), Geoid 03, ellipsoid 

World Geodetic System 1984 (WGS 84). Data from the base station collected throughout the study were 

submitted to the National Geodetic Survey’s Online Positioning User Service (OPUS) Web site for processing 

(http://www.ngs.noaa.gov/OPUS/, accessed October 19, 2011). All survey data were recomputed to reflect the 

OPUS solution correction.

POST PROCESSING AND DATA ANALYSIS

The motion sensor in the multibeam sonar records the instrument’s motion in terms of pitch, roll, and 

heave. Pitch is the alternating rise and fall of the boat’s bow and stern, roll is rotation of the boat about its 

main axis, heave is the vertical rise and fall of the entire vessel, and yaw is the rotation of the sonar from the 

boat’s main axis. The ambient offsets of pitch, roll, and yaw were corrected using a calibration test, otherwise 

known as a patch test, as the instrument was not installed exactly vertical and in line with the boat’s main axis. 

From the patch test, the pitch offset angle was determined to be 10 degrees and the roll 0.40 degree. The yaw 

offset angle was not corrected as no consistent value could be determined from the patch test. On days 2 and 3 

of the study, navigational and heave data were not collected due to instrument malfunction. As a result, pitch, 

roll, and yaw information was not collected during those days. After further examination, it was determined 

that the malfunction may have led to a positive bias of up to 1.5 percent or a total of 140 acre-feet (ac-ft) in 

reservoir volume based on the area surveyed on day two and three. Therefore, the volumes determined at each 

stage will reflect a positive bias of up to but no more than 140 ac-ft.

 The post processing of the multibeam echo sounder bathymetry data was performed using HYPACK 

2011 from HYPACK, Inc. (HYPACK, Inc., 2011). All the data were filtered using the automated search and 

filter tool. Then, manual filtering was performed on the multibeam echo sounder bathymetry data to edit any 

spikes or inconsistencies that the automated filter overlooked. Once all the data were filtered, the data were 

separated by the observation day. The multibeam echo sounder bathymetry data were interpolated onto a grid, 

with individual grid cells having sides of 6.56 ft, by computing the average depth in each cell and positioning 

the mean at the cell center. 

The RTK GPS water-surface elevation dataset was filtered manually to remove errant or poor data. Then, 

the daily mean water-surface elevation was computed from the edited dataset. The multibeam echo sounder 

bathymetry data, which initially used the water-surface elevation as a datum, were converted to elevation 

(NAVD 88) using the daily mean water-surface elevation. The result was one dataset with a common horizontal and vertical datum (NAD 83 and NAVD 88). 

The bathymetry dataset was imported into Environmental Systems Research Institute (ESRI) ArcMap 

9.3.1 (Environmental Systems Research Institute, Inc., 2010) so the elevation contours and reservoir area and 

volume could be determined from the lake bottom to approximately 2–3 ft above the top of dam. Once the 

6.56-ft grid was created using HYPACK 2011, it was merged with the RTK GPS dataset in ArcMap by 

generating a triangulated irregular network (TIN) from both datasets. The TIN was then converted to a 6.56-ft 

horizontal grid, and using the contour tool in ArcMap, contours at intervals of 10 ft were produced. The areas 

and volumes at various elevations were computed using the surface volume tool in ArcMap (figs. 1–3, and 

table 1).

In figure 1, some of the upper contours are not continuous; this is mainly due to the spillway and irrigation canal not allowing for the continuation of the contours as the spillway and canal inverts are below the 

elevation of the top of the dam. However, in the northwest end of the reservoir and east of the Nash Creek 

inlet, sufficient data were not collected to resolve the contours causing the breaks. Where breaks in contours 

occurred, it was assumed the areas and volumes above that point were computed as if a vertical wall existed at 

that point. That assumption provided better results than using a high resolution digital elevation model. When 

comparing the 10-meter digital elevation model (DEM) (Gesch and others, 2002) at the reservoir with the data 

collected, it appeared that the DEM did not provide sufficient accuracy to improve the dataset collected for 

this report. Therefore, no DEM was used to supplement the dataset and the results of this report were generated only from field data.

