The Hydrologic History of the 

San Carlos Reservoir, Arizona, 1929-71, 

with Particular Reference to 

Evapotranspiration and Sedimentation

GEOLOGICAL SURVEY PROFESSIONAL PAPER 655-N

The Hydrologic History of the 

San Carlos Reservoir, Arizona, 1929-71, 

with Particular Reference to 

Evapotranspiration and Sedimentation

By FRANK P. KIPPLE

GILA RIVER PHREATOPHYTE PROJECT

GEOLOGICAL SURVEY PROFESSIONAL PAPER 65 5-N

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1977
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Kipple, Frank P.

The hydrologic history of the San Carlos Reservoir, Arizona, 1929-71, with particular reference to evapotranspiration 

and sedimentation.

(Gila River phreatophyte project) (Geological Survey Professional Paper 655-N)

Bibliography: p. N40.

1. Hydrology~Arizona~San Carlos Reservoir. 2. Evapotranspiration-Arizona-San Carlos Reservoir. 3. Sediments

(,geology)-Arizona-San Carlos Reservoir. I. Title: The hydrologic history of the San Carlos Reservoir... II. Series.

III. Series: United States Geological Survey Professional Paper 655-N. 

QE75.P9 no. 655-N [GB705.A6] 557.3'03s [551.4'82'0979'54] 77-608085

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

Stock Number 024-001-02976-6
CONTENTS

Page 

Conversion factors ................................................................................ V

List of symbols ........................................................................................ V

Abstract ................................................................................................... Nl

Introduction ............................................................................................ 1

Purpose and scope ........................................................................ 1

Acknowledgments ............................................................................ 1

History of the San Carlos Project ............................................ 2

Definitions ......................................................................................... 2

Reservoir sediment ................................................................................ 3

Sediment deposition and capacity surveys ........................... 3

Surface-water storage losses from sediment 

accumulation ............................................................................. 5

Sediment distribution ................................................................... 6

The effect of phreatophytes on inflow channels ................. 7

Sediment trap efficiency ............................................................ 11

Analysis of sediment data ....................................................... 13

Interpolation of elevation-capacity ratings between 

capacity surveys .................................................................... 14

Pare 

Hydrology ...............................................................................................Nl4

Water-budget equation ................................................................ 14

Surface flow .................................................................................... 15

Gila River and San Carlos River inflow ......................... 15

Tributary inflow .................................................................... 16

Gila River outflow ................................................................... 18

Ground-water inflow ..................................................................... 18

Precipitation ..................................................................................... 19

Evaporation .................................................................................... 19

Water stage and surface-water storage at San Carlos 

Reservoir ....................................................................................... 2.2

Bank storage ..................................................................................... 24

Water-budget analyses ................................................................ 27

Evapotranspiration .............................................................................. 27

Development of surface-water storage-capacity ratings .......... 31

Simulation of the sediment depositional process .............. 31

Interpretation of simulation results ....................................... 37

Procedure to develop ratings ...................................................... 37

Summary and conclusions ................................................................. 3n

References cited ...................................................................................... 40

ILLUSTRATIONS

6-11.

Pare

FIGURE 1. Map showing reservoir boundary, location of gaging stations, and centerline of San Carlos Reservoir ............................N3

2-5. Graphs showing:

2. Area curves for San Carlos Reservoir .................................................................................................................................................. 4

3. Capacity curves for San Carlos Reservoir ........................................................................................................................................ 5

4. Vertical distribution of the volume of sediment deposition with 5-ft (1.5 m)-elevation intervals at San Carlos

Reservoir for the periods 1928-47 and 1947-66 ........................................................................................................................ 8

5. Bottom profiles of San Carlos Reservoir along principal longitudinal axis as determined by the five capacity

surveys ......................................................................................^^ 9

Photographs showing:

6. Aerial view looking downstream (west) on April 20, 1965, showing the sediment flats at the confluence of the

Gila and San Carlos Rivers within the San Carlos Reservoir .......................................................................................... 10

7. Aerial view looking upstream (north) on April 20, 1965, showing the San Carlos River on the left and the

Gila on the right .............................................................................................................................................................................^ 10

8. Aerial view of channel conditions of the Gila River upstream from the San Carlos River in 1935 7 years

after the completion of Coolidge Dam ...................................................................................................................................... 10

9. Aerial view of conditions of a part of the Gila River channel above the San Carlos River in June 1962 ................. 10

10. View looking downstream (south) on August 18, 1965, at the Gila River flood plain from a point 2 mi (3.2 km)

upstream from the mouth of the San Carlos River ..................................................... ............ ..................................... 11

11. Flood-plain conditions after the area was inundated by the reservoir pool ........................................................................ 11

12. Map showing extent of channel plugging in the Gila River on indicated dates ..._..................................................................... 12

13. Graph of profiles along the longitudinal axis of San Carlos Reservoir showing changes in bed elevations and slope

within the reach of channel plugging .................................................................................................................................................. 13

14. Photograph showing view of the Gila River looking north from a point 2.6 mi(4.2 km) upstream from the mouth

of the San Carlos River photographed in September 1964 ............................................................................................................ 13

15. Photograph showing downstream view on July 15, 1965, of a reach of the Gila River channel which had been

plugged in 1964 ................................................................................ 13

16. Photograph showing aerial view of channel plugging in the Gila River on July 22, 1964 ...................................................... 13

ill
IV CONTENTS

Page

FIGURES 17-31. Graphs showing:

17. Relation between accumulated decrease in storage capacity and accumulated streamflow by capacity

survey periods ...................................................................................................................................................................... N15

18. Annual combined Gila River and San Carlos River inflows to San Carlos Reservoir for water years

1929-71 ........................................................................................................................................................................................ 15

19. Frequency of occurrence of water-year inflow volumes, 1929-71 .................................................................................. 16

20. Mean monthly flow as a percent of mean annual for the Gila and San Carlos Rivers ........................................ 17

21. Relations between measured rainfall and tributary runoff for summer seasons 1964-71 from 72.6 mi2

(188 km2) of area tributary to San Carlos Reservoir .................................................................................................. 18

22. Evaporation at San Carlos Reservoir for water years 1931-71 ...................................................................................... 20

23. Five-year moving averages of pan evaporation at San Carlos Reservoir and the mean of pan evaporation at Mesa, Roosevelt Lake, and at Tucson for water years 1931-71 .............................................................. 21

24. Double-mass diagram of cumulative pan evaporation for San Carlos Reservoir and mean of Mesa,

Roosevelt Lake, and at Tucson for water years 1931-71 .......................................................................................... 21

25. Beginning-of-month lake stage of San Carlos Reservoir for water years 1929-71 .................................................. 23

26. Number of days in which lake stage was within a particular elevation interval for water years 1929-71 .. 24

27. Percentage of time lake stage of San Carlos Reservoir equaled or exceeded a given elevation for

water years 1929-71 ................................................................................................................................................._............ 24

28. Percentage of time usable surface-water storage of San Carlos Reservoir equaled or exceeded a

given volume for water years 1929-71 ............................................................................................................................ 25

29. Relations of cumulative ASg to cumulative AS# and cumulative AS# to cumulative ASy for the

periods January through April 1965 and December 1967 through May 1968 .................................................. 26

30. Relation between the computed change in bank-storage capacity and elevation at San Carlos Reservoir

for water years 1931-47 and 1948-71 .............................................................................................................................. 27

31. Elevation-capacity relation for usable surface-water storage, usable bank storage, and total usable

storage in 1966, for San Carlos Reservoir ...................................................................................................................... 29

32. Sketch showing relative magnitude of inflow and outflow water-budget components ..................................................... 31

33-36. Graphs showing:

33. Annual evapotranspiration from exposed area of reservoir, computed by reservoir water budget .................. 32

34. Annual depth of evapotranspiration from the exposed surface of San Carlos Reservoir for water

years 1931-71 .......................................................................................................................................................... 33

35. Computed monthly evapotranspiration at San Carlos Reservoir for water years 1931-71 ................................ 33

36. Mean monthly evapotranspiration depths computed for 10-year periods .................................................................. 33

37. Sketch identifying terms used in simulation of suspended sediment distribution ............................................................ 35

38. Sketch showing distribution of suspended sediment, Ss., when distance, D^, was computed as greater than

the distance from inflow point (A) to the dam ........................................................................................................................ 36

39. Graph showing simulated sediment deposition, in percent, downstream from point of inflow .................................... 37

40. Graphs showing comparison between the volumes of deposits measured and estimated for each incremental

reservoir, 1929-66, and volumes of estimated and measured deposits cumulative by 5-ft-elevation increments 

upward from lowest part of reservoir, 1929-66 water years ................................................................................................ 38

TABLES

Page

TABLE 1. Results of surface-water storage-capacity surveys .................................................................................................................................. N5

2. Storage capacities, sediment deposition, and streamflow data ............................................................................................................ 7

3. Volumes of sediment deposited by 5-ft-elevation intervals in San Carlos Reservoir during different periods...................... 7

4. Maximum and minimum volumes of water supplied by streamflow into San Carlos Reservoir for different time

durations .......................................................................................................................................................................................................... 16

5. Mean monthly inflows for the Gila River, the San Carlos River, and for both rivers, 1930-71 .............................................. 16

6. Tributary inflow into the San Carlos Reservoir along a reach of the Gila River ........................................................................ 17

7. Estimated tributary inflow into San Carlds Reservoir .......................................................................................................................... 17

8. Mean monthly discharge of the Gila River below Coolidge Dam, 1931-71...................................................................................... 18

9. Annual (water year) precipitation at San Carlos Reservoir, 1931-71 ................................................................................................ 19

10. Mean monthly precipitation and monthly extremes (1931-71) at San Carlos Reservoir .......................................................... 20

11. Evaporation at San Carlos Reservoir by water year, 1931-71 ............................................................................................................ 22

12. Mean monthly evaporation and mean monthly pan coefficients for San Carlos Reservoir .................................................... 23

13. Percent of time that available monthly surface-water storage was less than amount shown ................................................ 25

14. Example of water budget used to determine change in bank storage .......!...................................................................................... 25

15. Results of procedures to determine bank storage capacity at San Carlos Reservoir .................................................................. 28

16. San Carlos Reservoir elevation-capacity tables of usable bank storage, usable surface-water storage, and

total usable storage ...................................................................................................................................................................................... 29
CONTENTS v

TABLE 17. Summations of San Carlos Reservoir inflow-outflow components by water year, 1931-71 ....................................................N30

18. Water-budget summations ......................................................................................... 32

19. Total evapotranspiration for San Carlos Reservoir, by water year .................................................................................................. 33

20. Mean monthly evapotranspiration computed from 4 periods of 10 years each, and median monthly

evapotranspiration for 41 years................................................................................................................................................................ 34

21. Chart of notation used to identify time and inflow location of computed proportional sediment weights, Sy^ ............... 34

22. Optimum values of variables from simulation of the sediment depositional procedure ..........................................'.................. 36

23. Volumes of sediment deposits measured (Sjjffe) and computed (Sep for 1937-47, and the SMk /Sek ratios ............................ 38

24. Estimated change in surface-water storage capacity, Z, from start of period 3 to 1942, by 5-ft-elevation increments .... 39

25. Computed capacity ratings of surface-water storage used for 1938 through 1942 ........................................................................ 39

26. Comparison of segments of surface-water storage capacity ratings made by curve fitting and by computer .................. 39

CONVERSION FACTORS

English

acre-feet (acre-ft)

miles (mi)

feet (ft)

cubic feet per second (ftVs)

acres

pounds per cubic feet (Ib/ft3)

acre-feet per square mile (acre-ft/mi2)

gallons per day per square foot [(gal/day)/ft2]

inches (in.)

parts per million (ppm)

Multiply by

1.233 > 10-3

1.609 

.3048

2.832

4.047 

16

0.476

4.074 

25.4

l.OO1

10-2 

10-3

io-2

Metric (SI)

cubic hectometers (hm3)

kilometers (km)

meters (m)

cubic meters per second (mVs)

square kilometers (km2)

kilograms per cubic meter (kg/m3)

cubic hectometers per square kilometer (hmVkni2)

cubic meters per day per square meter [(m3 /day)/rr 2 ]

millimeters (mm)

milligrams per liter (mg/1)

LIST OF SYMBOLS

is 

IT

OG

IGW 

IP

OE 

°ET

Q 

K 

m 

u 

w 

P

Q

Gila River inflow at Calva

San Carlos River inflow at Peridot

Tributary inflow into the San Carlos Reservoir

Gila River outflow below Coolidge Dam

Ground-water inflow

Precipitation input over the lake surface

Evaporation from the lake surface

Evapotranspiration from the exposed surface of the

reservoir 

Change in surface-water storage

Change in bank storage

Ground-water flow

Hydraulic conductivity

Thickness of aquifer

Ground-water slope

Aquifer width

Rainfall

Stream discharge

Change in total reservoir storage

Weight of sediment discharge 

Mean sediment concentration

E

SB

Weight of sediment trapped by reservoir

Sediment trap efficiency of a reservoir 

Mean winter sediment concentration

Mean summer sediment concentration 

Winter streamflow summations 

Summer streamflow summations

Ratio of summer to winter concentrations 

Computed value proportional to weight of sediment

deposited in a reservoir 

Incremental storage reservoir 

Water year 

The ratio of the weight of larger sediment particles

to the weight of total sediment 

Distance estimated by simulation procedure from

location of river discharge into the reservoir pool

to the point where deposition is complete 

Distance from any point along DA to point where

distribution of sediment is complete 

Exponent of suspended sediment distribution 

Computed amount proportional to weight of sediment

deposited between point of inflow and any other

downstream point

1 See "Water Resources Data for Arizona, Part 2, Water Quality Records, 1973" for conversion factors when sediment concentration exceeds 8,000 ppm (factor varies depending on 

specific gravity of sediment and density of water).
VI

DA

CONTENTS

Estimated distance from location of river discharge 

into incremental reservoir i to the point where 

deposition is complete

Incremental reservoir of deposition

Distance from upstream point of incremental reservoir k, in which simulated deposition is occurring, 

to the point where deposition is complete

A computed quantity which is proportional to total 

sediment weight entering an incremental reservoir 

from the stream

A computed quantity which is proportional to weight 

of suspended sediment inflow

A computed quantity which is proportional to weight 

of suspended sediment deposited in reservoir k

Sdk,j

SM 

RATIO

ZkJ

A computed quantity which is proportioral to the 

suspended sediment weight deposited in reservoir k 

during water year ;

A computed quantity which is proportioral to the 

sediment weight deposited in an incremental reservoir during a period

The sum of all SR

The volume of sediment measured

A proportionality constant between sediirent measured and sediment estimated

Estimate of the absolute sediment volume for incremental reservoir k

The estimated decrease in storage for an incremental 

reservoir k from the start of a period to water year ;
GILA RIVER PHREATOPHYTE PROJECT

THE HYDROLOGIC HISTORY OF THE SAN CARLOS RESERVOIR,

ARIZONA, 1929-71, WITH PARTICULAR REFERENCE TO

EVAPOTRANSPIRATION AND SEDIMENTATION

By FRANK P. KIPPLE

ABSTRACT

Reservoir data records were used in an investigation of evapotranspiration from the land area of San Carlos Reservoir and 

evaporation from the water-surface area. A water-budget analysis 

indicates that the evapotranspiration loss was 11.3 percent and 

the evaporation loss was 10.5 percent of the total outflow from the 

reservoir during 1931-71.

The water-budget computations were used to develop ratings 

relating lake stage to usable bank storage. The rating developed 

for the 1948-71 period indicates that usable bank storage is 

approximately 159,000 acre-ft (196 hm3), or about 14 percent of 

total usable storage capacity, if the reservoir is filled to the 

spillway level of 2,511 ft (765 m).

A procedure was developed to simulate sediment deposition in 

the reservoir. The procedure was used to estimate the change in 

storage capacity between five reservoir capacity surveys made 

during the period 1914-66.

INTRODUCTION 

PURPOSE AND SCOPE

Once a reservoir is put into operation, a number of 

progressive changes are produced which affect the 

hydrology of the reservoir. Prior to inundation, water 

vaporization from the reservoir area is from plant 

transpiration and from soils and off-channel ponding. During inundation vaporization is evaporation 

from the ponded water surface. The soils and topography are changed by the deposition of sediment 

and sometimes by bank erosion. The water table 

adjacent to the reservoir rises, and the bank storage 

of water is increased. Vegetation on exposed parts of 

the reservoir may be altered because of changes in 

soils and water availability. These changes are 

particularly significant for reservoirs where the 

streams convey large quantities of sediment and 

where fluctuations of the reservoir water level and 

the water surface areas are large. The records of 

inflow, outflow, surface-water storage, and sediment 

deposition provide the data for evaluating some of 

the changes for the San Carlos Reservoir.

An investigation of these changes, and of the 

reservoir hydrology in general, was made to evaluate 

reservoir evapotranspiration (ET) and the change in 

ETfrom 1929 to 1971. The investigation was made as 

part of the Gila River Phreatophyte Project, a study 

of the hydrologic effect of phreatophyte control by 

the U.S. Geological Survey (Culler and others, 1970). 

The evaluation of ET is made by use of a waterbudget equation in which ETis the residual in the 

equation.

Secondary objectives included investigations of 

reservoir sediment deposition, lake evaporation, and 

reservoir bank storage. These investigations were 

essential prior to compilation of the water budget.

Data sources for this report include five surveys of 

reservoir capacity which provide a history of capacity change and sediment accumulation. Investigations of tributary runoff, precipitation, evapotranspiration, and lake evaporation were made as part of 

the Gila River Phreatophyte Project and furnish 

information for this report. Other sources of data are 

U.S. Geological Survey surf ace-water records (issued 

annually), Gila River Water Commissioner reports 

(issued annually), log books of precipitation andp^n 

evaporation at Coolidge Dam, and climatic data 

published by the National Weather Service (issued 

annually).

ACKNOWLEDGMENTS

Much of the planning and data interpretation for 

this report was contributed by R. C. Culler, project 

chief of the Gila River Phreatophyte Project. Preprration of the report was under the direction of R. L. 

Hanson. Assistance and cooperation were received 

from personnel of the Arizona District office, U.S. 

Geological Survey, and of the San Carlos Project of 

the U.S. Bureau of Indian Affairs, supervised by 1 1. 

D. Young, general engineer. Computations of lake

Nl
N2 GILA RIVER PHREATOPHYTE PROJECT

evaporation by energy-budget and mass-transfer 

methods were principally by J. Stuart Meyers, U.S. 

Geological Survey.

HISTORY OF THE SAN CARLOS PROJECT

The San Carlos Project was established to provide 

irrigation water to the Middle Gila District, Gila 

River Basin, Ariz. The district was defined by Davis 

(1897, p. 17) as "that portion from the mouth of Salt 

River to The Buttes above Florence and including the 

Pima Indian Reservation and the great Casa Grande 

Valley." Diversions of water from the upstream 

reaches of the Gila River by farmers during the 

period 1870-86 imperiled the water rights of the 

Indians on the Pima Reservation. An investigation 

was made by the Geological Survey to examine the 

possibility of providing a firm water supply to the 

Indians as reported by Davis (1897, p.-71). The 

construction of a dam on the Gila River at The Buttes 

14 mi (23 km) east of Florence was recommended. 

Numerous other feasibility studies were made during 

the next 15 years, culminating in a report by the U.S. 

Army Corps of Engineers (1914) recommending a 

dam on the Gila River.

On June 7, 1924, Congress approved legislation 

that authorized the Secretary of the Interior through 

the Indian Service to construct a dam at the San 

Carlos site as part of the San Carlos Project. The 

construction of Coolidge Dam was started in January 1927 and completed in October 1928. Water impoundment began on November 15, 1928. The area 

within the boundary of the reservoir is administered, 

and the facilities at Coolidge Dam are operated, by 

the San Carlos Project, an agency of the Bureau of 

Indian Affairs.

Coolidge Dam is located in sec. 17, T. 3 S., R. 18 E., 

Gila County, Ariz., in the San Carlos Indian Reservation (fig. 1). The dam is a multidomed structure 

having a length, including two spillways, of 850 ft 

(259 m). Each of the three domes has a span of 180 ft 

(55 m) and a base thickness of 28 ft (8.5 m). The thickness decreases to 4 ft (1.2 m) at the top. The dam rises 

203 ft (62 m) to the spillway at elevation 2,511 ft (765 

m) above mean sea level and approximately an additional 25 ft (7.6 m) to the highway on top of the dam. 

A spillway is located on each side of the dam. Each 

spillway has three gates 50 ft (15 m) wide and 12 ft 

(3.7 m) high. Maximum storage capacity of 1,267,000 

acre-ft (1,560 km3) at elevation 2,523 ft (769 m) is 

reached when the gates are raised. The gates are now 

inoperative in the lowered position. The maximum 

safe release from spillways and outlets is 122,000 

ftVs (3,455 mVs). The sill of the lowest outlet gate is 

at elevation 2,382.63 ft (726 m), providing an operating range, outlet to spillway, of 128.37 ft (39.13 m).

The principal purpose of the reservoir is to store

water for irrigation of 100,000 acres (405km2 ) of land 

within the San Carlos Project. Fifty thousand acres 

(202 km2) are Indian lands within the Gila River 

Reservation, and 50,000 acres (202 km2) are privately 

owned lands in the Florence-Casa Grande Valley. 

Water released from Coolidge Dam is diverted from 

the Gila River channel at the Ashurst-Hayder Diversion Dam 68 mi (109 km) downstream. A power plant 

at Coolidge Dam contains two generators having a 

combined output of 10,000 kilo volt-amperes. Power 

generation is subordinate to irrigation requir?ments 

and is stopped when irrigation demands are curtailed. The reservoir is also used for recreation, and 

the recreational facilities are operated under lease by 

the San Carlos Apache Indian Tribe. Flood protection provided by the reservoir is an incidental 

benefit.

Water in the Gilla River was adjudicated by a court 

decree entered in 1935 (U.S. vs. Gila Valley Irrig. 

Dist. et al., 1935). Briefly, the decree divides the 

water between the upstream users in the Safford and 

Duncan Valleys, the Gila Valley Irrigation District, 

and the downstream users of the San Carlos Irrigation Project, on the basis of priority of appropriation. 

