GEOLOGICAL SURVEY CIRCULAR 346 

FIRST FOURTEEN YEARS 

OF LAKE MEAD . 

Based on the report of the Lake Mead Comprehensive 

Survey of 1948--49 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Douglas McKay, Secretary 

GEOLOGICAL SURVEY 

W. E. Wrather, Director 

GEOWGICAL SURVEY CIRCULAR 346 

FIRST FOURTEEN YEARS OF LAKE MEAD 

By Harold E. Thomas 

Based on the report of the Lake Mead Comprehensive 

Survey of 1948-49 

Washington, D. C., 1954 

Free on application to the Geological Survey, Washington 25, D. C. 
PREFACE 

An intensive investigation of Lake Mead, the reservoir formed by Hoover Dam on the Colorado River, 

was begun in January 1948 and completed late in 1950. 

The investigation was conducted under the general direction of a project staff composed of representatives 

of the three principal participating agencies: William 

0. Smith, Geological Survey; Carl P. Vetter, Bureau 

of Reclamation; and George B. Cummings, Bureau of 

Ships, Navy Department. The technical report on this 

investigation, which is being prepared by the Geological Survey, presents the analytical data and conclusions in detail. An advance draft was reproduced by 

the Bureau of Reclamation for official use and review 

by the cooperating agencies and others concerned. 

The comprehensive survey of Lake Mead required 

the talents of a diverse group of specialists and effective coordination of their individual efforts. These men 

collected data pertaining to their specialties, and analyzed and interpreted those data with the aid of other 

data collected during the survey, as well as information obtained in earlier work in this part of the Colorado River basin. The technical report will be comprised of contributions by these men--27 authors in 

ali-in which the data of the comprehensive survey will 

be described, analyzed, and interpreted. 

This circular constitutes a summary of the principal 

findings of the comprehensive survey; details of procedures and equipment used have been omitted. The 

author took no part in the field work of the comprehensive survey, and his chief involvement has been as 

technical editor of the advance report. The ideas and 

conclusions expressed in the circular are those of the 

authors of the pertinent sections of the technical report, but the author of the circular accepts responsibility for any errors of judgment in quoting or otherwise summarizing the parent material. 

III 
CONTENTS 

Page 

Abstract..................................................... 1 

Environment of Lake Mead .......................... .. 

Geologic setting of Lake Mead, by C. R. 

Longwell.............................................. 1 

Tributary drainage basin, by H. E. Thomas.. 3 

Scope and program of the comprehensive 

survey.................................................. 3 

Geodetic surveys...... .. . . . . . . . . . . . . . .. . . . . .. . ... ... . .. . . 5 

Precise leveling, by E. J. Parkin............... 5 

Interpretation of the leveling data, by C. R. 

Longwell.............................................. 5 

Hydrographic surveys................................... 5 

Lake and Lower Granite Gorge portions of 

the reservoir, by Gunnar Liefson and 

F. C. Ames.......................................... 5 

Method of hydrographic surveying of the 

lake, by Gunnar Leifson.. ... .... ... .. .. .. .. .. .. .. 6 

Horizontal control, by R. M. Wilson............ 6 

Vertical control, by F. C. Ames................. 6 

Evaluation of the echo sounding, by G. B. 

Cummings............................................ 6 

Summary of results, by F. C. Ames............ 6 

Survey of the LowerGranite Gorge, by 

L. C. Pampel.. ... . .. ... .. ... .... ......... ... .... ... 7 

Reservoir storage........................................ 7 

Significance of area, capacity, and sediment tables, by J. W. Stanley.................. 7 

Computations and tables, by J. L. Speert, 

F. C. Ames, F. W. Kennon, and W. B. 

Langbein.............................................. 7 

Page 

Reservoir storage-Continued 

Water budget, by W. B. Langbein................. 8 

Limnology.................................... . . . . . . . . . . .. .. . 10 

Physical and chemical characteristics of 

the inflowing water, by C. S. Howard......... 10 

Density currents, by C. S. Howard............... 13 

Chemical limnology, by C. S. Howard........... 13 

Physical limnology, by E. R. Anderson 

and D. W. Pritchard............................... 15 

Sedimentology.............................................. 15 

Methods of investigation, by H. R. 

Gould.................................................... 15 

Characteristics of the accumulated sediment, by H. R. Gould.............................. 17 

Bacteriology and biochemistry of the sediments, byF. D. Sisler ............................ 18 

Amount of sediment, by H. R. Gould............. 20 

Transportation of sediment by turbidity 

currents, by H. R. Gould......................... 20 

Sedimentation in relation to reservoir 

utilization, by H. R. Gould....................... 21 

Life of the reservoir, by H. E. Thomas, 

H. R. Gould, and W. B. Langbein............. 23 

Projection of observed rates of sediment 

accumulation .......................... ,................ 23 

"Normal" rate of sediment accumulation........ 23 

Problems for future study.............................. 26 

Literature cited............................................ 26 

ILLUSTRATIONS 

Page 

Plate 1. Annual profiles of the delta along Colorado River channel through Lake Mead .......................... Facing 18 

Figure 1. Map of Colorado River basin, showing location of Lake Mead ................................................... ,. vi 

2. Sediment carried by Colorado River in water years 1931-50.................. .................................... 2 

3. Contours showing changes in altitude, 1935-50............................................... ......................... 4 

4. Stage and usable storage of Lake Mead, 1935--48...................................................................... 9 

5. Relation of annual runoff to sediment load at Grand Canyon........................................................ 11 

6. Periods of reported density currents and elevation of top of sediment near Hoove1· Dam, 

1935-50.................................................................................................................... .. ... 12 

7. Typical seasonal salinity distribution and circulation patterns.................................................... 16 

8. Specific weight of sediment in the Colorado delta..................................................................... 19 

9. Actuarial projections for Lake Mead...................................................................................... 25 

TABLES 

Table 1. Comparative summary, 1935 and 1948-49 ............................................................................... . 

2. Sediment in individual basins of Lake Mead ............................................................................. . 

v 

Page 

8 

20 
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Figure 1. --Map of Colorado River basin, showing location of Lake Mead. 
FIRST FOURTEEN YEARS OF LAKE MEAD 

By Harold E. Thomas 

ABSTRACT 

This circular summarizes the results of recent 

studies of Lake Mead and its environs. Area-capacity 

tables, prepared on the basis of a hydrographic survey 

of the lake in 1948-49, show that the capacity of the 

reservoir was reduced 4. 9 percent during the first 14 

years after Hoover Dam was completed, but the usable 

capacity was reduced only 3. 2 percent. Practically all 

of this reduction was caused by accumulation of sediment in the reservoir. Studies of inflow and outflow 

indicate that the reservoir has a total storage capacity 

about 12 percent greater than that shown by the areacapacity table, because of "bank" storage, or groundwater storage in the bottom and sides of the reservoir. 

Thus the total capacity in 1949 was greater than the 

quantity shown by the original area-capacity table,. 

even though large quantities of sediment had been deposited in the reservoir during the 14 years. 

According to computations of the volume and weight 

of the accumulated sediment, about 2, 000 million tons 

were deposited in the reservoir by the Colorado River 

in 14 years; this is within 2 percent of the amount calculated from measurements of the suspended sediment 

carried by the inflowing rivers. It is estimated that the 

sediment capacity of the reservoir, when filled to the 

level of the permanent spillway crest, is about 75, 000 

million tons. The sediment contributed by the Colorado River averages about 45 percent sand and 55 percent silt and clay. 

If the sediment carried by the river in the years 

1926-50 represents the long-term average rate of accumulation in Lake Mead, it will be a century before 

the sediment at the dam reaches the level of the lowest 

gates in the intake towers, and more than 4 centuries 

before the reservoir is filled with sediment to the level 

of the permanent spillway crest. The rate of sedimentation since the first year of Lake Mead (1935) has been 

about 20 percent lower, and if that rate continues in 

the future, the life of the reservoir will be correspondingly greater. Construction of upstream reservoirs to 

capture some of the inflowing sediment, or transportation of sediment in the outflow through Hoover Dam, 

would also increase the life of the reservoir. 

In the first 12 years of Lake Mead, the dissolved 

mineral matter in the outflowing water was significantly greater than the average in the inflowing water, owing in part to solution of gypsum and rock salt from the 

bed of the reservoir. Currently the increased dissolved solids in the outflowing water can be accounted 

for almost entirely by evaporation from the reservoir, 

which is about 5 to 7 percent of the annual inflow. The 

water from Lake Mead is habitually of better quality 

than that diverted from the river for irrigation prior 

to regulation by Hoover Dam, because it represents an 

average of the poor water of low stages and the excellent water from melting snow. 

Geodetic surveys of the Lake Mead area show that the 

weight of water has caused subsidence of the earth's 

crust amounting to about 120 millimeter at Hoover 

Dam, and an even greater amount in the principal area 

of storage in the reservoir. 

ENVIRONMENT OF LAKE MEAD 

Lake Mead is the reservoir formed by Hoover Dam 

on the Colorado River, and it covers parts of southeastern Nevada and northwestern Arizona (fig. 1). The lake 

is in the Lower Basin of Colorado River, as defined by 

the Colorado River Compact of 1922 1/. Hoover Dam 

is about 355 river-miles downstream from Lees Ferry, 

Ariz., which marks the lower limit of the Upper Basin 

of Colorado River. 

Hoover Dam provides the regulation necessary for effective operation of the downstream reservoirs created 

by Davis Dam, Parker Dam, and Imperial Dam. Lake 

Mead also provides practically all the water that is developed from the river in Lower Basin, for there is 

negligible inflow from Gila River, Bill Williams River, 

and other tributaries that enter the Colorado River below Hoover Dam. 

Geologic Setting of Lake Mead, by C. R. Longwell 

From the mouth of the Grand Canyon the Grand Wash 

Cliffs extend far northward and southeastward, marking 

the abrupt western edge of the Colorado Plateaus. Between the cliffs and Hoover Dam there are mountain 

ridges that trend generally north, and intervening wide 

valleys. The Colorado River has cut deep canyons 

1/ The agreement concerning the apportionment of 

the use of the waters of the Colorado River System dated 

November 24, 1922, executed at Santa Fe, N. Mex •• by 

Commissioners for the States of Arizona, California, 

Colorado, Nevada, New Mexico, Utah, and Wyoming, 

approved by Herbert Hoover, representative of the 

United States of America, and proclaimed effective by 

the President of the United States of America, June 25, 

1929. 
r.n 5 Entire column represents suspended load 

~ measured in Colorado River at Grand 

~ Canyon, Arizona (1943 estimated) 0300~--------------~-----------------------------------------~ 

r.n Part of column below dashed line is Upper 

Z Basin contribution as measured at Lees 

9 Ferry, Arizona (1931- 33; 1948- 50) ~ d Data from Geological Survey Water- Supply 

~ 250 Papers 

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SUSPENDED LOAD IN 

MAJOR TRIBUTARIES 

Little Colorado • River at Cameron, 

Arizona (beginning 1949) 

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San Juan River near Bluff, Utah 

~ Green River at Green River, Utah 

Colorado River ~ near Cisco, Utah 

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1931 1935 1940 

Figure 2. --Sediment carried by Colorado River in water years 1931-50. 

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SCOPE AND PROGRAM OF THE COMPREHENSIVE SURVEY 3 

through the mountain ridges, whereas in the adjacent 

basins the river valley is comparatively wide and low 

walled. Accordingly, the main body of Lake Mead consists of alternating narrow and wide segments. 

The rocks of the highlands are in general older and 

more resistant than those flooring the basins. These 

older rocks are well exposed in the canyon sections of 

Lake Mead. The younger basin deposits are in general 

inconspicuous, since they are either covered by the 

lake water or form slopes and low hills bordering the 

lakeshore. 

The most extensive of the sedimentary beds in the 

basins are those of the Muddy Creek formation, chiefly 

sand, silt, and clay in thin regular layers that suggest 

deposition in shallow lake water. The formation includes beds of gypsum and rock salt, and these soluble 

salts crop out in the floor of the reservoir in extensive 

areas. 

The Colorado River was dammed during the Pleistocene epoch and formed a lake that was deeper and more 

extensive than Lake Mead. The location of this prehistoric dam-or other cause of ponding-is not known, 

but Longwell's description of the Chemehuevi formation indicates a pattern of delta deposition similar to 

that which has begun since the completion of Hoover 

Dam. The lower part of the formation consists chiefly 

of banded clay, presumably deposited as bottomset 

beds in a deep lake. This clay is thin in Iceberg Canyon and thicker downstream; extensive remnants occur 

near Davis Dam, about 60 miles below Hoover Dam. 

Sand overlies the clay, and comprises most of the 

thickness of the formation in upstream areas. The upper part of the formation consists of river gravel and 

cobbles, presumably deposited after the lake was 

filled. Some of these gravels are about 300 feet above 

the high-water level of Lake Mead. The cobbles in the 

upper part of the Chemehuevi formation were transported by a larger and more powerful stream than the 

present Colorado River, which apparently carries 

nothing coarser than fine gravel into the deltaic fill in 

the Lower Granite Gorge. 

Tributary Drainage Basin, by H. E. Thomas 

The water and sediment that accumulate in Lake 

Mead come from an area of about 168, 000 square 

miles, or 5 percent of the United States. This tributary basin includes some areas that have very high 

rates of runoff, and other areas that yield very little 

water to the Colorado River; in certain areas the 

streams are generally clear, whereas other areas 

contribute much sediment to Lake Mead. 

Most of the runoff is derived from melting of snow 

in the higher mountains, but considerable quantities 

of water move for at least a short distance through the 

soil or underlying ground-water zones before reaching 

the river. Most of the dissolved material in the river 

is derived from ground water rising as springs and 

seeps along the main stem and tributary channels, and 

return flow of irrigation water. 

The geologic formations cropping out within the 

drainage basin are also the ultimate sources of the 

sediment carried by the Colorado River and its tributaries. Only an insignificant part of the sediment, 

however, comes directly from rock outcrops. The 

main stream and tributaries rarely come in contact 

with bedrock except in their headwaters, for generally, 

the bed and banks of the larger channels are formed of 

stream-borne sediment, or alluvium. These alluvial 

deposits constitute the immediate source of much of the 

sediment now moving toward Lake Mead. The bulk of 

Lake Mead sediment comes from the arid parts of the 

drainage basin in New Mexico, southern Utah, Arizona, 

and Nevada. 

