GIBSON RESERVOIR

1996 SEDIMENTATION

SURVEY

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U.S. Department of the Interior

Bureau of Reclamation
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1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

August 1997 Final _____________________________

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Gibson Reservoir PR

1996 Sedimentation Survey

6. AUTHOR(S)

Ronald L. Ferrari ___________________________

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

REPORT NUMBER

Bureau of Reclamation, Technical Service Center, Denver CO 80225-0007 ____________________________

9. SPONSORINGIMONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

Bureau of Reclamation, Denver Federal Center, P0 Box 25007,

Denver CO 80225-0007 DIBR

11. SUPPLEMENTARY NOTES

Hard copy available at Bureau of Reclamation Technical Service Center, Denver, Colorado

12*. DISTRJBU11ON!AVAILABILIIY STATEMENT 12b. DISTRIBUTION CODE

Available from the National Technical Information Service,

Operations Division, 5285 Port Royal Road, Springfield VA 22161 ___________________________

13. ABSTRACT (Maximum 200 words)

The Bureau of Reclamation (Reclamation) surveyed the underwater area of Gibson Reservoir in July 1996 to develop a

topographic map and compute a present storage-elevation relationship (area-capacity tables). The data were also

used to calculate reservoir capacity lost due to sediment accumulation since dam closure in 1929 and the last reservoir

survey in 1973. The survey used sonic depth recording equipment interfaced with a global positioning system (GPS)

that gave continuous sounding positions throughout the underwater portion of the reservoir. Underwater topography

was developed by a computer program using the collected data. Above-water topography was determined by digitizing

contour lines from the U.S.Geological Survey quadrangle (USGS quad) maps of the reservoir area developed from

aerial photography obtained in 1955. The new topographic map of Gibson Reservoir is a combination of digitized

contour and 1996 underwater measured topography.

As of July 9, 1996, at top of active conservation elevation (feet) 4,724.0, the surface area was 1,296 acres and the total

capacity was 96,477 acre-feet. Since initial filling in 1929, about 8,383 acre-feet of capacity has been lost in Gibson

Reservoir below elevation 4724.0 (top of radial gates), resulting in a 7.99-percent loss in reservoir volume. Since the

last sediment survey in 1973, about 2,590 acre-feet of sediment has accumulated below elevation 4724.0. Since 1929,

the estimated average annual rate of reservoir capacity lost is 125.7 acre-feet. Since 1973, the estimated average

annual rate of reservoir capacity lost to sediment accumulation is 113.1 acre-feet.

14. SUBJECT TERMS

______________________

15. NUMBER OF PAGES

19

reservoir area and capacity! sedimentation! reservoir surveys! sonar/ sediment distribution!

contour area/ reservoir area! sedimentation survey! global positioning system 16 PRICE CODE

17. SECURITY CLASSIFICATION 18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF REPORT OF THIS PAGE OF ABSTRACT

UL UL UL UL

NSN 754001-280-5500 Standard Form 298 (Rev. 2-89

Prescrtb.d by ANSI Sad. 239-1

298-102
GIBSON RESERVOIR

1996 SEDIMENTATION SURVEY

by

Ronald L. Ferrari

Sedimentation and River Hydraulics Group

Water Resources Services

Technical Service Center

Denver, Colorado

August 1997
ACKNOWLEDGMENTS

The Bureau of Reclamation's (Reclamation) Sedimentation and River Hydraulics Group

of the Technical Service Center (TSC) prepared and published this report. Ronald

Ferrari and James Melena of the TSC conducted the hydrographic survey. Special thanks

to Mr. Lee Caribon, dam tender, Greenfields Irrigation District, for his support during the

hydrographic survey. Ronald Ferrari completed the data processing needed to generate

the new topographic map and area-capacity tables. Sharon Nuanes of the TSC completed

the contour digitizing and was consulted during the map development. James Melena

of TSC performed the technical peer review of this documentation.

U.S. Department of the Interior

Mission Statement

The mission of the Department of the Interior is to protect and provide access to our

Nation's natural and cultural heritage and honor our trust responsibility to Tribes.

The information contained in this report regarding commercial products or firms may not

be used for advertising or promotional purposes and is not to be construed as an

endorsement of any product or firm by Reclamation.

The information contained in this report was developed for the Bureau of Reclamation;

no warranty as to the accuracy, usefulness, or completeness is expressed or implied.

11
CONTENTS

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Introduction .

Summary and Conclusions .

Reservoir Operations .

Hydrographic Survey Equipment and Method

GPS Technology and Equipment

Survey Method and Equipment

Reservoir Area and Capacity

Topography Development

Development of 1996 Contour Areas

1996 Storage Capacity

Reservoir Sediment Analyses

References

TABLES

Table

1 Reservoir sediment data summary

2 Summary of 1996 survey results

FIGURES

Figure

Page

1

2

3

3

3

5

6

6

8

8

9

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1 Gibson Reservoir location map 15

2 Gibson dam, plan and section 16

3 Gibson 1996 underwater collected data 17

4 Gibson Reservoir topographic map 18

5 1996 area and capacity curves 19

111
INTRODUCTION

Gibson Dam arid Reservoir, principal features of the Sun River Project, are located on the North

Fork of the Sun River about 24 miles northwest of Augusta, 75 miles west of Great Falls, and

within Teton, and Lewis and Clark Counties in Montana (fig. 1). Additional features are the

Willow Creek Dam and Reservoir, Pishkun Dikes and Reservoir, Sun River Diversion Dam, and

Fort Shaw Diversion Dam. The dam, operated by the Greenfields Irrigation District, releases

water to supplement natural river flows used for irrigation of Sun River Project lands.