In addition to priority rights, upstream users are also 

entitled to apportioned rights, which are dependent 

on the amount of water stored in San Carlos Reservoir. As defined in the decree, the rights are determined as follows: On January 1 of each yeer, or as 

soon thereafter as there is water stored in the San 

Carlos Reservoir, which is available for release for 

use on lands of the San Carlos Project, the Gila Water 

Commissioner, who is appointed by the court to 

enforce the decree, shall apportion for the ensuing 

irrigation year to irrigated lands above the San 

Carlos Reservoir from the natural flow of the Gila 

River an amount of water equal to that stored in the 

San Carlos Reservoir less losses. It is also provided 

that if and when at any time, or from time to time, 

during the year storage in the reservoir shall be 

increased and made available to downstream users, 

the Commissioner shall make further and additional 

apportionments to upstream users which fhall be 

equivalent in amount to the newly available stored 

water supply.

DEFINITIONS

The following are definitions of terms as used 

throughout this report:

Water year........................~October 1 to September 30

Surface-water storage The above-ground volume of a reservoir

capacity available to store water 

Dead storage capacity, The above-ground volume of a reservoir 

surface water below the invert of the lowest reservoir outlet, which cannot be evacuated 

by gravity
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 I T3

110

0 50 100 Kl LOMETERS 

\

AREA

32 ^- Tucson 

INDEX MAP

0 50 100MILES

110 30'

Base from U.S. Geological Survey 

Mesa 1:250,000, 1954

2345 KILOMETERS

CONTOUR INTERVAL 200 FEET

WITH SUPPLEMENTARY CONTOURS AT 100-FOOT INTERVALS 

DATUM IS MEAN SEA LEVEL

FIGURE 1. Map showing reservoir boundary, location of gaging stations, and centerline of San Carlos Reservoir.

Usable surface-water 

storage capacity

Usable bank-storage 

capacity

The difference between surface-water 

storage capacity and dead storage capacity, defined as the volume available 

for release below the stage of maximum controllable level

The below-ground volume in the banks of 

a reservoir available for storage which 

can be evacuated by gravity

RESERVOIR SEDIMENT

SEDIMENT DEPOSITION AND CAPACITY SURVEYS

Sediment deposition in a reservoir, and the resulting reduction in water-storage capacity, affects 

water supply and water management, installations 

within the reservoir, and recreational activities. 

Management can, in turn, influence the sediment 

distribution and sediment compaction within the

reservoir and the volume of sediment which passes 

through the reservoir by (1) regulating the stage, rate 

of water release, and frequency of sediment wetting 

and drying, (2) vegetative clearing, and (3) channel 

dredging. An inventory of sediment deposition can 

be obtained by use of data from reservoir-capacity 

surveys. In addition, these surveys provide the data 

used to establish the elevation-capacity and elevation-area ratings, which are needed for water 

management.

The volume of sediment deposited in the £<m 

Carlos Reservoir was calculated from the results of 

five surveys. The first survey was made during 1914 

and 1915 for the Indian Irrigation Service to determine potential storage capacity of the proposed 

reservoir. The second and third surveys were com-
N4 GILA RIVER PHREATOPHYTE PROJECT

pleted in 1935 and 1937 by the Soil Conservation 

Service, the third at the request of the Gila Water 

Commissioner. A fourth survey was made in 1947 by 

the Corps of Engineers to assess changes in the 

capacity of the reservoir, mainly resulting from 

above-normal inflow in 1941-42 (Thorp and Brown, 

1951). A map of the reservoir, scale 1:7,200, was 

produced by photogrammetric methods from the 

1947 survey. Changes during the period 1947-66 

were assessed by a fifth survey made in 1966 by the 

U.S. Geological Survey.

The survey of 1966 was made primarily to provide 

better water-surface area and storage-capacity data 

for the water-budget analysis of evapotranspiration. 

An above-normal lake level during the summer of 

1966 provided an opportunity to obtain an economical survey of the reservoir. Control points at-theends

of 51 range lines were established near the water's 

edge, and a recording fathometer was used to obtain 

a continuous record of the ground profile along each 

range line. The shorelines shown on aerial photography taken during 1966-67 were used to check 

some of the topographic data. Topographic changes 

within the study area of the Gila River PhreatcT>hyte 

Project above the maximum 1966 pool leve? were 

obtained from cross-valley profiles repetitively surveyed (Burkham, 1972). Elevation data were plotted 

on copies of 1947 reservoir topographic maps of the 

U.S. Army Corps of Engineers and were used to 

locate 5-ft (1.5 m) contours for 1966.

Area curves from the five capacity surveys are contained in figure 2. Reservoir capacities between consecutive 5-ft (1.5 m) contours were computed using 

the elevation-area data of each survey. The curves of

WATER-SURFACE AREA, IN THOUSANDS OF SQUARE HECTOMETERS 

1 234567

2530

2520

2510

2500

-" 2480

LU

_* 2470

LU

^ 2460

LU

O 2450

DO

I- 2440

LU 

LU 

LL

z 2430

O 2420

H

> 2410

LU

^ 2400 

2390 

2380 

2370

Top of fully raised gates, 2, 523 ft 

Top of spillway crest, 2, 511 ft

- 770

765

- 760 -1

755 LU W

Z

 Elevation at top of dead storage pool, 2, 382.63 ft

I I I | I I

-745

- 740

- 735

 730

- 725

- 720

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 

WATER-SURFACE AREA, IN THOUSANDS OF ACRES

FIGURE 2. Area curves for San Carlos Reservoir.
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N5

figure 3 show the elevation-capacity relations for all 

surveys. Water-surface areas and surface-water 

storage capacities for the surveys are listed by 5-ft 

(1.5 m)-elevation increments in table 1. Much of table 

1 data is from Thorp and Brown (1951, table 1). 

The earlier capacity surveys were not as detailed 

as later surveys because the needs were different. 

The 1914-15 reservoir survey was designed to describe agricultural lands and land use on the Gila 

and San Carlos River flood plains. The topography 

outside the flood plain area was included in this 

survey but was with less detail. More attention was 

given to defining flood-plain topography in the 1935 

and 1937 surveys. Not until the 1947 survey, however, was the upland topography well defined. Capacity ratings for the first three surveys were adjusted to the more accurate 1947 survey by Thorp and 

Brown (1951, p. 9, 10) and are shown in table 1. The 

errors remaining after adjustments are significant 

in the analyses of this report. As an example of the 

errors remaining in the ratings, it is seen in table 1 

that capacities computed for 5-ft-elevation increments in 1937 were greater than for corresponding 5- 

ft increments in 1935, in the range of 2,435 to 2,495 ft 

(742-760 m). Scour or compaction might have produced some of the increases, however, the increases 

extend about 25 ft (7.6 m) above the maximum water 

stage recorded prior to the 1937 surveys. The maximum reservoir elevation through 1937 was 2,471.56 ft 

(753 m), April 5, 1932. Another example of error in 

the first two surveys is indicated by no measured 

capacity change between surveys above certain 

stages, although considerable inflow occurred when 

the reservoir water level exceeded those stages. T N e 

1935 capacity table was identical to the 1914-15 table 

above 2,435 ft (742 m) elevation, yet about 668,010 

acre-ft (824 hm3 ) of inflow occurred above this 

elevation during 1928-35.

SURFACE-WATER STORAGE LOSSES FROM 

SEDIMENT ACCUMULATION

The maximum surface-water storage capacity of 

the San Carlos Reservoir in November 1928 w*s 

1,266,837 acre-ft (1,562 hm3) at 2,523 ft (769 m). Py 

1966, sediment deposition had reduced the capacity 

by 96,719 acre-ft (119 hm3) to 1,170,118 acre-ft (1,443 

hm3 ), a 7.6 percent loss. The loss in usable surfacewater storage capacity was 72,476 acre-ft (89 hm3) for 

the same period.

These storage losses indicating that the total

CAPACITY, IN CUBIC HECTOMETERS 

500 600 700 800 900 1000 1100

2530

Elevation of maximum capacity, 2, 523 ft

Spillway crest, 2, 511 ft -- -.. ^^-'^^-^

EXPLANATION

Elevation at top of dead storage pool, 2, 382.63 ft

2360 720

100 200 300 400 500 600 700 800 900 

CAPACITY, IN THOUSANDS OF ACRE-FEET

1000 1100 1200 1300

FIGURE 3. Capacity curves for San Carlos Reservoir.
N6 GILA RIVER PHREATOPHYTE PROJECT

TABLE 1. Results of surface-water storage-capacity surveys

1914-15 survey (adjusted)1 1935 survey (adjusted) 1937 survey (adjusted)

Elevation 

above 

mean sea 

level 

(ft)

2308 ........ 

2365 ..........

2370 ..........

2375 ..........

2380 ..........

2385 ... . .... 

2390 .........

2395 ..........

2400 ..........

2405 ..........

2410 ..........

2415 ..........

2420. ....... 

2425 ..........

2430 ..........

2435 ..........

2440 . ........

2445 . ........

2450 . ........ 

2455 ... ......

2460 ..........

2465 . ........ 

2470 . ........ 

2475 . ........

2480 .... .....

2485 . .. .....

2490 . .. .....

2495 . .. .....

2500 . .. ... .

2505 . ........

2510 . ........ 

2515......... 

2520 ..........

2525 ..........

25113 .. .. ..

25234 ..... ..

Area

(acres)

Orignal b<

... 489

... 830

... 1,116

... 1,511

... 1,825 

... 2,048

... 2,309

. 2,681

... 3,079

... 3,397

... 3,706

... 4,152 

... 4,612

(2)

(2)

... 6,250

... 6,845

.. 7,467 

.. 8,060

.. 8,841

.. 9,778 

.. 10,501 

.. 11,187

.. 11,887

.. 12,682

.. 13,516

.. 14,320

.. 15,250

.. 16,116

. 17,027 

.. 17,991 

.. 18,974

.. 19,985

Capacity 

to next 

lower Cumulative 

contour capacity Area 

(acre-ft) (acre-ft) (acres)

ottomatdam 

6,060 

3,307 

4,946 

6,523 

8,162 

9,396 

10,626 

12,347 

14,522 

16,434 

18,315 

20,559 

23,984 

25,547 

28,303 

29,735 

32,727 

35,769 

38,809 

42,238 

46,529 

50,688 

54,212 

57,677 

61,413 

65,485 

69,582 

73,914 

78,407 

82,849 

87,536 

92,403 

97,389

0 0 

6,060 66 

9,367 428 

14,323 768 

20,846 1,127 

29,008 1,362 

38,404 1,624 

49,030 1,824 

61,377 2,253 

75,899 2,651 

92,333 3,020 

110,648 3,481 

131,207 3,922 

155,191 4,537 

180,738 5,197 

209,041 5,649 

238,776 6,250 

271,503 6,845 

307,272 7,467 

346,081 8,060 

388,319 8,841 

434,848 9,778 

485,536 10,501 

539,748 11,187 

597,425 11,887 

658,838 12,682 

724,323 13,516 

793,905 14,320 

867,819 15,250 

946,226 16,116 

1,029,075 17,027 

1,116,611 17,991 

1,209,014 18,974 

1,306,403 19,985 

1,046,203 ...............

1,266,857 .................

Capacity 

to next 

lower 

contour

(acre-ft)

0 

5 

1,103 

2,949 

4,709 

6,213 

7,456 

8,615 

10,174 

12,247 

14,168 

16,239 

18,497 

21,129 

24,317 

27,108 

29,735 

32,727 

35,769 

38,809 

42,238 

46,529 

50,688 

54,212 

57,677 

61,413 

65,485 

69,582 

73,914 

78,407 

82,849 

87,536 

92,403 

97,389

Cumulative 

capacity Area

(acre-ft) (acres)

0 0 

5 55 

1,108 363 

4,057 622 

8,766 1,021 

14,979 1,264 

22,435 1,570 

31,050 1,835 

41,224 2,214 

53,471 2,717 

67,639 3,067 

83,878 3,430 

102,375 3,797 

123,504 4,455 

147,821 5,084 

174,929 5,648 

204,664 6,296 

237,391 6,896 

273,160 7,531 

311,969 8,145 

354,207 8,884 

400,736 9,808 

451,424 10,516 

505,636 11,183 

563,313 11,905 

624,726 12,685 

690,212 13,519 

759,793 14,320 

833,708 15,250 

912,114 16,116 

994,963 17,027 

1,082,499 17,991 

1,174,902 18,974 

1,272,291 19,985 

. 1,012,091 ...............

. 1,232,725 ................

Capacity 

to next 

lower 

contour

(acre-ft)

0 

4 

932 

2,434 

4,069 

5,704 

7,071 

8,504 

10,108 

12,306 

14,451 

16,234 

18,060 

20,609 

23,831 

26,818 

29,846 

32,969 

36,057 

39,180 

42,560 

46,712 

50,801 

54,238 

57,709 

61,466 

65,500 

69,589 

73,914 

78,407 

82,849 

87,536 

92,403 

97,389

Cumulative 

capacity

(acre-ft)

4 

936 

3,370 

7,439 

13,143 

20,215 

28,719 

38,827 

51,133 

65,584 

81,819 

99,879 

120,487 

144,318 

171,136 

200,982 

233,951 

270,008 

309,188 

351,748 

398,460 

449,261 

503,498 

561,207 

622,673 

688,174 

757,763 

831,677 

910,084 

992,933 

1,080,468 

1,172,872 

1,270,260 

. 1,010,061

. 1,230,695

Area

(acres)

1947 survey

Capacity 

to next 

lower 

contour

(acre-ft)

Cumulative 

capacity Area

(acre-ft) (acres)

Original bottom at dam 

00 00 

61 10 

227 450 450 13 

524 1,827 2,277 174 

803 3,293 5,570 291 

1,182 4,932 10,502 599 

1,454 6,578 17,080 968 

2,083 8,796 25,876 1,508 

2,493 11,425 37,301 2,065 

2,902 13,475 50,776 2,562 

3,315 15,531 66,307 3,061 

3,741 17,630 83,937 3,319 

4,391 20,309 104,246 3,859 

5,099 23,703 127,949 4,538 

5,564 26,650 154,599 5,095 

6,143 29,256 183,853 5,839 

6,731 32,174 216,029 6,372 

7,381 35,268 251,297 7,123 

7,971 38,371 289,669 7,751 

8,900 42,157 331,826 8,654 

9,617 46,282 378,108 9,668 

10,522 50,332 428,439 10,460 

11,133 54,131 482,570 11,078 

11,832 57,405 539,975 11,690 

12,689 61,291 601,266 12,524 

13,528 65,533 666,799 13,224 

14,320 69,612 736,411 14,212 

15,250 73,914 810,325 15,250 

16,116 78,408 888,732 16,116 

17,027 82,849 971,581 17,027 

17,991 87,536 1,059,116 17,991 

18,974 92,403 1,151,520 18,974 

19,985 97,389 1,248,908 19,985 

. .. 988.709

. 1,209,343 ...... ......

1966 survey

Capacity 

to next 

lower 

contour

(acre-ft)

0 

0 

22 

394 

1,150 

2,180 

3,880 

6,140 

8,897 

11,545 

14,039 

15,945 

17,929 

20,970 

24,069 

27,314 

30,519 

33,721 

37,174 

40,992 

45,783 

50,308 

53,839 

56,914 

60,525 

64,363 

68,576 

73,914 

78,407 

82,849 

87,536 

92,403 

97,389

Cumulative 

capacity

(acre-ft)

0 

0 

22 

414 

1,564 

3,744 

7,624 

13,764 

22,661 

34,206 

48,245 

64,190 

82,119 

103,089 

127,158 

154,472 

184,991 

218,712 

255,886 

296,878 

342,661 

392,969 

446,808 

503,722 

564,247 

628,610 

697,186 

771,100 

849,507 

932,356 

1,019,892 

1,112,295 

1,209,684 

949,484 

. 1,170,118

1 Use of these tables began in 1928 with closure of dam.

2 Area on area-elevation curves interpolated from 2425 and 2440 contours.

3 Spillway crest.

4 Spillway crest with gates fully raised.

volume of sediment deposited was 96,719 acre-ft (119 

hm3) also indicate that the mean annual volume for 

the period November 1928 to August 1966 was 2,553 

acre-ft (3.1 hm3 ), which is equivalent to 0.20 percent 

of the original surface-water storage capacity. Table 

2 includes storage capacity data, sediment deposition amounts and mean rates of deposition, and a 

comparison of sediment volume deposited to streamflow volume for all periods between surveys.

SEDIMENT DISTRIBUTION

The data of table 1 were used to calculate the 

volumes of sediment deposition for 5-ft (1.5 m)-elevation increments for the periods 1928-47,1947-66, and 

1928-66. (The capacity tables used in 1928 were 

obtained from the 1914-15 survey.) The computed 

volumes are given in table 3 and are illustrated in the 

vertical distribution graph of figure 4. Changes in 

elevation along the centerline of the reservoir resulting from sediment deposition are evident from the 

profiles shown in figure 5 for all surveys. The 

longitudinal centerline, as established on maps of 

the 1947 survey, was changed to more nearly center it

with respect to the flood plain at a distance of 42,000 

to 58,000 ft (12,800 to 17,700 m) upstream f-om the 

dam. The centerline, as revised, is shown in figure 1. 

Centerline distances from the dam to intercepts of 

centerline and contour lines were scaled from maps 

of all surveys except the 1937 survey, for which no 

maps were available. Centerline distances for the 

1937 survey were based on the 1937 capacity-survey 

data and the 1935 centerline distances.

Longitudinal slope of the reservoir bottom below 

the 2,380-ft (725 m) contour has decreased from 

0.00246 to 0.00057 during 1929-66. Above the 2,380-ft 

(725 m) contour the slope has decreased from 0.00167 

to 0.00138. Over the entire 22-mi (35.4 km) reservoir 

length, the mean slope was 0.00188 in 1928, 0.00140 

in 1935 and 1937, 0.00135 in 1947, and 0.00131 in 

1966. The longitudinal slope of the San Carlo,*? arm of 

the reservoir is about 0.00275. Changes in slope due 

to channel plugging are discussed in th«* next 

section.

A large sediment accumulation occurred in the 

lower parts of the reservoir, as indicated in table 3 

and figures 4 and 5, which corresponds to the most
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71

TABLE 2. Storage capacities, sediment deposition, and streamflow data 

A. Storage capacities

! T7

Year of survey 1928' 1935 1937 1947 1966

Surface-water storage capacity in acre-ft, at 2,523 ft (gates fully raised) ...................................... 1,266,837 1,232,725 1,230,695 1,209,343 1,170,118

Surface-water storage capacity in acre-ft, at 2,511 ft (spillway crest) ............................................ 1,046,203 1,012,091 1,010,061 988,709 949,484

Usable surface-water storage capacity in acre-ft, at 2,511ft .......................................................... 1,021,060 1,000,916 999,231 984,875 948,584

Dead storage capacity in acre-ft, at 2,382.63 ft .............................................................................. 25,143 11,175 10,830 3,834 900

Elevation of zero surface storage at time of capacity survey, in feet .... ................................... 2,309 2 2,354 2 2,354 2,370 2,374

B. Sediment deposition

Period

Sediment deposition per period in percent of original surface-water storage capacity 

at 2,523 ft ...... .. ..... .. .. . ....... ................... .. .................. ......... .. ........................... ... .. ......................... .. . Mean annual sediment deposited per period as percent of original surface-water

Dead storage loss by sediment deposition per period, in percent of original dead storage

1928-351

6.28

04 119

5,431

2.69

.43

13,968

55.55

. .. 1,722,985

.0198

1935-37

1.91

2,030

1,063

.16

.08

345

1.37

345,275

.0059

1937-47

10.00

21,352

2,135

1.69

.17

6,996

27.83

2,541,463

.0084

1947-66

19.70

39,225

1,991

3.09

.16

2,934

11.67

3,579,217

.0110

1928-661

37.89

96,719

2,553

7.63

.20

24,243

96.42

8,188,940

.0118

'Beginning November 15, 1928.

^Approximate.

3Inflow of Gila River and San Carlos River.

TABLE 3. Volumes of sediment deposited by 5-ft-elevation intervals in San Carlos Reservoir during different periods

Reservoir elevation

(mean sea level)

2308 .. . ...... . ..............

2365 ...... .. ....... . ...... .....

2370 ...... .........................

2375 ... .. .. ......... . ..........

2380 .. .. .. .........................

2385 .............................

2390 ...... ...... ... .. ... ......

2395 ...... .. .. .......... . .... .

2400 .. .. .. . ............. ..........

2405 ...... .. ..........................

2410 ................... .........

2415 . . .. ..........................

2420 ............................ ..

2425 ... .. .. . ..... ........ .........

2430 .............................. .

2435 ..... ........ ............ .

2440 ... .. .. ........................

2445 ..............................

2450 ..... .. ....................... ..

2455 ... .. .. ....... .. ...... .......

2460 . ..... .. ...................

2465 ... .. . ................ ..... ..

2470 ....... .......................

2475 .. .. ........................

2480 ..... . .......................

2485 . .. .. ............... ..........

2490 ..... . .... .....................

2495 .. .. .............................

2500 ... .. .. ...... ..... ........ ..

2505 ... .. .. ................. ..... .

2510 ...... . . ... ... ................

2515 . .. .. .. .......................

2520 ... .. .. ............. . ........

2525 ...... .. ................ ..... ..

Total .......... . .... ..