Long-term records of suspended sediment are available at four stations in the tributary drainage basin. 

These records indicate that the sediment transported 

during the water year 1948 was close to the average annual load in the period 1930-48. Probably more than 

95 percent of the sediment entering Lake Mead passes 

through the Grand Canyon. The annual·variations in 

suspended load at Grand Canyon and the proportions of 

the load that pass the three upstream stations are depicted in figure 2. 

In the period of record at Grand Canyon, the year of 

highest runoff (1929) was also the year of greatest suspended sediment load, and the year of least runoff 

(1934) was the year of least sediment transportation. 

There are striking variations in the proportions of the 

load that pass the three upstream stations from year 

to year, although it is generally true that the San Juan 

River carries less water and more sediment than either 

the Green River or the Colorado River above its junction with the Green River. 

SCOPE AND PROGRAM OF THE COMPREHENSIVE 

SURVEY 

The Lake Mead survey was essentially a reservoir 

survey needed primarily for operational purposes, but 

its scope far surpassed that of other reservoir surveys 

undertaken in the past. The Office of River Control,. 

of the Bureau of Reclamation, is responsible for operation and constru-ction for physical control of the Colorado River in Lower Basin, and is authorized by Congress 

to sponsor investigations that will provide the data 

needed for these activities. Although the need for surveys of sedimentation and salinity was recognized before the lake first filled to capacity in 1941, no definite 

plans could be made until World War II had ended. 

The basic requirements for surveys of sediment and 

salinity developed into the comprehensive survey. 

Thus, a hydrographic survey was undertaken to provide 

new area-capacity tables and also to derive, by difference from the pre-Lake Mead data, the volume of accumulated sediment. The characteristics of the sediment and the processes by which it was transported 

and deposited were also investigated, and some of these 

characteristics led to intensive studies by the Scripps 

Institution of Oceanography of the biochemistry and 

bacteriology of sediments near Hoover Dam. Data concerning the salinity were obtained largely during monthly cruises under the direction of the Navy Electronics 

Laboratory, and the circulation in the lake was established from the salinity distribution. A preliminary 

energy budget was also developed, and from it an estimate of evaporation was obtained. Precise leveling 

by the Coast and Geodetic S,urvey provided the basis 

for estimating the deformation of the earth's crust 

caused by storage in Lake Mead. 
4 

'P 

Miles 

Contours showing chonqes in altitude 

1935-1950 

Contours in Millimeters 

N1pton 

! 

FIRST 14 YEARS OF LAKE MEAD 

Cone Springs 

Figure 3. -Contours showing changes in altitude, 1935-50. 

Kingman 

Beaverdam 

Creek 

0 

0 

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Hockberr 
HYDROGRAPHIC SURVEYS 5 

GEODETIC SURVEYS 

Precise Leveling, by E. J. Parkin 

The precise leveling in the Lake Mead area was done 

by the Coast and Geodetic Survey for the purpose of 

measuring the deformation of the earth's crust resulting from the load of impounded water. The Hoover 

Dam level net was established in 1935, and levels were 

first run in March and April of that year, when the 

reservoir had just begun to fill. The elevations therefore represent the conditions at the time of nearly minimum load. The level net was rerun in 1940-41, when 

stor.age in the reservoir was about 85 percent of capacity. The latest leveling was in 1949-50; the reservoir 

storage at that time was less at all times than during 

the 1940-41 leveling. For determination of the effect 

of reservoir load upon the earth's crust, a special adjustment was made of the data obtained in each period 

of leveling, holding fixed the elevation of only one 

bench mark. All three sets of elevations derived by 

this special adjustment are based on the same elevation 

of bench mark R1 at Cane Springs, Nev. 

Interpretation of the Leveling Data, by C. R. Longwell 

The changes in elevation during the intervals between 

leveling are shown graphically in figure 3. A conspicuous feature is a broad basinlike depression centering 

in the vicinity of Boulder Canyon. The general form of 

this depression remained unchanged from 1940-41 to 

1949-50, although the area as a whole continued to subside during that interval, and a pronounced regional 

tilt extended apparently beyond the southern limit of the 

level net. During the interval 1935-41 the general tilt 

was southeastward. Between 1941 and 1950 the total 

area affected by subsidence was considerably enlarged, 

with overall tilting toward the southwest. The basin 

centering near Boulder Canyon was retained but not appreciably deepened. 

The basinlike subsidence centering about Boulder 

Canyon appears to have resulted from storage of water 

in Lake Mead. More than 60 percent of this water, as 

much as 25 billion tons, is stored in the wide Virgin 

and Boulder Basins that lie directly east and west of 

Boulder Canyon. The total area of these basins is 

about 85 square miles, and the load added to this area 

averages more than 10 tons per square foot. Since the 

two long arms of the lake diverge at a large angle from 

the Virgin Basin east of Boulder Canyon, the center of 

gravity of the lake lies in the vicinity of this basin in 

a very real sense. The extensive tilting subsidence, 

involving the entire lake basin and extending at least 

tens of miles farther south, is probably connected with 

a regional movement that may have. been in progress 

before the dam was built. 

With reference to bench mark R1 at Cane Springs, 

Nev. , the ten bench marks on Hoover Dam dropped an 

average of 60 millimeters between 1935 and 1941, and 

62 millimeters farther between 1941 and 1950, a total 

subsidence of 0. 4 foot. If there had been no change in 

the elevation of the reservoir bottom, this lowering of 

the dam would have had the effect of reducing the capacity of the reservoir at spillway level by approximately 60, 000 acre-feet. However, subsidence has 

exceeded 120 millimeters (the amount of subsidence 

at the dam) in an area that comprises more than 60 

percent of the total area of the reservoir; nearly 75 

percent of the water is stored within this area when 

Lake Mead is filled to spillway level, and the proportion increases with decreasing lake stage. Thus the 

change in overall capacity due to subsidence is doubtless of very small magnitude. 

Since 1941 the dam has been lowered more than any 

of the six locations where the level net provides data 

as to elevations near the shore of the reservoir. The 

leveling of 1941 occurred while the reservoir was filled 

nearly to capacity, and the differential rise at these 

six locations by 1950 may be an indication of elastic 

rebound with reduced load. 

HYDROGRAPHIC SURVEYS 

Lake and Lower Granite Gorge Portions of the Reservoir, by Gunnar Lief son and F. C. Ames 

The hydrographic survey of Lake Mead was undertaken to obtain the data necessary for determination of 

the volume and distribution of sediment, and the occurrence and extent of other changes in reservoir 

boundaries; and for the preparation of new reser:voir 

area and capacity tables. 

Lake Mead is filled to its capacity for controlled 

storage when it reaches the level of the spillway gates 

in raised position, 1, 221.4 feet above sea level. The 

highest lake level yet reached was 1, 220.4 feet on July 

29 and 30, 1941, at which time the lake formed a level 

pool extending 120 miles upstream from Hoover Dam 

to the Bridge Canyon rapids in the Lower Granite Gorge 

of Grand Canyon. At lower lake stages the river flows 

into the lake farther and farther downstream in the 

Lower Granite Gorge and in Pierce BaE?in. River rather 

than lake conditions prevail in the Lower Granite Gorge 

the greater part of the year. Within the Lower Granite 

Gorge there is an abrupt change at the foot of the Bridge 

Canyon rapids from a relatively steep, canyon..,type 

stream, to a stream with a typically wide, shallow 

cross section and meandering alinernent. Near Bridge 

Canyon these meanders are still controlled largely by 

the canyon alinement; but in the lower reaches the lowwater channel, 200 to 400 feet wide, meanders across 

a sediment "flood plain" as much as 2, 200 feet wide. 

The water in the canyon section is always sediment 

laden. The lower 65 miles of the reservoir (80 rivermiles) between Pierce Ferry and Hoover Darn is essentially a chain or series of relatively wide basins, connected by short, narrow canyon sections. In this part 

of the reservoir, lake conditions exist at all stages, 

with depths ranging up to 450 feet and widths ranging 

from a minimum of 650 feet in Boulder Canyon to about 

5 miles in the basin areas. The water is generally 

clear and the only velocities are the very low ones resulting from circulation of water in the lake and those 

induced by density-current action (p. 13, 20). 

The lake is divided naturally into two parts, each 

requiring different surveying techniques and different 

types of equipment. In the Lower Granite Gorge the 

methods were essentially those of land surveying, with 

soundings across the river channel to complete the 

valley cross section. In the main part of the lake the 

methods were those of deep-water hydrographic surveying. 
6 FIRST 14 YEARS OF LAKE MEAD 

Method of Hydrographic Surveying of the Lake, 

by Gunnar Leifson 

In the hydrographic survey of the lake, depths were 

measured by automatic recording echo sounders, and 

the position of the sounding boat was fixed at regular 

intervals along the sounding line. Lake Mead provided 

an exceptional opportunity for comparison of hydrographic with topographic surveys. The echo-sounding 

profile and the corresponding cross section derived 

from topographic maps that had been prepared prior to 

the filling of the lake are in close agreement. 

Horizontal Control, by R. M. Wilson 

The triangulation during 1947-48 had several purposes: first, to recover triangulation stations set by 

Fairchild Aerial Surveys for control of the 1935 topographic maps made for the Soil Conservation Service 

(U. S. Department of Agriculture, 1935} and to test the 

precision of their coordinates; second, to tie in points 

of other previous surveys wherever possible, so that 

all these surveys might be coordinated on a common 

datum; and third, to establish convenient reference 

points for the 1948-49 hydrographic survey and to mark 

them permanently so that future surveys can be referred to the same points. 

Altogether, 336 new reference points and triangulation stations were established, most of them just above 

the high-water mark of the reservoir. About 35 navigation lights or other objects were located by intersection, 15 stations of the Coast and Geodetic Survey were 

recovered and occupied, and 33 stations established in 

previous surveys were tied in. The position closures 

involved in all these ties indicate excellent work in the 

earlier triangulation in this area. Analysis discloses 

that the triangulation of 1935 is dependable and accurate, and entirely adequate for the purposes of this investigation, some of which required a higher order of 

accuracy than that required for use in making the maps 

under the 1935 contract. 

Vertical Control, by F. C. Ames 

Vertical control was required in the hydrographic 

survey of Lake Mead for conversion of the sounded 

depths to the corresponding bottom elevations above a 

selected reference plane or datum. All soundings were 

referenced to the water surface at the time and place 

of sounding, and a prerequisite for vertical control was 

the collection of adequate records of the water-surface 

elevation at those times and places. A permanent gage 

at Hoover Dam provides a continuous record of watersurface elevations at the dam. This record was sufficiently accurate to be used without adjustment for conversions of sounded depths to bottom elevations. 

Evaluation of the Echo Sounding~ by G. B. Cummings 

The sonar equipment and methods used to determine 

the depth of water and thickness of sediment in Lake 

Mead are based on the echo-sounding principle; a pulse 

of sound energy is emitted from a projector, travels 

through the water, is reflected and returns as an echo 

to the projector which has been arranged in the meantime to receive the echo. This echo can be recorded 

in various ways, such as on a cathode ray screen or on 

the paper chart of a recorder. 

The equipment operating at 50 kilocycles gave a consistent indication of the top of the sediment, within 2 

feet of accuracy. Waves of this frequency did not penetrate the sediment layer. Where the sediment was very 

thin, the trace of the interface was slightly fuzzy. Independent determinations showed that the transition 

layer was 1 to 4 inches thick. The instruments operating at 14. 25 kilocycles produced an echo from the sediment. surface and, where the sediments were dominantly clay, an echo from the bottom of the old river 

bed. In the Boulder Basin, penetration of 100 feet of 

soft sediment was obtained. The depths obtained by the 

Navy sonar depth-sounding equipments were in close 

agreement with the results obtained by cable-and-reel 

depth measurements where the depths permitted the 

latter observations to be made. 

Summary of Results, by F. C. Ames 

The sounding records revealed a range of depths up 

to about 74 fathoms (444 feet}. The elevation of the 

sediment surface was lowest at the dam. The thickness 

of sediment along the center line of the former river 

channel was more than 100 feet at the dam, diminishing 

to 45 feet in the Temple Bar area, and then increasing 

to a maximum of 270 feet near Pierce Ferry. In the 

Boulder Basin, where both the sediment surface and 

the prelake topography were recorded on the sound 

traces, the indicated elevations of the prelake topography checked the 1935 map elevations very closely. 

Nearly all of the significant changes in reservoir 

bottom between 1935 and 1948 have resulted from deposition of sediment by the Colorado River; in the reservoir west of longitude 113° 57', cove red by the deepwater or "lake" survey, the accumulated sediment from 

the river is about 850, 000 acre-feet. The Virgin River 

has built up a delta containing 34, 000 acre-feet of sediment. In contrast to the Colorado River, however, 

there was no evidence of any significant amount of sediment in the old Virgin River channel below the toe of 

the delta. No measurable quantity of sediment was 

found in any part of the old Muddy Creek channel. 

Aside from sediment accretion Lake Mead has, in 

general, remained relatively unchanged since the original survey in 19'35. Only four slump areas of any consequence were noted, three in Boulder Basin and one 

in Virgin Canyon. The largest of these involved an 

area along the north shore of Boulder Basin, where 

lateral displacements between original and new contours 

amounted to several hundred feet. About 12,000 acrefeet of earth between elevations 925 and 1, 220 feet 

slumped downward, of which some probably came to 

rest below the present sediment surface at elevation 

750 feet. 

Stream erosion since 1935 has been great enough in 

only one locality to cause an appreciable increase in 

the reservoir area. At the mouth of Detrital Wash the 

channel above the average lake level has been scoured 

in places to depths of about 10 feet. Some minor alterations of shoreline are clearly attributable to wave action, particularly along a few steep-sided headlands of 

rather loosely cemented sand and gravel, which are 

subject to undercutting and caving. Except for these 
RESERVOIR STORAGE 7 

headlands and the rock-walled canyons, the lakeshore 

is moderately sloping and not subject to serious undercutting and caving. Caving above the high-water line 

at elevation 1, 220 feet is rare, and the reservoir capacity has not been decreased significantly by wave 

action. 