BATHYMETRY OF GROUNDHOG RESERVOIR

From the RTK GPS data, the top of the dam was approximately 8,681 ft above NAVD 88, the spillway 

invert was 8,671 ft above NAVD 88, and full-pool elevation was 8,669 ft above NAVD 88. At the spillway 

elevation, the surface area of the lake is 738 acres (ac) and the volume is 25,800 ac-ft. The minimum elevation of the reservoir is located 100 ft below the spillway elevation. As shown in figure 3, the volume increases 

very rapidly with depth initially, but at elevations greater than 8,640 ft above NAVD 88, the stage-volume 

curve is approximately linear.

SUMMARY

A better understanding of available water supply aids water managers in their operation of reservoirs 

during periods of drought and (or) high demand, and, should another bathymetric survey be conducted in the 

future, provides a dataset that also could be used to determine sedimentation infill rate and the useful lifespan 

of the reservoir. The U.S. Geological Survey, in cooperation with the Dolores Water Conservancy District, 

carried out a bathymetry study of Groundhog Reservoir, Dolores County, Colorado, from June 27–29, 2011. 

The study was performed using a man-operated boat-mounted multibeam echo sounder integrated with GPS 

navigation and RTK GPS. The vertical and horizontal precision of the multibeam echo sounder GPS as rated 

by the manufacturer are ±0.065 ft and ±0.032 ft, respectively. The vertical and horizontal precision of the 

RTK GPS as rated by the manufacturer are ±0.066 ft and ±0.033 ft, respectively. From the RTK GPS data, the 

ABSTRACT

In order to better characterize the water supply capacity of Groundhog Reservoir, Dolores County, 

Colorado, the U.S. Geological Survey, in cooperation with the Dolores Water Conservancy District, conducted 

a bathymetric survey of Groundhog Reservoir. The study was performed in June 2011 using a man-operated 

boat-mounted multibeam echo sounder integrated with a global positioning system and a terrestrial real-time 

kinematic global positioning system. The two collected datasets were merged and imported into geographic 

information system software. A bathymetric map of the reservoir was generated in addition to plots for the 

stage-area and the stage-volume relations.

INTRODUCTION

Groundhog Reservoir is owned and operated by Montezuma Valley Irrigation Company, and water from 

the reservoir is used for the irrigation of agricultural land in Dolores and Montezuma Counties in southwestern 

Colorado. Because the storage capacity of Groundhog Reservoir is not documented and storage capacity 

information would aid water-resources managers in their operation of the reservoir during periods of drought 

and (or) high demand, the U.S. Geological Survey (USGS), in cooperation with the Dolores Water Conservancy District, conducted a bathymetric study of Groundhog Reservoir to determine the stage-surface area and 

stage-volume relations of the reservoir. This report provides water managers with a better understanding of 

available water supply in the reservoir and, should another bathymetric survey be conducted in the future, a 

dataset that also could be used to determine sedimentation infill rate and the useful lifespan of the reservoir. 

This report supports one of the USGS strategic directions of water supply by providing information related to 

the water census of the United States.

PURPOSE AND SCOPE

This report presents the results of the bathymetric evaluation of Groundhog Reservoir. The report 

contains a description of the data collection and analytical methods used to survey the reservoir and to develop 

datasets which aid water managers. A bathymetric map of the reservoir is presented with plots for the stagearea and the stage-volume relations. 

DESCRIPTION OF STUDY AREA

Groundhog Reservoir is located in southwest Colorado at 37° 47' 26" N. latitude and 108° 17' 29" W. 

longitude relative to North American Datum of 1983 (NAD 83) at the confluence of Groundhog and Nash 

Creeks in eastern Dolores County (U.S. Geological Survey, 2011). Groundhog Reservoir is a man-made lake 

created by the impoundment of Groundhog Creek (Montezuma Valley Irrigation Company, 1938). Construction of the dam was completed in 1908, and it was enlarged in 1938 to its current dimensions (Montezuma 

Valley Irrigation Company, 1938). The drainage basin of the reservoir is 16.2 square miles (U.S. Geological 

Survey, 2011).

The rights to the water in the reservoir are owned by Montezuma Valley Irrigation Company; however, 

recreational activities are permitted on the reservoir. Approximately 6 miles downstream from Groundhog 

Reservoir, Groundhog Creek flows into the West Dolores River, a branch of the Dolores River, which is a 

tributary of the Colorado River (U.S. Geological Survey, 2011).