Volume to next lower contour,

1928-47

0

6,060

3,306

4,506

4,696

4,869

4,464

4,048

3,551

3,097

2,959

2,784

2,929

3,675

1,844

1,653

479

553

501

438

81

247

356

81

272

122

-48

-30

0

0

0

0

0

0

57,493

1947-66

0

0

1

428

1,435

2,143

2,752

2,698

2,656

2,528

1,930

1,492

1,685

2,380

2,733

2,581

1,942

1,655

1,547

1,197

1,165

499

24

292

491

766

1,170

1,036

0

0

0

0

0

0

39,226

in acre-ft

1928-66

0

6,060

3,307

4,934

6,131

7,012

7,216

6,746

6,207

5,625

4,889

4,276

4,614

6,055

4,577

4,234

2,421

2,208

2,048

1,635

1,246

746

380

373

763

888

1,122

1,006

0

0

0

0

0

0

96,719

common range of stage of the reservoir pool. As a 

consequence, the surface-water dead storage capacity of the reservoir was reduced 96 percent from 

25,143 acre-ft (31 hm3) to 900 acre-ft (1.1 hm3) from 

November 1928 to August 1966 (see table 2). The 

distribution of sediment is partly regulated by the 

physical features of the reservoir, such as longitudinal slope, cross-sectional dimensions, shape, ve^etation on exposed ground surface, and inflow channel geometry. The location of deposition is also regulated by concentration and particle size of sediment 

inflow, by rate of streamflow, and by the stage and 

volume of water in storage.

THE EFFECT OF PHREATOPHYTES ON INFLOW CHANNELS

Phreatophytes can significantly reduce the conveyance of reservoir inflow channels, particularly if 

the stage of the water in the reservoir fluctuates 

widely. The flat fertile plain at the upstream end of 

the reservoir pool has a shallow water table and is 

periodically inundated, creating an ideal environment for phreatophytes such as saltcedar. Inundation may kill the plants, but the prolific seed production and rapid growth of saltcedar quickly recreates a dense thicket. When these sediment flats 

are exposed for extended periods of time, the saltcedar narrows the inflow channel by encroachment 

and can eliminate a continuous channel. During the 

period 1962-65, the inflow channel of the Gila River 

into the San Carlos Reservoir was blocked by a 

combination of conditions including encroachment 

by phreatophytes on the sediment, a reduction in 

channel gradient, and plugging by the deposition of 

floating debris.

Figures 6 and 7 show the sediment flats formed by 

deposition in the area above and below the confluence of the Gila and San Carlos Rivers. Tie 

upstream end of the reservoir pool was located in the 

area shown in these photographs during much of the 

period 1945-65.

During the period 1935-62 the location and alinement of the Gila River channel did not change sigrif-
N8 GILA RIVER PHREATOPHYTE PROJECT

VOLUME OF SEDIMENT DEPOSITED WITHIN 1.52-METERELEVATION INCREMENTS, IN CUBIC HECTOMETERS

2520 

2510 

2500 

2490 

HJ 2480

LU

_J

< 2470 

LU W 

Z 

< 2460

LU

LU 

> 2450 

0

CO

LU 244°

LU 

LL

z

0 

^ 2420

LU

LU 2410 

2400 

2390 

2380

2370 

2365^ 

2308*1

I III I'

 Elevation of maximum storage: Spillway

gates fully raised 2,523 ft _

EXPLANATION _

R&fMtVrl _

" :: ":i- ::- .:>. '.:..;::. :.. ,:^ -x j

1958-47 _

J _ 1947-66 

ill

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m

3 * *: *>£* mi\

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|

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5'j| if :; ; ;: , .-;; ;:; Jj'g .J. Q j; :? .J 5:S: j ::J: -f m'm&S &£&&£&$ S *'?: M M -¥S ;* S I^'SS^^M

 770 

 765

 760 

 7^5 

 750

 740 

 735 

 730

 725 

 721

 703

 750 <

1000 2000 3000 4000 5000 6000 7000 

VOLUME OF SEDIMENT DEPOSITED WITHIN 5-FT-ELEVATION INCREMENTS, IN ACRE-FEET

8000

FIGURE 4. Vertical distribution of the volume of sediment deposited within 5-ft (1.5 m)-elevation intervals at San Carlos

Reservoir for the periods 1928-47 and 1947-66.

icantly as indicated by comparing figure 8 with 

figure 9. The width of the channel was appreciably 

reduced and natural levees had formed during this 

period, however. Figure 10 shows the levees along 

the banks of the inflow channel of the Gila River 

which had developed by August 1965. The riverbanks with abundant water supply and extensive

exposure to sunlight are an excellent environment 

for saltcedar. This vigorous growth encroaches on 

the channel and thus reduces the conveyance capability of the channel during flood flows. Wh«m flows 

exceed the conveyance capacity of the channel, the 

excess water overflows the banks and inundates the 

adjacent flood plain. Sediment is then deposited on
LEVEL SEA MEAN ABOVE FEET IN ELEVATION, 

oo H 

n

Q>

-r 01 

T

3 o 

«' § 

(D 0} 

Ul Ul to -> 

CO -> 

I I 

H ,!' ! 

O -» 

> oo 

H O 

X 

O

«!

m

H

10 -> -> 

O Ul O (Jl O Ul O o 01 o 

LEVEL SEA MEAN ABOVE METERS IN ELEVATION, 

OIOO1OHQAH AHOXSIH dO NVS SO1HVO 'HIOAH3S3H 'VNOZIHV IL-6Z61 6N
N10 GILA RIVER PHREATOPHYTE PROJECT

FIGURE 6. Aerial view looking downstream (west) on April 20, 

1965, showing the sediment flats at the confluence of the Gila 

and San Carlos Rivers within the San Carlos Reservoir. The 

reservoir pool, with water-surface elevation at 2,419.00 ft (737 m) 

above sea level and usable contents of about 60,000 acre-ft 

(74 hm3), is shown in the upper right center of the photograph. 

The Gila River channel is at the lower right and the San Carlos 

River channel is at the right center. The dark areas are wellestablished saltcedar.

FIGURE 7. Aerial view looking upstream (north) on April 20, 

1965, showing the San Carlos River on the left and the Gila 

River on the right. The lower center part of this photograph 

overlaps the right center part of figure 6. The area shown in 

figures 6 and 7 includes the area periodically inundated by 

reservoir water during the period 1945-65.

the flood plain because of decreased velocity of the 

overbank flow. Intermittent channels and pools are 

formed on the flood plain and provide a surface 

irrigation for the saltcedar.

FIGURE 8. Aerial view of channel conditions of the Gila River 

upstream from the San Carlos River in 1935 7 years after the 

completion of Coolidge Dam. The upstream end of the reservoir 

pool is shown in the lower left. The Gila River flood plain shown 

in the photograph had been inundated by the reservoir pool 

during the period 1929-35. Soil Conservation Service photograph.

FIGURE 9. Aerial view of conditions of a part of the Gila River 

channel above the San Carlos River in June 1962. Flow in the 

river is from upper right to lower left. The channel is continuous, 

but a comparison of this photograph with figure 8 indicates that 

the width has been reduced and the riverbed has been considerably elevated. Extensive ephemeral off-channel ponds are 

shown in the vicinity of the abandoned railroad crossing.

Figure 11 shows a saltcedar thicket after the 

reservoir water has receded following an inundation
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 Nil

FIGURE 10. View looking downstream (south) on August 18, 

1965, at the Gila River flood plain from a point 2 mi (3.2 km) 

upstream from the mouth of the San Carlos River. The former 

Gila River inflow channel extends from the lower left corner 

along the right side of the photograph. The natural levees and 

vigorous bankside saltcedar are apparent. Deposition elevated 

the channel above the surrounding flood plain. In the lower 

foreground numerous channels cross the former main channel 

at right angles. The water surfaces shown near the center of the 

photograph are ephemeral off-channel ponds. The roadbed fill 

approach to the abandoned railroad bridge shows as a dark line 

in the center of the photograph.

FIGURE 11. Flood-plain conditions after the area was inundated 

by the reservoir pool. The water had been higher than the treetops, thus killing the saltcedar. The white bands on the dead 

tree stems are mud deposited during the reservoir recession.

of sufficient depth to kill the plants. The soggy 

ground conditions, obvious in the photograph, are

ideal for subsequent germination and establishment 

of saltcedar growth.

The plugging of the inflow channel of the Gila 

River during 1962-65 occurred within the 4.1-mi (6.6 

km) reach shown in figure 12.

The deposition of sediment and reduction in slope 

of the flood plain since construction of Coolidge Dam 

in 1928 are shown in figure 13. The constriction 

caused by the railroad fill at the abandoned railroad 

bridge site was partly responsible for the exceptional 

depth of sediment deposits and the resulting decrease in slope in this reach of flood plain. Deposition 

of sediments at the mouth of the San Carlos River 

may also have been responsible. The combination of 

a reduction in channel width due to encroachment by 

vegetation and a reduction in channel velocities due 

to reduced slopes ultimately caused the deposition of 

debris shown in figure 14. Logs and sediment formed 

a dam practically eliminating the conveyance capabilities of the channel. Saltcedar then became established in the debris and sediment to create an 

erosion-proof channel plug as shown in figure 15.

The formation of a channel plug causes channel 

filling which quickly progresses upstream as indicated in figure 12. The upstream end of the debris 

plug deposited by a flood discharge during the period 

July 15-22,1964, is shown in figure 16. Large concentrations of debris are a characteristic of summer 

flood flows on the Gila River. As described by Burkham (1970), these flows originate from intense individual thunderstorms which produce high rates of 

runoff from small tributary watersheds but only 

rarely do they produce large rates of flow on the main 

channel. The high rates on the tributaries strip 

debris from the watersheds, convey the debris to the 

Gila River, and thence downstream on what is 

generally a moderate flow in the Gila channel. The 

progress of the plugging phenomenon as shown in 

figure 12 was 1.5 mi (2.4 km) in 1963,1.8 mi (2.9 km) in 

1964, and 0.8 mi (1.3 km) in 1965 and is primarily the 

result of summer flows.

In the summer of 1965, a channel was excavated by 

dragline parallel to the plugged natural channel. The 

high discharge in the Gila River during December 

1965 to May 1966 raised the water level in San Carlos 

Reservoir to an elevation which inundated most of 

the Gila River flood plain shown in figure 12. An 

excavated inflow channel has been maintained for 

the Gila River since 1966.

SEDIMENT TRAP EFFICIENCY

Sediment trap efficiency is the ability of a reservoir 

to retain sediment and is expressed in percent of total 

incoming sediment (Gottschalk, in Chow, 1964). A
N12 GILA RIVER PHREATOPHYTE PROJECT

110 25'

33 15'

33 10'

C A E L O S

Plugging started 1962 ~

Base from U.S. Geological Survey, San Carlos 

Reservoir 1:62,500,1962

2 MILES

0 1 3 KILOMETERS

CONTOUR INTERVAL 80 FEET

DOTTED LINES REPRESENT 20-FOOT CONTOURS 

DATUM IS MEAN SEA LEVEL

EXPLANATION

60

 

Distances along longitudinal axis of 

reservoir from Coolidge Darn, in 

thousands of feet

Oct. 1964

\

Location of channel plug on date shown 

FIGURE 12. Extent of channel plugging in Gila River on indicated dates.

sediment trap efficiency of 83 percent was predicted 

for San Carlos Reservoir by the U.S. Army Corps of 

Engineers (1914, p. 30). The remainder of the sediment was expected to pass through in suspension or 

by sluicing.

There are no records of sediment inflow or outflow
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N13

DISTANCE FROM COOLIDGE DAM, 

IN KILOMETERS

13

2460

14 15 16 17 18 19 20 21

<2450

111 W

Z

W 2440

LLJ 

>

O

CD 2430

<

LU

2420

2410

-Abandoned railroad crossing

2400

43

748

- 746 

744

- 742

- 740

- 738

- 736 

734 

732

50 60 70

DISTANCE FROM COOLIDGE DAM, 

IN THOUSANDS OF FEET

FIGURE 13. Profiles along the longitudinal axis of San Carlos 

Reservoir showing changes in bed elevations and slope within the reach of channel plugging.

FIGURE 14. View of the Gila River looking north from a point 

2.6 mi (4.2 km) upstream from the mouth of the San Carlos River 

photographed in September 1964. The debris in the channel is 

part of a recently deposited channel plug. Flow is from right to 

left.

for San Carlos Reservoir, so trap efficiency is not 

known; it probably exceeds the predicted 83 percent. 

Trap efficiency should be 96 percent or more at San

FIGURE 15. Downstream view on July 15, 1965, of a reach of the 

Gila River channel which had been plugged in 1964. Saltcedar 

is becoming established in the former channel bottom.

FIGURE 16. Aerial view of channel plugging in the Gila 

River on July 22, 1964. Flow is from right to left.

Carlos Reservoir, according to Gottschalk (in Chow, 

1964, fig. 17-1-6).

ANALYSIS OF SEDIMENT DATA

The U.S. Army Corps of Engineers reported (1914, 

p. 29-30) predictions of the volume of sediment 

deposition based in part on streamflow records and 

on 15 sediment samples collected from deposits 

along the Gila River. Streamflow records of 1890 and
N14 GILA RIVER PHREATOPHYTE PROJECT

1895-1912 indicate that the mean annual streamflow 

was 346,000 acre-ft (427 hm3). In the 1914 report, the 

predicted specific weight of deposited sediments was 

70 lb/ft3 (1,120 kg/m3), predicted volume of deposition at 100 percent trap efficiency was 1.3 percent of 

total streamflow, and the predicted trap efficiency 

was 83 percent. Based on these predictions and 

measured streamflow, the estimated mean annual 

volume of reservoir sediment deposits was 3,740 acreft (4.6 hm3).

Records of streamflow from November 1928 

through August 1966 show the mean annual streamflow was 216,120 acre-ft (266 hm3), or 63 percent of 

the predicted flow. The mean annual volume of 

sediment deposition was 2,553 acre-ft (3.15 hm3 ), or 

68 percent of the predicted volume.

A mean sediment concentration for stream discharge can be estimated from measured volumetric 

changes in reservoir deposits because a reservoir is a 

collector for fluvial sediment moved by all transport 

methods. The mean sediment concentration computed from the information in the 1914 report is 

14,478 ppm (14,615 mg/1). The mean concentration 

from November 1928 through August 1966 is 13,684 

ppm (13,808 mg/1) when computations are made 

using measured streamflow and sediment deposition 

and when estimates of trap efficiency, specific 

weight of deposits, and specific gravity are 96 percent, 70 lb/ft3 (1,120 kg/m3), and 2.65, respectively.

On a volumetric basis, sediment accumulated at 

an average rate of 0.0118 (volume of sediment deposited/volume of streamflow) from November 1928 to 

August 1966. The rates for the periods between 

surveys, chronologically, are 0.0198, 0.0059, 0.0084, 

and 0.0110, as listed in table 2.

INTERPOLATION OF ELEVATION-CAPACITY 

RATINGS BETWEEN CAPACITY SURVEYS

Elevation-capacity relations were defined for each 

reservoir survey. Significant changes in these relations between surveys required the development of a 

systematic method of interpolating changes during 

the periods.

The simplest method of interpolation is to pro-rate 

storage capacity change by time between consecutive surveys. This method of interpolation can be 

applied either to change in total storage capacity or 

to change by increments of the total reservoir 

storage.

Interpolation can also be made by pro-rating the 

change in storage capacity according to streamflow. 

Because the loss in storage capacity is the volume of 

sediment deposition, this method is basically the use 

of a mean sediment concentration computed for a

period. The storage loss for any time interval is 

estimated as the product of this mean concentration 

and the interval streamflow. (See table 2 for period 

rates for San Carlos Reservoir.) This storage loss can 

be obtained graphically for San Carlos Reservoir by 

using accumulated streamflow and figure 17. The 

inset in figure 17 shows measured change in storage 

compared to measured streamflow for each of the 

periods between surveys.

The method of interpolating storage change 

adopted for use in this report employs the equivalency of the loss in surface-water storage capacity 

and the change in sediment deposits. A procedure for 

simulating deposition was developed and used to 

estimate the sediment volume deposited each water 

year. The development of this procedure and its 

application in providing yearly capacity ratings is 

described in the section "Development of Surf aceWater Storage-Capacity Ratings."

HYDROLOGY 

WATER-BUDGET EQUATION

A water budget is used in this report for presentation of an historical accounting of the hydrology of 

San Carlos Reservoir for water years 1931-71. The 

water-budget equation for San Carlos Reservoir is

JG + 'S JT~ G

~~ C-/Z7* "" C/ZT'T1 

FJ FJ /

± ASp ± AS» = 0. (1)

The components of the water budget are identified 

as follows:

IS 

Ip

OG =

=Gila River inflow at Calva,

-San Carlos River inflow at Peridot, 

^precipitation input over the lake surface,

-ground-water inflow,

tributary inflow downstream from the

Gila and San Carlos River

gaging stations,

Gila River outflow below Coolidge Dam, 

^evaporation from the lake surface, 

=evapotranspiration from the exposed

land surface of the reservoir, 

=change in surface-water storage, and 

^change in bank storage.

Neither the evapotranspiration nor the change in 

bank storage was measured. Before evapotranspiration could be computed by the water-budget equation, estimates of bank storage were necessary. A discussion of the evaluation of each water-budget component is presented in the following sections.
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N15

120

ACCUMULATED STREAMFLOW, Q, IN CUBIC KILOMETERS

345678

I______I______I

- 140

I I I I I

Accumulated streamflow, Q, in cubic kilometers 

01 2345

I I I

01234 

Accumulated streamflow, Q, in millions of acre-feet

1947

EXPLANATION

1935

o

Year of capacity survey

ACCUMULATED STREAMFLOW, Q, IN MILLIONS OF ACRE-FEET

FIGURE 17. Relation between accumulated decrease in storage capacity and accumulated streamflow by capacity survey 

periods. Inset shows relation of storage capacity change and streamflow for each period.

SURFACE FLOW

GILA RIVER AND SAN CARLOS RIVER INFLOW

Records of streamflow into the reservoir for the 

1929-71 water years were taken from U.S. Geological 

Survey Water-Supply Papers 1313 and 1733 and from 

U.S. Geological Survey annual state reports of 

Arizona streamflow. Inflow records used in this 

study are primarily those for the Gila River at Calva 

and the San Carlos River at Peridot. Some of the 1929 

inflow data were estimated from reservoir outflow 

data and changes in reservoir storage. Additional 

sources of Gila River streamflow information include reports by Burkham (1970) and the U.S. Army 

Corps of Engineers (1914).

Figure 18 shows the total annual streamflow into 

the reservoir for water years 1929-71. The streamflow data are included in table 17. The mean annual 

inflow for this period was 214,940 acre-ft (265 hm3), 

and the annual median was 159,000 acre-ft (196 

hm3 ). The large difference between the mean and 

median values is caused by infrequent years of 

extremely high flow. Of the annual totals 67 percent

WATER YEAR

FIGURE 18. Annual combined Gila River and San Carlos River 

inflows to San Carlos Reservoir for water years 1929-71.

is less than the mean because of the influence of 

these infrequent extreme annual totals.

Mean annual streamflow, 1929-71, into the reservoir was 33,450 acre-ft (41 hm3 ) for the San Carlos 

River and 181,490 acre-ft (224 hm3 ) for the Gila River.
N16 GILA RIVER PHREATOPHYTE PROJECT

The San Carlos River, with 8.6 percent of the contributing area, produced 15.6 percent of the total 

streamflow into the reservoir. Annual streamflow of 

the San Carlos River ranged from 6.2 percent of the 

total streamflow into the reservoir in 1959 to 40 

percent of the total streamflow in 1956.

A generally declining trend in annual streamflow 

coincided with a similar declining trend in annual 

precipitation from about 1920 to 1962 (Burkham, 

1970, fig. 7). The trend is not as evident during the 

period of reservoir operation included in this report 

(1929-71). The many years of low runoff during the 

1940's and 1950's are distinct in the 10-year moving 

average flow graphed in figure 18, but the average 

annual runoff was higher near the beginning and 

end of the study period. Annual streamflow exceeded 

the mean only twice (1949 and 1952) during the 15- 

year period 1943-57. By contrast, annual streamflow 

exceeded the mean 14 times during the 43 year period 

1929-71 and 5 times during the 11 year period 1958- 

68. A histogram of annual inflow volumes is shown 

in figure 19 for water years 1929-71.

Of special interest to users of San Carlos Reservoir 

water is the probable water supply over time durations of a year or more. Table 4 shows the 1930-71 

historical extremes in both maximum and minimum 

supply for several time durations.

Table 5 gives the computed mean monthly inflows 

for the Gila and San Carlos Rivers and their combined total inflows, based on 1930-71 data. Figure 20 

compares the mean monthly streamflows of the two 

rivers in percent of their mean annual totals. In 

general, the mean monthly discharges for both 

rivers follow similar seasonal patterns. The greatest 

monthly volumes typically occur during several 

months of the winter season as the result of frontal 

storms passing over most or all of the basin. The 

period of increased runoff generally begins in 

December and extends through March. The mean 

winter and spring (November to June) inflow is 

155,480 acre-ft (192 hm3 ), but flow is highly variable 

from year to year. The variability is demonstrated in 

that the average deviation from the November to 

June mean is 137,000 acre-ft (169 hm3), an 88 percent 

deviation.

Increased streamflow from summer storms normally begins in July, peaking in August, and may 

continue high into October. Individual summer 

storms are smaller in areal extent than winter 

storms and are more highly variable in precipitation 

intensity, but the total summer streamflow is slightly more consistent than the total winter and spring 

streamflow. The mean streamflow from July

INFLOW, IN CUBIC HECTOMETERS 

200 400 600 800 1000 1200

i u -

CO Q 

DC 9

< 8 - 

>. 1 -

u. 6-

0 5 -

DC 4 _

LU

CQ 3 -

1 2 -

z 1 -

n

c

ro

T3

0)

I

 I

ro 

<u

I . , . , . n

INFLOW, IN THOUSANDS OF ACRE-FEET

FIGURE 19. Frequency of occurrence of inflow volumes, 

water years 1929-71.

TABLE 4. Maximum and minimum volumes of water supplied by 

streamflow into San Carlos Reservoir for different time 

durations, in acre-ft

Duration of 

period ........ 1 year 3 years 5 years 7 years 10 years

Maximum inflow 

for specific period 

and dates of 

period ... ... ....

Minimum inflow 

for specific period 

and dates of

1,005,180 

(1941)

1,518,810 

(1940-42)

1,775,650 

(1937-41)

2,309,870 

(1936-42)

3,041,280 

(1932-41)

period

TABLE

Oct .......

Dec .....

Feb ......

Mar .......

July .....