Gibson Dam was constructed between 1926 and 1929 with first storage in December 1929. The

drainage area above the dam is 575 square miles, ranging from elevation (feet)' 4,557.5, top of

dead pool, to greater than 8,800 at its head waters. The reservoir length, at elevation 4,724, is

around 5.2 miles with an average width of 0.4 mile. The dam is a massive concrete arch

structure, whose dimensions are (fig. 2):

• Hydraulic height2 195 feet

• Top width 15 feet

• Crest elevation 4,725.5 feet

Structural height 199 feet

• Crest length 960 feet

• Top of parapet wall 4,729 feet

The design maximum controllable water surface (top of spiliway radial gates) is elevation

4,724.0. The maximum historical water surface, elevation 4,732.23, occurred on June 8, 1964.

A record inflow of 60,000 cubic feet per second (cfs) caused Gibson Dam to overtop where

water flowed over the parapet wall for 20 hours without seriously damaging the dam. After this

event, modification work was done to provide additional protection to the dam and foundation

for safe overtopping of greater than 100-year frequency flood flows.

Gibson Dam's spillway, located through the left abutment, is a drop-type structure controlled by

six 34-foot-wide by 12-foot-high radial gates. The radial gates were installed in 1938 to provide

15,000 acre-feet of storage. The spiliway crest is at elevation 4,712.0, with top of the spillway

radial gates at elevation 4,724.0. The spillway provides a discharge of 30,000 cfs at reservoir

elevation 4,724, top of active conservation, and 50,000 cfs at reservoir elevation 4,729.0, top of

the parapet. The discharge enters a 29.5-foot-diameter vertical shaft and turns into a horizontal

tunnel before discharging into the river below.

The outlet works consist of a trashrack, two 72-inch-diameter steel-lined conduits through the

base of the dam, and a 60-inch jet-flow gate in each conduit that controls flows. The jet-flow

gates, installed in 1971-72, replaced two original installed needle valves. Five-foot-square,

hydraulically operated high-pressure emergency slide gates are located upstream from the jetflow gates. The discharge capacity is 3,050 cfs at reservoir elevation 4,724.0.

1Elevation levels are shown in feet.

2The definition of such terms as "hydraulic height,11 "structural height," etc. may be

found in manuals such as Reclamation's Design of Small Dams and Guide for Preparation of

Standing Operating Procedures for Dams and Reservoirs, or ASCE's Nomenclature for

Hydraulics.
SUMMARY AND CONCLUSIONS

This Reclamation report presents the 1996 results of the survey of Gibson Reservoir. The

primary objectives of the survey were to gather data needed to:

• develop reservoir topography

• compute area-capacity relationships

• estimate storage depletion caused by sediment deposition since dam closure

• estimate storage depletion caused by sediment deposition since last reservoir survey in 1973

The bathymetric survey was run using sonic depth recording equipment interfaced with a

differential global positioning system (DGPS) capable of determining sounding locations within

the reservoir. The system continuously recorded depth and horizontal coordinates of the survey

boat as it was navigated along grid lines covering Gibson Reservoir. The positioning system

provided information to allow the boat operator to maintain course along these grid lines. Water

surface elevations recorded by a Reclamation gage during the time of collection were used to

convert the sonic depth measurements to true reservoir bottom elevations.

1996 underwater surface areas at predetermined contour intervals were generated by a computer

graphics program using the underwater collected data. Above-water reservoir contours were

digitized from U.S. Geological Survey 7.5 minute quadrangle (USGS quad) maps of Gibson

Reservoir. The new topographic map of Gibson Reservoir is a combination of the digitized and

underwater-measured topography. The area and capacity tables were produced by a computer

program that uses measured contour surface areas and a curve-fitting technique to compute area

and capacity at prescribed elevation increments. The 1996 area and capacity tables were

generated using 1996 areas at elevation 4,720.0 and below and 1973 areas for elevation 4,724.0

and greater.

Table 1 contains a summary of the Gibson Reservoir's watershed characteristics for the 1996

survey. The 1996 survey determined that the reservoir has a storage capacity of 96,477 acre-feet

and a surface area of 1,296 acres at reservoir elevation4,724.0 feet. Since closure in 1929, the

reservoir has had a volume change of 8,383 acre-feet below reservoir elevation 4,724.0. This

volume represents a 7.99 percent loss in capacity and an average annual loss of 125.7 acre-feet.

2
RESERVOIR OPERATIONS

Gibson Reservoir is primarily an irrigation facility. The July 1996 area-capacity tables show

96,477 acre-feet of active conservation storage below elevation 4,724.0. The 1996 survey

measured a maximum depth of elevation 4,557.1, but there is no volume below the dead pool

of elevation 4,557.5, bottom of outlet conduits.

The Gibson Reservoir inflow and end-of-month stage records in table 1, operation period

October 1973 through July 1996, show the inflow and annual fluctuation since the last reservoir

survey in 1973. The average inflow into the reservoir for this operation period was 590,210 acrefeet per year. Also shown for this operation period are the extreme storage fluctuations of

Gibson Reservoir, ranging from elevation 4,575.7 in 1988 to elevation 4,724.2 in 1976. The

records show that in more than 50 percent of the years, the reservoir fills to the near-full reservoir

condition of elevation 4724.0, maximum conservation level, and in some years, while supplying

irrigation water downstream, drops more than 100 feet in elevation. The maximum reservoir

water surface on record, elevation 4,732.23, occurred on June 8, 1964 when a major inflow

caused overtopping flows of Gibson Dam.

HYDROGRAPHIC SURVEY EQUIPMENT AND METHOD

The hydrographic survey equipment was mounted in the cabin of a 24-foot tn-hull aluminum

vessel equipped with twin in-board motors. The hydrographic system contained on the survey

vessel consisted of a global positioning system (GPS) receiver with a built-in radio and an

omnidirectional antenna, a depth sounder, a helmsman display for navigation, a plotter, a

computer, and hydrographic system software for collecting underwater data. Power to the

equipment was supplied by an on-board generator.