BATHYMETRIC MEASUREMENT AND STORAGE ANALYSIS

The USGS performed a bathymetric survey of Groundhog Reservoir using a man-operated boat-mounted 

multibeam echo sounder integrated with a global positioning system (GPS) and a terrestrial real-time 

kinematic (RTK) GPS in June 2011. For the purposes of this report, data collected from both the boatmounted multibeam echo sounder and terrestrial RTK GPS will be considered bathymetric data. The multibeam echo sounder collected data at lake depths of approximately 3 feet (ft) and greater, whereas a terrestrial 

topographic survey was performed with the RTK GPS in shallow areas near the shore not navigable by boat to 

the elevation of the top of the dam plus about 2 to 3 additional feet.

BATHYMETRIC MEASUREMENTS USED IN DATA COLLECTION

Bathymetric data from the multibeam echo sounder were collected June 27–29, 2011, using a Teledyne 

Odom Hydrographic ES3PT–M integrated multibeam echo sounder and motion sensor (Teledyne Odom 

Hydrographic, Inc., 2011) equipped with a Trimble SPS461 GPS receiver using procedures described in 

Wilson and Richards (2006). The vertical and horizontal precision of the multibeam echo sounder GPS as 

rated by the manufacturer are ±0.065 ft and ±0.032 ft, respectively (Trimble Navigation Limited, 2009b). The 

multibeam echo sounder has a swath width of 120 degrees capable of collecting data to depths of 197 ft. The 

echo sounder collects 240 data points at a sample frequency of 12 hertz (Hz); however, this varies with depth. 

The GPS generates position data at a rate of 5 Hz and navigational data at a rate of 1 Hz. A Teledyne Odom 

Hydrographic Real Time Appliance (RTA) was used for data synchronization of all aforementioned instruments resulting in a data string recorded at 1 Hz. The bathymetry data from the multibeam echo sounder were 

compiled and stored using the hydrographic survey software, HYPACK 2010 from HYPACK, Inc. (HYPACK, 

Inc., 2010). 

The multibeam echo sounder emits a pulse at a frequency of 240 kilohertz, which is reflected off the lake 

bed and detected by the receiver. The velocity of the pulse is affected by the lake temperature and salinity. 

For Groundhog Reservoir, a freshwater lake, the effect of salinity was assumed to be negligible (U.S. Army 

Corps of Engineers, 2002). Velocity profiles of Groundhog Reservoir were recorded using a Teledyne 

Odom Hydrographic Digibar-Pro velocity meter throughout the study and were used to correct the data during 

post-processing (Teledyne Odom Hydrographic, Inc., 2001). Calibration points were obtained with the RTK 

GPS while the multibeam echo sounder was simultaneously recording to confirm the results.

The dam, principle outlet structure, and spillway also were surveyed using the RTK GPS. The terrestrial 

topographic survey was performed June 27–29, 2011, using a Trimble R8 GNSS RTK GPS receiver, a Trimble 

HPB450 radio modem, and a Trimble TSC2 controller. The vertical and horizontal precision of the RTK GPS 

as rated by the manufacturer are ±0.066 ft and ±0.033 ft, respectively (Trimble Navigation Limited, 2009a). 

The RTK GPS setup consisted of a base station, which included a receiver and radio, and a rover which 

consisted of a receiver and controller. The base station was located at a fixed position on the east end of the 

dam and receives information from satellites and transmits data to the rover’s receiver via the radio. The base 

top of dam was approximately 8,681 ft above North American Vertical Datum 1988 (NAVD 88) and the 

spillway invert was 8,671 ft above NAVD 88. The two collected datasets were merged and imported into 

geographic information system software. The results of the study include a stage-surface area and stagevolume relations and bathymetric map. At the spillway elevation, the surface area of the lake is 738 acres and 

the volume is 25,800 acre-feet.

ACKNOWLEDGMENTS

Several U.S. Geological Survey (USGS) employees helped with the data collection, including Colin G. 

Aanes, Jennifer L. Moore, Katherine S. Sams, and Jesse R. Zacher. Kristine L. Verdin and Barbara C. Ruddy 

(USGS) helped generate the map and associated graphics. The author would like to thank Paul J. Kinzel 

(USGS) for the technical support he provided during the data collection and analysis, as well as comments 

provided as a technical reviewer. Cory A. Williams (USGS) provided valuable comments as a technical 

reviewer. The data collection was made easier by Rich Van Loenen and Aryol Brumley who granted access to 

the shoreline of their property, and Don Magnuson of Montezuma Valley Irrigation Company for providing 

previous survey information and helping to coordinate the study. 