... .... 34,740 198,880 444,140 755,440 1,301,020 

(1956) (1969-71) (1944-48) (1950-56) (1944-53)

5. Mean monthly inflows for the Gila River, tf ? San 

Carlos River, and for both rivers 1930-71

Mean monthly Mean monthly 

Gila River San Carlos River 

Month inflow inflow 

(acre-ft) (acre-ft)

. ......... ...... 8,587 821

.. .. .. ..... .. 5,845 809

..... .. .................. . 14,948 5,782

.. .. .. ........ . ..... .... 26,594 5,611

.. .. .... .......... ... .. . 28,826 6,843

. .. ...... .... ..... 31,669 6,307

14,966 1,035

.. .. ...................... . 6,702 265

.. .. .......... ...... ...... 972 95

................... ... 6,041 1,126

.. . . ............. ........ 23,272 3,461

.. ................. ...... 12,933 1,255

Combined total 

mean monthly 

inflow 

(acre-ft)

9.4C8 

6,6£4 

20,7fO 

32.2C5 

35,6f9 

37,976 

16.0C1 

6,9f7 

1,067 

7.1P7 

26,7f3 

14.1P8

through October is 59,480 acre-ft (73 hm3 ), and the 

mean deviation of 37,450 acre-ft (46 hm3 ) is 63 percent of mean summer streamflow.

TRIBUTARY INFLOW

Tributary discharge from 390 mi2 (1,010 km2 ) flows 

directly into the reservoir between the stream gaging 

stations (Gila River at Calva and San Carlos River at 

Peridot) and the dam. It is convenient for waterbudget computations that flow seldom occurs in the 

tributaries and that mean annual tributary flow is 

probably less than 2 percent of the mean annual 

input from all sources. Significant tributary runoff
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N17

MONTH

FIGURE 20. Mean monthly flow as a percent of mean annual 

flow for the Gila and San Carlos Rivers.

usually occurs only during the summer storm season, July through October.

Tributary inflows from 72.6 mi2 (188 km2) of the 

gaged tributary area entering the reservoir below the 

Calva gaging station were determined for the summer storm seasons of 1964-71 (Burkham, 1976). 

Totals of seasonal runoff from this area, and runoff 

per unit area, are listed in table 6.

Several methods were used to estimate seasonal 

runoff from all areas tributary to the reservoir. In the 

first method, runoff was assumed to be spatially constant. The seasonal runoff values per square mile for 

1964-71, from table 6, were multiplied by the 390-mi2 

(1,010 km2 ) tributary area and the results listed in 

column 2 of table 7.

The tributary runoff estimates of column 3 in table 

7 are results of a rainfall-runoff correlation. Rainfall 

was measured for the Gila River Phreatophyte Project in gages located near the downstream ends of the 

tributary streams. The tributary runoff data is from

TABLE 6. Tributary inflow into the San Carlos Reservoir along 

a reach of the Gila River

TABLE 7. Estimated tributary inflow into San Carlos Reservoir

Estimated seasonal runoff, in acre-ft

Water year

(1)

1931 ... .. . .....

1932 ..... .......

1933 ..............

1934 ..............

1935 ....... .....

1936 .. ..........

1937 .. .. .. ......

1938 ... ...........

1939 .............

1940 ... .. .. .. ...

1941 ................

1942 ............

1943 ................

1944 . .. .........

1945 ...............

1946 . ..........

1947 ... .. .. .......

1948 ..............

1949 ..............

1950 . ...........

1951 ..... .... .. .

1952 ...............

1953 .............

1954 .. .. .. .......

1955 ................

1956 . . .. ...... ....

1957 .. .. . ..........

1958 .. .. ....... .. .

1959 .. .. ............

1960 .................

1961 .. .. . ... ....

1962 .. ...............

1963 .. .. ............

1964 ... .. ....... ....

1965 .. .... .... .....

1966 .. ............. .

1967 .................

1968 .. .. ......... .

1969 ............ ...

1970 ................

1971 .. .... ..........

Mean .....

Based on runoff Prom rainfallmeasured for runoff relation,

part of the area equation 2

(2) (3)

............ . .................. 3,660

..... ............ .......... 0

................... .. .............. 1,420

................................. 1,800

....... ... . .. .. ........... .. 4,520

.............................. 0

...... ........ .... ........ 0

............. .... ........... 0

... ........................... 0

........ ...................... 0

....... .. .. .. .. ....... ... 10,470

1,320

..... . .......... .. ............. 890

............................. 8,640

. ..... . . .. ............ 2,520

. ....... ......... .. .............. 6,190

............. . .. .. .............. 3,600

..... .. .... ..... .. . ............ 1,650

................................. 1,860

.... . ... .. .. . .............. 1,550

.... .... . . .. .............. 2,340

.. ..... . . ..... ..... 1,720

................ ............. 0

................................. 11,460

............................... 10,420

........... ... .. .. .............. 0

.. . ......... .. .. . ............ 0

.......... .... .. ................. 9,330

.. ............ .. .. .............. 8,330

............... .. . .............. 4,370

.. ......... .. . ............... 6,670

......... . .. . ............. 2,460

3,660

3,320 11,730

3,860 370

1,600 13,660

8,380 8,100

1,210 0

1,250 1,460

470 3,620

11,930 4,310

1 3,998 2 3,758

Based on 9

acre-ft/mi2

(4)

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

3,510

2 3,510

Average of

columns 3

and 4

(5)

3,580

1,760

2,460

2,660

4,020

1,760

1,760

1,760

1,760

1,760

6,990

2,410

2,200

6,070

3,010

4,850

3,550

2,530

2,680

2,530

2,930

2,620

1,760

7,430

6,960

1,760

1,760

6,420

5,920

3,940

5,080

2,980

3,580

7,620

1,940

8,580

5,800

1,760

2,480

3,560

s.gi')

2 3,6? 1

Water year

Seasonal tributary runoff 

(acre-ft)

Mean seasonal runoff 

(acre-ft/mi2 )

1964

1965

1966

1967

1968

1969

1970

1971

616

1 732

301

1,560

228

229

86

2,220

8.5

9.9

4.1

21.5

3.1

3.2

1.2

30.6

'Eight (8) year mean. 

2Forty-one (41) year mean.

table 6. The regression equation relating rainfall and 

runoff data was

Q = -0.382 + 0.093P, where P > 4, (2)

Includes an additional gaged area of 1.0 mi2 (2.59 km2 ).

with rainfall, P, and runoff, Q, given in inches. The 

regression line and the data used to develop equrtion 

2 are plotted in figure 21. The equation was used to 

estimate runoff from the 390-mi 2 (1,010 km2 ) tributary area using seasonal precipitation at San Carlos 

Reservoir. The correlation between seasonal rainfall 

measured on the Gila River Phreatophyte Project 

area and seasonal rainfall at the San Carlos Reservoir weather station is poor. Runoff estimates in 

column 3 of table 7 are probably poor partly because 

of this high spatial variability in rainfall intensities. 

Burkham (1974) indicated that the mean seasonal 

runoff from all the study watersheds for the period 

1963-71 was about 9 acre-ft/mi2 (0.004 hmWm2 ). 

Seasonal tributary runoff into the reservoir using
N18 GILA RIVER PHREATOPHYTE PROJECT

SUMMER RAINFALL,P, IN MILLIMETERS 

0 50 100 150 200 250 300

UJ

I

o

z

z

-

LL

LL 

O

Z

D

o:

o:

HI

->

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

i i i i i i ^ ......... .

/

/

Q=-0.382 + 0.093P S

(Q and P in inches) /

-

- /

.I i V i i i i i

- 150 Ol

-

u-

- 100 3

cc

« -50

5

D 

W

L- n

12

SUMMER RAINFALL,P, IN INCHES

FIGURE 21. Relations between measured rainfall and tributary 

runoff for summer seasons 1964-71 from 72.6 mi2 (188 km2) of 

area tributary to San Carlos Reservoir.

this rate is 3,510 acre-ft (4.33 hm3), as shown in 

column 4 of table 7.

These methods do not accurately estimate seasonal tributary runoff to San Carlos Reservoir for 

each year. However, the individual means at the 

bottom of column 2 through column 4 of table 7 

deviate less than 10 percent from the average of the 

means. The close agreement of mean values suggests 

that the 41-year water budget is improved by the 

addition of tributary runoff estimates.

The average of the estimates from columns 3 and 4 

of table 7 was selected somewhat arbitrarily to define 

the seasonal totals to include in the water budget and 

column 5 of table 7. The estimates in column 3 

assume that a seasonal variability of precipitation is 

uniform in space. The estimates in column 4 assume 

runoff is uniform in space and time.

GILA RIVER OUTFLOW

About 78 percent of all inflow into San Carlos 

Reservoir is released downstream into the Gila 

River. Releases are based on reservoir supply and 

the needs of the San Carlos Project near Coolidge, 

Ariz. Records of releases are based on river-stage 

data obtained at a flume 0.4 mi (0.6 km) downstream 

from Coolidge Dam, at the U.S. Geological Survey 

gaging station, Gila River below Coolidge Dam, 

Ariz. Streamflow records have been available at this 

station from 1939 to 1971 and near this location for 

much of the period 1899-38. 1

Discharge data for water years 1931-71 are in-

'Quoting from the 1971 annual report of U.S. Geological Survey surface-water records, 

Gila River below Coolidge Dam, Arizona, "Records available. July to October 1899, 

April 1900 to March 1902, July to September 1902, December 1902 to December 1904, 

January to May 1905 (gage heights only), June to November 1905, August 1910 to February 1911 (gage heights only), April 1914 to current year. Published as 'at San Carlos' 

1899-1911, as 'near San Carlos' 1914-1926, and as 'at Coolidge Dam' 1927-38."

eluded in the water budget. Mean monthly discharges of the Gila River below Coolidge Dam are 

shown in table 8 and are an indication of seasonal 

demands of water for irrigation.

The peak instantaneous discharge since the dam 

was constructed was 1,350 ftVs (38 mVs) on July 28, 

1952. No flow occurred several times prior to 1938, 

when the gaging station was about 0.2 mi (0.3 km) 

upstream from its present location. The minimum 

flow after 1938 was about 0.4ftVs (0.011 mVs) which 

includes the discharge of Warm Springs, a tributary 

to the Gila River.

GROUND-WATER INFLOW

From 1963 to 1972, ground-water data were collected and analyzed as part of the Gila River Phroatophyte Project. The part of the Gila River F^reatophyte Project area within the boundaries of the 

reservoir (fig. 1) included an 8 mi (13 km) reach of the 

Gila River flood plain below the Calva gaging 

station. Computations of ground-water inflow into 

the reservoir were based on results of the project's 

ground-water analyses (Hanson and others, 1972; 

Hanson, 1972).

The geologic water-bearing units along the Gila 

River have been identified as alluvium and basir fill. 

The alluvium was deposited in a trench incised in the 

basin fill. Two components compose the alluvium: 

flood-plain alluvium and terrace alluvium. Terrace 

alluvium covers the basin fill in the trough and 

extends above the flood plain on the adjoining 

slopes. Flood-plain alluvium overlies the terrace 

alluvium in the flood-plain region and averages 

about 50 ft (15 m) in depth and 5,000 ft (1,500 n) in 

width. The basin fill extends a considerable distance 

up the slopes beyond the terrace alluvium.

Water enters the basin fill on the mountain slopes 

and moves generally toward the flood plain. Where 

the basin fill is in contact with the alluvium, rufficient head exists to create a slow upward movement 

of water from basin fill to the overlying alluvium. 

Another ground-water source into the reservoir is

TABLE 8. Mean monthly discharge of the Gila River below 

Coolidge Dam, 1931-71

Month

Oct

Feb ... . ... ......... .. . -..........--.-- .. ..................

July .... .... ..... .. .. ............................ .....................

Discharge, in acre-ft

............... .................. 8,580

............ .. .................. 5,980

............ .. .................. 6,640

............ ..................... 3,160

......... .. .. .................. 6,920

...... .. .. .................... 16,330

......... .. ..................... 21,140

......... .. ..................... 20,800

......... .. ..................... 24,050

......... .. ..................... 25,880

......... .. .................... 21,840

............................... 18,110
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N19

downvalley flow through the saturated flood-plain 

alluvium. A small amount of ground water enters the 

reservoir from the alluvium deposited by tributary 

streams but is considered insignificant.

Downvalley ground-water movement in the Gila 

River flood plain alluvium has been computed at 5.1 

acre-ft (0.0063 hm3 ) per day (Hanson, 1972, p. 25). 

Hanson, Kipple, and Culler (1972, p. 317) estimated 

basin-fill inflow along the Gila River at 0.82 acre-ft 

(0.0010 hm3 ) per day per 1,000 acres (4.05 km2 ). This 

is equivalent to about 0.50 acre-ft (0.00062 hm3 ) per 

day per downvalley mile, or 10.5 acre-ft (0.0129 hm3 ) 

per day over the 21-mi (33.8 km) reach from the Calva 

streamflow station to the dam. The sum of the 

alluvial inflow and basin-fill inflow is 15.6 acre-ft 

(0.0192 hm3 ) per day in the Gila River part of the 

reservoir.

Flood-plain alluvial flow (q) for the San Carlos 

River at Peridot was computed from the equation

q = K»m (3)

where K is the hydraulic conductivity estimated for 

Gila River flood-plain alluvium at 5,200 gal/day/ft2 

(212 mVday/m2 ) and is assumed the same for the 

flood-plain alluvium along the San Carlos River, m is 

the depth of alluvium and is estimated to be 30 ft (9 

m), wis the alluvial width of about 2,800 ft (850 m), u 

is the slope of the downvalley ground-water surface 

and is estimated to be equal to the downvalley floodplain slope of 0.00275. Equation 3 gives q equal to 3.7 

acre-ft (0.0046 hm3 ) per day. The reach of the San 

Carlos River from the Peridot gaging station to the 

river mouth is 9.2 mi (14.8 km). Assuming that the 

San Carlos River flood plain has the same basin-fill 

inflow per unit area as the Gila River flood plain, the 

inflow per downvalley mile per day is 0.50 acre-ft 

(0.00062 hm3 ) times the ratio of alluvium widths 

(2,800 ft/5,000 ft) for the two flood plains, or 0.28 acreft (0.00034 hm3 ) per mile per day. Basin-fill inflow 

along the 9.2 mi (14.8 km) San Carlos River reach is 

therefore about 2.6 acre-ft (0.0032 hm3 ) per day. The 

sum of the basin-fill and alluvial ground-water contribution from the San Carlos River basin is 6.3 acreft (0.0078 hm3 ) per day.

PRECIPITATION

Records of precipitation at San Carlos Reservoir 

were obtained from National Weather Service publications and log books at Coolidge Dam. Table 9 

includes annual precipitation totals for water years 

1931-71. The mean annual precipitation was 13.87 

in. (352 mm) and ranged from 6.46 in. (164 mm) in 

1934 to 30.53 in. (775 mm) in 1941.

Mean monthly precipitation at San Carlos ReserTABLE 9. Annual (water year) precipitation, in inches, at S«.n 

Carlos Reservoir, 1931-71

Water 

year

1931 ........

1932 ........

1933 .........

1934 ........

1935 .........

1936 ..........

1937 ..........

1938 ...........

1939 ...........

1940 ...........

1941 ...........

1942 ...........

1943 ...........

1944 ...........

Precipitation

............ 15.57

........... 14.19

............ 13.49

............. 6.46

........... 19.64

........... 11.61

........... 12.98

......... 11.75

............ 10.65

............ 12.50

............ 30.53

............ 12.94

........... 11.82

........... 14.96

Water 

year

1945 ...........

1946 ...........

1947 ..........

1948 ..........

1949 ...........

1950 ...........

1951 ..........

1952 ..........

1953 ...........

1954 ...........

1955 ...........

1956 ...........

1957 ...........

1958 ...........

Precipitation

........... 13.53

........... 11.56

.............. 9.45

.............. 9.27

........... 13.63

....... .. ....11.14

............ 11.87

............ 18.61

............ 10.81

............ 19.16

............ 14.08

.............. 9.05

............. 11.17

............. 19.87

Water 

year

1959 ...........

1960 ...........

1961 ...........

1962 ...........

1963 ...........

1964 ...........

1965 ...........

1966 ...........

1967 .........

1968 ...........

1969 ..........

1970 ..........

1971 ..........

Precipitation

............. 11.74

............ 15.36

............. 11.02

............ 13.34

.- ....... 14.87

............ 12.94

............. 14.00

............. 27.17

............. 13.34

............. 16.53

............ 13.41

............ 13.28

............... 8.85

voir for the period ranged from 0.21 in. (5.3 mm) in 

May to 2.19 in. (56 mm) in August. Seasonal trends 

are evident in the mean monthly precipitation data 

of table 10. The monthly extremes, also listed in table 

10, indicate the large variation in precipitation. Tl e 

mean precipitation for the summer season (July 

through October) is 5.89 in. (150 mm), or 42 percent of 

the mean annual total. Precipitation for the remainder of the year averaged 7.98 in. (203 mm) during the 

41 years.

The volume of daily precipitation into the reservoir 

was computed as the product of daily precipitation 

measured at the San Carlos Reservoir weather station and the surface area of the water in storage for 

the day. Water year volumes of precipitation fallirg 

onto the water in storage are listed in table 17. From 

1931 through 1971, the total accumulated volume of 

precipitation falling directly onto the water surface 

of the reservoir was computed as 203,900 acre-ft (251 

hm3 ), about 2.2 percent of reservoir input from z 11 

sources.

EVAPORATION

Evaporation from San Carlos Reservoir is significant because of the warm, dry environment in which 

the reservoir is located. Evaporation, as used in tb«s 

San Carlos Reservoir water budget, is computed as 

direct loss from the surface area of the reservoir pool.

Daily pan evaporation data for water years 1931- 

71 were available for the weather station at San 

Carlos Reservoir. Saturday and holiday pan evaporation readings were omitted from 1930 through Apr I 

1948. The history of changes to the evaporation pan 

at the dam is available in the National Weather 

Service publication, "Substation History for 

Arizona."

Reservoir pool evaporation is computed as tl ^ 

product of measured pan evaporation and the evaporation pan coefficient. A daily volume of computed 

pool evaporation is the product of daily pan evaporation, the pan coefficient, and daily surface area of th<*
N20 GILA RIVER PHREATOPHYTE PROJECT 

TABLE 10. Mean monthly precipitation and monthly extremes (1931-71) at San Carlos Reservoir

Mean monthly 

precipitation,

Minimum monthly 

precipitation,

Maximum monthly 

precipitation,

Oct.

0.88

.0

4.28

Nov.

1.07

.0

3.73

Dec.

1.80

.0

8.53

1.44

.0

4.00

Feb.

1.33

.0

3.26

Mar.

1.28

.0

6.02

0.52

.0

2.29

May

0.21

.0

.98

0.33

.0

1.40

July

1.62

.08

4.68

2.19

.25

5.99

Sept.

1.20

.0

3.61

pool. Records of daily pool evaporation volumes have 

been published in annual reports of the Gila River 

Water Commissioner beginning in 1936. A pan coefficient of 0.7 was used by the Gila River Water Commissioner in computations of pool evaporation.

In December 1963, the U.S. Geological Survey 

established a station about 350 ft (107 m) from the 

San Carlos Reservoir weather station to collect 

radiation and air temperature data for use in computing pool evaporation by energy-budget and masstransfer methods. Wind movements and watersurface temperature data were recorded on raftmounted instruments at either one or two locations 

on the lake; the number and location of rafts was 

determined by the surface area of the pool. Streamflow temperature profiles (thermal surveys) of the 

pool were made every 2 or 3 weeks to measure 

changes in stored energy.

The energy-budget equation is based on the principle of conservation of energy. Measurements are 

made of most of the incoming and outgoing energy 

components and of changes in stored energy in the 

water body. The unmeasured energy remaining as a 

residual of the energy-budget equation includes 

energy for the evaporation process, energy of sensible-heat exchange, and energy advected by evaporated water. The energy-budget method has been 

described by Anderson (1954).

In the mass-transfer method, evaporation is 

treated as the turbulent transport of water vapor in 

the boundary layer overlying the water surface. The 

method requires data of wind speed, vapor pressure 

of the air, and vapor pressure of saturated air at 

water-surface temperature. A detailed description of 

the mass-transfer method is available in Marciano 

and Harbeck (1954).

Computations of lake evaporation were made by 

energy-budget and mass-transfer methods for 93 

periods during 1964-71. Occasional incomplete data 

reduced the number of periods of reliable data to 72. 

A pan coefficient was computed for each period. The 

average pan coefficient was 0.80 for the 8 years of 

record.

Annual pan evaporation at San Carlos Reservoir

is shown in the upper portion of figure 22 for water 

years 1931-71. Mean annual (water year) pan evaporation was 97.3 in. (2,470 mm), and water-year 

extremes ranged from the 1941 low of 83.6 in. (2,120 

mm) to the high in 1939 of 111.4 in. (2,830 mm).

At San Carlos Reservoir, annual pan evaporation 

appears to have followed a downward trend, as 

indicated by the plot of 5-year moving averages 

shown in figure 22. During this same 41-year period, 

annual precipitation also indicates a decreasing 

trend (Burkham, 1970). This seems contradictory 

because the decrease in precipitation implied that 

evaporation potential might have increased. T N US, it 

was necessary to investigate further pan evaporation data at San Carlos Reservoir.

Pan evaporation is affected by the conditions of 

both the pan and the immediate environment, conditions which can change independently of char ges in 

climate. In an attempt to evaluate the reliability of 

the pan evaporation data at San Carlos Reservoir, 

the data were compared with pan evaporation data 

at other Arizona stations. Figure 23 compares the 5- 

year moving average of pan evaporation at San 

Carlos Reservoir with the 5-year moving average of

O)

WATER YEAR 

EXPLANATION

Annual pan evaporation

5-year moving average of pan evaporation

Adjusted annual evaporation for reservoir

FIGURE 22. Evaporation at San Carlos Reservoir for water years

1931-71.
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N21

WATER YEAR

FIGURE 23. Five-year moving averages of pan evaporation at 

San Carlos Reservoir and the mean of pan evaporation at Mesa, 

Roosevelt Lake, and at Tucson (Univ. of Ariz.) for water years 

1931-71.

the mean of annual pan evaporation for Mesa, 

Roosevelt Lake, and Tucson (Univ. of Ariz.). The 

trend at San Carlos Reservoir is downward for much 

of the 1931-71 period as opposed to an upward trend 

at the other stations. Changes in the relation between the San Carlos Reservoir data and the me,<m of 

the other three stations were found to have occurred 

in about 1938 and 1950 by use of a double-mass curve 

(fig. 24). Accordingly, three separate time periods 

were used in computing evaporation from the reservoir: (1) 1931-38, (2) 1939-50, and (3) 1951-71. 