Survey of the Lower Granite Gorge, by L. C. Pampel 

The upper section of Lake Mead, the Lower Granite 

Gorge, is approximately 40 miles long, extending between Bridge Canyon and a point just above Pierce 

Ferry. At the maximum designed pool elevation of 

1, 229 feet, the water-surface area in the Lower Granite Gorge is about 3 percent of the whole lake, and the 

original storage capacity in this section was about 2 

percent of the total lake capacity. However, the 1948-

49 survey indicates that this gorge section contains 

about one-third of the total sediment deposited in Lake 

Mead up to that time. 

In the Lower Granite Gorge survey, the position of 

the sediment surface was determined with respect to 

an overhead reference plane corresponding to the maximum designed water-surface elevation, and the computations led first to a determination of the amount of 

original capacity that was not filled with sediment in 

1948. The volume determinations were based on 175 

cross sections. Excepting the six sections farthest upstream and nearest to Bridge Canyon rapids, these 

sections were spaced at intervals of 620 to 2, 260 feet 

(averaging 1, 220 feet) throughout the Lower Granite 

Gorge. 

As in the lake survey, adequate horizontal and vertical control was essential in the Lower Granite Gorge 

for accurate determinations of the positions of the sediment surface at each point of measurement. The horizontal control was based on the Fairchild triangulation 

net established in 1935 and on a net established by the 

Bureau of Reclamation in 1942 for use in the Bridge 

Canyon investigations. The vertical control was based 

on bench marks set by the Bureau of Reclamation for 

use in its Bridge Canyon investigations. 

The total remaining storage capacity in the Lower 

Granite Gorge up to elevation 1, 229 feet was found to 

be 219,000 acre-feet. The original storage capacity of 

the Lower Granite Gorge was computed by the Soil 

Conservation Service to be 760, 000 acre-feet. The 

amount of sediment deposited in the Lower Granite 

Gorge since the closure of Hoover Dam was determined 

by subtraction to be 541,000 acre-feet. 

The thickness of sediment varies from none at the 

foot of the Bridge Canyon rapids to 16 feet at the 

Bridge Canyon dam site and to a maximum of 270 feet 

near the junction of the canyon section with the main 

lake. The profile of the center of the stream in 1948-

49 had a uniform slope of about 1. 25 feet per mile in a 

27-mile reach below Separation Canyon. The profile 

of highest sediment deposition in the Lower Granite 

Gorge also has a slope of about 1. 25 feet per mile. 

Many of the highest sediment surfaces in the gorge 

probably were formed in 1941 and 1942, when Lake 

Mead reached maximum elevations of 1, 220.4 and 

1, 213.5 feet respectively. 

RESERVOIR STORAGE 

Significance of Area, Capacity, and Sediment Tables, 

by J. W. Stanley 

The functions of river regulation and flood control 

are in the first priority among the purposes for which 

Hoover Dam was authorized. To assure that the desired 

flood control can be accomplished, the upper 59 feet of 

controlled storage in the original reservoir was reserved for flood control, and storage for other purposes 

must not reach into this reserved zone as of April 1 of 

each year. By August 1 only the upper 8 feet was reserved for protection against floods. Any sediment 

deposited in this upper part of the reservoir since 1935, 

even in the zone of uncontrolled storage, reduces the 

lower limit of the flood-control reserve, and therefore 

the upper limit of the storage available for other purposes. Accurate area-capacity data for the upper levels 

of Lake Mead are thus necessary to determine the lower 

limit of the flood-control reserve. 

Second in priority among the purposes of Hoover Dam 

and Lake Mead is the combined function of irrigation 

and domestic uses. In 1950 the annual release required 

from Hoover Dam for downstream use was approximately 6, 600,000 acre-feet, which includes losses in 

transit by evaporation, transpiration, and seepage. It 

may become necessary during years of minimum inflow 

to limit releases from Hoover Dam to those required 

by downstream interests, thus curtailing power generatiun. 

Last in priority among the functions of Lake Mead 

and Hoover Dam is hydroelectric power generation. 

In spite of this low priority, the revenue derived from 

the sale of electrical energy must repay the cost of the 

dam and appurtenant works. Accurate area-capacity 

information for all elevations of Lake Mead is necessary to translate forecasts of future inflow into terms 

of hydrostatic head on the powerplant and thereby into 

water releases necessary to generate the energy required to meet estimated future loads. Further, the 

amount of water stored in the reservoir in the elevation 

band usable for power generation must be known in 

order to maintain at all times enough reserve storage 

water to permit generation of the firm obligation under 

conditions established in the contracts with the several 

power allottees. 

Computations and Tables, by J. L. Speert, 

F. C. Ames, F. W. Kennon, and W. B. Langbein 

Tables in the technical report show the lake-surface 

area in acres for 1-foot increments in water-surface 

elevation; the usable capacity (above lowest outlets at 

intake towers) for similar increments of elevation; the 

reservoir volumes and sediment deposition in individual 

basins of the reservoir, by 10-foot increments of elevation; and comparison of the areas and capacities in 

1935 and 1948 and the accumulation of sediment during 

the 14-year interval. 

Table 1 summarizes the significant items of change 

between 19.35 and 1948. It shows the elevations above 

mean sea level of various features of operational significance, both with respect to powerhouse datum and 
8 FIRST 14 YEARS OF LAKE MEAD 

Table 1. -Comparative summary, 1935 and 1948-49 

Changes in elevation and capacity 

Elevation Capacity 

(feet above mean sea level) (acre-feet) 

Powerhouse Datum of 1929 

datum levels of 1935 Original 1948-49 survey 

Maximum designed water surface ........................•.......... 1229.00 1229.55 32,471,000 31,047,000 

Top of spillway gates-raised ...................••.................... 1221.40 1221. 95 31,250,000 29,825,000 

Flood control level August 1, 1935 .................•..............• 1213.17 1213.72 29,965,000 . .................... Flood control level August 1, 1948. ··························•···•· 1213. 14 1213.69 ................ 28,547,000 

Permanent spillway crest ................................•.....•.....•. 1205.40 1205.95 28,794,000 27,376,000 

Flood control level April 1, 1935 ........ ··························· 1162.83 1163.38 22, 958,000 ····················· Flood control level April 1, 1948 ................................... 1162. 19 1162. 74 ................ 21,547,000 

Sill of upper outlet in intake towers ...•....................•....... 1045.00 1045.55 11, 162, 000 10,222,000 

Sill of lowest outlet in intake towers ............................•.. 895.00 895.55 3,223,000 2,620,000 

Storage of water and sediment, in acre-feet 

Flood-control reserve, August 1 ..................••............... 

Permissive usable contents, August 1 ...............•...........• 

Flood-control reserve, April 1 ....•...................•.......•..... 

Permissive usable contents, April 1 ..•......•...•..•....•......... 

Dead storage ......... M .................................................... 

Total storage below permanent spillway crest .................. 

Total storage below spillway gates in raised position ......... 

Usable storage below permanent spillway crest ................ 

Usable storage below spillway gates in raised position ....... 

the datum of 1929, leveling of 1935. The significant 

changes in storage of water and sediment are also 

summarized. Thus the maximum storage controlled by 

the spillway gates in raised position decreased from 

31, 250,000 acre-feet in 1935 to 29, 825, 000 acre-feet 

in 1948. The required August 1 flood-control reserve 

extended less than half an inch lower in 1948 than in 

1935, although the usable storage below that level decreased 815, 000 acre-feet because of sediment deposition. However, the April 1 flood-control reserve by 

1948 required an additional 0. 64-foot layer of the reservoir as a result of deposition of 99, 000 acre-feet of 

sediment in the upper part of the reservoir, chiefly in 

the Lower Granite Gorge. The dead storage, below the 

lowest outlet in the intake towers, was decreased 

603,000 acre-feet during the 14-year interval, because 

of deposition of sediment. 

Water Budget, by W. B. Langbein 

The water budget of Lake Mead is an account of its 

operation for the basic function of regulating the highly 

variable flow of Colorado River for the benefit of flood 

control, irrigation, domestic use, and hydroelectric 

power generation. This regulation of the flow is 

brought about through storage of water in years of high 

runoff to supplement the flow during years of low runoff (fig. 4). 

Lake Mead in 1935 had a usable capacity of more 

than 28,000,000 acre-feet, which by 1948 had been reduced to 27, 200, 000 acre-feet because of sediment 

deposition. The annual inflow into the lake in the 14-

1935 1948 

Water Sediment Water Sediment 

2,500,000 0 2,500,000 8,000 

26,748,000 0 25,927,000 815,000 

9, 500,000 0 9,500,000 99,000 

19,748,000 0 18,927,000 724,000 

3,223,000 0 2,620,000 603,000 

28,794,000 0 27,376,000 1,418,000 

31,250,000 0 29,825,000 1,425,000 

25,571,000 ................ 24,756,000 .......................... 28,027,000 ................ 27,205,000 ....................... 

year period since Hoover Dam was completed (1935-48 

inclusive) averaged about 13, 000, 000 acre-feet. The 

usable capacity, therefore, has been equal to 215 percent, and is still 210 percent of the average yearly flow 

through the reservoir. This storage ratio is indicative 

of a long detention time for river regulation and 

control. 

Setting up the water budget of the lake requires an 

accounting of all items of inflvw, outflow, and storage. 

The shortest practical period for which the budget can 

be set up is a year. Most of the inflow to the lake is 

measured at the gaging station on Colorado River near 

Grand Canyon, 145 river-miles above Lake Mead, and 

the inflows from Bright Angel Creek and Virgin River 

are also measured. The sum of these three constitutes 

the measured inflow. There is about 25,000 square 

miles of tributary area from which the runoff into the 

lake is unmeasured; this unmeasured inflow is estimate1 

by statistical analysis to be a maximum of 3 percent 

and to average about 1 t percent of the measured inflow. 

The precipitation on the lake is computed from the rainfall recorded at three locations around the lake, multiplied by the mean surface area during each year. In the 

year of greatest annual precipitation, 1941, the computed precipitation on the reservoir was considerably 

less than 1 percent of the measured inflow from 

tributaries. 

The water released through Hoover Dam constitutes 

nearly all of the outflow from Lake Mead, and this is 

measured at a gaging station 1 mile below the dam. 

Evapotranspiration constitutes the only other important 

item of outflow from the lake. Evaporation is small in 
1300 

25,000 000 - ACRE-_FEEj ) L ~~- I~ -- -I- -I- - 1-- -

t--- _____ ~ __ -l----r:-~~[7~.ooo!._o~oL ACRE~b-d'E~~k- -~~;;:-;r..::::::_-l- -~~ 1200 

1100 

1000 

900 

800 

700 

600 

15,000 000 ACRE- FEET 1'-..J" -~ '-""' ---~ 1----- ----1-----1--- -- 1--- -~---------- -------1--- -~ --- ----

f-- __ f-- _ -lj --=~ ___ >-- ____ __oD,000,£0!'__~CRE:"_EET __ ---- ___ f----_______ >-- __ _ 

.c= 5,000 000 ACRE-FEET _ 0-u Dead storage below this level '0 l - ,___ 1-- - -- - - ~ 1-- - -

J 

I 

1935 1936 1937 1938 1939 1940 1941 

TIME, IN 

1942 

YEARS 

1943 1944 

Figure 4. --Stage and usable storage of Lake Mead, 1935-48. 

---- ------ -

1945 1946 1947 1948 

!l:l 

tr1 

til 

tr1 

!l:l 

<! 

0 

..... 

!l:l 

til 

~ 

0 

~ 0 

tr1 

co 
10 FIRST 14 YEARS OF LAKE MEAD 

comparison with the measured outflow-about 5 to 7 

percent of the annual discharge through the dam. 

Nevertheless, it is one of the largest unmeasured 

items in the water budget, for it is considerably greater than the unmeasured inflow and 10 to 30 times as 

great as the addition by precipitation. 

The estimates of evaporation used in the water budget are obtained by statistical analysis of available data; 

they assume a constant annual rate of evaporation of 

6. 0 feet per year, which ignores the variation in climate from year to year. The inadequacy of these estimates is fully recognized, and they were used in the 

technical report as an interim measure, with the assurance that they will be superseded by more accurate 

determinations on the basis of recent research at Lake 

Mead. 

The water budget shows that on an annual basis more 

water enters into or is withdrawn from storage in Lake 

Mead than is indicated by the capacity table. In other 

words, Lake Mead has a significant amount of storage 

space in addition to the volumes calculated from the 

hydrographic survey of 1948-49, or even the original 

reservoir survey of 1935. The unsurveyed storage in 

Lake Mead represents the volume of water that can be 

stored in the reservoir bottom and sides as determined 

during the hydrographic survey of 1948-49. It corresponds to bank storage along a river, with all the phenomena of filling and draining ofinterstices as the lake 

rises and recedes. It includes the water in the volume 

that has been filled by sediment during the interval 

from 1935 to 1948, and it includes voids in the gravel 

and sand, and other rock materials that underlie the 

bed and sides of the reservoir. 

The magnitude of this bank storage, or ground-water 

storage, is indicated by comparison of the reservoir 

storage as indicated by capacity table and the total 

storage as computed by difference between outflow and 

inflow. As of September 30, 1941, the total storage 

computed from inflow-outflow differences was 

33, 000, 000 acre-feet, compared with reservoir contents of 29, 400, 000 acre-feet as compiled from the 

original capacity curve. The difference of about 

3, 600,000 acre-feet is presumed to represent the 

ground-water storage plus sediment storage. 

In general under present conditions and with the 

rough estimates now available for evaporation, precipitation, and unmeasured inflow, the annual change in 

gross storage averages about 12 percent more than the 

change in reservoir contents indicated by the capacity 

table. At the spillway level of 1, 221.4 feet, the computed gross capacity of Lake Mead is about 35,000, 000 

acre-feet, or 3, 750, 000 acre-feet more than is indicated by the capacity of the table. The sediment particles carried into the reservoir between 1935 and 1948 

have a total estimated volume of 550, 000 acre-feet, 

and the water storage beneath the reservoir sides and 

bottom when the lake is filled to capacity is there.fore 

about 3, 200, 000 acre-feet. 