REFERENCES CITED

Environmental Systems Research Institute, Inc., 2010, ArcGIS—A complete integrated system: Redlands, 

Calif, ESRI. Accessed Oct. 17, 2011, from http://www.esri.com/software/arcgis/. 

Gesch, D., Oimoen, M., Greenlee, S., Nelson, C., Steuck, M., and Tyler, D., 2002, The National Elevation 

Dataset: Photogrammetric Engineering and Remote Sensing, v. 68, no. 1, p. 5-11.

HYPACK, Inc., 2010, HYPACK® 10.0.5.31, Hydrographic survey software user manual: Middletown, Conn., 

HYPACK, Inc. [variously paged], accessed Oct. 5, 2011, from 

http://www.hypack.com/new/Support/DocumentsManualsTechnicalArticlesetc/tabid/81/Default.aspx

HYPACK, Inc., 2011, HYPACK® 11.0.1.49, Hydrographic survey software user manual: Middletown, Conn., 

HYPACK, Inc. [variously paged], accessed Oct. 5, 2011, from 

http://www.hypack.com/new/Support/DocumentsManualsTechnicalArticlesetc/tabid/81/Default.aspx

Montezuma Valley Irrigation Company, 1938, Map of the Groundhog Reservoir: Cortez, Colo., Montezuma 

Valley Irrigation Company.

Trimble Navigation Limited, 2009a, Trimble® R8 GNSS receiver user guide, version 4.11, revision A: 

Dayton, Ohio, Trimble Navigation Limited, 50 p., accessed Nov. 4, 2011 from http://www.altop.com/sites/default/files/0Trimble_R8-R6-R4-5800M3_ENG.pdf

Trimble Navigation Limited, 2009b, Trimble® SPS461 specifications sheet: Dayton, Ohio, Trimble Navigation Limited, 2 p., accessed Nov. 4, 2011, from http://trl.trimble.com/docushare/dsweb/Get/Document445955/SPS461%20DGPS.pdf

Teledyne Odom Hydrographic, Inc., 2001, Digibar-Pro profiling sound velocimeter 1.0, Operation manual: 

Baton Rouge, La., Teledyne Odom Hydrographic, Inc. [variously paged], accessed Nov. 4, 2011, from 

http://www.odomhydrographic.com/media/pdf/product_resources/84.pdf

Teledyne Odom Hydrographic, Inc., 2011, ES3PT 1.4, User manual: Baton Rouge, La., Teledyne Odom 

Hydrographic, Inc. [variously paged], accessed Nov. 2, 2011, from 

http://www.odomhydrographic.com/media/pdf/product_resources/85.pdf

U.S. Army Corps of Engineers, 2002, Engineering and design, hydrographic surveying, Manual No. 1110-2-

1003: Washington, D.C., 510 p.

U.S. Geological Survey, 2011, Colorado StreamStats: U.S. Geological Survey, accessed Oct. 17, 2011, from 

http://streamstatsags.cr.usgs.gov/co_ss/default.aspx?stabbr=co&dt=1323879434710

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, p. 2–7.

Base from U.S. Geological Survey digital data, 2011

Universal Transverse Mercator projection, Zone12 North

Horizontal coordinate information is referenced to the North American Datum 

of 1983 (NAD 83)

Figure 1. Bathyetric contours of Groundhog Reservoir, Dolores County, Colorado, 2011 (not for navigational use).

EXPLANATION

CONTOUR INTERVAL 10 FEET

NORTH AMERICAN VERTICAL DATUM OF 1988 

Bathymetric contour—Line of equal lake-bottom elevation,

 in feet above NAVD 88. Interval is 10 feet. Hachures

 indicate depression

Lake boundary—Edge of water at lake-surface elevation,

 8,669 feet above NAVD 88

8,580

8,590

8,590

8,680

8,680

8,650

8,680

8,600

8,6108,620

8,6308,640

8,650

8,670

8,660

8,680

8,680

8,680

8,680

8,680

8,670 8,660

8,650

8,6408,630

8,620

8,610

8,600

8,670

8,660

8,650

8,640

8,630

8,620

8,610

8,680

8,670

8,680

8,670

8,680

8,670

8,660

8,650

8,640

8,630

8,660

108°18'30"

108°17'30"

108°17'30"

108°18'00"

108°18'30"

37°48'30"

37°48'30"

37°47'30"

37°47'30"

37°48'00"

37°48'00"

Dam

Spillway

Beaver Creek Irrigation Ditch

Groundhog Creek

Nash Creek

108°18'00"

0 200 400 500 METERS

0 400 800 1,200 1,600 2,000 FEET

100 300

0 100 200 300 8,560

8,580

8,600

8,620

8,640

8,660

8,680

8,700

400 500 600

Reservoir surface area, in acres

Stage elevation, in feet (NAVD 88)

700 800 900 1,000

Figure 2. Stage-surface area curve of Groundhog Reservoir, Dolores County, Colorado, 2011. Figure 3. Stage-volume curve of Groundhog Reservoir, Dolores County, Colorado, 2011.