For period 3, a pan coefficient of 0.78 was derived

10

320

CUMULATIVE PAN EVAPORATION-SAN CARLOS RESERVOIR, IN METERS

15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90

I .I I. I.I.I .I I I , I.I.I .I

95

40 60 80 100 120 140 160 180 200 220 240 260 280

CUMULATIVE PAN EVAPORATION-SAN CARLOS RESERVOIR, IN FEET

300 320 340

FIGURE 24. Double-mass diagram of cumulative pan evaporation for San Carlos Reservoir and mean of Mesa, Roosevelt Lake,

and at Tucson (Univ. of Ariz.) for water years 1931-71.
N22 GILA RIVER PHREATOPHYTE PROJECT

from computed lake evaporation at San Carlos Reservoir and the average of pan evaporation for Mesa, 

Roosevelt Lake, and Tucson (Univ. of Ariz.). This 

coefficient was assumed applicable in periods 1 and 2 

also, when computing San Carlos Reservoir evaporation on the basis of pan evaporation at the other 

three stations. San Carlos Reservoir pan coefficients 

for periods 1 and 2 were 0.61 and 0.66, respectively, 

obtained from the relation: San Carlos Reservoir pan 

coefficient for a period equals computed lake evaporation divided by measured pan evaporation. Periods 

1 and 2 pan coefficients were applied to annual pan 

evaporation measured at San Carlos Reservoir to 

compute annual lake evaporation. The annual pan 

evaporation, pan coefficients, and corresponding 

computed lake evaporation amounts for the 41-year 

study period are included in table 11. Computed lake 

evaporation is compared with the San Carlos Reservoir pan evaporation in figure 22. The annual lake 

evaporation expressed as volumetric loss is given in 

table 17. Mean annual depth of evaporation was 

69.95 in. (1,780 mm) and represents 10.5 percent of 

the outflow from the reservoir. Mean monthly depths 

of evaporation from the lake are shown in table 12. 

Monthly differences between the computed pan 

coefficients shown in table 12 are caused primarily 

by changes in available energy. There was not 

sufficient confidence in the computed monthly pan 

coefficients for use in computations of lake evaporation, although it is evident that the relation between 

pan evaporation and lake evaporation is subject to 

seasonal change.

WATER STAGE AND SURFACE-WATER STORAGE 

AT SAN CARLOS RESERVOIR

Prior to January 15, 1937, water stage was determined by use of reference points of known elevation 

on a series of stakes. A continuous record of lake 

stage was made from January 1937 through 1971 

using a water-stage recorder. Daily stage records 

were applied to the elevation-capacity relations (fig. 

3) to obtain daily reservoir storage contents. Daily

TABLE 12. Mean monthly evaporation and mean

Month ........................................... Oct. Nov. Dec. Jan. Feb.

Mean monthly lake 

evaporation in inches, 

1931-71 ................................. 5.01 2.66 1.57 1.73 2.52

Mean monthly pan 

coefficients from energybudget computations, 

1964-71 ........... ...................... .88 1.00 1.05 .74 .66

Mean monthly pan 

coefficients from masstransfer computations, 

1964-71 .......... ................... . .90 1.10 .93 .71 .68

records of stage and usable surface-water contents 

are published in the annual reports of the Gila River 

Water Commissioner. Usable surface-water storage 

data are also contained in annual U.S. Geological 

Survey reports. Figure 25 shows beginning-of-month 

reservoir stages for water years 1929-71. Total 

surface-water storage and change in surface-water 

storage were obtained from the yearly elevationcapacity ratings and are shown in table 18 by water 

year. The derivations of these ratings are described 

in the section "Development of Surf ace- Water 

Storage-Capacity Ratings."

TABLE 11. Evaporation at San Carlos Reservoir by water year 

1931-71

Pan Computed 

evaporation lake evaporation 

Water year (in.) Pan coefficient (in.)

1931 ..... .

1932 ........

1933 ........

1934 . ......

1935 ..........

1936 .........

1937 .... ...

1938 .. . 

1939 .. .. .. . 

1940 ...........

1941 ...........

1942 . . ....

1943 ..........

1944 .... ....

1945 ...........

1946 .... . 

1947 ........

1948 .....

1949 ..........

1950 ... . 

1951 ..... ..

1952 ..........

1953 ..........

1954 ..........

1955 ..........

1956 ...........

1957 ..........

1958 ......... 

1959 ......... 

1960 ..........

1961 .........

1962 ..........

1963 .........

1964 . .......

1965 .........

1966 ........

1967 ........

1968 .........

1969 ......

1970 .. ....

1971 .........

............... .. 94.47

..... .. 96.46

105.45

109.86

95.64

103.00

107.54

... . .... .... .. . 107.56 

..... .. ...... 111.35 

101.83

83.64

.. .. .. .... ..... 94.34

...... ... .... 101.13

98.73 

100.81

....... ... 104.80 

.......... .. ... 104.20

106.18

.. . .. 98.30

.. ..... .. .. 98.68 

............. ....... 98.82

90.55

94.84

.. ........ . 93.73

.. .. ............... ... 91.49

100.35

.. .. ............ ... 96.54

. .. . ... 89.03

. . ................ ... 97.63

. .............. .. ... 99.30

. ................ .. .. 94.60

........... ... 94.63

.. . .................... 89.13

94.81

86.74

...... 88.58

93.12

......... ........ ... 91.39

....... . .... 92.56

......... ... 91.19

. ................ . .. 97.47

0.61 

.61 

.61 

.61 

.61 

.61 

.61 

.61 

.66 

.66 

.66 

.66 

.66 

.66 

.66 

.66 

.66 

.66 

.66 

.66 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80 

.80

57.63 

5884 

6432 

67.01 

5834 

62.83 

6560 

6561 

7349 

67.21 

5520 

62.26 

66.75 

65.16 

66.53 

69.17 

68.77 

70.08 

64.88 

65.13 

7S.06 

72.44 

75.87 

74.98 

73.19 

8C.28 

77.23 

71.22 

IS. 10 

7C.44 

7E.68 

7S.70 

71.30 

75.85 

6£.39 

7C.86 

74.50 

7S.11 

74.05 

72.95 

77.98

monthly pan coefficients for San Carlos Reservoir

Mar.

4.42 

.68 

.66

Apr. May June

6.57 9.03 10.58 

.65 .69 .75 

.66 .70 .71

July Aug.

10.28 8.46 

.79 .79 

.83 .82

Sept.

7.12 

.86

.79
FIRST-OF-MONTH ELEVATION, IN FEET ABOVE MEAN SEA LEVEL

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N24 GILA RIVER PHREATOPHYTE PROJECT

The highest reservoir stage of record (1929-71 

water years) was 2,501.62 ft (762.49 m) above mean 

sea level on March 18, 19422 . Usable surface-water 

storage at that elevation was 819,200 acre-ft (1,010 

hm3) using 1947 capacity tables. This storage is 

about 83 percent of the usable surface-water storage 

capacity at 2,511 ft (765 m). Other peak stages 

recorded were 2,491.17 ft (759.31 m) in 1968 and 

2,477.30 ft (755.08 m) in 1966. Usable surface-water 

storages at these elevations were 643,324 acre-ft (793 

hm3 ) and 417,673 acre-ft (515 hm3 ) using 1966 

elevation-capacity tables.

The minimum stage has fallen to, or below, the 

lowest outlet elevation of 2,382.63 ft (726.22 m) at 

some time during 14 of the 43 years of record. At other 

times, inflow was barely sufficient to keep the pool 

level above the stage of zero usable surface-water 

storage. The pool level was below 2,385 ft (727 m) 

elevation about 15 percent of the time and can occur 

at any time of the year.

A summation of the number of days in which mean 

daily stage was within a prescribed elevation interval was needed prior to frequency analyses of storage. Figure 26 is a graph and listing of the number of 

daily occurrences within each 5-ft (1.5-m) stage 

interval.

The cumulative number of days for which the lake 

stage was above a specified elevation is expressed as 

a percentage of the total days of record in figure 27. 

Figure 27 shows that stage exceeded elevation 2,439 

ft (743 m) only 25 percent of the time. Elevation 2,416 

ft (736 m) was exceeded 50 percent of the time, and 

elevation 2,392 ft (729 m) was exceeded 75 percent of 

the time.

Figure 28 is a time-storage curve which shows the 

percentage of time that usable surface-water storage 

was equal to or greater than a given volume, based 

upon first-of-month storages. Usable surface-water 

storage exceeded 176,000 acre-ft (217 hm3 ) only 25 

percent of the time, 64,000 acre-ft (78.9 hm3) 50 

percent of the time, and 11,000 acre-ft (13.6 hm3 ) 75 

percent of the time.

Table 13 shows the usable surface-water storage 

available at the beginning of each month at 25, 50, 

75, and 100 percent of the time, as an indicator of 

seasonal availability.

BANK STORAGE

A part of the total storage in a reservoir in addition

2500 

1- 2490

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NUMBER OF DAYS IN INTERVAL

FIGURE 26. Number of days in which lake stage was within a 

particular elevation interval for water years 1929-71.

LAKE STAGE, IN METERS ABOVE MEAN SEA LEVEL 

730 735 740 745 750 755 760 765

Q

l UU | | 1 I I I 1 1 1 1 | I | I 

1

UJ \

< 90

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2 Prior to January 1,1948, gage datum was 0.72 ft (0.22 m) below mean sea level. (See 

U.S. Geological Survey, 1954, p. 636). No adjustments were made in this report for the 

datum change except that the peak stage listed has been adjusted to datum used after 

January 1948. The error in the water budget introduced by datum change is small 

because the computed annual change in surface-water storage is not significantly 

affected.

LAKE STAGE, IN FEET ABOVE MEAN SEA LEVEL

FIGURE 27. Percentage of time lake stage of San Carlos 

Reservoir equaled or exceeded a given elevation for water 

years 1929-71.

to surface-water storage is bank storage. Ir some 

reservoir water budgets, the change in bank storage 

has been treated as being equal to the residual of a 

water-budget equation, even when the residual included significant evapotranspiration losses, 

ground-water inflow, and so forth. For this investigation the change in bank storage is considered a separate water-budget component. The symbols for the 

reservoir storage terms are as follows:
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N25

USABLE SURFACE-WATER STORAGE, 

IN CUBIC HECTOMETERS

0 100 200 300 400 500 600 700 800 900 1000 1100

ii.i I i I I I I J I L_J I J \ L, I l__l I I,

USABLE SURFACE-WATER STORAGE, 

IN 100,000 ACRE FEET

FIGURE 28. Percentage of time usable surface-water storage at 

San Carlos Reservoir equaled or exceeded a given volume for 

water years 1929-71.

Sy is the total water in storage, 

fig is water contained in surface-water storage, 

fig is the water contained in bank storage, and 

A indicates the change in an associated storage term.

Inflow and outflow from bank storage can be 

computed by use of combined water-budget and 

modeling methods for some reservoirs, as demonstrated by Simons and Rorabaugh (1971). At San

Carlos Reservoir insufficient ground-water data 

were collected to model aquifer response. However, 

by applying the water budget of the reservoir to 

selected short periods during the winter, estimates of 

bank storage were made.

The change in bank storage, ASg , at San Car'os 

Reservoir was estimated for short budget periods by 

solving a modified form of equation 1. The periods 

selected included winter months when ASg was a 

significant budget component and when evapotrpnspiration, O^y, was insignificant. Tributary flow 

was not included because no flow was assumed in 

winter (Burkham, 1974). Equation 1 as applied was

(4)

The rate of change in bank storage is dependent 

upon the change in surface-water storage, ASg, so it 

is important that ASg be small for a month or more 

before and after the evaluation period. Table 14 

illustrates the application of equation 4 in computing 

ASg for the winter period January through April of 

1965.

ASg was determined for each of 23 winter periods 

of significant ASg increases. ASg was not determined for periods of decreasing Sg because all 

periods of a significant decrease in Sg corresponded 

to periods of high evapotranspiration rates. 

Several procedures were used to investigate the

TABLE 13. Percentage of time that available monthly surface-water storage was less than amount shown

Percentage 

of time

25

50 .............. ..................

75 ........... .. .. ....................

100 ...................................

Usable surface-water storage, in acre-ft

Oct.

6,820 

50,300

..... .. 113,000

...... 689,000

Nov.

3,150 

48,000 

103,000 

744,000

Dec.

11,200 

46,000 

107,000 

752,000

Jan.

8,700 

55,000 

138,000 

802,000

Feb.

20,100 

93,400 

198,000 

835,000

Mar.

29,100 

104,000 

194,000 

843,000

Apr.

35,700 

113,000 

260,000 

841,000

May

21,000 

115,000 

235,000 

816,000

June

12,700 

97,000 

203,000 

779,000

July

950 

70,100 

165,000 

727,000

Aug.

5,000 

44,000 

140,000 

676,000

Sepf..

8,2"^ 

47,0^ 

116,0^ 

659,0^

TABLE 14. Example of water budget used to determine change in bank storage

Month

Water surface

(water year 1965) stage

Feb ........... . .... .... . .. ..

Mar . ............ ............ ..

Jan-Apr

AsB/ft

AsB/AsT

ASR/ASn

(ft)1

2,407.47

2,413.35

2,418.63

2,418.%

2,418.51

= 759 acre-ft/ ft

= .210

= .266

Change

in stage

(ft)

5.88

5.28

.33

-.45

11.04

Surface-water 

storage,

Sp

(acre-ft)

29,790

45,254

61,700

62,769

61,311

Average stage

Average Sn

Elevation corresponding to

average S_

Change in 

surface-water

storage, A°S#

(acre-ft)

15,464

16,446

1,069

-1,458

31,521

= 2,412.99 ft

= 45,550 acre-ft

= 2,413.45ft

Inflow2

(acre-ft)

18,986

24,840

17,327

10,642

Outflow3

(acre-ft)

619

4,810

14,715

11,748

Change in 

total storage, Asy4

(acre-ft)

18,367

20,030

2,612

-1,106

39,903

Change in 

bank storage,

As^j 5

(acre-ft)

2,903

3,584

1,543

352

8,382

'Values for beginning and end of month.

2Inflow is the sum of Gila River inflow, San Carlos River inflow, precipitation on water surface, and ground-water inflow.

'Outflow is the sum of Gila River outflow and evaporation.

4 A/Sy equals inflow minus outflow.
N26 GILA RIVER PHREATOPHYTE PROJECT

relation between bank-storage capacity, Sg, and 

water-surface stage. In the first procedure, the ratio 

ASg/ ASg was compared to stage, where ASg and 

ASg values were from the water budgets. In the 

second procedure, the ratio ASg/ ASy was compared to stage. ASy is the change in total storage, 

computed in the water budget as the difference 

between inflow and outflow components. Relations 

of ASg to ASft and ASg to ASy are shown in figure 

29 for two winter budget periods. The need for 

extending the time period of the water budget past 

the period of rapidly increasing surface-water storage is obvious in the upper ends of the curves in 

figure 29A. These curves show S# and Sy decreased 

during April but Sg increased because of the time lag 

between inflow into surface-water storage and subsequent movement into bank storage. Figure 29B is 

included to show the small increase in Sg , when 

compared to changes in Sg and Sy, in December 

1967 and January 1968. This condition occurred because gravity drainage from bank storage was incomplete at the start of the period. As a result, only 

the February through May period of 1968 was used in 

the analyses of bank storage. Figure 29 A also shows 

that the cumulative plots of ASg versus ASg and 

ASy1 define the ratios ASg/ ASy and ASg/ ASg 

from the slopes of lines drawn from the start to end of 

the period.

The third and principal procedure of analyzing 

bank storage at San Carlos Reservoir was based on 

the relation between a change in bank storage and a 

change in stage. The computed ASg for a budget 

period was divided by the range in stage, giving the 

rate ASg /ft. Because each rate, ASg /ft, is related to 

a specific range of stage, the rate must be associated 

with the stage which seems most representative of 

the budget period. A representative water-surface 

stage is easily obtained from either of two calculations. The first calculation simply determines the 

mean of the beginning and ending stages for the 

period. In the second calculation, considered better, 

the mean value of the beginning and ending surfacewater storages for the period is applied to elevation 

versus surface-water capacity tables to obtain the 

corresponding stage. The stage for each period is 

plotted against the corresponding ASg /ft of the 

period to define the ratings of ASg and stage as 

shown in figure 30 for San Carlos Reservoir data. 

The winter stage and storage data obtained from the 

water budget of the reservoir and used in the above 

three procedures are tabulated in table 15.

The 1931-47 stage and storage data define one 

curve of figure 30, and the other was defined by data

of 1948-71. The shift in the rating with time reflects 

an increase in bank storage due to sediment accumulation in the reservoir. The data were inadequate to 

define more than the two ratings shown in figure 30. 

Much of the scatter exhibited by points from the 

1931-47 data is due to inaccuracies in the early

CUMULATIVE CHANGES IN SURFACE-WATER 

STORAGE(A5^) AND TOTAL STORAGE (AST),

IN CUBICHECTOMETERS 

0 5 10 15 20 25 30 35 40 45 50 55

A. January through April 1965 _

0 5 10 20 25 30 35 40 45 50

CUMULATIVE CHANGES IN SURFACE-WATER 

STORAGE (ASR ) AND TOTALSTORAGE (ASr),

IN THOUSANDS OF ACRE-FEET

CUMULATIVE CHANGE IN SU RF ACE-WATER

STORAGE (&SR ) AND TOTALSTORAGE (&ST),

IN CUBIC HECTOMETERS

0 100

uj 50 

0

£ uj 40 

Z °

CQ Uz O 30

- c/) 

uj Q

i<

200 300 400 500 600

i i i i i i__i i i 60

40

DC 

0 «

iu H

Z O UJ

UJ I

30

20

I- CC

< ^a 

10 -J<

D

D 

n O

0 100 200 300 400 500

CUMULATIVE CHANGE IN SURFACE-WATER

STORAGE (&SR ) AND TOTALSTORAGE (&ST),

IN THOUSANDS OF ACRE-FEET

FIGURE 29. Relations of cumulative AS#to cumulative AS/? and 

cumulative AS#to cumulative ASyfor the periods (A) January 

through April 1965 and (B) December 1967 through May 1968.
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 J T27

0.5

COMPUTED CHANGE IN BANK STORAGE, IN 

CUBIC HECTOMETERS PER METER

2 4 681012

2510 765

- 760 UJ

Number (66) indicates water year

2380

1 00 1 000

COMPUTED CHANGE IN BANK STORAGE, 

IN ACRE-FEET PER FOOT

3000

FIGURE 30. Relation between the computed change in bankstorage capacity and elevation at San Carlos Reservoir for 

water years 1931-47 and 1948-71.

capacity surveys. The data points for 1931 and 1932 

include an increment of water which went into 

nonretrievable bank storage when the reservoir initially filled.

Table 16 includes usable bank storage capacity 

ratings for the 1931-47 and 1948-71 periods based on 

figure 30. It was not practical or necessary to make 

an elevation-capacity rating of total bank storage 

because an estimate of bank dead storage would 

have been required.

The usable bank-storage capacity at the spillway 

elevation of 2,511 ft (765 m) was about 152,800 acre-ft 

(188 hm3) for 1931-47 and about 159,200 acre-ft (196 

hm3) for 1948-71 (table 16). At this elevation, usable 

bank-storage capacity is about 14 percent of total 

usable storage capacity. At lower reservoir elevations, table 16 shows that usable bank storage 

sometimes exceeds usable surface-water storage.

Sg is never static because of the time of response 

required to adjust to changes in Sg. However, the 

quantity of water required to place SB and S# in 

equilibrium at the end of a water year is usually

small in comparison to water-year budget totals and 

for expediency was assumed zero.

The fact is stressed that reservoir water availability is more than just the amount in usable 

surface-water storage. Figure 31 shows this difference by comparison of 1966 ratings of usable surfacewater storage capacity and total usable storage 

capacity.

WATER-BUDGET ANALYSES

The water budget utilizes the conservation of mass 

equation

I-O= AS, (5)

where /is inflow, O is outflow, and AS is change in 

storage. Identification of all the /, O, and AS components included in the water budget of San Carlos 

Reservoir is given in equation 1.

The water budget was computed by months and by 

water years for the 41 years of record using equat; on 

1. Data for all the yearly inflow and outflow budget 

components except Ojgp are recorded in table 17. 

Analyses of computed OET data are included in the 

following section on evapotranspiration. Table 18 

lists the storage values, ASg and ASg, the summation of the inflow and outflow components giver in 

table 17, and the evapotranspiration, Ogy, for each 

water year during the 41-year period of record.

The data in tables 17 and 18 were used to compare 

the magnitude of each inflow component with the 

total inflow for the period 1931-71. The outflow components were compared similarly. Gila River streamflow contributed 78.2 percent of the total inflow; the 

San Carlos River, 14.5 percent; ground water, 3.5 percent; precipitation, 2.2 percent; and tributary flow, 

1.6 percent. The outflow components and percent of 

total outflow are: Gila River, 78.2 percent; evapotranspiration, 11.3 percent; and lake surface evaporation, 10.5 percent. Figure 32 compares the relat;ve 

magnitude of each component.

EVAPOTRANSPIRATION

Water loss by evapotranspiration (ET) from the 

exposed surface of San Carlos Reservoir occurs by 

plant transpiration and by evaporation from soil, 

litter, and ephemeral ponds. Water in the reservoir 

area becomes available for ET by movement from 

streams, ground water, reservoir surface-water storage, and by direct precipitation on the exposed 

surface. Annual ET computed by equation 1 is listed 

in table 18 and plotted in figure 33.