Under present conditions there is a considerable 

variation in usable ground-water storage with change 

in lake levels, which adds materially to the degree of 

river regulation afforded by the reservoir. From September 1941 to September 1946 there was a net decline 

of 52. 2 feet in reservoir level. .. Reservoir contents 

declined 7, 140,000 acre-feet, but the gross storage 

contributing to the discharge at Hoover Dam amounted 

to 7, 800,000 acre-feet, indicating a recovery of 660, OOC 

acre-feet from ground-water storage during this 5-year 

drawdown. Because of the time required for water to 

saturate or drain from porous material, it is likely 

that the ground-water storage changes only slightly during short-time or seasonal changes in reservoir level. 

LIMNOLOGY 

Thls study is concerned especially with the physical 

and chemical characteristics of the lake water, and 

with interpretations and conclusions based on those 

characteristics. In part, these characteristics are 

products of the environment of the lake: the hot and 

dry climate; the reservoir bed of ridges, canyons, and 

broad basins; and the soluble rocks that form the reservoir floor in some places. In large part, however, 

these physical and chemical characteristics are inherited from the water that flows into the lake. A knowledge of the characteristics of the inflowing water is 

therefore prerequisite to an understanding of the characteristics of the lake water. Accordingly, there follows first a discussion of the physical and chemical 

characteristics of the inflow and second sections de -

scribing the chemical and physical limnology of the 

reservoir. 

Physical and Chemical Characteristics of the Inflowing 

Water, by C. S. Howard 

About 95 percent of the inflow to Lake Mead comes 

from the Colorado River and is measured at the Grand 

Canyon gaging station, 190 miles above the uppermost 

basin (Pierce Basin) of the lake. The inflow from side 

canyons and tributaries below Grand Canyon, including 

the Virgin River, is probably not in sufficient volume 

to have any significant effect on the physical and chemical characteristics of the water in Lake Mead. 

In the first 14 years after the beginning of storage in 

Lake Mead, 1, 995 million tons of suspended sediment 

passed the Grand Canyon station. Thus the Colorado 

River has carried sediment toward Lake Mead at an 

average rate of 142 million tons a year. Since 1926 the 

annual suspended load measured at the Grand Canyon 

station has varied from about 50 million tons in 1934, 

when annual runoff was less than 5 million acre-feet, 

to nearly 500 million tons in 1929, when annual runoff 

was more than 19 million acre-feet. Plotting of the annual runoff against sediment load (fig. 5) shows that 

the sediment load is not directly proportional to the 

runoff, but increases more rapidly with increasing discharge. In the years 1941-50 the sediment load has 

generally been less at equivalent runoff then in the preceding 15 years. The concentrations and sediment loads 

of the inflow during the 1948-49 survey were near the 

average for the period since 1935 at Grand Canyon. 

In the winter months, November to March, the river 

discharge is usually near the minimum for the year, 

ranging from 3, 000 to 10,000 cfs. Sand particles 

rarely comprise more than 10 percent of the suspended 

load, and clay is likely to make up half or more of the 

total. During the period of greatest volume of flowApril to July-sand ordinarily constitutes at least 20 

percent and may be more than 50 percent of the sediment load. From August to October the discharge of 
LIMNOLOGY 11 

500~--~------------------,-------------------~-----------------yl 

400~--+--------------------}--------------------r---------~~------~ 

I til 

I z 

0 

~ 

I Iii 0 

til 

z 300 g 

...:I 

...:I 

~ .41 

~ .32 

~ .. 

z .30 .38 ~ 1-1 

0 

fil 

til 200 

Q 

fil 

Q 

z 

fil 

~ 

til 

:::> 

til 

5 10 15 20 

RUNOFF. IN MILLIONS OF ACRE-FEET 

Figure 5. -Relation of annual runoff to sediment load at Grand Canyon. 
_J 780 

w 

> 

w 

_J 

<( 760 

w 

(/) 

w 

> 0 740 

al 

<( 

1-

w 

w 720 

LL 

z 

-

z 0 700 

1-

<( 

> 

w 

~ 680 

660 

640 

PERIODS OF DENSITY CURRENTS 

In 11 I 1 I I 1l11 1 I 11 I •I 1 I 1 [ 1--r-l1 ----.-l-1 -----.-l-1 -~ -.---I 1--------.--11------.---l-~--. Reported by Grover No record 

and Howard (1938) Reported by Bureau of Reclamation (1941, 1947, and 1949) 

A 

\ Top of sediment ot river-mile 353.5 ~ ~ I J~ 

Top of sediment at river-mile 354 N '-~, ~~ J ~~~~ 

\/ · "N ~~ 

J

"f.J ~r\~~' / 

l..J "'"N ~ 

"---Top of sediment 

, 

at intake towers, 

//v / 1 river-mile 354.61 

r-----+-----~---1~~._ __ _, ____ --+Note:-i------+I----~I~----+-----·~-----+----~------+-----~-----+------ Top of sediment was measured monthly at intake towers throughout period, 

except for 1944 and 1945 when measurements were made at river-mile 

No record 353.5. Measurements weTre also ma9mlle 354 in I! 37-40 

'---~. - I I 

1935 1940 1945 1950 

Figure 6. -Periods of reported density currents and elevation of top of sediment near Hoover Dam, 1935-50. 

,_. 

t>:) 

t:tj 

S3 

Ul 

1-j 

,_. 

~ 

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t::rJ 

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:;tl 

Ul 

0 

t:tj 

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LIMNOLOGY 13 

the river may drop to less than 5, 000 cfs, or it may 

increase to 30. 000 cfs or more for short periods because of flood runoff from cloudburst storms. Regardless of the discharge the distribution of particle sizes 

in the suspended load is generally similar to that in the 

winter, with less than 20 percent sand, and a predominance of clay and very fine silt. 

Most of the dissolved mineral matter in Lake Mead 

is carried in by the Colorado River. The river has the 

lowest concentration of dissolved solids in the spring 

during the period of flood stages resulting from melting 

snow, and the maximum concentration is found during 

the periods of low flow when the runoff consists almost 

entirely of ground-water inflow. The concentration of 

dissolved solids may also be relatively high during 

floods which come after heavy rains on the arid portions of the drainage area. During the 1948-49 survey, 

the dissolved solids ranged from 800 to 1, 000 parts per 

million (ppm) in early 1948, decreased to a minimum 

of 276 ppm in the period June 11-20, rose to 1, 380 

ppm October 11-20. and then fluctuated between 800 

and 1, 200 ppm during the winter of 1948-49. These 

variations were less than have been recorded in some 

years since 1935. In 1942 the dissolved solids reached 

a 10-day minimum of 225 ppm on June 11-20, and a 

maximum of 1, 350 ppm on September 21-30. In 1940 

the range was even greater, from a minimum of 334 

ppm in the 10-day period June 1-10 to a maximum 

1, 720 ppm on September 1-10. The maximum 10-day 

average concentration in the period of record was 

1, 890 ppm on September 21-30, 1934, before Hoover 

Dam was completed. 

The average annual concentration of dissolved solids 

in the Colorado River has varied from 491 ppm in water year 1928 to 960 ppm in 1934. Since Hoover Dam 

was completed in 1935 the weighted average concentration of dissolved solids at Grand Canyon ranged from 

749 ppm in calendar year 1940, the year of minimum 

runoff, to 505 ppm in 1942, following the year of greatest runoff. This weighted average concentration has 

varied greatly from year to year; it decreased about 

25 percent from 1940 to 1941, and increased by an even 

larger percentage from 1938 to 1939. 

Density Currents, by C. S. Howard 

A density current has been defined as a gravity flow 

of a fluid through, under, or over a fluid of approximately equal density (Bell, 1942b). Apparently this 

flow takes place with very little mixing of the two 

fluids. For the density currents in Lake Mead, the 

difference in density may result from differences in 

temperature or in content of dissolved material or suspended material, or a combination of these three factors. Generally the difference in density has been 

caused partly by particles in suspension, so that the 

water in the density current is turbid (p. 20). 

The water stored in the lake forms the medium in 

which density currents can flow; the causative factor 

is the inflowing water, because of its different density, 

which in turn is dependent upon its suspended load, 

dissolved solids, and temperature. Thus the characteristics of the inflow are the characteristics which 

under favorable conditions produce density currents in 

the lake. 

The first and still the best evidence of density currents flowing through the entire length of Lake Mead 

was obtained during the first 15 months of operation of 

Hoover Dam (Grover and Howard, 1938). Subsequently, 

laboratory studies of density currents were made by the 

Soil Conservation Service (Bell 1942a, b). The Bureau 

of Reclamation and the Geological Survey made field 

observations of density currents in Lake Mead, which 

have been reported by the Bureau of Reclamation (U. S. 

Dept. Interior, 1941, 1947, 1949, 1953). During the 

Lake Mead survey the observational and samplecollecting program was expanded, with considerable 

emphasis being given to the collection of data in the 

upper end of the reservoir. 

Density currents have been reported in each year 

since 1938, and in all months except January. Some of 

these density currents occurred during or immediately 

prior to recorded increases in the amount of sediment 

at the face of the dam, but others are not clearly related to such increases. Since August 1937 the sediment 

level has risen more than 15 feet in each of eight periods; the amount of rise was more than 35 feet in the 

fall of 1941, and almost as much in the fall of 1947 

(fig. 6). Following each of these eight major rises the 

sediment level has declined, rapidly at first and then 

more gradually, presumably owing in part to ·compaction. Several minor rises or interruptions in this general downward trend may indicate minor accretions of 

sediment at the dam. It is possible also that there is 

some redistribution of sediment in the Boulder Basin 

subsequent to deposition by density currents. 

Chemical Limnology, by C. S. Howard 

During the late winter the concentration of dissolved 

solids in the lake water is fairly uniform throughout the 

lake, both in depth and areal distribution. In March 

1948 there was less than 30 percent variation in the 

concentration of dissolved solids from Pierce Ferry 

to the intake towers and at practically all depths. 

The freshets of April to July bring in water of lower 

concentration than the lake water. As a result the surficial water is more dilute than the deeper water in 

parts of the upper lake, and water with a dissolvedsolids concentration as low as 300 ppm is found as far 

down lake as Gregg Basin in late May and the eastern 

part of Virgin Basin in late July. In the Boulder Basin 

the concentration of dissolved solids in surficial water 

never reaches the minimum observed in the eastern 

part of the lake, showing that some mixing and probably 

some increase in solids through evaporation and solution have taken place. On the other hand, by late summer the surficial water at the head of the lake is more 

concentrated than that in the Boulder Basin. 

Inflow from the Colorado River and evaporation 

losses from the lake are least in November, December, and January. As the air temperature lowers, the 

surface water cools, becomes more dense, and moves 

to greater depth in the lake. This causes a mixing of 

the water of the upper layers, and monthly samples 

during the survey of 1948-49 show a mixing to progressively greater depths. The extent of the mixing in any 

season depends upon the extreme air temperatures 

reached. In January 1949 the mixing extended the en-
14 FIRST 14 YEARS OF LAKE MEAD 

tire depth at the intake towers, but in 1943 only the upper 150 to 200 feet of the lake was affected. By the 

close of the winter season, just before the spring inflow, the lake water is as thoroughly mixed as it can 

be for the year. 

Throughout the year, except during a short period in 

late winter or early spring, there is definite stratification of the lake water, as shown by the existence of two 

or more layers of water at different concentrations. 

At the bottom of the lake the dissolved solids, alkalinity, and temperature of the water are higher than in the 

main body of water. Usually samples of this water 

have large quantities of sediment. The greater 

dissolved-solids content is mostly in calcium and bicarbonate, and it is likely that these constituents have 

been dissolved from the sediment. Prolonged contact 

of water with the sediments produces a further increase in alkalinity and dissolved solids. Samples collected at the intake towers and stored in the laboratory 

increased 50 ppm in alkalinity in a 2-month period and 

178 ppm in a year. 

The temperatures of water recorded at the surface 

in the main body of the lake have ranged from 53° to 

90°F. These surface temperatures are affected by 

wave action, wind currents, and evaporation, but the 

trends reflect the changes in atmospheric temperature. 

Large diurnal and seasonal fluctuations occur only in 

the surficial zone of the reservoir. Water in the main 

part of the lake and at most depths has a fairly uniform 

temperature of about 52°F. At the intake towers the 

range in temperature at depths greater than 150 feet 

(except at the bottom) is less than 4°F. The temperature of the water throughout the lake approaches uniformity in the winter both in depth and geographic 

distribution. 

Analyses of water released from the reservoir show 

a variation generally less than 100 ppm of dissolved 

solids in the course of a single year. This variation is 

very small in comparison with that of the inflowing water, which commonly varies by more than 1, 000 ppm 

of dissolved solids during a year. Thus it is evident 

that there is a great amount of mixing of waters in 

Lake Mead (p. 15). 

Changes within the lake result in changes in content 

of dissolved solids in the stored water. The white precipitates along the shores below the high-water line of 

the reservoir are conspicuous, although they represent 

a very small volume of material. The disappearance 

of soluble rock materials from the bed or banks of the 

reservoir has also been noted in some places. Finally, 

significant quantities of water have been lost from the 

reservoir by evaporation, with a resultant increase in 

concentration of dissolved solids in the water remaining in the reservoir. The effects of these chemical 

phenomena within the reservoir are shown by comparison of the analyses of the inflowing and the outflowing 

water. 

The dissolved solids carried past the Grand Canyon 

station, based on annual weighted average analyses, 

has ranged from 6, 100,000 tons in the water year 1934 

(a year of serious drought) to 12, 900, 000 tons in 1941. 

In the water year 1934 the dissolved solids in the river 

below Hoover Dam amounted to 6, 500, 000 tons, slightly greater than that measured at Grand Canyon. A total of 140,000,000 tons was carried by the river in the 

14 water years 1935-48, and it is estimated that an additional 8, 000, 000 tons was carred in the unmeasured 

inflow to Lake Mead in this period. In the 14 years the 

dissolved solids in the outflowing water totaled 

136, 000, 000 tons, or about 12, 000, 000 tons less than 

that carried in the inflow. 