0 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 8,560

8,580

8,600

8,620

8,640

8,660

8,680

8,700

Reservoir volume, in acre-feet

 Stage elevation, in feet (NAVD 88)

Table 1. Stage-surface area and stage-volume relation for selected elevations

 of Groundhog Reservoir, Dolores County, Colorado, 2011.

Stage elevation,

in feet (NAVD 88)

Stage-volume,

in acre-feet

8,571 0.000 0.000

8,572 0.009 0.001

8,573 0.100 0.048

8,574 0.221 0.202

8,575 0.378 0.507

8,576 0.519 0.951

8,577 0.663 1.54

8,578 0.787 2.26

8,579 0.924 3.11

8,580 1.35 4.21

8,581 1.83 5.80

8,582 2.37 7.89

8,583 2.89 10.5

8,584 3.24 13.6

8,585 3.48 17.0

8,586 3.99 20.7

8,587 5.21 25.2

8,588 7.74 31.5

8,589 11.1 41.0

8,590 14.6 53.8

8,591 18.6 70.3

8,592 23.2 91.3

8,593 27.4 117

8,594 30.9 146

8,595 34.7 178

8,596 39.0 215

8,597 44.8 257

8,598 50.3 305

8,599 58.4 359

8,600 66.8 421

8,601 73.4 491

8,602 80.9 568

8,603 88.8 653

8,604 96.2 746

8,605 103 845

8,606 110 952

8,607 117 1,060

8,608 126 1,190

8,609 135 1,320

8,610 143 1,460

8,611 149 1,600

8,612 156 1,750

8,613 163 1,910

8,614 170 2,080

8,615 178 2,250

8,616 184 2,440

8,617 191 2,620

8,626 252 4,600

8,627 259 4,850

8,618 197 2,820

8,619 203 3,020

8,620 209 3,220

8,621 215 3,430

8,624 237 4,110

8,622 222 3,650

8,625 244 4,350

8,623 229 3,880

8,628 267 5,120

8,629 275 5,390

8,630 283 5,670

8,631 292 5,950

8,632 301 6,250

8,633 301 6,560

8,634 319 6,870

8,635 328 7,190

8,636 338 7,530

8,637 347 7,870

8,638 357 8,220

8,639 366 8,580

8,640 376 8,960

8,641 386 9,340

8,642 396 9,730

8,643 406 10,100

8,644 416 10,500

8,645 427 11,000

8,646 437 11,400

8,647 447 11,800

8,648 458 12,300

8,649 469 12,800

8,650 479 13,200

8,651 489 13,700

8,652 499 14,200

8,653 510 14,700

8,654 522 15,200

8,655 533 15,800

8,656 545 16,300

8,657 556 16,800

8,658 567 17,400

8,659 579 18,000

8,660 591 18,600

8,661 603 19,200

8,662 616 19,800

8,663 629 20,400

8,664 643 21,000

8,665 656 21,700

8,666 668 22,300

8,667 682 23,000

8,668 695 23,700

8,669 709 24,400

8,670 723 25,100

8,671 738 25,800

8,672 754 26,600

8,673 771 27,400

8,674 788 28,100

8,683 904 35,700

8,684 915 36,700

8,675 801 28,900

8,676 813 29,700

8,677 826 30,600

8,678 838 31,400

8,681 878 34,000

8,679 851 32,200

8,682 892 34,900

8,680 864 33,100

Stage elevation,

in feet (NAVD 88)

Stage-surface

area, in acres

Stage-surface

area, in acres

Stage-volume,

in acre-feet

COLORADO

Index map

Dolores

County

Groundhog Reservoir

Rico

Prepared in cooperation with the

Dolores Water Conservancy District

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3202

Kohn, M.S., Bathymetry of Groundhog Reservoir, Dolores County, Colorado, 2011