Large errors in the computed .ETlosses occur when 

one or more of the hydrologic components of the 

reservoir are in a state of rapid transition at the end 

of a budget period. The annual ET shown in figure 33
N28 GILA RIVER PHREATOPHYTE PROJECT 

TABLE 15. Results of procedures to determine bank storage capacity at San Carlos Reservoir

Startir 

AS/ A% ASB end 

Water Period eleva 

year included (acre-ft) (acre-ft) (acre-ft) (f

949

1931 ... . Nov.-Mar. 92,277 71,812 20,465 f"

94^ 1932 . . ... Dec.-Apr. 323,063 270,007 53,056 

240 1935 ... .. Dec.-Apr. 136,282 121,428 14,854 ;," 24o

1936 . Jan.-Apr. 88,530 71,804 16,726 ^ i4o 

oqn

1937 .... ...... Jan.-Apr. 264,946 239,274 25,672 ~~

i4o

OOQ

1939 .. ... .. Nov.-Apr. 22,826 18,513 4,313 ^°

£,A\J 

OOQ

1940 .. . Jan.-Mar. 49,777 40,906 8,871 °

246 1941 ......... Mar.-June 359,660 312,877 46,783 

948

1942 ............ Sept.-Mar. 215,359 190,272 25,087 ^ ioO 

OOQ

1946 .. ........ Jan.-Feb. 17,718 14,577 3,141 ~^

OOQ

1948 .. .. .. . ... Jan.-Apr. 18,858 12,962 5,896 ~° io9

OOQ

1949 .. .......... Dec.-Apr. 312,140 251,306 60,834 ~?245

oqo

1952 ... ......... Dec.-Mar. 196,070 155,216 40,854 ^43

OOQ

1953 . ............ Jan.-Mar. 14,686 10,544 4,142

239

OOQ

1954 ... . ..... .. Mar. 51,082 36,336 14,746 ^4°

OOQ

1957 .. . .... . Jan.-Feb. 11,715 7,422 4,293 f**

£iA\J

941 

1958 ........ Feb.-Apr. 204,404 167,062 37,342 ^

1960 .... ...... ... Nov.-Mar. 196,941 163,768 33,173 ^44

OOQ

1962 .... .... Nov.-Apr. 181,480 141,049 40,431 ~* i4o

9QQ

1963 .. ....... Oct.-Dec. 39,560 30,557 9,003 !~

240 1965 . . ....... .. Jan.-Apr. 39,904 31,521 8,383 * 

OOQ

1966 ...... ... Nov.-May 520,735 414,683 106,052 "7

1968 . .. . ...... Feb.-May 320,492 279,289 41,203 ^49

for 1941 is an example of this condition. This error is 

compensated, however, by an error of equal magnitude but of opposite sign during the following year(s) 

and is of no significance in the 41 -year mean annual 

ET rate. 

Determination of the size of the "exposed surface 

area of the reservoir" is a prerequisite to computing 

the ET by depth. The reservoir area at 2,525 ft (770 m) 

is 19,925 acres (8,064 hm2). This area excludes approximately 60 acres (24 hm2 ) which lie within the 

reservoir boundary but are upstream from the Gila 

River Calva station. Added to the reservoir area, 

however, are 925 acres (374 hm2) in the San Carlos 

River flood plain between the Peridot gaging station 

and the reservoir boundary, giving a total of 20,850 

acres (8,438 hm2) as the maximum area possible for 

ET loss. At any specific time, the exposed surface

igngan<1 Mean Mean Elevation of ^ ^ ^

tions elevation Sp mean S/j 

t) (ft) (acre-ft) (ft) AS T ASR ft

L76 2428.71 163,059 2429.20 0.222 0.285 1,472 3.66

7~ 2454.31 338,814 2456.68 .164 .196 1,600 J.JH)

7 2422.03 122,006 2424.65 .109 .122 520

3.ol

^ 2427.52 137,102 2428.15 .189 .233 1,118

' 7ft 

° 2424.12 153,975 2431.80 .097 .107 487 3.46

!';f 2390.18 19,935 2390.86 .189 .233 355 3.26

7 '^ 2397.95 35,185 2400.03 .178 .217 444

 t 09 

''"2 2481.00 578,220 2482.04 .130 .150 1,811

3 QA

 " 2495.39 753,257 2495.24 .116 .132 1,903

..JO

R7

'°' 2396.23 21,236 2396.69 .177 .215 360 j.oy

3 AO

_T~ 2389.18 10,521 2390.08 .313 .455 479

Xoo

3 1 ft

'*° 2416.94 129,452 2430.58 .195 .242 901

D 00

 ;: 2409.94 80,936 2420.16 .208 .263 768 3.55

x53 2390.59 9,690 2391.44 .282 .393 409 

5.65

1 10

.,:. 2395.34 21,043 2394.74 .289 .406 607 .oO

~) CA

: " 2387.68 5,795 2389.00 .366 .578 414

i.OO

J04 ' 2430.17 133,899 2433.59 .183 .224 1,090

J f)0

:'"^ 2426.64 117,198 2430.93 .168 .203 891 >.^o 

? 17 

'' 2414.70 77,624 2421.96 .223 .287 897

i f>7 

^' 2405.42 29,343 2406.40 .228 .295 657

7 47 

'' 2412.99 45,550 2413.46 .210 .266 799 s.ol

j 17 

,' ' 2430.24 210,140 2448.77 .204 .256 1,260 L.A£I

T ffi

j^ 2478.15 488,088 2479.03 .129 .148 1,700

area available for ETloss is 20,850 acres (8,438 hm2 ), 

less the lake surface area. 

Surface conditions on the exposed area rrnged 

from open bodies of shallow water to dense phreatophytes and from wet to very dry soil. Optimum 

surface conditions for high ET exist over a large 

reservoir area following a major lake stage recession 

such as occurred in 1942-45. 

The computed volume of monthly ET was divided 

by the mean monthly exposed area providing a value 

of monthly ET depth. Water year totals of these 

monthly ET depths are listed in table 19 and are 

plotted in figure 34. The computed mean annual 

depth of ET was 1.47 ft (0.448 m). 

For each month, 41 values of .ETdepths were available from the water budget. All monthly values were 

used to indicate the most common range in FT for
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N29

TABLE 16. San Carlos Reservoir elevation-capacity tables of usable bank storage, usable surface-water storage, and total usable

storage

Period 1931-47

Cumulative usable storage, in acre-ft

Elevation

(ft)

2382.631 ..... ............ ... ........ ..

2385 .. .. .. .. ......................... .. ..

2390 ....... ..................... . .. ..

2395 .................................

2400 .. . .. ... ............ ...... .. .. .

2405 .. .. .. .. ... . .............. .. .. .

2410 .............. ....................

2415 ............. ....................

2420 .. .. .. ..... ................. . .. ..

2425 .. .. .. ..... ................. .. ..

2430 ............ ....................

2435 ..................................

2440 . ................................

2445 ..... .. .. ............ ........ .. .. ..

2450 ..... .. .. ....... ............ ... .. ..

2455 ...................... .............

2460 .... .... .... ....................

2465 ........ .............. .........

2470 .. ................. ............

2475 .. .. .. .. .................... .... ..

2480 ............... ..................

2485 .. .. .. ............... .... .. .. ..

2490 .... .. .. . ...................... .. .

2495 ....................................

2500 ...... ...... .. . .... . .

2505 ....... ............. ..... ..

2510 ..... .. .. . .... . . ........... .. ..

2511 .. .... .............. .... .....

2515 ....... .........................

Bank

SB

0 

770 

2,520

4,483 

6,670 

9,095 

11,795 

14,820 

18,185 

21,945 

26,095 

30,670 

35,720 

41,270 

47,295 

53,720 

60,495 

67,620 

75,120 

83,020 

91,320 

100,020 

109,145 

118,720 

128,770 

139,345 

150,470 

152,750 

162,170

Surface water

SR

2 0 

1,735 

6,668 

13,249 

22,041 

33,468 

46,941 

62,474 

80,102 

100,411 

124,115 

150,760 

180,020 

212,197 

247,463 

285,832 

327,993 

374,278 

424,606 

478,736 

536,141 

597,432 

662,965 

732,575 

806,491 

884,898 

967,746 

984,874 

1,055,286

Total 

ST

0 

2,505 

9,188 

17,732 

28,711 

42,563 

58,736 

77,294 

98,287 

122,356 

150,210 

181,430 

215,740 

253,467 

294,758 

339,552 

388,488 

441,898 

499,726 

561,756 

627,461 

697,452 

772,110 

851,295 

935,261 

1,024,243 

1,118,216 

1,137,624 

1,217,456

Period 1948-71

Cumulative usable storage, in acre-ft

Bank

SB

0 

1,006 

3,269 

5,769 

8,531 

11,581 

14,931 

18,581 

22,531 

26,806 

31,431 

36,431 

41,831 

47,606 

53,706 

60,131 

66,906 

74,031 

81,531 

89,431 

97,731 

106,431 

115,556 

125,131 

135,181 

145,756 

156,881 

159,161 

168,581

Surface water

SR

3 o

660

2,845 

6,728 

12,865 

21,749 

33,308 

47,343 

63,284 

81,217 

102,202 

126,269 

153,572 

184,059 

217,811 

254,977 

295,976 

341,707 

392,073 

445,822 

502,827 

503,363 

627,710 

696,272 

770,200 

848,607 

931,456 

948,584 

1,018,996

Total 

ST

0 

l,66f 

6,114 

12,497 

21,39f 

33,330 

48,23? 

65,924 

85,815 108,02" 

133,63? 

162,700 

195,403 

231,665 

271,517 

315, IOF 

362,882 

415.73F 

473,604 

535,25? 

600,55F 

669,794 

743,266 

821,40? 

905,381 

994,363 

1,088,337 

1,107,745 

1,187,577

'Elevation of zero usable storage.

2From 1947 tables of usable surface storage.

^From 1966 tables of usable surface storage.

CAPACITY, IN CUBIC HECTOMETERS 

200 400 600 800 1000 1200 1400

2520

2510

2500

2490

2480

2470

2460

2450

2440

2430

2420

2410

2400

2390

2380

(From 1966 Capacity Survey)

Usable surface-water, 

storage capacity

- 765

-760 <

Shaded area equals usable 

bank-storage capacity

755

750

745

740

- 735

- 730

1 2 3 4 5 6 7 8 9 10 11 12 

CAPACITY, IN 100,000 ACRE-FEET

FIGURE 31. Elevation-capacity relations for usable surfacewater storage, usable bank storage, and total usable storage 

in 1966, for San Carlos Reservoir.

each month, to show the monthly extremes, and to 

illustrate the seasonal ET trend. In figure 35, the

range of the central two-thirds of the values for each 

month is bracketed. This approximately corresponds 

to the number of values included in one standard 

deviation. The seven values above and seven b?low 

the bracketed range were plotted for each month. The 

monthly medians have also been identified in figure 

35. The occurrences of extreme values of computed 

ET usually coincide with periods of vigorous change 

in one or more water-budget components. An unrealistic extreme resulted when bank storage had not 

adjusted to the change in stage, although changes in 

lake stage and changes in bank storage wero assumed simultaneous in water-budget computations. 

The mean annual ET of the monthly mediars in 

figure 35 is 1.45 ft (0.442 m). This value corresponds 

to the mean of the annual ET values in figure 34, 

computed as 1.47 ft (0.448 m).

The seasonal trend ofETis obvious in figure 35 in 

spite of the large scatter exhibited. After excluding 

extreme values (outliers) the data were examined for 

possible changes in rates over the life of the resei*voir 

and for obtaining more realistic values of mean 

monthly ET depths. The monthly data were separated into 4 periods of 10 years each, beginning with 

1931. Within a 10-year period, the maximum and 

minimum extremes were omitted from the 10 values 

for each month, and a mean monthly ET was 

obtained from the 8 remaining values. Figure 36
N30 GILA RIVER PHREATOPHYTE PROJECT

TABLE 17. Summations of San Carlos Reservoir inflow-outflow components by water year, 1931-71

Inflows for year in acre-ft

Water year

1931 ............ ...... ...

1932 ... ...... .. ........ ....

1933 ............. .... ..

1934 ...................... ....

1935 ...... ..................

1936 ....... ... ..........

1937 ...... .. ..... .

1938 ........ .. . ...... ...

1939 ........ ... .. . ..... ..

1940 .............. ........

1941 .. ......... . .. .. ..

1942 . .......................

1943 .........................

1944 ... .... ...... .............

1945 .......... .. ........

1946 .......... .. . ... . ..

1947 ... ..... .. .. . .. ..

1948 ............ .. ..... .

1949 . ............ .. .... . ...

1950 . . .... . .. ... .. . ...

1951 ............. ..

1952 ..... ..... .. .. .... .....

1953 ........ .. .. .............

1954 . ........ . . .... ....

1955 ...... .... . . .... ..

1956 ............... .. ..

1957 ............. ..... .....

1958 .......................

1959 ............ .........

1960 ....... ... . .. ...

1961 .... . .. . ... ..

1962 ............ . ........ 

1963 ........ .. .. .. 

1964 ...... ... ... ... ..

1965 .......................

1966 . ... . .. .. .............

1967 ............ . 

1968 ...... .. ......... 

1969 ........ ... . 

1970 . ... .... . ... 

1971 ..... . ...........

1931-71 

totals .

Gila 

River

289,917 

442,175

149,072 

160,085

149,423

150,165

317,944

106,170

91,500

158,275

803,991

314,222

102,531

80,725

131,705 

55,627

45,607 

63,511 

422,114

37,441

35,304 

191,624 

42,937

116,372 

123,593 

20,858 

128,264 

296,372 

97,574

193,693 

45,499 

275,405 

175,069 

94,351 

90,948

533,205

148,375 

579,171 

60,545 

31,214 

59,775

7,412,348

San Carlos 

River

36,677 

51,950 

16,802 

13,733 

87,670 

44,559 

46,630 

15,713 

18,378 

15,858 

201,192 

25,273 

30,191 

13,161 

17,038 

15,233 

11,141 

10,390 

23,192 

5,908 

9,134 

80,840 

8,384 

42,254 

26,899 

13,886 

9,136 

47,883 

6,419 

64,990 

6,189 

33,567 

34,123 

10,301 

24,550 

119,391 

11,295 

72,822 

15,599 

12,502 

19,247

1,370,100

Precipitation

5,850 

9,565 

7,399 

1,895 

6,684 

3,659 

3,717 

2,505 

1,362 

1,891 

22,378 

16,250 

10,954 

7,512 

3,863 

1,245 

796 

620 

3,863 

2,709 

808 

4,658 

861 

2,345 

3,194 

2,053 

586 

5,676 

2,290 

3,809 

461 

2,650 

3,340 

1,697 

2,803 

13,948 

7,697 

12,339 

11,171 

5,889 

861

203,853

Ground water

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994 

8,015 

7,994 

7,994 

7,994

327,964

Tributary

3,580 

1,760 

2,460 

2,660 

4,020 

1,760 

1,760 

1,760 

1,760 

1,760 

6,990 

2,410 

2,200 

6,070 

3,010 

4,850 

3,550 

2,530 

2,680 

2,530 

2,930 

2,620 

1,760 

7,430 

6,960 

1,760 

1,760 

6,420 

5,920 

3,940 

5,080 

2,980 

3,580 

7,620 

1,940 

8,580 

5,800 

1,760 

2,480 

3,560 

3,920

148,890

Total inflow

344,018 

513,465 

183,727 

186,368 

255,791 

208,157 

378,046 

134,142 

120,994 

185,799 

1,042,546 

366,150 

153,870 

115,483 

163,610 

84,949 

69,088 

85,066 

459,843 

56,583 

56,170 

287,758 

61,936 

176,396 

168,639 

46,572 

147,740 

364,345 

120,197 

274,447 

65,223 

322,596 

224,106 

121,984 

128,234 

683,118 

181,162 

674,107 

97,789 

61,159 

91,786

9,463,160

Outflows for year in acre-ft

Gila 

River

223,920 

256,150 

334,910 

184,660 

184,553 

228,090 

279,621 

189,720 

102,426 

160,720 

215,724 

352,030 

365,140 

297,430 

216,290 

74,550 

59,983 

68,270 

254,061 

148,860 

36,347 

229,435 

49,319 

68,964 

97,710 

109,062 

52,123 

243,220 

147,870 

256,896 

25,312 

246,730 

141,430 

107,170 

121,999 

226,476 

255,250 

281,340 

315,376 

218,860 

57,463

7,485,460

Evaporation

23,008 

44,455 

36,244 

15,416 

21,554 

22,162 

29,436 

14,321 

9,365 

10,323 

54,381 

74,465 

60,302 

34,186 

20,734 

7,049 

5,467 

5,913 

29,914 

13,692 

4,866 

22,710 

5,983 

10,579 

12,342 

12,572 

5,288 

28,118 

15,965 

26,884 

2,449 

21,971 

18,074 

12,697 

13,688 

53,587 

44,067 

70,871 

59,744 

28,413 

5,777

1,009,032

To'al 

outflow1

246,928 

300,605 

371,154 

200,076 

206,107 

250,252 

309,057 

204,041 

111,791 

171,043 

270,105 

426,495 

425,442 

331,616 

237,024 

81,599 

65,450 

74.183 

283.975 

162.552 

41.213 

252 145 

55.302 

79,543 

110,052 

121.634 

57,411 

271.338 

163 835 

283 780 

27,761 

268 701 

159 504 

119867 

135 687 

280063 

299,317 

352.211 

375 120 

247,273 

63240

8,494.492

'Does not include evapotranspiration losses.

shows the resulting monthly means for each 10-year 

period. The seasonal trend is apparent for all four 

periods, but no obvious changes in ET rates can be 

detected during the life of the reservoir.

The average of the annual mean ET values computed from these four periods is 1.51 ft (0.460 m), 

which is slightly higher than that obtained when the 

maximum and minimum extremes are included in 

the computation. The monthly mean and median ET 

values and the water-year means from the data 

shown in figures 35 and 36 are listed in table 20.

Two characteristics can be noted about the seasonal trends in the computed ET values shown in 

figures 35 and 36. First, the computed ET is essentially zero during the winter months from December 

through February in part because zero ET was used 

in the development of the bank-storage ratings. 

Second, the reservoir surface-water storage normally 

increases during August, and the bank storage 

response to this increase does not approach equilibrium until sometime in September. As a result,

August .ET was often underestimated and September 

ET overestimated.

Vegetation increased on the exposed areas cf the 

reservoir flood plain, especially following the 1941- 

42 floods (Turner, 1974, fig. 4). If an increasing trend 

in ET could have been conclusively shown, chs nges 

in ET would have been correlated with the increase 

in vegetation. Data used in figure 34 were examined 

for an indication of increased ET, since no increasing 

trend was evident in figure 36. Mean annual ET was 

computed for the selected periods shown in figure 34. 

Only those periods of least stage change were used, 

thereby eliminating periods when soil-moisture content was high over large areas. Thus, a greate^ percentage of the total ET should have been through 

transpiration and not by evaporation from exposed 

areas of bare ground. Mean annual ET depths were 

computed as 0.93 ft (0.28 m) from 1934 to 1940,1.07 ft 

(0.33 m) from 1945 to 1957, and 1.48 ft (0.45 m) from 

1959 to 1965. This shows an apparent increase in ET, 

but as figure 34 indicates, the range in arnual
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N31

Precipitation 2.2 percent^ 

Ground water 3.5 percent

/Tributary flow 1.6 percent

INFLOW

Evaporation 10.5 percent

Evapotransp [ration 

1 1.3 percent

OUTFLOW

FIGURE 32. Relative magnitude of inflow and outflow waterbudget components.

amounts during each period is too great to state 

definitely that the increase is related to vegetation 

changes.

The total exposed surface area of the reservoir 

extends outside the flood plain because much of this 

surface area was occasionally inundated. Generally, 

however, the highest rates of ET are from the floodplain portion of the area. Use of the total exposed 

surface area prevented making a more accurate 

evaluation of vegetative ET losses.

Flood-plain vegetation in the reservoir area was 

removed beginning in the 1967 water year. A comparison of monthly before-clearing and afterclearing data was made for periods 1961-66 and 

1967-71 to evaluate vegetative consumptive use 

(transpiration). The results of these comparisons 

were inconclusive, however, because the ET rates

were too greatly affected by the large fluctuations 

in lake stage from 1966 to 1971.

Although precipitation falling on the exposed a rea 

of the reservoir is a reservoir inflow, a distinction is 

made between the relation of this inflow to ET and 

the relation of inflow from other sources to ET. The 

ET loss computed by the water budget (table 18) 

represents a loss in usable reservoir water. Normally, only a minimal amount of the input from direct 

precipitation on the exposed areas is ever a par4 of 

"usable" contents. The fact that precipitatior is 

distributed on the surface enhances the possibility 

for immediate and total ET. Vaporization of the 

precipitation depletes the energy available for vaporizing water from other sources, but this effect is 

temporary in arid regions because of the infrequent 

and limited quantity of precipitation.

All precipitation on the exposed ground area of the 

reservoir is an additional .ETloss. This added ETis 

assumed equal to precipitation measured at the Pan 

Carlos Reservoir weather station. ET from this 

precipitation source was added to the ET computed 

from the water budget of the reservoir (table 18) to 

give the total ET from the reservoir water-surface 

area and adjacent exposed ground area. The average 

annual total ETfor the 1931-71 period is 2.62 ft (C .80 

m). A comparison of this value with the computed 

water-budget ET values in table 20 indicates that 

precipitation on the exposed area of the reservoir 

contributed an average of about 1.2 ft (0.37 m) per 

year to the total ET.

DEVELOPMENT OF SURFACE-WATER 

STORAGE-CAPACITY RATINGS

SIMULATION OF THE SEDIMENT DEPOSITIONAL PROCT.SS

Interpolations of capacity changes between capacity surveys were made by using a procedure which 

included simulating the sediment depositional 

process in the San Carlos Reservoir. The simulation 

I procedure was structured about three basic phases of 

the depositional process:

1. Sediment inflow

2. Sediment distribution

3. Sediment compaction

Equations were developed to represent each phrse. 