The shore equipment included a second GPS receiver with a built-in radio and an

onmidirectional antenna. The GPS receiver and antenna were mounted on a survey tripod over

a known datum point. The power for the shore unit was provided by a 12-volt battery.

GPS Technology and Equipment

The positioning system used at Gibson Reservoir was Navigation Satellite Timing and Ranging

(NAVSTAR) GPS, an all-weather, radio-based, satellite navigation system that enables users to

accurately determine three-dimensional position. The NAVSTAR system's primary mission is

to provide passive global positioning and navigation for land-, air-, and sea-based strategic and

tactical forces and is operated and maintained by the Department of Defense (DOD). The GPS

receiver measures the distances between the satellites and itself and determines the receiver's

position from intersections of the multiple-range vectors. Distances are determined by accurately

measuring the time a signal pulse takes to travel from the satellite to the receiver.
The NAVSTAR system consists of three segments:

• The space segment is a network of 24 satellites maintained in precise orbit about 10,900

nautical miles above the earth, each completing an orbit every 12 hours.

• The ground control segment tracks the satellites, determining their precise orbits.

Periodically, the ground control segment transmits correction and other system data to all

the satellites, and the data are then retransmitted to the user segment.

• The user segment includes the GPS receivers which measure the broadcasts from the

satellites and calculate the position of the receivers.

The GPS receivers use the satellites as reference points for triangulating their position on earth.

The position is calculated from distance measurements to the satellites that are determined by

how long a radio signal takes to reach the receiver from the satellite. To calculate the receiver's

position on earth, the satellite distance and the satellite's position in space are needed. The

satellites transmit signals to the GPS receivers for distance measurements along with the data

messages about their exact orbital location and operational status. The satellites transmit two

"L" band frequencies (called Li and L2) for the distance measurement signal. At least four

satellite observations are required to mathematically solve for the four unknown receiver

parameters (latitude, longitude, altitude, and time); the time unknown is caused by the clock error

between the expensive satellite atomic clocks and the imperfect clocks in the GPS receivers (for

hydrographic surveying the altitude, Gibson's water surface elevation parameter was known,

which realistically meant only three satellite observations were needed to track the survey vessel;

during the Gibson Reservoir survey, the best 6 available satellites were used for position

calculations).

The GPS receiver's absolute position is not as accurate as it appears in theory because of the

function of range measurement precision and the geometric position of the satellites. Precision

is affected by several factors--time, because of the clock differences, and atmospheric delays

caused by the effect on the radio signal of the ionosphere. Geometric dilution of precision

(GDOP) describes the geometrical uncertainty and is a function of the relative geometry of the

satellites and the user. Generally, the closer together in angle two satellites are from the receiver,

the greater the GDOP. GDOP is broken into components: position dilution of precision (x,y,z)

(PDOP), and horizontal dilution of precision (x,y) (HDOP). The components are based only on

the geometry of the satellites. The PDOP and HDOP were monitored during the Gibson

Reservoir Survey, and for the majority of the time they were less than 3, which is within the

acceptable limits of horizontal accuracy for Class 1 and 2 level surveys (Corps of Engineers,

1991).

An additional and larger error source in GPS collection is caused by false signal projection,

called selective availability (S/A). The DOD implements S/A to discourage the use of the

satellite system as a guidance tool by hostile forces. Positions determined by a single receiver

when S/A is active can have errors of up to 100 meters.

4
A method of collection to resolve or cancel the inherent errors of GPS (satellite position or S/A,

clock differences, atmospheric delay, etc.) is called differential GPS (DGPS). DGPS was used

during the Gibson Reservoir survey to determine positions of the moving survey vessel in real

time. DGPS determines the position of one receiver in reference to another and is a method of

increasing position accuracies by eliminating or minimizing the uncertainties. Differential

positioning is not concerned with the absolute position of each unit but with the relative

difference between the positions of two units, which are simultaneously observing the same

satellites. The inherent errors are mostly canceled because the satellite transmission is essentially

the same at both receivers.

At a known geographical benchmark, one GPS receiver is programmed with the known

coordinates and stationed over the geographical benchmark. This receiver, known as the master

or reference unit, remains over the known benchmark, monitors the movement of the satellites,

and calculates its apparent geographical position by direct reception from the satellites. The

inherent errors in the satellite position are determined relative to the master receiver's

programmed position, and the necessary corrections or differences are transmitted to the mobile

GPS receiver on the survey vessel. For the Gibson Reservoir, position corrections were

determined by the master receiver and transmitted via a ultra-high frequency (UHF) radio link

every 3 seconds to the survey vessel mobile receiver. The survey vessel's GPS receiver used the

corrections along with the satellite information it received to determine the vessel's differential

location. Using DGPS resulted in positional accuracies of 1 to 2 meters for the moving vessel

compared to positional accuracies of 100 meters with a single receiver.

The Technical Service Center (TSC) mobile and reference GPS units are identical in construction

and consist of a 6-channel Li coarse acquisition (C/A) code continuous parallel-tracking

receiver, an internal modem, and a UHF radio transceiver. The differential corrections from the

reference station to the mobile station are transmitted using the industry standard Radio

Technical Commission for Maritime Services (RTCM) message protocol via the UHF radio link.

The programming to the mobile or reference GPS unit is accomplished by entering necessary

information via a notebook computer. The TSC's GPS system has the capability of establishing

or confirming the land base control points by using notebook computers for logging data and

post-processing software. The GPS collection system has the capability to collect the data in

1927 or 1983 North American Datums (NAD) in the surveyed area's state plane coordinate

system's zone. For Gibson Reservoir, the data were collected in Montana's 1927 NAD central

state plane zone.