Since April 1, 1936, the dissolved solids in the outflow have never exceeded 825 ppm, whereas the concen~ 

tration of dissolved solids in the inflow has for short 

periods exceeded 1, 700 ppm. On the other hand, the 

sulfate concentration in the outflowing water in every 

year from 1937 through 1949 has been greater than in 

the inflowing water and in several years has been more 

than 50 percent greater. The calcium content of the 

outflowing water has been generally 20 to 35 percent 

greater than that of the inflow, and the sodium content 

in the outflowing water has also been as much as 25 

percent greater in several years. These changes within 

the reservoir are correlative with the solution and precipitation of mineral matter by the lake water and with 

the overall concentration resulting from evaporation. 

The evaporation losses from Lake Mead are about 

5 to 7 percent of the average inflow to the lake, or 

about 700,000 to 900,000 acre-feet a year. The soluble 

salts from the evaporated water are left in the lake, 

thereby increasing the concentration of dissolved solids 

in the lake water. 

It is estimated that during the first 14 years of storage more than 1, 000, 000 tons of silica and 9, 000, 000 

tons of calcium carbonate have been precipitated in the 

lake. Most of the white deposit above the waterline and 

around the lake edge is calcium and presumably carbonate, probably deposited by precipitation following 

loss of carbon dioxide. Silica comprises about 20 percent of the material. The marked decreases in silica 

and bicarbonate shown in the weighted average analyses 

of the outflowing water constitute additional evidence 

of precipitation from the lake water. 

The water released from Lake Mead has shown in 

almost every year a higher concentration of dissolved 

solids than the average for the inflow. Assuming that 

the 24, 600,000 acre-feet of water in Lake Mead on 

September 30, 1948, carried 0. 91 ton of dissolved 

solids per acre-foot, the total solids in the lake would 

be about 22, 300, 000 tons, of which about 12,000, 000 

tons is accounted for by the 14-year cumulative difference between inflow and outflow. The increase of more 

than 10, 000,000 tons represents only part of the dissolving action within the lake during the first 14 years 

of storage, because an estimated 10, 000, 000 tons of 

calcium carbonate and silica have been precipitated 

from the stored water. The total increase, about 

20,000,000 tons, has been chiefly calcium and sulfate, 

derived from gypsum and anhydrite forming the lake 

bottom and shores. 

Although there has been an increase in dissolved 

solids through evaporation and solution, there has also 

been a stabilization of the chemical quality during the 

period of storage which has been of considerable value 

to the users of water below Hoover Dam. As a result 

of that stabilization a lower tonnage of soluble salts 

has been delivered to the irrigated lands below Hoover 

Dam than would have been delivered if there had been 

no storage. This is because the concentration of soluble salts in the unregulated river water (as indicated 
SEDIMENTOLOGY 15 

by the Grand Canyon records) is higher than the concentration in the released water during the periods 

when most of the water is taken from the river for irrigation. Thus the "alkali" problem of the lands irrigated by the Colorado River below Lake Mead has been 

decreased to an appreciable extent. 

Physical Limnology, by E. R. Anderson and 

D. W. Pritchard 

The Navy Electronics Laboratory's objectives in the 

Lake Mead survey were: 1) To determine the general 

circulation of the lake from the distribution of temperature and salinity, and 2) to establish a preliminary 

energy budget from which an estimate of evaporation 

could be made. These authors have published (1951) 

a detailed description of the methods and techniques 

employed, a complete analysis of the data with respect 

to circulation and evaporation, and a summary of the 

basic data taken on the various limnological cruises. 

The technical report of the comprehensive survey contains only a summary of these studies. 

Oceanographic techniques were utilized to establish 

the general features of the circulation in Lake Mead. 

Analysis of the data collected during 12 monthly 

cruises on Lake Mead indicates a distinct and orderly 

cyclic progression in distribution of properties and in 

the implied circulation which can be related to the seasonal changes of weather and river inflow. 

In the winter the salinities in the lake are uniform, 

ranging from 600 to 700 ppm (fig. 7). The inflowing 

denser Colorado River water flows along the lake bottom, appearing to influence bottom salinities well into 

Boulder Basin. A single cellular circulation exists, 

resulting in an uplake flow of surface waters from Virgin Basin. 

The spring runoff results in high inflow from Colorado River which, because of its low salinity of 200 to 

300 ppm, flows out over the lake water, producing a 

layer of low salinity over most of the lake. The flow 

along the surface sets up a cellular circulation below 

150 feet which gives rise to flow uplake along the 

bottom. 

In summer. with decreasing inflow and increasing 

salinity of inflow, the downlake spread of Colorado 

River water occurs below the su~face at about 80 feet. 

Two distinct cellular circulations occur above Virgin 

Canyon, one in the surface flow and the other below 

80 feet, resulting in an uplake flow in deeper waters. 

There is some evidence of a third cell, producing 

downlake flow along the bottom, being caused by the 

sinking of some sediment-laden inflow water along the 

bottom. 

In the fall the decrease in temperature of inflow 

from Colorado River is associated with a greater sinking of the inflowing river water. During this season 

there is downlake flow along the bottom until, at about 

170 feet in depth, the flow spreads horizontally downlake and then slopes up slowly toward the surface. 

Two large cellular circulations, one producing uplake 

movement of surface water and the other uplake movement of bottom water exist above the Virgin Basin. 

The seasonal cycle is completed when, with further 

increase in density, the in flowing water flows all the 

way down the slope of the delta along the bottom. The 

deep cellular circulation is eliminated and the single 

cell of the winter season remains. 

Overton Arm appears to reflect primarily the water 

conditions found in Virgin Basin. The influence of flow 

from Virgin River is seen only in the upper few miles 

of the arm. Sedimentation below Lower Narrows in 

Overton Arm appears to result from the flow into Overton Arm of water of Colorado River origin. Turbid water from this source was observed to extend into Overton Arm during the May survey. Characteristic of 

all seasons is the relative uniform character of the 

Boulder Basin, indicating that the Virgin Basin acts as 

a sort of large "mixing bowl," in and above which the 

large seasonal variations in salinity of ·the inflowing 

waters are smoothed to nearly their mean value. 

That conditions differ from year to year is evident 

from the differences found between February 1948 and 

the same month of 1949, which was abnormally cold. 

The temperature of inflow in February 1949 was some 

8°F less than in 1948, and the temperatures in the lake 

ran from 3° to 5° less at all levels. The inflow during 

February 1949 was only 4, 500 cfs as compared with 

13,000 cfs a year earlier. Despite these differences 

the major character of the salinity pattern and the indicated circulation did not differ materially between 

the two winter seasons. It is believed that the important features of the circulation for all seasons as presented above is repeated from year to year. 

In their estimate of evaporation from Lake Mead, 

Anderson and Pritchard stress the preliminary nature 

of their energy budget. Since the completion of the 

comprehensive survey of Lake Mead the Geological 

Survey, Navy Electronics Laboratory, Bureau of Reclamation, and Weather Bureau have cooperated in further research in this field at Lake Mead and elsewhere, 

with the result that data are now being collected which 

permit determination of evaporation from Lake Mead 

with a high degree of accuracy. The basic pattern of 

the energy budget as outlined by Anderson and Pritchard 

still holds, but their estimates of certain parameters 

in this budget, particularly solar radiation and longwave radiation, would be modified by the subsequent 

refinements and instrumentation. 

SEDIMENTOLOGY 

Methods of Investigation, by H. R. Gould 

The conclusions concerning the accumulated sediment 

are based largely on studies of more than 1, 800 samples, of which about 1, 500 were 6- to 12-inch segments 

of 46 cores, and the others were collected by Foer.st 

sampler, dipper, snapper-sampler, or pick. Of the 

cores, 38 were obtained by a gravity corer. which penetrated to a maximum depth of 95 feet, with an average 

recovery of 35 percent. A piston corer used at 8 other 

locations penetrated to a maximum depth of 79 feet, 

with recovery generally close to 100 percent (Smith, 

W. 0.. report in preparation). Physical properties of 

the sediments were measured by standary laboratory 

procedures. Sherman (1951, 1953) describes some of 
16 FIRST 14 YEARS OF LAKE MEAD 

100 

,:::-

~200 

:1: 

t: 

l!l300 

400 

0 

STATUTE MILES 

0 

100 

SPRING SUMMER 

10 

STATUTE MILES 

STRONG CONVERGENCE 

BOULDER BOULDER VIRGIN 

BASIN CANYON BASIN 

AUTUMN WINTER 

Figure 7. -Typical seasonal salinity distribution and circulation patterns. 

VIRGIN 

CANYON 

800 

STRONG CONVERGENCE 

ICEBERG PIERCE'S 

CANYON FERRY 
SEDIMENTOLOGY 17 

these procedures in his discussions of textural analyses and of the flocculent structure of sediments in 

Lake Mead. 

Characteristics of the Accumulated Sediment, by 

H. R. Gould 

The sediment accumulated in Lake Mead has formed 

two deltas, one of which extends from Bridge Canyon 

to Hoover Dam along the thalweg of the Colorado River. and another of much smaller size along the inundated Virgin River channel. Most of the sediment 

brought into Lake Mead is supplied by the Colorado 

River and accumulates in the Colorado delta. 

Deltas are characteristically formed by sedimentladen streams where they enter the ocean or other 

bodies of still water. The growth of the delta may be 

analyzed conveniently with reference to the point at 

which the flowing water meets the still water: Sediment is dropped in the river bed as water velocities 

are reduced; additional sediment is carried to the edge 

of this graded slope and dropped into the deeper stillwater body to form foreset beds; finer material is carried farther out into the still-water body and there 

forms the bottomset beds. As deposition continues the 

foreset beds are built out progressively farther over 

the bottomset beds, and in turn are covered by topset 

beds; also the gradient of the stream is modified and 

sediment is deposited upstream from the original 

mouth. Thus the delta grows outward in the still-water 

body, by deposition of foreset and bottomset beds; by 

the deposition of topset beds it grows upward, and it 

also projects backward into the original channel of the 

stream that provides the source material. 

The Lower Granite Gorge and the ea::;tern part of 

Pierce Basin have been filled with sediment to a level 

higher than the average lake level (approximately 1, 170 

feet altitude). The Colorado delta progressively increases in thickness from Bridge Canyon to Pierce 

Basin, where it attains a maximum thickness of 270 

feet. Topset beds make up the surface of this part of 

the delta. Although some of the topset beds in the Lower Granite Gorge are above the level of the permanent 

spillway crest (1, 205.4 feet altitude). the delta in its 

present stage of development does not extend upstream 

from the original eastern end of the reservoir. 

West of Pierce Basin the delta lies entirely beneath 

the average lake surface, and is confined to the region 

of the submerged Colorado River channel. Foreset 

beds make up the surface of the delta in part of Pierce 

Basin, but from that basin to Hoover Dam, a distance 

of about 75 river-miles, the delta surface is composed 

of bottomset beds. In Iceberg, Virgin, Boulder, and 

Black Canyons steep walls bound the sediment on either side, but in the intervening basins the delta 

spreads out over the terraces that flank the inundated 

Colorado River channel. 

The topset beds that make up the surface of the Colorado delta in the Lower Granite Gorge have an average slope of 1. 2 to 1. 3 feet per mile, over which the 

Colorado River flows into Pierce Basin. Because of 

extreme fluctuations in river discharge and changes 

in lake level, the topset beds are reworked extensive ... 

ly, but observations during 1948 and 1949 show that 

the average gradient of 1. 25 feet per mile is virtually 

unchanged throughout the year. 

The foreset beds that make up the delta front in 

Pierce Basin dip sharply beneath the lake surface for 

a distance of about 1 t miles, measured along the course 

of the submerged Colorado River channel. Near its 

southern margin the delta front ranges in slope from 

about 300 feet per mile near the top to less than 25 feet 

per mile near the foot, and averages about 100 feet per 

mile. The central and northern part of the delta front 

is traversed by a valley 300 to 1, 000 feet wide, and as 

much as 25 feet below the average delta front. The axis 

of this valley extends toward the mouth of the Colorado 

River. At the foot of the delta front the valley broadens 

out onto the gently sloping surface of the bottomset 

beds. The average slope of the delta front measured 

along the valley axis is about 55 feet per mile. 

From the foot of the delta front to Hoover Dam, the 

average slope of the bottomset beds is about 4 feet per 

mile, ranging from an average of 9 feet per mile between the delta front and the mouth of Iceberg Canyon 

to less than 1 foot per mile in the southern part of 

Boulder Basin. There is a marked reduction in slope 

of the bottomset beds beginning in the vicinity of Virgin 

Canyon and extending westward to the dam. The bottomset beds in 1948 had a minimum thickness of 45 feet 

in the Temple Bar area, increasing progressively to 

106 feet at the dam. 

The Virgin delta occupies only the upper 14 miles of 

the Overton Arm, and there it is restricted chiefly to 

the inundated channel and flood plain of the Virgin 

River. Except for their subdued relief, the surface 

features of the Virgin delta are generally similar to 

those of the Colorado delta. The topset beds make up 

the surface of the upper 8 miles of the Virgin delta, 

and they have an average slope of 10.7 feet per mile. 

A break in slope at altitude 1, 140 feet marks the delta 

front, where the topset beds grade into the foreset 

beds, which have an average downstream slope of 29 

feet per mile. Farther south the delta is composed of 

bottomset beds with an average slope of 14 feet per 

mile, which grade into the profile of the former Virgin 

River channel at the toe of the delta. 

The growth of deltas in Lake Mead is necessarily 

more complex than those of streams debouching into 

an ocean or into a lake having relatively constant level. 

During the first 6! years of the lake history the point 

at which the river entered still water migrated from 

the Boulder Basin to Bridge Canyon in the Lower Granite Gorge (a distance of more than a hundred rivermiles), with variations each year corresponding to the 

fluctuations in lake stage. Since July 1941 the lake has 

generally been at elevations ranging from 1, 150 to 

1, 200 feet, reaching a minimum level of 1, 134 feet in 

April 1947, and the river has entered the lake either 

in Pierce Basin or in the western part of the Lower 

Granite Gorge. A general idea of the growth of the Colorado delta from 1937 to 1948 may beobtained from 

the profiles of plate 1. 