The "best fit" values of variables in the equations 

were selected by minimizing the difference between 

the volumes of sediment measured and estimatei.

Measurements of sediment accumulation were 

available from only four separate periods between 

1929 and 1966. However, volumes of sediment deposited within 5-foot-elevation increments were defined 

from each survey, and these volumes provided the
N32 GILA RIVER PHREATOPHYTE PROJECT

TABLE 18. Water-budget summations

Water year

Water-surface 

elevation

(ft)

Surface-water 

storage 

(acre-ft)

Surface-water

storage change

(acre-ft)

Bank storage 

(acre-ft)

Change in

bank storage

(acre-ft)

Inflow 

(acre-ft)

Outflow1 

(acre-ft)

Evaiotranspiration- 

(acre-ft)

	2424.40 139,361

1931 2430.66 163,401

932 2453.53 309,714

|£ 2420.26 110,631

| 934 2411.85 73,647

J 93^ 2421.78 108,450

9* 2402.50 44,980

1937 2418Q4 9121Q

*° 2388.26 16,138

940 -- -- 2389.28 16,465

i»w 2386.48 11,276

1941 ...... 2491.64 696,666

194^ 2483.99 594,180 194d .......... 2459.22 329,562

944 ------- - 2424.70 105,620

1945 2399.67 26,578

1946 ............ 2393.68 15,211

1947 2386.02 6,260

948 ...................... 2382?5 3j542

iq^n 2426.77 109,452 1950 ........... 2383.36 3,694

1951 2382.81 3,060

1952 ........... 2384-.00 3,499

1953 2382.57 2,581

9^4 -------- 2414.91 59,024

1955 2421.74 81,836

I95b ............... 2382.30 2,004

957 .................... 241556 5gln

1900 ....................... 2426.49 97,988

1959 ................ 24Q6 5Q gl 53g

itwu ............ 2382.89 1,830

9bl ............. 239651 n92()

u«w 2398.57 14,064 18W ...... 2415 lg 51)66()

I»M ............... 2408.87 33,078

19b5 23911? 446g

i»DD 2462.23 316,729

19b7 2447n 19573g

I9ba ............ 2477.86 471,372

I9b9 ...................... 2448g2 2Q5212

.', .... 24Q4 54 2Q 7gl

	............... 2412.27________37,542

'Does not include evapotranspiration losses. 

2Excludes precipitation from exposed area of reservoir.

-24,040

-146,313 

199,083 

36,984

-34,803 

63,470

-46,230 

75,072

-327 

5,189

-685,390

102,486

264,618

223,942

79,042

11,367

8,951

2,718

-105,910

105,798

634

-439 

918

-56,443

-22,812 

79,832

-56,107

-39.877 

66,450 

29,708

-10,090 

-2,144

-37,596 

18,582 

28,609

-312,260 

120.990

-275,633 

266,160 

184,431

-16,761

'

asqn 

\oq\4 

\qvtn

7000 7' 8_T

®q

2268 '

112286 '

00449

94 nw

'*"* iM

28184 '

'~"

_ 

'

in '""(tot 4

04170 '

,oo Ib.ooo

"5' 204

-25,132

33' 441

5' 476

~6' 616

11,647

~8' 989 

14,961

~357 

980

-110,998

14 ' 023 

38' 825 

37' 718 

15' 194 

2,561

2,497

1,384

-28,360

28' 109 

226

-488 

586

-18,506

~5' 503 

24' 120

-19,124

~9' 161 

15,598

12,445

-6,462

-1,138

-10,962

4' 549

10 ' 320

-66,230 

19,904

-43,998

41' 912

40' 966

-5. 288

344,018

513,467

183,727

186,368

255,791

208,158

378,046

134,142

120,994

185,800

1,042,546

366,150

153,870

115,484

163,610

84,949

69,088

85,067

459,843

56' 583 

56,170

287,759

61,936

176,396

168,639

46' 573 

147,740

364,345

120,197

274,448

65,223

322,596

224,106

121,985

128,234

683,118

181,162

674,108

97' 789 

61,159

91,786

246,928

300,605

371,154

200,076

206,107

250,252

309,057

204,041

111,791

171,043

270,105

426,495

425,442

331,616

237,024

81,599

65,450

74,183

283,975

162,552

41,213

252,145

55,302

79,543

110,052

121,634

57,411

271,338

163,835

283,780

27,761

268,701

159,504

119,867

135,687

280,063

299,317

352,211

375,120

247,273

63,240

67,846 

41,417 

45,077 

28,751

8,236 

33,024 

13,739 

20,134

8,519 

20,926 

-23,947 

56.1S4 

31,871 

45,528 

20,822 

17,278 

15,0?7 

14,9?<5 

41,598 

27,898 

15,8' 7 

34,6^7

8,138 

21,904 

30,272 

28,891 

15,098 

43,970 

38,410 

32,821 

20,910 

50,613 

16,024 

25,250 

31,476 

24,565 

22,738

2,235 

30,741 

39,2^3

6,4:98

UJH

WATER YEAR

FIGURE 33. Annual evapotranspiration from exposed area of 

reservoir computed by reservoir water budget.

basis for developing and testing the simulation of the 

sediment depositional process. In the simulation, 

each storage unit identified by a 5-foot-elevation

increment is considered a separate surface-vater 

storage reservoir.

The sediment inflow phase of the simulation procedure provided estimates of weight of incoming 

sediment by assuming that sediment inflow is a 

function of streamflow. The limitation of this assumption is the large range of sediment concentrations which occur for any given water discharge. 

However, for long time periods it is assumed that use 

of mean sediment concentrations is acceptable, if 

adjustments are made for seasonal differences in 

concentration.

According to the U.S. Army Corps of Engineers 

(1914, par. 92, p. 30), the total sediment discharge in 

summer averaged about 2.5 percent, by weight, of the 

discharge of the water-sediment mixture. In winter, 

the ratio of discharges was approximately 0.5 percent, so the summer-to-winter concentration ratio 

was about 5:1. Burkham (1972, p. 8) derived a similar 

ratio based on 1965-70 U.S. Geological Survey data
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N33

TABLE 19. Total evapotranspiration (al) for ban Carlos Reservoir, by water year

ET from precipitation 

ET as residual of exposed 

of water budget reservoir surface Total ET 

Water year (ft) (ft) (ft)

1931 ....

1932 .....

1933 . ...

1934 . ..

1935 .....

1936 .. . 

1937 .....

1940 .....

1941

1942 ....

1943 ... 

1944

1945 .... 

1946 ....

1947

1948 .. .. 

1949 . . 

1950 .....

1951 . 

1952 ....

1953 ....

1955 ....

1956 .....

1957 .....

1958 .....

1959 .....

1960 ......

1961 ..... 

1962 ....

1963 . .. 

1964 . ..

1965 .....

1966 .....

1967 ....

1968 . . 

1969 ....

1970 ...

1971 .. .

PIRATION, IN FEET 

J* CO0)MOl vj0

00 1 

Z

< 0 -

DC

EVAPOT i i CO 10_

... . 3.55 1.30 4.85

3.18 1.18 4.36

2.88 1.12 4.00

1.08 .54 1.62

.20 1.64 1.84

.... 1.81 .97 2.78 

.86 1.08 1.94

..... .97 .98 1.95

.29 .89 1.18

............ .93 1.04 1.97

-3.08 2.54 -.54

7.86 1.08 8.94

.... . . .... 2.95 .98 3.93 

2.71 1.25 3.96

.. .... .. .97 1.13 2.10

.. .... .58 .96 1.54

........... .61 .77 1.38 

...... ... 2.41 1.14 3.55 

............ 1.42 .93 2.35

.63 .99 1.62

1.86 1.55 3.41

.31 .90 1.21

.66 1.60 2.26

1.20 1.17 2.37

........... 1.48 .75 2.23

. .. .61 .93 1.54

2.25 1.66 3.91

. ........ ... 1.78 .98 2.76

1.66 1.28 2.94

.75 .92 1.67 

.. ...... 2.69 1.11 3.80

......... .62 1.24 1.86 

.91 1.08 1.99

........... 1.54 1.17 2.71

............ 2.31 2.26 4.57

1.13 1.11 2.24

.37 1.38 1.75

......... 2.48 1.12 3.60

2.21 1.11 3.32

.05 .74 .79

Ml M M M M M \ M ' M 1 1 i M M 1 1 1 M M ' | 00 

I *

EXPLANATION 2.0 £

, ___ , - LU

 5 Mean annual ET depth -- 1.5 ^ 

for period bracketed 

\( .-1.0 I

\ A \ A A ,,/WUA /Y°-5 \

i (/)

 _ o 2

*

-" ~~°' 5 o

1 °-

1 -1.0 <

III

«- o o o o«- 

co ^ in co r^r^

0) 0) 0) O) 0)0)

WATER YEAR

FIGURE 34. Annual depth of evapotranspiration from the 

exposed surface of San Carlos Reservoir for water years 1931-71.

1.0 

0.9 

0.8 

0.7 

h 0.6

LU 

LU

" 0.5

z

. 0.4 

| 03

DC 0.2

CL

jjj 0.1

DC 0

2-o.i

> -0.2 

-0.3 

-0.4 

-0.5 

-0.6

-5.0

i

FIGURB

2 0.4 

O 

P 0.3

Q- LU

H~ ° 

O Q- -0.1

Lu -0.2

1 1 1 1 1 1 1 1 1 1 °4.4 °1.7 1.2° o 1.4

° Q 8

- 3 ' 2 A j'2 8-

8 g o 2t

"I ^ fT "1~\ " i;

+2 -i | | { 3± 212 £2 \ 24-

! s I ]_ I 1 g 2° t 2

-0.9

MONTH ~" W 

EXPLANATION

! 4 -?- Number indicates actual evapotranspiration, in

Range of 2/3 of monthly values 

+ Median of monthly values

-L Monthly values outside bracketed range 

2 Number indicates number of values

: 35. Computed monthly evapotranspiration 

Carlos Reservoir for water years 1931-71.

^^^

MONTH 

EXPLANATION 

................ 1931-40

1 JH- 1 3U

1OR 1 ~rn

- 0.30 

- 0.25

- 0.20 oo

DC 

LU hLU 

- 0.15 %

Z

-0.10 § 

h 

- o.or cc

0. 

00

- 0 <

DC

1-

o

- -0.05 <

LU

- -0.10 

- -0.15

r 1.52

feet 

at Fan

-0.12 z 

-0.10 ° 

- 0.08 <

- o.oe o:

_ _ f Q- m

- 0.04 oo j_ 

- 0.02 <^

- 0.00 £ 2 

-0.02O" 

-0.0'' <

- -0.0~LU

of suspended sediment for the Gila River station at 

the head of Safford Valley, which is located about 50 

mi (80 km) upstream from San Carlos Reservoir. The 

change in this ratio from the Safford Valley site to

FIGURE 36. Mean monthly evapotranspiration depths computed 

for 10-year periods.

the reservoir is not known, nor is it known how the 

ratio would be affected by using total sediment
N34 GILA RIVER PHREATOPHYTE PROJECT

TABLE 20. Mean monthly euapotranspiration (ET) computed from 4 periods of 10 years each, and median monthly evapotransp'.ration

for 41 years

Oct. Nov. Dec. Jan. Feb.

Mean monthly 

from 4 periods 

of 10 years 

each .. .. ........... 0.123 0.092 -0.009 0.003 0.004

Median of 

41 monthly 

values ............ .109 .071 .044 - .018 .005

Computed monthly ET 

(ft)

Mar. Apr. May June July Aug. Sept. Annual

0.062 0.168 0.212 0.274 0.239 0.166 0.176 1.510

.099 .142 .193 .300 .198 .155 .150 1.448

discharge instead of solely the suspended sediment 

discharge. Estimates of the magnitude of deposition 

in San Carlos Reservoir were made with ratios of 

summer-to-winter concentrations in the range of 1:1 

to 10:1 for the four periods prior to implementing the 

total simulation procedure. Comparison between 

estimates and measurements of deposition showed 

that results using the suggested 5:1 concentration 

ratio were only slightly better than those using a 1:1 

ratio.

A general equation relating the weight of sediment 

discharged by a stream to the stream inflow volume 

during a fixed time interval is

S=CQ , (6)

where Sis the weight of sediment discharged, Cisthe 

mean sediment concentration expressed as the 

weight of sediment per unit volume of inflow, and Q 

is the volume of stream inflow. The weight of se.diment retained by the reservoir, Sj, is equal to S times 

the trap efficiency, E. The weight of trapped sediment is therefore

When equation 7 is expanded to include seasonal 

terms for C and Q, it becomes

where Cw and Cs are the mean winter and summer 

sediment concentrations, respectively, and Qw and 

Qs are the winter and summer streamflow summations, respectively.

The ratio r, where r = CS/CW, was one of the 

variables for which an optimum value was sought in 

defining the sediment deposition equations. Substituting r into equation 8 gives

which eliminates Cs from the equation. A value of r 

was assumed for the initial trial through the simulation procedure. The value of r was adjusted in subsequent trials to improve the results from the simula-' 

tion procedure. 

E and Cw were assigned values of unity because

both are constant during each trial of the simulation 

which compares the estimated with the measured 

sediment deposition. Equation 9 was modified for 

these assigned values, and the resulting equation 

provided estimates, designated Sy, which were proportional to Si- The modified equation is

sy = +0w (10)

Each daily streamflow amount was assigned to 

the 5-foot-elevation increment (incremental storage 

reservoir) into which streamflow occurred; the proper 

incremental reservoir was determined by the lake 

stage at the end of the day. Daily streamflow data 

was summed according to water year, season of the 

year, and incremental reservoir. The proportional 

sediment weight, Sy, for each year and each incremental storage reservoir was computed by inserting 

the appropriate Qs and Q^ sums into equation 10. 

Table 21 shows a generalized chart of Sy- into a

reservoir, where i = I to n designates the increir^mtal 

storage reservoirs and j - 1 to m designates the water 

years.

In table 21, the proportional sediment weight for 

any given year is determined by summing the Syj ;

values in the column corresponding to that year. 

Similarly, a summation of Syj values for a particular row of the table is the proportional sediment 

weight for an elevation-increment of storage. The 

total Sy for a period is therefore

TABLE 21. Chart of notation used to identify time and inflow 

location of computed proportional sediment weights, £ -,- 

Water year, j

Incremental storage 

reservoir, i

3 

3 to 1i-"

1 2 m -1 m

sn,i syi,2 * * s-vi, m -i syi,m 

s s s s

vn-l,l yn -1,2 vn-l, m-l >'n-l, m

s»u syn,2 " * syn ,m-i syn , m
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N35

n m

2 s: (ID

The second phase in the sediment depositional 

process is the distribution of sediment inflow within 

a reservoir. As a stream merges with pooled reservoir 

water, water velocities decrease, resulting in deposition of sediment.

In this preliminary simulation the reservoir is 

assumed uniform in width and bottom configuration. Another assumption is that the larger sediment 

particles are deposited immediately upon entry into 

the surface-water storage pool of the reservoir.

The weight of larger sediment particles to the total 

sediment weight was defined as a. Selection of an 

optimum a was done in the simulation procedure by 

comparing estimated with measured sediment deposition. The Syij values were divided into two parts by 

use of the equation

Sy =aSy +(1.0-a)Sy. (12)

The quantity (1.0 - a)Sy is the part of Sy made up of 

the smaller (suspended) sediment particles. Some of 

this quantity of smaller particles is assumed to be 

deposited within the incremental storage reservoir 

where the stream enters the storage pool, and the 

remainder is assumed to move to lower incremental 

reservoirs and is then deposited.

The computed weight of smaller particles, (1.0 - 

a)Sy, was designated S§ to reduce equation symbolism. Ss was exponentially distributed over the distance from the point of inflow to a point where deposition is considered complete. Referring to figure 37, 

the deposition of suspended sediment was distributed from point A, the point of inflow, to point C, 

over the distance DA. Bis a point along DA, and DB 

is the distance from point B to point C. The ratio of 

the weight of suspended sediment passing point B to

(

U Z> v 

jr~ \ , where x is the 

r__. __ ___. A/

In the simulation procedure, the proportional 

weight of suspended sediment passing B is equal to

FJ 00 

/ L* ID v

Ss | TT~ I. The difference between quantities pass- (/) \DA/

ing A and B is the proportional weight of suspended 

sediment deposited between A and B, designated 

SS r>. Accordingly,

SB (13)

Suspended sediment deposited 

in reservoir

FIGURE 37. Sketch identifying terms used in simulation of 

suspended sediment distribution.

The optimum values for the distance DA and the distribution exponent x were found in the procedure 

which selected best estimates of sediment deposition.

To simulate distribution of suspended sediment 

into incremental reservoirs, DA and DB of equation 13 were redesignated DAI and DB^ respectively. The subscript i represents the incremental 

reservoir into which sediment entered the reservoir 

pool, and k represents the incremental reservoir for 

which deposition is being computed. Point A is the 

upstream limit of reservoir i, and point B is the upstream limit of reservoir k. Syj is the computed

quantity proportional to sediment weight entering 

incremental reservoir i at point A, so the suspended 

sediment fraction of Sy - is Ss -, The suspended sediment which passes into reservoir k at point B is consequently computed as

g x

*'(%) ' The sediment

passing into the next lower incremental reserved,

D

k + 1, is

Bk+l

x

)

The difference between

these two amounts, Ss^ .is proportional to the suspended sediment weight deposited in incremental 

reservoir k and is given as

The S§i total for each incremental reservoir was 

distributed by SSfc amounts into the appropriate 

incremental reservoirs by equation 14. Distribution 

of Ssi started with k-\ and continued through successively lower incremental reservoirs to either the 

lowest incremental reservoir or to the end of DA^ (at 

point C ), whichever came first. Incremental reservoirs were numbered from 1, for the upper, reservoir, 

to n, for the lowest, and SSj was distributed into all 

incremental reservoirs from i = 1 to i = n.
N36 GILA RIVER PHREATOPHYTE PROJECT

If the optimal value selected for D^- was greater

than the distance from point A to the dam, a part of 

the SSj quantity could not be distributed by equation

14. This remaining quantity was distributed uniformly over the distance from point A to the dam as 

shown in figure 38.

For San Carlos Reservoir, distribution was made 

by water year. Thus, within each of the four periods, 

the deposition was categorized by the incremental 

storage reservoir of deposition and by water year. 

For incremental reservoir k and for any water year j, 

designation of the computed amount deposited 

was

The simulating process continues with the compaction phase after the sediment is distributed and 

deposited. Lane and Koelzer (1943) presented a compaction equation to estimate unit weight -of sediments at a specified time following deposition. This 

equation requires knowing the "in place" composition of the sediment and its specific weight 1 year 

after deposition. This information was unavailable 

for San Carlos Reservoir. However, the Lane and 

Koelzer equation was applied to the reservoir using 

estimates of percentages of sand, silt, and clay 

deposited after 1 year and an estimate of the specific 

weight of the deposit. The amount of compaction estimated by this method was found to be small compared to the probable error in the simulation of sediment deposition. Also, the addition of a compaction 

equation to the simulation procedure resulted in unacceptable parameter values in the sediment inflow 

and distribution equations. For these reasons, the 

compaction of reservoir sediments was deleted from 

further computations.

With deletion of the compaction phase from the 

simulation, the results of only the sediment inflow 

and sediment distribution phases were used in making estimates of sediment deposited. The computed 

proportional weights for distributed sediments the 

amounts were summed for each incremental

reservoir and for each of the periods including the 

1929-66 period. The incremental reservoir sums are 

designated SR^ , and the total SR^ amount of a

period is SR. SR can be expressed as

n m

SR =

<<-* y ±-i T

k = lj=l

(15)

where the range of incremental reservoirs is from k=l

to k=n and the water years range from j = 1 to j = m.

The relation between simulated deposition and

Distance from inflow point to dam

istribution of sediment 

by equation 14

" Uniform distribution of remaining 

sediment after distribution of 

first part by equation 14

FIGURE 38. Sketch showing distribution of suspended sediment, 

Ss ., when distance DA was computed as greater than the distance from inflow point (A) to the dam.

measured deposition was established for each period 

by the following equation:

SM=RATIO (16)

where SM is the total volume of sediment deposited 

in the period. RATIO is a proportionality constant 

whose value is affected by E and Cw of equation 9, 

which had been set equal to unity for the computation procedure. RATIO also includes a unit conversion to relate the measured volume of SM to SR. Each 

repetition of the simulation produced a different 

value for RATIO.

The estimate of absolute volume of deposited 

sediment, Sej^, was made for each 5-foot incremental 

reservoir by multiplying the periods RATIO times 

the SRfc of the incremental reservoir:

Sek =RATIO (SRk). (17)

The value of one or more of the variables, r, a, x, or 

DA, was changed slightly for each trial of the 

simulation procedure. The optimum values of variables were naturally those which produced the best 

sediment estimates. A listing of the variables and 

optimum values are shown in table 22 for the four 

periods between capacity surveys and for the 1929- 

66 period.

TABLE 22. Optimum values of variables from simulation of the 

sediment depositional procedure

Period number 

and dates

No. 1, 1929-35 ..... ................

No. 2, 1935-37 .................... .

No. 3, 1937-47 .................... .

No. 4, 1947-66 .................... .

No. 5, 1929-66 .................... .

Variables

r

13.20

1.80

2.35

1.35

4.65

a

0.25 

.40 

.38 

.55 

.35

X

1.30 

.88 

.82 

.82 

1.60

*>A

60000ft 

40 000 ft 

72 000 ft 

16 000 ft 

56 000 ft

Symbols:

r is the seasonal ratio of summer to winter sediment concentration.

a is the percent, by weight, of total sediment load deposited when streamflow 

reaches the reservoir pool.

x is the exponent of the expression for the distribution of suspended sediment.

DA is the distance along the reservoir centerline from the point of inflow to the point 

where no sediment remains to be deposited.
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 N37

INTERPRETATION OF SIMULATION RESULTS

The range of the summer-to-winter concentrations 

(r = 1.35-13.2) in table 22 is not unexpected for the 

time periods and geographical area considered but 

may not be meaningful because varying discharge, 

velocities, and so forth, were not considered. A 

comparison of differences between the summer and 

winter suspended sediment concentrations at several southern Arizona locations shows that, for short 

periods, r can vary more than that shown in table 22. 