Survey Method and Equipment

The Gibson Reservoir hydrographic survey collection was conducted on July 8 through July 9,

1996, at water surface elevations 4,723.1 and 4,723.3, respectively. The bathymetric survey was

run using sonic depth recording equipment interfaced with a DGPS capable of determining

sounding locations within the reservoir. The survey system software continuously recorded

reservoir depths and horizontal coordinates as the survey boat moved across close-spaced grid

lines covering the reservoir area. Most of the transects (grid lines) were run primarily in a

perpendicular direction to the original river thaiweg. Data were also collected along the shore

5
as the boat traversed to the next transect. The survey vessel's guidance system gave directions

to the boat operator to assist in maintaining course along these predetermined lines. During each

run, the depth and position data were recorded on the notebook computer hard drive for

subsequent processing by TSC personnel. The underwater data set includes 22,038 data points.

A graph plotter was used in the field to track the boat and ensure adequate coverage during the

collection process.

For stationing the master GPS unit there were no initial, known benchmarks or datums that

overlooked the reservoir. For the underwater collection, the hydrographic survey crew

established an initial datum near the center of the dam using a precision lightweight GPS receiver

(PLGR). The PLGR unit utilizes the precise positioning service of the DOD GPS that is

available to federal users only and has a horizontal accuracy of ±4 meters. At the time the datum

coordinates were collected, the position accuracy was ±4.5 meters. The shore-based master

GPS unit, which transmits the correction information to the mobile GPS unit on the survey

vessel, was stationed at this site at the start of the survey. This location was chosen because it

was accessible, near the reservoir, and overlooked the reservoir. The location allowed for good

radio transmission of the differential corrections to the mobile survey vessel for the lower portion

of the reservoir survey. Additional datums were established when the radio-transmitted

differential corrections were blocked by the reservoir terrain. This was accomplished by using

the hydrographic GPS units and software with a horizontal accuracy of ±1 to 2 meters. The

initial datum, at the center of the dam, was used as the reference for establishing the land-based

network of datums that were used for locating the shore-based master GPS units during the

survey of the reservoir. During-post processing of the collected data, the few collected points

without differential correction were removed.

The underwater data were collected by a depth sounder which was calibrated by lowering a

deflector plate below the boat by cables with known depths marked by beads. The depth sounder

was calibrated by adjusting the speed of sound, which can vary with density, salinity,

temperature, turbidity, and other conditions. The collected data were digitally transmitted to the

computer collection system via a RS-232 port. The depth sounder also produces an analog hardcopy chart of the measured depths. These graphed analog charts were printed for all survey lines

as the data were collected and recorded by the computer. The charts were analyzed during postprocessing, and when the analog charted depths indicated a difference from the recorded

computer bottom depths, the computer data files were modified. The water surface elevation

recorded by a Reclamation gage during the time of collection was used to convert the sonic depth

measurements to true lake-bottom elevations.

RESERVOIR AREA AN]) CAPACITY

Topography Development

The topography of Gibson Reservoir was developed from 1996-collected underwater data and

USGS quad maps. The upper contours of Gibson Reservoir were developed by digitizing the

contour line, marked spiliway elevation 4,725.0, from the USGS quad maps that covered the

Gibson Reservoir area. For the purpose of this study, this contour was assume to be elevation

6
4,724.0, top of movable radial gates. The USGS quad maps were developed from aerial

photography dated 1955. ARC/INFO V7.0.2 geographic information system (GIS) software was

used to digitize the USGS quad contours. The digitized contours were transformed to Montana's

NAD 1927 central state plane coordinates using the ARC/INFO PROJECT command. The

resulting digitized area from the USGS quads was 1,312 acres, or 101.2% of the 1,296 acres

reported in the 1973 survey of Gibson Reservoir at elevation 4,724.0.

As part of the 1996 sediment analysis of Gibson Reservoir, the elevation 4,725.0 contour of the

1973 aerial maps was also digitized. The 1973 reservoir survey completed an aerial analysis on

September 22, 1973 of Gibson Reservoir at water surface elevation 4,597.8. A total of seven

Reclamation maps, drawing numbers 28-600-58 through 64, were developed at intervals of 1

inch equals 200 feet, with 5-foot contour. The resulting digitized area of the 4,725.0 contour,

from the seven maps, was 1,317 acres, or 101.2% of the 1,302 acres reported in the 1973 survey

of Gibson Reservoir at elevation 4,725.0. This is a very small difference that can be attributed

to the different methods of measuring the areas. To match the northeast tick marks of the 1973

aerial contour maps with the USGS quad northeast tick marks and the 1996 underwater data

required a slight rotation along with a shift of around 6,200 feet to the north and 4,430 feet to the

west. It appears the coordinate system for the 1973 aerial control was a local system established

by Reclamation. Using the local control system had no bearing on the 1973 results, but it did

not allow the digitized contour to be used as the clip for the 1996 study.

The Gibson Reservoir contour outline that was digitized from USGS quad maps was used to

perform a clip of the Gibson Reservoir triangular irregular network (TIN) such that interpolation

was not allowed to occur outside of this contour that was assigned an elevation of 4724.0. This

contour was selected since it was the closest complete elevation to represent the reservoir at the

time of the survey. This clip was performed using the hardclip option of the ARC/INFO

CREATETIN command. Using ARCEDIT, the underwater collected data and digitized contours

from the quad maps were plotted. The plot found that the underwater data did not completely

lie within this clip, which required slight modification in order to incorporate the entire

underwater data set. Modifications were also made to the clip so the 1996 computed area at

elevation 4,724.0 matched the 1,296 acres computed by the 1973 survey. Using select and move

commands within ARCEDIT, the vertices of the 4724.0 clip were shifted to enclose all the

collected underwater data and to reduce the area to match the 1973 study. The 1996 underwater

data are presented on figure 3.