The Lake Mead deltas are made up almost entirely 

of particles of sand, silt, and clay. Gravel is present 

only in the easternmost 2 miles of the reservoir and 

is confined to the center of the channel; only a very 

small part of the detrital load carried into the lake by 

the Colorado River is composed of gravel. The sand 

has been deposited at or near the mouths of the Colorado and Virgin Rivers and is confined, therefore, to 

the topset and foreset beds of the deltas. On the other 
18 FIRST 14 YEARS OF LAKE MEAD 

hand, the silt and clay particles have been carried into 

the deeper parts of the lake and deposited in the bottomset beds. As a result, the Lake Mead deltas are 

geographically graded, the coarsest particles accumulating at the heads of the deltas and the finest particles 

at the greatest distance from the source. This gradation in particle size has produced a gradational change 

in most other physical properties of the deltas. For 

example, the sand of the topset and foreset beds, because of its small volume of interstitial water, is hard 

and firmly packed, but the silt and clay of the bottomset beds have an extremely high water content which 

has given them the consistency of a soft ooze. Only 

samples of the most deeply buried layers are solid 

enough that they do not flow under their own weight. 

There are numerous gas cavities which in some zones 

give the bottomset beds a sponge like appearance. 

The water content and porosity are of particular interest since they provide the best indication of the amount of compaction that has taken place in the accumulated sediment. When the silt and clay of the bottomset 

beds are deposited, the water content is commonly 

more than 80 percent of the wet weight of the accumulated sediment, and the porosity exceeds 90 percent. 

As additional layers are deposited on top of those in 

place, the solid particles of the buried material pack 

together more tightly, thus driving out part of the water. The water content and porosity of the accumulated 

sediment are also related to the size, shape. and. sorting of the particles. The relationship is well illustrated in the Colorado delta where the water content 

ranges from 16 percent in the sand of the topset beds 

to 75 percent in the silt and clay of the bottomset beds. 

Expressed in terms of volume, the porosity ranges 

from 34 percent to 89 percent. The water content in 

all sections decreases with increasing depth. 

Sand and gravel when they are first deposited have a 

high specific weight because of the dense packing of the 

constituent particles. Silt and clay have a very lQw 

specific weight when they are initially laid down because of the loose arrangement of the solid grains. In 

the Colorado delta the specific weight ranges from 108 

pounds per cubic foot for coarse sand to as little as 18 

pounds per cubic foot for silt and clay (fig. 8). 

According to laboratory experiments by Terzaghi 

(1925), the consolidation or increase in specific weight 

of sand is insignificant when subjected to pressures · 

less than about 100 pounds per square inch. Since the 

weight of the superimposed load reaches this value only 

in the most deeply buried sand layers east of the delta 

front, the specific weight of the topset and foreset beds 

probably does not increase appreciably with increasing 

depth, and the specific weight of the surface material 

is characteristic of all depths. Consequently, the specific weight of all the sand in the topset and foreset 

beds east of the delta front is estimated at 93.8 pounds 

per cubic foot, which is the average specific weight of 

12 samples collected from the surface of the topset and 

fore set beds. 

It has been assumed that the deeply buried bottomset 

beds eastofthe Colorado delta front are similarin texture to the bottomset beds immediately west of the delta 

front. Data from the bottomset beds immediately west 

of the delta front are available, however, only to a 

sediment depth of 95 feet. Extrapolation of these data 

to greater depths gives an estimated mean specific 

weight of 100 pounds per cubic foot for the deeply 

buried bottomset beds in the vicinity of the delta front. 

It is estimated that the Colorado delta has an average 

calcium carbonate content of 10. 3 percent. This quantity is far greater than can be accounted for by precipitation from the lake, and it is concluded that large quantities of calcium carbonate are carried into the lake as 

part of the suspended load of the Colorado River. This 

conclusion is confirmed by analyses by Sykes (1937) of 

the suspended load of the Colorado River deposited in 

the delta at the head of the Gulf of California, which indicate that the suspended load has an average calcium 

carbonate content of 10 percent. 

The average organic content of samples from the bottomset beds is 0. 93 percent. and of samples from the 

topset and foreset beds, 0. 14 percent. Within the. bottomset beds the organic content shows a general but 

somewhat irregular decrease from 1. 12 percent in 

Pierce Basin to 0. 68 percent at Hoover Dam. 

Bacteriology and Biochemistry of the Sediments, by 

F. D. Sisler 

Bacteriological examinations were made of samples 

of the sediment midway between the intake towers at 

Hoover Dam. The samples were collected from depths 

of 0. 3, 46, and 99 feet below the sediment surface, or 

approximately the top. middle, and bottom of the accumulated sediment. The samples indicated a progressive decrease in water content with depth, from 71 percent at the top to 62 percent at the bottom, and an increase in temperature from 52°F at the top to 58. 4°F 

at the bottom. The median particle diameter of a sample from 93-foot depth was 0. 0006 millimeter and 97 

percent of the particles were smaller than 0. 008 

millimeter. 

The total bacterial population was found to exceed 

1, 000,000 bacteria per gram at all depths, and is thus 

comparable with raw sewage. Near the mud surface 

there is a minimum of 10. 000,000 bacteria per gram. 

By contrast the water within a foot above the sediment 

surface contained only 100 bacteria per gram. By comparison with marine and natural lake muds. the Lake 

Mead sediments are unique in containing a high bacterial population distributed uniformly throughout a deep 

and loosely packed layer. 

Laboratory tests indicate that the abundance and activity of the microflora have contributed to the heating 

of the fresh sediment layer in Lake Mead, but radioactivity was found to be negligible as a possible source 

for the heating of the sediment in Lake Mead. It is estimated that the amount of heat from radioactivity is 

less than 10- 10 calories per hour per gram, whereas 

the heat obtained from the biochemical oxidation of 

oxidizable material in Lake Mead mud is about 10-6 

calories per hour per gram. 

The evolution of methane from the mud was demonstrated in the laboratory. Methane production is a 

common occurrence in swamps and some lakes where 

conditions are anaerobic, and where abundant organic 

matter such as cellulose is present. Although the mud 

appeared to be a stable colloidal mixture, changing the 

reaction in the laboratory from pH 7. 25 to pH 10 
...J 

Ill 

> 

Ill 

...J 

tIll 

Ill 

.... 

z 

0 

i= 

<i 

> 

Ill 

...J 

w 

PROF ILE WITH NO VERTICAL EXAGGERATION j_ ~--------------------~------------------------~--------------------------------------------------------------------------------------~------------------------------------------------------------------------------------------------'fe32FT 

HOOVER DAM NOTE : WIDTH NOT TO SCALE 

1941 

MAXIMUM LAKE LEVEL- 11137 

MAXIMUM LAKE LEVEL.- 1838 

TOPSET AND rORESET 

BASIN------->'1 

--------

ORIGINAL COLORADO RIVER PROFILE (1935) 

i*------------------------------- LOW£R GRANITE GORG£--------------------------------------..j 

1000 

aoo 

EXPLANATION 

ANNUAL PR OFIL ES OF SED IMEN T DISTR I BUT ION 

1945 

lt47 

1148 

184& 

11144 

1843 

1842 

1841 

DISTANCE A L ONG COLORADO RIVER CHANNEL MEASURED IN MILES FROM U.S.G.S. CONCRETE GAGE WELL OPPOSITE MOUTH OF PARIA RIVER, ARIZONA 

ANNUAL PROFILES OF SEDIMENT DISTRIBUTION ALONG COLORADO RIVER CHANNEL THROUGH LAKE MEAD 
1200 

1100 I 

WT. Of 

~ ~1000 ~ 

<( 

0 

I f900 

(l: 

w 

> 

0 

0 

:I: 

800 

700 

360 350 340 

MEAN LAKE LEVEL 

DRY SEDIMENT IN Las/cu. FT. NO DATA 

~ ( 30 AVAILABLE 

30- 40 

40- 50 

50- 60 

60-70 ii COLORADO 

70-80 RIVER PROFILE 

80-90 

>90 

SPECIFIC WEIGHT OF SEDIMENT 

SHOWN IN LONGITUDINAL SECTION THROUGH 

THE COLORADO DELTA 

330 320 310 300 290 280 270 260 250 240 230 

01 STANCE1 IN RIVER MILES 

Figure 8. --Specific weight of the sediment accumulated in the Colorado delta along the thalweg. 

00 

lzJ 

t:1 

..... 

~ 

lzJ 

z 

~ 

0 

s 

0 

t<l 

..... 

co 
20 FIRST 14 YEARS OF LAKE MEAD 

brougfit about a 22-percent volume reduction or compaction. The pH of sediments in situ may be a significant factor in compaction of those sediments. 

Amount of Sediment, by H. R. Gould 

The amount of sediment accumulated in Lake Mead 

may be expressed either in terms of volume-the space 

occupied by both the solid constituents and the interstitial water--or of weight, which includes only the 

solid particles. The weights, volumes, and certain 

other features of the sediment in individual basins of 

Lake Mead are summarized in table 2. 

The average annual rate of sedimentation during the 

14-year period, 1935-1948, is computed to be 144 million tons. Most of the material in the Colorado delta 

and the Virgin delta has been supplied by the Colorado 

and Virgin Rivers, respectively. Consequently, the 

mean annual load of the Colorado River was 140 million tons, and that of the Virgin River only 4 million 

tons, in the 14-year period. During the same period 

the suspended sediment carried by the Colorado River 

averaged about 142 million tons a year or approximately 2 million tons more than the estimated average annual deposition of sediment from the Colorado River in 

Lake Mead in the same period. Such close agreement 

between the mean annual suspended load of the Colorado River and the average annual rate of sedimentation 

in the Colorado delta is striking. 

Any material carried as bed load has not been included in the measurement of sediment passing the 

Grand Canyon station, whereas it is .included in the estimate of the amount of sediment accumulated in the 

Colorado delta. If the quantity of sediment carried in 

the bed load were appreciable, the total amount of sediment in the Colorado delta should be somewhat greater than the total suspended load passing the Grand 

Canyon station. Since this is not the case, it appears 

that the bed load of the Colorado River is extremely 

small, and that practically all of the sediment supplied 

by the Colorado River is carried in suspension. 

Transportation of Sediment by Turbidity Currents, 

by H. R. Gould 

Some density currents (p.13) are produced solely by 

differences in temperature or salinity, whereas others 

may originate, at least in part, from differences in 

their amount of suspended sediment. The type of density current in which suspended sediment accounts for 

part of the difference in densl.ty is referred to here as 

a turbidity current. Some quantitative aspects of Lake 

Mead turbidity currents have been described elsewhere 

by Gould (1951). 

A turbidity current may be rather simply defined as 

a gravity flow of turbid water through, under, or over 

water of different density. Part of the difference in density is produced by suspended sediment, but suspended 

sediment is not always the dominant factor. In the 

overflow type of turbidity current (Bell, 1942a) the lower density of the current is due to higher temperature 

or lower salinity or both. Similarly in the interflow or 

underflow type of current, suspended sediment may not 

account in some cases for as great a part of the higher 

density as does lower temperature or higher salinity. 

A density current moves through still water with very 

little mixing and remains identifiable because of the 

lack of mixing. 

Turbidity currents are always present near the mouth 

of the Colorado River, but only rarely do they flow the 

entire length of the lake. The three types of currents 

designated above as overflow, interflow, and underflow 

all occur in Lake Mead. Overflows occur only during 

the late spring and early summer (usually from late 

April to early July) when the incoming riv.er water is 

less saline than the water at the surface of the lake. 

Turbid overflows seldom extend west of the Virgin Basin and they apparently never reach the dam. 

InterfloW's during 1948 were most prominent in August 

and September, when the temperatures of the river and 

the lake surface water were about the same, but the 

salinity of the incoming water was considerably greater 

than ~he salinity of the lake surface layer. The susTable 2. --Sediment in individual basins of Lake Mead 

Volume Weight 

Area Percent Percent Millions Percent Mean specific Median 

of total Acre-feet of total of tons of total weight particle dia- : 

area (lbs/ cu ft) meter 

(microns) 

Colorado delta: 

Boulder Basin ..••................•.......... 26.9 295,000 20.7 219 10.9 34.1 0.95 

Virgin Basin ...........•..................... 28.4 142,000 10.0 123 6. 1 39.8 1. 25 

Temple Bar Area .......................... 8.7 59,000 4. 1 54 2.7 41.8 1. 40 

Gregg Basin ...........•..................... 6.7 114,000 8.0 122 6. 1 49.4 2.45 

Grand Bay .........•........•................. 2.7 97,000 6.8 111 5.5 52.3 6.60 

Pierce Basin ................................ 2.5 144,000 10. 1 214 10.6 68.2 25.0 

Lower Granite Gorge ......•......•....... 3.0 541,000 37.9 1, 113 55.3 94.5 150.0 

Total or mean ............................ 78.9 1,392,000 97.6 1, 957 97.2 64.6 47.0 

Virgin delta: 

Overton Arm ................................ 21. 1 34,000 2.4 57 2.8 78.2 31.0 

Total or mean, Lake Mead ........... 100,0 1,426,000 100.0 2,014 100.0 64.9 44.0 
SEDIMENTOLOGY 21 

pended sediment settles to the bottom within a tew 

miles of the river entrance, but the high salinity of 

such interflows can be identified for a considerable 

distance downstream and thus distinguishes the interflow from the lake water above and below it. 

Whenever the density of the inflow is greater than the 

density of the deeper lake layers, the turbid water 

plunges beneath the lake surface and travels downlake 

as an underflow. Water discharged by the Colorado 

River from October to April is colder, more saline, 

and more turbid than the deep water in the eastern part 

of the lake, hence it sinks rapidly beneath the clear 

water and travels along the submerged Colorado River 

channel. During this period the division between the 

turbid river water and the clear lake surface is sharp 

and distinct and is commonly marked by logs and other 

debris which collect at the convergence of the opposing 

surface currents. 

Most underflows do not reach the western part of the 

lake, but at least 12 conspicuous underflows have traveled along the submerged Colorado River channel from 

the river mouth to Hoover Dam. Of this number, 11 

occurred during the first 7 years of reservoir operation (1935-1941) when the lake was from 70 to 120 

miles long. The only other major flow of turbid water 

to reach the dam arrived in the fall of 194 7 when the 

distance between the river mouth and the dam was 

about 78 miles. Although these extensive currents are 

comparatively rare, they have transported a tremendous quantity of sediment great distances from the head 

of the lake. The periods of underflow are marked by a 

rapid rise in the level of the water-sediment interface 

behind the dam (fig. 6) whereas the intervals between 

periods of underflow are marked by a gradual but progressive lowering of the sediment surface due to compaction. 