The period 5 data suggest that the ratio of summer-towinter concentrations for the 38-year period 1929-66 

actually approached the 5:1 value discussed on page 

32.

The variable a ranges from 0.25 to 0.55 in table 22 

and averages 0.39, indicating that about 40 percent 

of the total sediment was deposited near the entry of 

the San Carlos and Gila Rivers into the reservoir 

pool. This 40 percent probably is the approximate 

bed material discharge of the San Carlos and Gila 

Rivers.

It was assumed prior to determining an optimum 

x, that x would be equal to, or greater than, 1.0; that 

is, the rate of suspended sediment deposition would 

be at least as great at the point of inflow as in any 

other part of the reservoir. The selection of the 

optimum x did not confirm this assumption for all 

periods. The values of x determine the curvature of 

the relations in figure 39. These relations show the 

proportional distributions of sediment deposits 

along the reservoir using the optimum values of a, 

DA, and x determined by the simulation procedure.

DISTANCE DOWNSTREAM FROM POINT OF 

INFLOW, IN THOUSANDS OF METERS

DISTANCE DOWNSTREAM FROM 

POINT OF INFLOW, IN FEET

FIGURE 39. Simulated sediment deposition, in percent, 

downstream from point of inflow.

In table 22 DA. is an approximation of the average 

distance over which sediment is deposited. For 

shorter time periods DA can be considerably different because the distance through which sediment is 

transported is affected by the rate of inflow, the 

volume of water in surface storage, constrictiors in 

reservoir, and so forth. It appears from optimizing 

DA that sediment distribution sometimes occurred 

throughout the reservoir, even when surface-water 

storage was a great amount.

Sediment inflow at San Carlos Reservoir does- not 

appear to move far into the pooled water under low 

streamflow conditions, as illustrated by period 4 

results in figure 39. Inaccuracies in the storage 

capacity ratings adversely affected all measurements and estimates of sediment deposition. ^ Iso, 

sediment distribution based on mean cross-sectional 

velocities or daily streamflow volume rather thar the 

mean distance, DA, may improve the sediment 

model. These indications emphasize the need for 

more development of the sediment distribution 

model.

Figure 40A shows a comparison between the volumes of deposits measured and computed during 

period 5 for all incremental reservoirs. In figure 405 

a comparison is made between cumulative volumes 

of measured and computed sediment deposits for 

period 5. The summations were determined by progressively adding the volume of deposit in an incremental reservoir to the sum of the deposits in all 

lower incremental reservoirs.

PROCEDURE TO DEVELOP RATINGS

Reservoir surface-water storage values for the 

water budget of water years 1929, 1935, 1937, 1947, 

and 1966 were obtained from surface-water capacity 

ratings of the five surveys. Ratings for water years 

1967 through 1971 were interpolated on the basis of 

estimated volumes of sediment deposited from the 

simulation procedure by using the parameter values 

of period 5. For all other years, surface-water storage 

ratings were developed by interpolating storage 

changes between capacity surveys.

The development of a rating of surface-water 

storage for each water year was begun by inserting 

the optimum parameter values of table 22 into the 

simulation procedure. Estimates of absolute sediment volumes, Se^, were obtained for all incremental

reservoirs during the periods between surveys. rr|he 

ratio of the measured to estimated volumes, 

SMk/Sefc, was computed for each incremental reservoir of each period. Table 23 lists the 

values for period 3.
N38 GILA RIVER PHREATOPHYTE PROJECT

MEASURED VOLUME OF SEDIMENT 

DEPOSITED, IN CUBIC 

HECTOMETERS

012345678910 24 25

u 

LU

H

O

a. i_

LU[U

D LU

I-HZLU

LU (T

lo5<

LU LU

W O

LU tf)

OD

LU ^

D W

pLU 

Irf*

5

Iw

LU

£. \

20

i

-

1*1 1 *7~S^

7

6

5

4

3

2

1

_

-

-

-

 

w A

-*/

/

I 0 

0 1

I I I i

/

 /

/

/

'/ '

/

Y *

i

2345

1

*/

x

'

i

i" n ' 7\

/ I 25

/ \

/ r 24

/ ^i-

/'

/ 

- 10

- 9

- 8

- 7

\ T 6

-

-

-

\ , i \

- 5

- 4 

- 3

- 2

- 1

n

6 7 8 ' 20 21

LJ

LU

H

O

Q.

lil

D

1- W

Z Ml

UJ |_

^ LU

D 2

LU O

w H

LL

°i

1^

_J O 

0"

>zD~

UJ1-

I

H

LU

MEASURED VOLUME OF SEDIMENT

DEPOSITED, IN THOUSANDS

OF ACRE-FEET

MEASURED CUMULATIVE VOLUME OF 

SEDIMENT DEPOSITED, IN CUBIC 

HECTOMETERS

TABLE 23. Volumes of sediment deposits measured (Sj^,) and 

computed (Se ) for 1937-47, and the SM k /Sek ratios

Elevation 

increment

(ft)

below 2380 

2380-2385

2385-2390 

2390-2395 ........... 

2395-2400 

2400-2405 . .. ...

2405-2410 . ......

2410-2415 .... ..... 

2415-2420 ....... ...... . .. ..

2420-2425 . .... ..... . 

2425-2430 ... .

2430-2435 ....

2435-2440 . ... .. ....... ..

2440-2445 . .......... .

2445-2450 .. .. .. ... 

2450-2455 ............. .

2455-2460 ...... ...... ..

2465-2470 . . .. ... .. 

2470-2475 ...... ....... .. 

2475-2480 ..... .. .

Above 2485 - ....... .

(acre-ft)

5,162 

2,411 

2,139 

1,926 

1,312 

881

976

703 

. 430 

300 

128

168

590

795

189 

809

403

430

469 

107 

304 

175

0

(acre-ft)

5,160 

718 

780 

2,189 

861 

740 

695 

854 

428 

1,042 

1,586 

661 

594 

572 

462 

515 

714 

439 

418 

321 

298 

760 

0

S"/S"

1.00 

3.36 

2.74 

.88 

1.52 

1.19 

1.40 

.82 

1.01 

.29 

.0!? 

.25 

.99 

1.39 

.41 

1.57 

.56 

.98 

1.12 

.33 

1.02 

.23

MEASURED CUMULATIVE VOLUME OF 

SEDIMENT DEPOSITED, IN THOUSANDS OF ACRE-FEET

FIGURE 40. A, Comparison between the volumes of deposits 

measured and estimated for each incremental reservoir, 

1929-66. B, Comparison between volumes of estimated and 

measured deposits cumulative by 5-ft-elevation increments 

upward from lowest part of reservoir, 1929-66 water years.

The sediment volume accumulated in each incremental reservoir was also estimated in the simulation from the beginning of a period to each subsequent year in the period. These volumes are designated Sefc j. Completion of these estimates concluded

the simulation procedure, but much of the interpolative computations remained to be done. The estimates were multiplied by the appropriate incremental reservoir ratio, SMk/Sefr This adjustment 

was required so that the interpolated storage change 

from sediment deposition over a period equaled the 

measured sediment deposition. The symbol Z distinguishes an interpolated volume from the estimated volume, Se . Therefore, the volume for any 

incremental reservoir is

(18)

The annual ZkJ quantities listed in table 24 for a 

part of period 3 are the estimated losses in surfacewater storage through interpolation from the beginning of the period in 1937 to the start of the year 

shown. Subtraction of ZkJ for an incremental reservoir from the surface-water capacity at the si art of 

the period gives an adjusted capacity. Adjusted 

surface-water storage capacities for each year were 

then assembled into an elevation-capacity rating for 

the year. The elevation and corresponding adjusted 

capacity values are shown in table 25 for water years 

1938-42.

A comparison was made between a computerdeveloped rating and a rating developed by curve 

fitting to determine whether computer-developed 

ratings were acceptable for the investigation. 

Surface-water storage capacities for 1966 from table 

1 by 5-foot-elevation increments were used ir both
HYDROLOGIC HISTORY OF SAN CARLOS RESERVOIR, ARIZONA, 1929-71 I T39

TABLE 24. Estimated change in surface-water storage capacity, 

Z, from start of period 3 to 1942, by 5-ft-elevation increments

Estimated loss in surface-water storage capacity, Z, in acre-ft

Elevation Water year

(ft) 1938 1939 1940 1941 1942

Below 2380 .......... 680 1,142 1,663 2,570 3,705

2380-2385 ........ ... 210 360 636 1,008 1,688 

2385-2390 . ........ 93 173 504 1,291 1,699 

2390-2395 .............. 133 363 575 864 1,291

2395-2400 .............. 93 141 327 542 850

2400-2405 ........ ..... 88 124 146 243 547

2405-2410 ... ...... 118 239 239 369 613

2410-2415 ........ .... 113 233 233 233 440

2415-2420 . . ..... .. .. 98 184 184 184 312

2420-2425 .......... .. 47 53 53 53 114 

2425-2430 . . ....... 20 20 20 20 70

2430-2435 .... ....... 28 28 28 28 112 

2435-2440 ..... .... 171 171 171 171 445 

2440-2445 ..... .... . 208 208 208 208 588 

2445-2450 ....... .. .. . 66 66 66 66 153

2450-2455 .......... .. 222 222 222 222 555 

2455-2460 ............... 0 0 0 0 216

2460-2465 ....... ... 0 0 0 0 228 

2465-2470 .... .... 0 0 0 0 329 

2470-2475 ............. 0 0 0 0 67

2475-2480 ...... ..0 0 0 0 237 

2480-2485 ....... 0 0 0 0 73 

Above 2485 . ..... ... 0 0 0 0 0

TABLE 25. Computed capacity ratings of surface-water storage 

used for 1938 through 1942

FWatinn Cumulative surface-water storage capacity in acre-ft by water year

(ft) 1938 1939 1940 1941 1942

2380 ......... 6,759 6,297 5,776 4,869 3,734

2385 ........... .. 12,253 11,641 10,844 9,565 7,750

OQQrt 1 Q 9Q1 1 ft ^QQ 1 1 A \ 1 1 ^ QX^ 1 Q 1 99

2395 ...... . .. .. 27,602 26,680 25,340 22,985 20,335

2405 ............ 49,835 48,829 47,281 44,614 41,352 

2410 ........... 64,168 63,041 61,493 58,696 55,190

OXOrt QQ 9£\1 Qfi Q1 Q Q£» Q'Trt QO E\TZ QQ 7Q9

2425 .......... 118,813 117,474 115,926 113,129 109,227 

2430 ...... .. . .. 142,624 141,285 139,737 136,940 132,988

2435 .............. 169,414 168,075 166,527 163,730 159,694 

2440 ........ . 199,089 197,750 196,202 193,405 189,095 

2445 ....... 231,850 230,511 228,963 226,166 221,476 

2450 ............ 267,841 266,502 264,954 262,157 257,380

2455 ...... ... 306,799 305,460 303,912 301,115 296,005

2460 ............ 349,359 348,020 346,472 343,675 338,349

(JACK QQft ft7l QQ/1 7Q9 QQQ 1 AA 3CU1 QSf7 QQ>1 QQQ

2470 ... ..... 446,872 445,533 443,985 441,188 435,305

2475 ............ 501,110 499,771 498,223 495,426 489,476

2480 .... .. 558,819 557,480 555,932 553,135 546,948 

2485 .. .. . ... . 620,285 618,947 617,398 614,601 608,341 

2490 ...... .... 685,785 684,446 682,898 680,101 673,841 

2495 .......... 755,274 754,035 752,487 749,690 743,430 

2500 .. .. . .... 829,288 827,949 826,401 823,604 817,344 

2505 . .. . ...... 907,695 906,356 904,808 902,911 895,751 

2510 ... .. . 990,544 989,205 987,657 984,860 978,600 

2511 .... ...... 1,007,672 1,006,333 1,004,785 1,001,988 995,728

2515 ........... 1,078,079 1,076,740 1,075,192 1,072,395 1,066,135

ratings. Attention was given to the rate of change in 

capacity with respect to 0.1-ft (0.03 m)-elevation 

changes in the curve-fitted rating. The computer 

rating was developed to 0.1-ft (0.03 m)-elevation 

intervals by use of a constant elevation-capacity 

ratio over each 5-foot-elevation increment. A sample 

of these ratings is given in table 26. The largest 

volumetric difference between ratings was 763 acreft (0.94 hm3 ) at 2,462.5 ft (751 m) elevation, which 

is 0.063 percent of the total surface-water storage 

capacity. The mean deviation of the 0.1 ft (0.03 m)

TABLE 26. Comparison of segments of surface-water storage 

capacity ratings made by curve fitting and by computer

Surface-water storage capacity, 

in acre-ft Difference

Elevation between rat-rigs, 

(ft) Developed by Computer acre-ft 

curve fitting developed

2410.0 ...................... .. ........ 33,308 33,308 0

.1 ............. .. .. .. ...... .... 33,563 33,589 26

.2 .... ........ ..... ...... 33,819 33,869 50

.3 ................. . .. .. ........ 34,076 34,150 74

.4 . . ..... .. .. .. .. .... ..... 34,334 34,431 97

.5 . ................. .. .. . ...... 34,593 34,711 118

.6 ...... ........ .... .. .. . ... 34,854 34,992 138

.7 ......................... 35,116 35,273 157

.8 . .................... .... ...... 35,379 35,554 175

.9 .................. .. .......... 35,643 35,834 191

.1 ........ ....... .... .. ........ 36,175 36,396 221

.2 . ................. .. .. ........ 36,443 36,676 233

.3 ......... ...... .. .. .. ....... 36,712 36,957 245

.4 . . .................. .. .. . ... 36,982 37,238 256

.5 ..... ................ .. ........ 37,253 37,518 265

.6 ......................... .. .... 37,526 37,799 273

.7 ..... .......... . .. .. ........ 37,800 38,080 280

.8 ..... ................ .. ........ 38,075 38,361 286

.9 ...................... . .. ..... 38,351 38,641 290

2412.0 .................. ... .. .. ..... 38,628 38,922 294

.1 .......... ..... . ...... .. ..... 38,906 39,203 297

.2 .... ..... .. .. ........ .. .. 39,185 39,483 298

.3 ................ . ... .. ....... 39,465 39,764 299 

.4 ....... ................. .. .. .. 39,746 40,045 299

.5 ............. ...... .... .. .. 40,028 40,325 297

.6 . .. .... ............... .. .. .. 40,310 40,606 296

.7 ........................... 40,593 40,887 294

.8 .............................. 40,877 41,168 291 

.9 . ..... ...... .... . .. .. .. 41,162 41,448 286

2413.0 .. ............... .... .... .. . 41,448 41,729 281

.1 ................ ....... .. .. 41,734 42,010 276

.2 ... ............. ........ .. .. 42,021 42,290 269

.3 .. .. ...... .............. .. .. 42,309 42,571 262

.4 ...... ...... ....... ...... .. .. 42,598 42,852 254

.5 ......................... 42,888 43,132 244

.6 .. .. ............ ........... .. 43,179 43,413 234

.7 .. ... ...... ..... ........ .. .. 43,471 43,694 223

.8 ........................... 43,764 43,975 211

.9 ............. ... . .. .. .. .. .. 44,058 44,255 197

2414.0 . ...... .......... ..... .. .. . 44,353 44,536 183

.1 ............................ 44,648 44,817 169

.2 .. .................. ........ .. 44,944 45,097 153

.3 . .. .. ........ ..... ... .. 45,241 45,378 137

.4 .. .. . ..... . ............. 45,539 45,659 120

.5 ... .......... ...... .. .. . 45,838 45,939 101

.6 .. .. ....... ... . ........... .. 46,137 46,220 83

.7 .. .. .... .......... . ... . 46,437 46,501 64

.8 .. . ........... .... ....... 46,738 46,782 44

.9 .. .. . ... . ..... .... .. 47,040 47,062 22

2415.0 .. ...... ..... ....... . .. 47,343 47,343 0

differences between ratings was 0.23 percent. F *om 

these comparisons it was concluded that use of either 

rating was acceptable.

SUMMARY AND CONCLUSIONS

Existing records of the hydrology of many reservoirs can be used to make an inexpensive evaluation 

of reservoir performance. The records may provide 

sufficient data to estimate available water in bank 

storage and to determine water losses by evaporation and transpiration within the reservoir boundary. It is frequently possible to investigate changes 

in storage capacity and in the volume of sediment 

deposition by use of existing capacity-survey ir formation. 

At San Carlos Reservoir, the water loss by evapo-
N40 GILA RIVER PHREATOPHYTE PROJECT

transpiration (ET) was greatest during and soon 

after large water-level recessions when considerable 

surface areas of the reservoir were uncovered. During these periods the potential for ET was high 

because the exposed soils had a high water content. 

ET computed by the reservoir water budget averaged 26,230 acre-ft (32.3 hm3 ) per year or 11.3 percent 

of the total reservoir outflow. Mean annual depth of 

ET from the exposed area was computed as 1.47 ft 

(0.448 m) in the water budget. When precipitation on 

the exposed surface of the reservoir was included, ET 

depth was 2.62 ft (0.800 m) annually. The changes in 

ET caused by the increasing amount of vegetation 

during the period of record were not readily apparent 

from the existing San Carlos Reservoir data.

Evaporation from the water surface of the lake 

averaged 24,611 acre-ft (30.3 hm3 ) per year and 

accounted for 10.5 percent of the total outflow from 

the reservoir. During the period 1964-71 when evaporation was computed by energy-budget and masstransfer methods, the computed pan coefficient was 

0.80.

Sediment deposition in the reservoir from 1929 to 

1966 totaled 96,719 - cre-ft (119 hm3) and averaged 

2,553 acre-ft (3.1 hm3 ) per year. The mean volume of 

sediment deposited was 1.2 percent of streamflow for 

this 38-year period.

Usable water in bank storage was computed by 

water budgets during winter periods when ET was 

considered minimal and changes in surface-water 

storage were large. Usable bank storage was found 

to be about 14 percent of total usable storage at the 

elevation of maximum storage capacity. At lower 

reservoir elevations, this percentage was even 

greater. Ratings were developed relating usable 

bank storage to stage.

The simulation of the sediment depositional process in a reservoir was developed primarily to aid in 

interpolating capacity changes between capacity 

surveys. The simulation was successful for the purposes of this study. For applications in which the 

primary objectives are to predict location and 

amounts of sediment deposited and to estimate compaction, the simulation model needs further testing 

and modification.

REFERENCES CITED

Anderson, E. R., 1954, Energy-budget studies, in Water-loss investigations Lake Hefner studies, technical report: U.f. Geol.

Survey Prof. Paper 269, p. 71-119. 

Burkham, D. E., 1970, Precipitation, streamflow, and majo-floods

at selected sites in the Gila River drainage basin above

Coolidge Dam, Arizona: U.S. Geol. Survey Prof. Paper 655-B,

33 p. 

 1972, Channel changes of the Gila River in Safford Valley,

Arizona, 1846-1970: U.S. Geol. Survey Prof. Paper 655-G,

24 p. 

-1976, Flow from small watersheds adjacent to the study

reach of the Gila River Phreatophyte Project: U.S. Geol. 

Survey Prof. Paper 655-1, 19 p.

Culler, R. C., and others, 1970, Objectives, methods, and environment Gila River Phreatophyte Project, Graham County, 

Arizona: U.S. Geol. Survey Prof. Paper 655-A, 25 p.

Davis, A. P., 1897, Irrigation near Phoenix, Arizona: U.S. Geol. 

Survey Water-Supply Paper 2, 98 p.

Gottschalk, L. C., 1964, Reservoir sedimentation, in Chow, Ven Te, 

Handbook of applied hydrology: New York, McGr^w-Hill, 

p. 17-1 to 17-34.

Gila Water Commissioner, issued annually, Distribution of waters 

of the Gila River.

Hanson, R. L., 1972, Subsurface hydraulics in the area of the Gila 

River Phreatophyte Project, Graham County, Arizona: U.S. 

Geol. Survey Prof. Paper 655-F, 27 p.

Hanson, R. L., Kipple, F. P., and Culler, R. C., 1972, Changing 

the consumptive use on the Gila River flood plain, southeastern Arizona, in Age of changing priorities for land and 

water: Am. Soc. Civil Engineers, Irrigation and Drainage 

Div. Specialty Conf., p. 309-330.

Lane, E. W., and Koelzer, V. A., 1943, A study of methods used in 

the measurement and analysis of sediment loads in streams: 

U.S. Army Corps Engineers rept. 9.

Marciano, J. J., and Harbeck, G. E., Jr., 1954, Mass-transfer 

studies, in Water-loss investigations Lake Hefner studies, 

technical report: U.S. Geol. Survey Prof. Paper 269, p 46-70.

National Weather Service, issued annually, Climatological data, 

Arizona: U. S. Dept. Commerce.

Simons, W. D., and Rorabaugh, M. L, 1971, Hydrology of Hungry 

Horse Reservoir, northwestern Montana: U.S. Geol. Survey 

Prof. Paper 682, 66 p.

Thorp, E. M., and Brown, C. B., 1951, Sedimentation in San Carlos 

Reservoir, Gila River, Arizona: Soil Conserv. Service Tech. 

Paper 91, 26 p.

Turner, R. M., 1974, Quantitative and historical evidence of vegetative changes along the upper Gila River, Arizona: U.S. 

Geol. Survey Prof. Paper 655-H, 19 p.

U.S. Army Corps of Engineers, 1914, San Carlos Irrigation Project, Arizona: U.S. 63d Cong., 2d sess., H. Doc. 791, 168 p.

U.S. District Court vs. Gila Valley Irrigation District, et al., 1935, 

Globe Equity No. 59: Decree entered June 29, 1935.

U.S. Geological Survey, issued annually, Water resources data for 

Arizona Part 1, Surface water records: U.S. Geol. Survey 

open-file reports.

 1954, Compilation of records of surface waters of the United 

States through September 1950; Part 9-Colorado Rive" basin: 

U.S. Geol. Survey Water-Supply Paper 1313, 733 p.

* U.S. GOVERNMENT PRINTING OFFICE: 1977 - 789-025/7^