Contours for elevations below 4,724.0 were computed from the 1996 collected underwater data

using the TIN surface modeling package within ARC/INFO (a TIN is a set of adjacent, nonoverlapping triangles computed from irregularly spaced points with x,y coordinates and z

values). TIN was designed to deal with continuous data such as elevations. The TIN software

uses a method known as Delaunay's criteria for triangulation. Triangles are formed between all

data points including all boundary points. This method preserves all collected survey points.

The method requires that a circle drawn through the three nodes of a triangle will contain no

other point, meaning that sample points are connected to their nearest neighbors to form triangles

using all collected data. Elevation contours are then interpolated along the triangle elements.

The TIN method is discussed in greater detail in the ARC/INFO V7.0.2 Users Documentation.

7
The linear interpolation option of the ARC/INFO T[NCONTOUR command was used to

interpolate contours from the Gibson Reservoir TIN. In addition, the contours were generalized

by weeding out vertices along the contours. This generalization process improved the

presentability of the resulting contours by removing very small variations in the contour lines.

This generalization had little bearing on the computation of surface areas and volumes for

Gibson Reservoir. The contour topography at 25-foot intervals is presented on figure 4, thawing

number 28-D-873. Contour topography at 5-foot intervals with a scale of 1 inch = 200 feet is

also available from Reclamation.

Development of 1996 Contour Areas

The 1996 contour surface areas for Gibson Reservoir were computed at 5-foot increments, from

elevation 4,560.0 to 4,724.0, using the Gibson Reservoir TIN discussed above. The 1996 survey

measured the minimum reservoir elevation at 4,557.1 feet. These calculations were performed

using the ARC/INFO VOLUME command. This command computes areas at user-specified

elevations directly from the TIN and takes into consideration all regions of equal elevation.

1996 Storage Capacity

The storage-elevation relationships based on the measured surface areas were developed using

the area-capacity computer program ACAP85 (Bureau of Reclamation, 1985). Surface areas at

5-foot contour intervals from minimum reservoir elevation 4,557.1 to elevation 4,724.0 and the

1973 surface areas at contour elevation 4,730.0 and 4,740.0 were used as the control parameters

for computing the Gibson Reservoir capacity. The program can compute an area and capacity

at elevation increments 0.01- to 1.0-foot by linear interpolation between the given contour

surface areas. The program begins by testing the initial capacity equation over successive

intervals to ensure that the equation fits within an allowable error limit, which was set at

0.00000 1 for Gibson Reservoir. This capacity equation is then used over the full range of

intervals fitting within this allowable error limit. For the first interval at which the initial

allowable error limit is exceeded, a new capacity equation (integrated from basic area curve over

that interval) tests the fit until it also exceeds the error limit. Thus, the capacity curve is defined

by a series of curves, each fitting a certain region of data. Final area equations are derived by

differentiating the capacity equations, which are of second order polynomial form:

y a1 + a2x + a3x2

where: y capacity

x = elevation above a reference base

a1 = intercept

a and a3 = coefficients

Results of the 1996 Gibson Reservoir area and capacity computations are listed in table 1 and

columns (8) and (9) of table 2. On table 2, columns (2) and (3) list the original surface areas and

8
recomputed capacity values and columns (4) and (5) list the 1973 surface areas and recomputed

capacity values. A separate set of 1996 area and capacity tables has been published for the 0.01-,

0.1-, and 1-foot elevation increments (Bureau of Reclamation 1996). A description of the

computations and coefficients output from the ACAP85 program is included with these tables.

Both the original, 1973, and 1996 area-capacity curves are plotted on figure 5. As of July 1996,

at elevation 4,724.0, the surface area was 1,296 acres with a total capacity of 96,477 acre-feet.

SEDIMENT ANALYSES

Since dam closure in December 1929 the measured total volume change at reservoir elevation

4,724.0 was 8,383 acre-feet. The average annual rate of capacity lost between closure and July

1996 (66.7 years) was 125.7 acre-feet per year. The storage loss in terms of percent of original

storage capacity was 7.99 percent. Tables 1 and 2 contain the Gibson Reservoir sediment

accumulation and water storage data based on the 1996 resurvey. Colunm 10 of table 2 gives

the measured volume change by elevation. This column is labeled "1996 measured sediment

volume," but it is assumed that a portion of this change is due to the accuracy of the original

surface areas that were projected from an area curve developed in the 1930's. Figure 5, a plot

of the original area data versus the 1973 and 1996 measured areas, shows the plots/areas in a

near-mirror image of each other, possibly indicating an elevation datum difference between the

original survey and the resurveys of Gibson Reservoir.

The geology of Gibson Dam and Reservoir area is described as a steep ridge of erosion-resistant

limestone and dolomite with a low probability of landslides (Goodson, 1981). During the 1996

survey, and noted in the 1973 resurvey report, no delta deposit of coarser-size sediments in the

upper reach of the reservoir was observed. The 1996 survey, at elevation 4,724.0, measured a

61-acre difference in surface area since the original survey. One suspects an elevation datum

difference between the original survey and the resurveys of Gibson Reservoir because of the low

probability of landslides and because no large landslides and upper reservoir deposits of coarsersize sediments were observed. The reservoir is located in a forest drainage usually conducive

to low sediment contribution except after major events within the drainage, such as forest fires.

Records for the period between 1929 and 1973 list a 1945 fire near the Bench Mark Area in the

South Fork drainage that burned approximately 2,270 acres. Since the 1973 survey, records

show two fires in 1988 with one burning 466 acres and the second burning 50,408 acres.