One of the most striking features of the underflows 

is their low velocity. Mean velocities of 1. 0 foot per 

second are common near the river mouth where the 

turbid water plunges down the face of the delta front, 

but by the time these currents reach the flat floor of 

the Boulder Basin, their velocities are less than 0. 25 

foot per second and are commonly too low for meas:-: 

urement. 

In most of Lake Mead, underflows are confined to 

the submerged channel of the Colorado River and are 

generally only a few feet thick. Between the entrance 

of the river in 1948 and the mouth of Iceberg Canyon 

(a distance of about 14 miles), abundant mixing of the 

muddy inflow and the lake water occurred several tens 

of feet above the reservoir bottom, but west of Iceberg 

Canyon the underflow of turbid water was· confined -to 

the very deepest part of the reservoir. In late November 1948, an underflow of muddy water extended into 

the eastern part of the Virgin Basin. Observations of 

this current showed a very sharp interface between the 

muddy flow at the bottom of the lake and the clear water above. The underflow west of Iceberg Canyon was 

about 3 feet thick and its density ranged from approximately 1. 001 at the top to 1. 20 at the bottom, 

It has been estimated that approximately 50 percent 

of the total weight of sediment accumulated in the Colorado delta is contained in the fine-grained bottomset 

beds, and practically all of this material has been 

transported by turbidity currents developed at the 

mouth of the Colorado River. Most of the sediment in 

Boulder Basin has been transported at least 70 miles 

before finally coming to rest. 

The Colorado delta deposits have a minimum thickness of 45 feet in the Temple Bar area about 40 miles 

above Hoover Dam. The progressively increasing 

thickness east of this point is normal for a sedimentladen stream entering quiet water. The increasing 

thickness to the west may be attributed to Hoover Dam, 

which has arrested the infrequent but farthest traveling 

turbidity currents and caused deposition of their suspended loads. In open water, some of these currents 

might have continued for tens or even hundreds of 

miles, and the total thickness of deposition would decrease progressively from the front of the delta near 

the point of entrance of the inflowing water. 

The suspended particles transported by the Colorado 

River turbidity currents consist of 67 percent clay, 32 

percent silt, and less than 1 percent sand. The coarsest particles are generally deposited near the river 

mouth and the finest particles at the greatest distance 

from the source. The sediment may not settle as discrete particles, but as flocculated masses that are 2 

to 20 times larger than the individual grains. Studies 

of suspended sediment in the Lake Mead underflows 

(Sherman, 1953) and in the Colorado River (Grover and 

Howard, 1938) show that the material also travels in 

a flocculated state. 

Most of the Virgin River turbidity currents have extended only a short distance into Lake Mead, but observations at Lower Narrows indicate that some underflows of turbid water have traveled through most, if 

not all, of the inundated Virgin River channel. Records 

obtained several times each year since January 1938 

show that suspended sediment has appeared periodically 

in the submerged channel between 1938 and 1947. Suspended sediment was observed in the channel at Lower 

Narrows in April 1948 but by January 1949 it had disappeared. The suspended sediment at Lower Narrows commonly appears prior to the period of Colorado 

River overflows, and the Virgin River is evidently its 

source. 

Sedimentation in Relation to Reservoir Utilization, 

by H. R. Gould 

This section forecasts the p_attern of sediment accu.., 

mulatioi1 in Lake Mead after evaluating ·the sedimentstorage capacity of the reservoir and the compaction of 

the sediment. 

The difference between the sediment-storage capacity 

of Lake Mead and its water-storage capacity is equal 

to the \folume between the level surface at the elevation 

of the spillway crest and the sloping sediment surface 

that will extend upstream from the dam when the reservoir becomes filled with sediment. If we assume that 

the ultimate slope of the topset beds will be at least as 

great as the slope in 1948 (roughly 1. 2 feet per mile). 

the topset beds when the reservoir is completely filled 

will intersect the old Colorado River p:r:ofile about 13.5 

miles upstream from the original head of the reservoir 

and will rise about 160 feet higher than the permanent 

spillway crest. The sediment-storage capacity above 

the permanent spillway level is estimated to be about 
22 FIRST 14 YEARS OF LAKE MEAD 

6, 900,000 acre-feet, and the total sediment-storage 

capacity is therefore 35, 700, 000 acre-feet. 

Sediment compaction has an extremely important 

bearing on the rate of depletion of water-storage 

volume and on the life expectancy of the reservoir. 

The sediment accumulated in Lake Mead consists of 

two major types of material; namely, the sand of the 

topset and foreset beds, and the silt and clay of the 

bottomset beds. The compaction of sand is probably 

insignificant and can be neglected. The potential compaction of silt and clay, however, is large. 

The degree to which silt and clay sediments are 

compacted is related primarily to their depth of burial 

and to the size, shape, and sorting of their constituent 

particles. In order to obtain an average empirical relation between these various factors, the depth and 

water-content data of all cores from the bottomset 

beds of both the Colorado and Virgin deltas were plotted, and a best-fit visual curve was drawn which indicated an approximately linear relation between decrease in water content and depth of burial of the bottomset beds throughout a depth interval of 95 feet. By 

the use of Hedberg's (1936) porosity data for finegrained sediments, it is possible to extend the watercontent curve obtained from the Lake Mead cores to 

considerably greater depths. 

The gates through which water can be discharged 

from the reservoir are at three levels. The uppermost of these are the spillways with their permanent 

crest at 1, 205 feet elevation. The gates at the two 

lower levels are situated in the four intake towers immediately uplake from Hoover Dam. The sills of the 

upper gates are at an elevation of 1, 045 feet, 310 feet 

above the 1948 sediment surface and 415 feet above 

the original floor of the reservoir. The sills of the 

lower gates are at elevation 895 feet, 160 feet above 

the 1948 sediment surface and 265 feet above the original lake bottom at the intake towers. These are the 

outlets through which water for generating power and 

for downstream users is normally discharged. 

All water stored below the 895-foot level is, of 

course, in dead storage. When water was first impounded behind Hoover Dam, the dead storage space 

measured 3, 223, 000 acre-feet, but by January 1949 

it had been reduced by the accumulation of sediment 

to 2, 620,000 acre-feet, or by 18.7 percent. Whenever 

the dead storage space becomes completely filled with 

sediment it will be necessary either to discharge great 

quantities of sediment downstream or to close the lower gates and allow them to become covered. Loss of 

the dead storage space will thus mark the first critical 

stage of reservoir filling. Similarly, the filling of the 

reservoir to the sills of the upper gates in the intake 

towers and to the permanent spillway crest will 

denote, respectively, the second and third critical 

stages. 

A total of about 2, 000 million tons of sediment (dry 

weight) has been deposited in Lake Mead during the 14-

year period 1935 to 1948. This quantity is taken as a 

convenient unit of measurement for discussion of the 

future accumulation of sediment in Lake Mead. This 

material consists of approximately 45 percent sand 

and 55 percent silt and clay. The total load of sand, 

therefore, is 900 million tons, and the total load of 

snt·and clay is 1, 100 million tons. For the few centuries under discussion in the following paragraphs, it 

is assumed that the size distribution will be similar to 

that in the 2, 000-million ton sample accumulated prior 

to the 1948 survey. 

Using a mean specific weight of 93.8 pounds per cubic 

foot for sand, the total sediment-storage capacity 

(35, 700, 000 acre-feet) has been reduced 441,000 acrefeet by the sand in the first 2, 000 million tons of sediment brought into the reservoir. Since it is assumed 

that sand will experience no further compaction, the 

sand in each 2, 000-million ton unit will reduce the total 

sediment-storage capacity by an equivalent amount. 

The rate of sediment-storage depletion resulting from 

the accumulation of silt and clay is not constant, but 

will become progressively less because of increased 

compaction during the filling of the reservoir. 

Based on curves of sediment compaction it is calculated that when Lake Mead reaches its sediment-storage 

capacity of 35, 700, 000 acre-feet, it will contain 74, 700 

million tons of sediment-37 times as much as in 1948--

with a mean specific weight of 96. 1 pounds .Jer cubic 

foot, 48 percent greater than the computed mean in 

1948. The sand of the topset and foreset beds will 

weigh 33, 600 million tons, with specific weight of 93. 8 

pounds. The silt and clay will have a mean specific 

weight of 98. 0 pounds per cubic foot, and a total weight 

of 41, 100 million tons. 

The usefulness of the reservoir will not be materially 

impaired until its water-storage capacity at the level 

of the permanent spillway crest is reduced to about 

13, 000, 000 acre-feet, the average annual flow into 

Lake Mead from 1935 to 1948. This will occur when 

the sediment accumulation becomes slightly more than 

18 times as great as the amount brought into the lake 

between 1935 and 1948. At that time the topset, foreset, and underlying bottomset beds will fill all the reservoir east of Virgin Basin and extend into that basin. 

The bottomset beds at the dam will reach almost to the 

level of the upper outlet gates. The water-storage capacity (not including ground-water storage) will be 

about 13,000, 000 acre-feet below the permanent spillway crest, comprising about 4, 500,000 acre-feet in 

Boulder Basin, 500, 000 in Boulder Canyon, 5, 000,000 

in Virgin Basin, and 3, 000, 000 acre-feet in Overton 

Arm. The total water storage can be increased to about 

14, 900, 000 acre-feet by raising the spillway gates. 

The sediment level at the dam will reach the elevation 

of the lower outlet gates when the total accumulation 

becomes about seven times as great as the amount 

brought into the lake from 1935 to 1948. At that time 

the reservoir east of Gregg Basin will be filled with 

sediment, but abundant storage space will still be 

available in the western part of the reservoir. The 

water-storage capacity below the permanent spillway 

crest will be about 22, 000,000 acre-feet, comprising 

6, 300, 000 acre-feet in Boulder Basin, 800, 000 in 

Boulder Canyon, 8, 000,000 in Virgin Basin, 3, 300, 000 

in Overton Arm, 2, 000,000 in the Temple Bar area, 

300, 000 in Virgin Canyon, and 1, 300, 000 acre-feet in 

Gregg Basin. By raising the spillway gates, the total 

storage can be increased to about 24, 100,000 acrefeet. 

These predictions are based on certain assumptions, 

of which an important one concerns reservoir operations. It is assumed that the average operating level 
LIFE OF THE RESERVOIR 23 

will be at spillway level 35 feet higher than at present, 

and on that assumption it is predicted that approximately 20 percent of the total sediment load will be 

deposited above the level of the permanent spillway 

crest. Lower average operating levels will reduce the 

proportion of sediment deposited in the upper part of 

the reservoir, extend the delta front downlake, reduce 

the area available for deposition of silt and clay, and 

result in a more rapid rise of the sediment level at the 

dam. 

In the foregoing discussion of the various stages of 

reservoir filling it has been assumed that all the sediment transported into Lake Mead will be deposited in 

the lake and that none will be discharged through the 

outlet gates or spillway tunnels of Hoover Dam. If the 

outlet gates are used for discharging sediment through 

the dam, however, it is likely that the reservoir will 

never become entirely filled with sediment but will 

eventually reach a terminal stage when all the sediment transported into the lake will be discharged 

through the outlet gates and spillway tunnels of Hoover 

Dam. 

LIFE OF THE RESERVOIR 

By H. E. Thomas, H. R. Gould, and W. B. Langbein 

The quantities of sediment that may eventually be 

deposited in Lake Mead-about 75, 000 million tons 

when the reservoir is completely filled, or 37,000 

million tons at the half-way point, as calculated by 

Gould-are esseqtial items in the computation of the 

life of the reservoir. The life, or the half-life, of the 

reservoir in years depends also upon the average annual rate of sediment accumulation. The life of the 

reservoir is forecast first on the assumption that the 

average rate of accumulation in the 14-year period 

1935-48 will continue in the future, and then on the basis of analyses of other hydrologic data for the purpose 

of evaluating the "normal" rate of accumulation. 

Projection of Observed Rates of Sediment Accumulation 

Some estimates of the probable life of Lake Mead 

have been based on projection of the observed rates of 

sediment accumulation at Hoover Darn. As shown by 

figure 6 the top of the sediment rose 100 feet in the 

first 3 years of operation of the dam, and then rose 

another 40 feet by the fall of 1941, less than 7 years 

after water was first impounded. This rapid rise of 

the sediment level-to a position about halfway between 

the original river bed and the lower gates at the intake 

towers-could lead to very pessimistic estimates as 

to the life of the reservoir. However, the rapid rise 

is readily explained by the shape of the reservoir: the 

volume in the lowest 100-foot layer of the reservoir 

(below elevation 720 feet) constitutes less than 0. 5 percent of the reservoir capacity. Records since 1941, 

too, show that sediment compaction is a very important factor at the dam, for by January 1950 the sediment level at the intake towers was 38 feet lower than 

it had been in 1941. Projections of the observed rate 

of sediment accumulation into the future necessarily 

lead to erroneous estimates of the life of the reservoir, if they neglect the factor of compaction. 

Other estimates of the life of Lake Mead have been 

based upon projection of the ratio of sediment volume 

to the water-storage capacity of the reservoir. In the 

1948-49 survey it was determined that the volume occupied by sediment was 1, 426, 000 acre-feet, and thus 

the average annual rate of accumulation in the 14-year 

period was 102, 000 acre-feet. By projection of this 

rate of accumulation, the time required for a sediment 

volume equivalent to the original water-storage capacity of Lake Mead would be 280 years. 

The maximum life of the reservoir will be considerably greater than this, however, because 1) the 

sediment-storage capacity is greater than the waterstorage capacity of the lake, and 2) the current average 

specific weight of the sediment will be increased by 

compaction. From consideration of the principles of 

stream gradation and the history of delta building in 

Lake Mead to date it has been concluded (p. 21) that 

the sediment-storage capacity of the reservoir is about 

25 percent greater than the water-storage capacity. 