The purpose of the 1996 survey was to measure the reservoir volume lost due to sediment

contribution from the major 1988 forest fire that occurred in the drainage since the 1973 survey.

Column 12 of table 2 gives the 1996 measured sediment volume, since the 1973 reservoir

sedimentation surveys, which was 2,590 acre-feet at elevation 4,724.0. The average annual rate

of sediment deposition since 1973 (22.9 years) was 113.1 acre-feet per year. It is assumed that

the majority of this sediment contribution came after the drainage fire. Field observation from

the dam tender during low reservoir elevation indicated that large sediment deposits existed in

the upper reservoir area. With the first few years after the fire, the inflow definitely had large

amounts of burn material, lessening each year.

9
A resurvey of Gibson Reservoir should be considered in the future if major sediment inflow

events are observed, such as those occurring after a major drainage fire, or if the average annual

rate of sediment accumulation requires further clarification.

10
REFERENCES

American Society of Civil Engineers, 1962. Nomenclature for Hydraulics, ASCE Headquarters,

New York.

Bureau of Reclamation, July 1975. Resurvey of Gibson Reservoir, Sun River Project, Great

Plains Region, Denver CO.

1981. Project Data, Denver Office, Denver CO.

_________________ 1985. Surface Water Branch, ACAP8S User's Manual, Technical

Service Center, Denver CO.

_________________ 1987(a). Guide for Preparation of Standing Operating Procedures for

Bureau of Reclamation Dams and Reservoirs, U.S. Government Printing Office,

Denver,CO..

Denver CO.

1987(b). Design of Small Dams, U.S. Government Printing Office,

____________ May 1996. Denver Office, Gibson Reservoir Area and Capacity Tables,

Sun River Project, Great Plains Region, Denver CO.

Corps of Engineers. March 1991. Application of Differential Global Positioning System for

Hydrographic Survey and Dredge Positioning, ETL 1110-7-5 (FR), Department of the

Army, Washington DC,.

Environmental Systems Research Institute, Inc., 1992. ARC Command References.

Goodson & Associates, Inc., May 22, 1981. Technical Data Workbook Abstract Report for

Gibson Dam, Sun River Project, Montana, Upper Missouri Region.

:1.1
RESERVOIR SEDIMENT

DATA SUMMARY Gibson Reservoir

NAME OF RESERVOIR 1

DATA SHEET NO.

O 1. OWNER Bureau of Reclamation 2. STREAM Sun River 3. STATE Montana

A 4. SEC. 4 TWP. 21 N RANGE 9 W S. NEAREST P.O. Augusta 6. COUNTY Teton/Lewis-Clark

H 7. LAT 47" 36' 09" LONG 112" 45' 39" 8. TOP OF DAM ELEVATION 4725.5 9. SPILLWAY CREST EL 4712.02

P. 10. STORAGE 11. ELEVATI ON 12. ORIGINAL 13. ORIGINAL 14. GROSS STORAGE 15. DATE

E ALLOCATION TOP OF POOL SURFACE AREA, AC CAPACITY, AF ACRE- FEET STORAGE

S BEGAN

a. SURCHARGE

v

b. FLOOD CONTROL

________________ _________________ 12/2

O c. POWER

________________ _________________ 9

I d. WATER SUPPLY 16. DATE

P. e. IRRIGATION NORMAL

f. CONSERVATION 4724.0 1357 104,768 104,784

OPERATION

BEGAN

g. DEAD 4557.5 18 16 16 12/29

17. LENGTH OF RESERvOIR 5.2 MILES AVG. WIDTH OF RESERVOIR 0.4 MILES

B 18. TOTAL DRAINAGE AREA 575 SQUARE MILES 22. MEAN ANNUAL PRECIPITATION 19 INCHES

A

19. NET SEDIMENT CONTRIBUTING AREA 575 SQUARE MILES 23. MEAN ANNUAL RUNOFF 20.1 INCHES

20. LENGTH 45 MILES MT. WIDTH 13 MILES 24. MEAN ANNUAL RUNOFF 615,480 ACRE-FEET

21. MAX. ELEVATION 8800 MIN. ELEVATION 4557.5 25. ANNUAL TEMP. MEAN 45"F RANGE -49"F to 106"F3

S 26. DATE OF 27. 28. 29. TYPE OF 30. NO. OF 31. SURFACE 32. CAPACITY 33. C/I

U SURVEY PER. ACCL. SURVEY RANGES OR AREA, AC. ACRE - FEET RATIO Ar/AX

R __________________________________

V 12/29 1,3576 104,8606 .17

E 8/73 43.8 43.8 Contour (D) 10-ft 1,2966 99Q576 .16

Y

7/09/96 22.9 66.7 Contour (D) 5-ft 1,296 96,477 .16

A

26. DATE OF 34. PERIOD 35. PERIOD WATER INFLOW, AcRE FEET WATER INFLOW TO DATE, AX

T

SURVEY ANNUAL

A

PRECIP. a. MEAN ANN. J

b. MAX. ANN. C. TOTAL a. MEAN ANN. b. TOTAL

8/73 17.62 628,690 984,500 27,536,7008 628,690 27,536,7008

7/09/96 19 590,210 918,300 13,515,800 615,480 41,052,500

26. DATE OF 37. PERIOD CAPACITY LOSS, ACRE-FEET 38. TOTAL SEDIMENT DEPOSITS TO DATE, AF

SURVEY

a. TOTAL b. AV. ANN. J

c. /MI."-YR. a. TOTAL b. AV. ANNUAL c. /MI.-YR.

8/73 5,7939 l32.3 .23 5,793 132.3 .23

7/09/96 2,59010 113.1 .20 8,38311 125.7 .22

26. DATE OF 39. AV. DRY 40. SED. DEP. TONS/MI.-YR. 41. STORAGE LOSS, PCT. 42. SEDIMENT

SURVEY MT. (#/FT")

_______________________________

a. PERIOD b. TOTAL TO a. AV. b. TOTAL TO a. b.

8/73 .126 5.52

7/09/96 .120 7.99

26.

DATE

43. DEPTH DESIGNATION RANGE BY RESERVOIR ELEVATION

OF

SURVEY

4580-

4550

4600-

4580

4620-

4600

4640-

4620

4660-

4640

4680-

4660

4700-

4680

4720-

4700

4724-

4720

PERCENT OF TOTAL SEDIMENT LOCATED WITHIN DEPTH DESIGNATION

7/96 3.3 13.2 7.5 8.4 9.1 12.2 22.9 19.7 3.7

26.

DATE

44. REACH DESIGNATION PERCENT OF TOTAL ORIGINAL LENGTH OF RESERVOIR

VEY

0-10 10-

20

20-

30

30-

40

40-

50

50-

60

60-

70

70-

80

80-

90

90-

100

100-

105

105-

110

110-

115

115-

120

120-

125

PERCENT OF TOTAL SEDIMENT LOCATED WITHIN REACH DESIGNATION

Table 1. - Reservoir sediment data summary (page 1 of 2).

12
45. RANGE IN RESERVOIR OPERATION12 ___________ ___________

YEAR MAX. ELEV. MIN. ELEV. INFLOW, AF YEAR MAX. ELEV. MIN. ELEV. INFLOW, AX

1974 4,723.9 4,595.1 709,600 1975 4,720.4 4,672.7 902,600

1976 4,724.2 4,673.4 824,000 1977 4,715.4 4,595.5 285,500

1978 4,724.0 4,596.8 739,600 1979 4,724.0 4,609.4 570,200

1980 4,724.0 4,618.1 596,000 1981 4,724.0 4,625.8 600,400

1982 4,723.5 4,637.8 677,500 1983 4,724.0 4,656.9 442,800

1984 4,723.9 4,626.3 419,000 1985 4,723.9 4,637.0 503,200

1986 4,716.3 4,610.2 657,600 1987 4,724.1 4,641.8 429,900

1988 4,722.0 4,575.7 357,000 1989 4,723.8 4,605.7 663,400

1990 4,724.0 4,653.0 737,100 1991 4,723.7 4,651.0 918,300

1992 4,720.9 4,621.8 416,000 1993 4,724.0 4,622.0 530,800

1994 4,720.0 4,607.5 435,400 1995 4,724.0 4,610.0 483,800

1996 4,724.0 4,677.0 616,10012

___________ ___________ ___________

46. ELEVATION - AREA - CAPACITY DATA FOR 1996 CAPACITY13

ELEVATION AREA CAPACITY ELEVATION AREA CAPACITY ELEVATION AREA CAPACITY

4557 0 0 4560 1.0 2 4565 5.2 17

4570 12.6 62 4575 26.9 160 4580 59.5 376

4585 89.0 748 4590 125.2 1,283 4595 147.2 1,964

4600 200.3 2,833 4605 240.9 3,936 4610 312.3 5,319

4615 374.9 7,037 4620 415.7 9,013 4625 456.7 11,194

4630 494.1 13,571 4635 525.2 16,120 4640 554.2 18,818

4645 580.4 21,655 4650 610.3 24,631 4655 651.2 27,785

4660 691.4 31,142 4665 733.4 34,704 4670 784.3 38,498

4675 853.2 42,592 4680 916.5 47,016 4685 967.6 51,726

4690 1,009.2 56,668 4695 1,048.7 61,813 4700 1,105.5 67,198

4705 1,160.2 72,862 4710 1,217.4 78,806 4715 1,255.0 84,987

4720 1,280.2 91,325 4724 1,296.2 96,477 4730 1,333 104,364

4740 1,394 117,999

____________ ___________ ___________

47. REMARKS AND REFERENCES

____________

1 Parapet elevation 4729.0.

2 Top of movable radial gates 4724.0.

Bureau of Reclamation Project Data Book, 1981.

' Calculated using mean annual runoff value of 615,480 AS, item 24, 1973-1996.

Computed annual inflows from 1929 through 7/96.

6 Original surface area and capacity at elevation 4,724.0. Original and 1973 capacity recomputed by Reclamation's

ACAP program using measured surface areas. Original areas projected from 1931 area curve. 1973 areas measured

from aerial photography.

Surface area and capacity at elevation 4724.0 computed by ACAP program using 1996 and 1973 surface areas. 1996

surveyed only underwater portion of reservoir below elevation 4724.0. Elevation 4724.0 and above from 1973 study.

Values from May 7, 1976 reservoir sedimentation data sumary table.

Capacity loss calculated by comparing original recomputed capacity and 1973 capacity at reservoir elevation

4724.0, top of conservation elevation. Portion of capacity difference due to accuracy of two surveys, original

areas projected from area curve and 1973 areas measured from aerial survey.

10 Capacity loss calculated by comparing 1973 recomputed capacity and 1996 capa city at reservoir elevation 4724.0.

Total capacity loss calculated by comparing original recomputed capacity and 1996 capacity at reservoir elevation

4724.0. Portion of capacity difference due to accuracy of two surveys. Original areas projected from area curve.

12 Maximum and minimum elevations and inflow values in acre-feet by water year, from October 1 973 through July 1996.

13 Capacities computed by ACAP computer program. Areas at elevation 4724 and above from 1973 survey.

48. AGENCY MAKING SURVEY Bureau of Reclamation

49. AGENCY SUPPLYING DATA Bureau of Reclamation DATE August 1997

Table 1. - Reservoir sediment data summary (page 2 of 2).