From analysis of core samples and other data, it has 

been estimated (p. 22) that the average specific weight 

of the sediment when the reservoir is completely filled 

will be nearly 50 percent greater than the average of 

the deposit in 1948. The 2, 000 million tons of sediment 

deposited in the 14-year period (1935-48) constitutes 

only 2. 7 percent of the calculated ultimate storage capacity of nearly 75, 000 million tons. By projection of 

this rate of accumulation, the probable maximum life 

of the reservoir is computed to be about 520 years. 

The time required to reach the half-way point, when 

accumulation of 37, 000 million tons of sediment will 

have reduced the water-storage capacity at the permanent spillway crest to 13, 000,000 acre-feet, would be 

about 260 years. 

:Normal" Rate o( Sediment Accumulation 

In using the record for a 14-year period to forec;;tst 

events 5 centuries in the future, we are using an exceedingly short base and projecting it for nearly 40 

times its length. Any inaccuracies in that base are 

similarly magnified in the forecasts. Thus, if on the 

average it takes only 12 years for streams to carry the 

·2, 000 million tons that reached Lake Mead in the 14-

year period, the probable life of the reservoir would 

be shortened by 75 years. The records at Grand Canyon 

show that there is sufficient variability in sediment load 

to justify a great range in forecasts as to the probable 

life of the reservoir. In the single year 1927 the suspended load of 480 million tons was nearly one-fourth 

of the total carried in the 14-year base period; and in 

the 8 years 1926-33 the river carried 2, 000 million 

tons of sediment past the Grand Canyon station--as 

much as was carried in the 14-year period 1935-48. 

Hydrologists are generally agreed that the period 

1935-48 was not a normal period, for the average runoff was appreciably less than averages for longer 

terms, and the sediment load was also less than the 

average for the entire period of record. The problem 

is complicated by the fact that the sediment-runoff relation since 1941 has been notably different from that 

recorded in earlier years. 
24 FIRST 14 YEARS OF LAKE MEAD 

The volumetric survey of 1948 provides only one 

point of correlation between runoff and sediment: that 

2, 000 million tons of sediment was carried into the 

reservoir by 176 million acre-feet of Colorado River 

water, plus the unmeasured flow of other tributaries. 

Any detailed analysis of the rate of sediment transportation in relation to stream discharge depends on records of suspended load, especially the record for the 

Colorado River at Grand Canyon. 

The record of sediment at Grand Canyon, beginning 

in 1926, provides information as to the suspended load 

and contemporaneous runoff for nearly a quarter of a 

century. Records of river discharge cover a longer 

period, and climatic fluctuations are inferred for a 

still longer period, but the sediment-runoff relationships in these extended periods must be extrapolated 

from the relatively short sediment-gaging records. 

The average annual load of suspended sediment passing the Grand Canyon gaging station in the 23-year period 1926-48 was 175 million tons, or about 23 percent 

greater than the mean annual load (142 million tons) 

during the 14-year period 1935-48. The mean annual 

runoff in the 23-year period was 12.8 million acrefeet, or 2 percent greater than the average for the period 1935-48. If the high suspended load recorded at 

Yuma in the years 1911-25 is representative of Upper 

Basin, it appears that the rate of sediment transport 

into the Lake Mead area during the 38 -year interval 

1911-48 may possibly be as much as 60 percent above 

the 1935-48 average. 

Schulman's (1945) tree-ring investigations in Upper 

Basin provide a basis for estimating the average rate 

of runoff of the Colorado River during the past several 

hundred years. Extrapolation of his results, together 

with available runoff records, suggests that the average annual virgin streamflow at the Grand Canyon station during the 658-year period 1288-1945 has been 

about 15,800, 000 acre-feet, slightly less than the estimated virgin flow in the period 1897-1948. However, 

this long-term average is about 7 percent greater than 

the estimated actual flow in the 52 years, 23 percent 

greater than the measured flow in the period 1926-48, 

and 27 percent greater than the average discharge in 

the 14-year period 1935-48. The depletions caused by 

water development, currently about 2, 200, 000 acrefeet a year, will doubtless insure that the average flow 

at Grand Canyon in the future will be less than the 

average as indicated by tree-ring records. With no 

increase in the amount of upstream diversions, this 

long-term future average should be about 13,600,000 

acre-feet, or 8 percent greater than the average in the 

14-year period 1935-48. 

In four years since 1926 the annual runoff at Grand 

Canyon has been cl-ose to the projected long-term mean 

of 13, 600,000 acre-feet: In 1930, 1944, 1947, and 

1948 the runoff ranged from 13,400, 000 to 13, 900, OUO 

acre-feet. These four years demonstrate the striking 

variation in sediment transport that is characteristic 

of the records to data. The measured sediment load 

was more than 235 million tons in 1930, but less than 

98 million tons in 1944. In both 194 7 and 1948 the load 

was close to the 142 million-ton average for the 14-

year period 1935-48. Similar inconsistencies are apparent throughout the record, including years of high 

as well as low runoff. Thus in 1950 less sediment was 

carried in 11, 200,000 acre-feet of water than was 

transported by 6, 700, 000 acre-feet in 1931; and 

17, 300,000 acre-feet of water carried 389 million tons 

of sediment in 1927, but only 230 million in 1942. 

As shown by figure 5 the sediment-runoff relation in 

the years 1942 to 1950 is as consistent as for earlier 

years of record, and the curve showing that relation is 

approximately parallel to that drawn on the basis of 

records for the years 1926 to 1940. However, in each 

of the later nine years the annual sediment load has 

been 50 to 100 million tons less than would be expected 

on the basis of the curve established by data for years 

prior to 1941. 

Neither the annual runoff nor the seasonal distribution of runoff since 1941 has been perceptibly different 

from that in earlier years, and thus neither offers an 

explanation for the change in sediment-runoff relationship that year. Since that year introduced a new and 

different relation that has persisted for at least 9 

years. the factor or factors causing the change must 

have been effective for a similarly long period. It is 

believed that a possible explanation may lie in longterm trends in regional precipitation. The southwestern 

United States since 1941 has recorded a drought which 

is recognized as one of the eight most severe droughts 

in 600 years. Lake Mead is within this drought area, 

but the principal sources of its water supply are far to 

the north, in a region where precipitation has generally 

been about normal. The drought-affected area includes 

most of the sediment-producing areil of the Colorado 

River basin: the basins of the Virgin, Little Colorado, 

San Juan, and Dirty Devil Rivers, and smaller tributaries that enter the Colorado below the mouth of the 

Green River. The change in sediment-runoff relation 

since 1941 may well be a product of drought; reduced 

streamflows in sediment-producing tributaries, and 

corresponding reduction in contribution of sediment 

from ~hem; normal streamflow from the high headwater 

areas in Colorado, Wyoming, and northern Utah. 

The life of Lake Mead will be prolonged if some of 

the sediment is permitted to pass through the reservoir 

and dam and continue downstream. At the present time, 

of course, there is no possibility of passing sediment 

through the intake towers and there is little likelihood 

of doing so in the near future, because the water is 

habitually clear at the lower gates of the intake towers. 

At some time in the future it is inevitable that the sediment surface will approach that level, and it will then 

become a question of operational policy whether to 

cease using the lowest reservoir outlets or to draw 

sediment-laden water through them. If passage of 

sediment-laden water through the lower gates is accepted as normal procedure, the outflow of sediment 

will be nil for several decades, while the sediment 

surface rises toward the 895-foot level. Then one may 

expect sediment outflow, occasionally at first as a result of exceptional turbidity currents, then more frequently and with greater volumes of sediment as the 

sediment surface in Boulder Basin continues to rise. 

From the foregoing discussion, the following conclusions are believed to be warranted as to the future 

rates of sediment accumulation Lake Mead: 

1. The average annual rate of suspended-load transport at Grand Canyon in the 14-year period 1935-48 is 

142 million tons, and this rate is well confirmed by the 

calculated total of sediment accumulated in Lake Mead 

FIRST 14 YEARS OF LAKE MEAD 

during the period. However, the quantities calculated 

by both methods are probably 3 to 5 percent less than 

the total sediment deposited in the lake in the 14-year 

period. The error in suspended load measurements 

results chiefly from the omission of unmeasured loads 

of the Virgin River and other minor tributaries to the 

lake, whereas the error in measurements of the quantity of sediment accumulated in the lake results from 

the omission of unm.easured amounts of sediment deposited during the 1935 and 1948 surveys. The uppermost graph of figure 9 is based on the assumption that 

sediment accumulation will continue at the rate of approximately 148 million tons a year, or 4 percent 

more than the average annual suspended load at Grand 

Canyon during 1935-48. 

2. The average annual rate of suspended-load transport at Grand Canyon in the last 7 years of this 14-

year period was only 104 million tons, but this reduced 

rate is attributed to exceptional climatic variations 

within the basin and is not considered to be indicative 

of future trends. Inclusion of the data for these 7 

years has of course lowered the average annual rate 

in the 14-year period. 

3. The average annual suspended-load transport at 

Grand Canyon in the 23-year period 1926-48 is 175 

million tons. and if a 4-percent allowance is made for 

unmeasured contributions to the river above Black 

Canyon (as in 1935-48), the estimated average rate of 

sediment inflow to the Lake Mead area would be increased to 182,000,000 acre-feet a year. Most of the 

graphs of figure 9 are based on a projection of this 

rate. 

4. Correlation of sediment records at Grand Canyon 

and at Yuma is considered to be too weak to justify 

conclusions as to rates in years prior to 1926. 

5. The average annual runoff in the 23-year period 

1926-48 was only 2 percent greater than in the 14-year 

period 1935-48. There is evidence from tree-ring 

studies that the long-term average virgin flow of the 

Colorado River at Grand Canyon is about 15,800. 000 

acre-feet annually. which is slightly less than the 

average obtained from extended records of streamflow 

in 1897·-1948. corrected for stream depletions. With 

current depletions. however. the estimated long-term 

average at Grand Canyon is about 13.6 million acrefeet a year. The 1926-40 curve on figure 5 offers a 

possible sediment-runoff relation. based on 15 years 

of record: with annual runoff of 13.6 million acre-feet 

the annual sediment load would be about 200 million 

tons. The basis for this estimate is weak. because of 

the great uncertainty as to the sediment-runoff relation. 

6. The construction of Glen Canyon reservoir will 

intercept more than 75 percent of the sediment moving 

toward Lake Mead. and together with Bridge Canyon 

reservoir will intercept about 95 percent of it, until 

the two reservoirs start spilling sediment. Upstream 

development and use of water may reduce the sediment 

inflow to these two reservoirs by about 25 percent. 

7. After the sediment level at Hoover Dam has risen 

to the level of the lower outlet gates. silt and clay can 

be passed through the turbines until the rate of sediment outflow approaches half the rate of sediment inflow. 

The graphs of figure 9 demonstrate the variation in 

estimates based on various assumptions as to the future. More significant, they show that the benefits of 

Lake Mead will continue for many centuries and that 

the usefulness of the reservoir will not be seriously 

impaired. 

PROBLEMS FOR FUTURE STUDY 

The Lake Mead survey has provided answers to many 

perplexing questions. but equally important. it has 

raised many new questions and serves to focus attention 

upon problems that require further study. Many of the 

conclusions throughout the technical report are necessarily hedged because of the incompleteness of our 

knowledge. This is particularly noticeable relative to 

the life of the reservoir. which is necessarily speculative because it is concerned with future centuries. 

but which must be satisfied with wide limits because of 

uncertainty both as to observed phenomena and basic 

principles of sedimentation. The Lake Mead survey 

serves as an introduction both to a discussion of problems related to reservoirs in general by Thomas Maddock. Jr. • and to discussions of the broad hydrologic 

and geologic problems of the Colorado River basin by 

C. B. Hunt and H. E. Thomas. 

LITERATURE CITED 

Anderson, E. R .• and Pritchard. D. W .• 1951, Physical limnology of Lake Mead: Navy Electronics Lab .• 

Rept. 258. 152 p. 

Bell, H. S .• 1942a, Density currents as agents for 

transporting sediment: Jour. Geology, v. 50, p. 

514. 

1942b, Stratified flow in reservoirs and its use 

in the prevention of silting: U. S. Dept. Agriculture 

Misc. Pub. 491. 46 p. 

Gould. H. R .• 1951, Some quantitative aspects of Lake 

Mead turbidity currents: Soc. Econ. Paleontologists 

and Mineralogist. Special Pub. 2, p. 34-52. 

Grover. N. C .• and Howard, C. S .• 1938, Th.e passage 

of turbid water through Lake Mead: Am. Soc. Civ. 

Eng. Trans .• v. 103, p. 720-732. 

Hedberg. H. D •• 1936. Gravitational compaction of 

claysandshales: Am. Jour. Sci., v. 31. p. 241-

287. 

Longwell, C. R .• 1936, Geology of the Boulder Reservoir floor; Geol. Soc. America Bull .• v. 47, p. 

1393-1476. 

Schulman. Edmund. 1945. Tree-ring hydrology of the 

Colorado River basin: Univ. Ariz. Bull .• v. 16. 

no. 4. 

Sherman, Irving, 1951. A rapid substitute for textural 

analysis: Jour. Sed. Petrology. v. 21. p. 173-177. 

1953. Flocculent structure of sediment suspended in Lake Mead: Am. Geophys. Union Trans .• v. 

34. p. 394-406. 
LITERATURE CITED 27 

Sykes, Godfrey, 1937, The Colorado delta: Am. Geog. 

Soc., Special Pub. 19, p. 132-133. 

Terzaghi, Karl, 1925, Principles of soil mechanics, 

elastic behavior of sand and clay: Eng. NewsRecord, v. 95, p. 989-990. 

U. S. Dept. Agriculture, Soil Conservation Service, 

1935, Topographic maps of Lake Mead area: 52 

sheets at scale 1:12,000 

U. S. Dept. Interior, Bureau of Reclamation, 1941, 

Lake Mead density currents investigations 1937-40, 

v. 1 and 2, 453 p. 

U. S. Dept. Interior, Bureau of Reclamation, 1947, 

Lake Mead density currents investigations 1941-46, 

v. 3, p. 454-904. 

1949, Report of river control work and investigations in 1948 and 1949: Office of River Control, 

Boulder City, Nev., p. 53-124. 

195 3, Report of river control work and investigations in 1950 and 1951: Office of River Control, 

Boulder City, Nev., p. 59-94. 

INT.-DUP. SEC .• WASH .. D.C683J3