13
(I)

Elevation

(feet)

(2)

Original

Area

(acres)

(3)

Original

Capacity

(acre-feet)

(4)

1973

Area

(acres)

_______________

(5)

1973

Capacity

(acre-feet)

_________________

(6)

1973

Measured

Sediment

Volume

(acre-feet)

(7)

1973

Percent of

Computed

Sediment

(8)

1996

Survey

Area

____________

(9)

1996

Survey

Capacity

(10)

1996

Measured

Sediment

Volume

(acre-feet)

(11)

1996

Percent of

Computed

Sediment

(12)

1973-1996

Measured

Sediment

Volume

(acre-feet)

(13)

1973-1996

Percent of

Computed

Sediment

(14)

Percent

Reservoir

Depth

4724.0 1,357 104,860 1,296 99,067 5,793 100.0 1,296

____________

96,477 8,383 100.0 2,590 100.0 100.0

4720.0 1,319 99,508 1,272 99,930 5,578 96.3 1,280 91,325 8,183 97.6 2,605 100.6 97.7

4710.0 1,263 86,598 1,214 81,500 5,098 88.0 1,217 78,806 7,792 93.0 2,694 104.0 92.0

4700.0 1,192 74,323 1,109 69,885 4,438 76.6 1,105 67,198 7,125 85.0 2,687 103.7 86.2

4690.0 1,091 62,908 1,016 59,260 3,648 63.0 1,009 56,668 6,240 74.4 2,592 100.1 80.5

4680.0 972 52,593 939 49,485 3,108 53.7 917 47,016 5,577 66.5 2,469 95.3 74.7

4670.0 870 43,383 825 40,665 2,718 46,9 784 38,498 4,885 58.3 2,167 83.7 69.0

4660.0 728 35,393 710 32,990 2,403 41.5 691 31,142 4,251 50.7 1,848 71.4 63.2

4650.0 666 28,423 636 26,260 2,163 37.3 610 24,631 3,792 45.2 1,629 62.9 57.5

4640.0 597 22,108 570 20,230 1,878 32.4 554 18,818 3,290 39.2 1,412 54.5 51.7

4630.0 534 16,453 510 14,830 1,623 28.0 494 13,571 2,882 34.4 1,259 48.6 46.0

4620,0 461 11,478 438 10,090 1,388 24.0 416 9,013 2,465 29.4 1,077 41.6 40.2

4610.0 385 7,248 377 6,015 1,233 21.3 312 5,319 1,929 23.0 696 26.9 34.5

4600.0 262 4,013 215 3,055 958 16.5 200 2,833 1,180 14.1 222 8.6 28.7

4590.0 168 1,863 126 1,350 513 8.9 125 1,283 580 6.9 67 2.6 23.0

4580.0 72 663 50 407 193 3.3 60 376 287 3.4 94 3.6 17.2

- 4570.0 24 183 18 130 53 0.9 13 62 121 1.4 68 2.6 11.5

4560.0 5 38 4 20 18 0.3 1 2 36 0.4 18 0.7 5.7

4557.0 4 24 3 10 14 0.2 0 0 24 0.3 10 0.4 4.0

4550.0

.l

0 0

'.

0 0 0 0.0 0

- -

0

-

0 0.0 0 0.0 0.0 -------- ,i,p i,cvniiuii en rvservujr water suriace. Tabie 2

(2) Original reservoir surface area.

(3) Original calculated reservoir capacity computed using ACAP from original measured surface areas projected by area curves.

(4) Reservoir surface area from 1973 survey.

(5) 1973 calculated reservoir capacity computed using ACAP, from 1973 surface areas.

(6) Measured sediment volume = column (3) - column (5).

(7) Measured sediment expressed in percentage of total sediment 5,793 acre-feet at elevation 4724.0.

(8) Reservoir surface area from 1996 survey for elevations 4720.0 and below. Areas for elevation 4724 and above from 1973 survey.

(9) 1996 calculated reservoir capacity computed using ACAP, from 1996 surface areas.

(10) 1996 measured sediment volume = column (3) - column (9).

(11) Measured sediment expressed in percentage of total sediment 8,383 acre-feet at elevation 4724.0.

(12) Measured sediment volume from 1973 to 1996 = column (5) - column (9).

(13) Measured sediment expressed in percentage from 1973 to 1996, sediment volume of 2,590 acre-feet at elevation 4724.0.

(14) Depth of reservoir expressed in percentage of total depth (174 feet).
.SW .TW. SW. N.5W.

t

28

____I _______ _______ _____

24

N.

TI 231 N,

,P/SNNUN

_______

-flE5ERVO-

$VN RIVER

DI VERSION DAM ,SUN RIVER SLOPE

T.

GREAT

Lw. R.3W. 12W 1.1W. 1 IC

C HOT E AU

GIma 0

Rub

,4_. POT SHAW

DIVERS (IDA 0

•1

R.!C R3C. 1.4

.8

''a

fRIGATEO AICA

SCAIC O MILES

0 0

Sun River Project

01
Space intentionally left blank due to security concerns 

North Arnericn Deturn of 1927

Montana Coordinate System. Centre] Zone

8 10g7 .V
Area-Capacity Curves for Gibson Reservoir

Area (acre)

1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100

4730---- - - --

4710

4690

4670

4650

Ui

4630

4610

4590

4570

m-a.

0,

ccr

__ __-__- ---- ---- ---__ __

Capacity Area

-4.i1_ -

I..

__ - __

-"-'-'I

0

I-a

0

4730

4710

4690

4670

4650

4630

4610

4590

4570

4550

7500 15000 22500 30000 37500 45000 52500 60000 67500 75000 82500 90000 97500 105000

Capacity (acre-feet)
RECLAMATION'S MISSION

The mission of the Bureau of Reclamation is to manage, develop, and protect water and related

resources in an environmentally and economically sound manner in the interest of the American

Public.