Comprehensive Survey of Sedimentation in Lake Mead, 1948-49

GEOLOGICAL SURVEY PROFESSIONAL PAPER 295

Prepared in collaboration with the U.S. Depart- ment of the Interior, Bureau of Reclamation; U.S. Department of the Navy, Chief of Naval Opera- tions', Bureau of Ships, Bureau of Ordnance, Bureau of Naval Personnel, Hydrographic Ojfice, and Navy Electronics Laboratory; U.S. Department ofCommerce, Coast and Geodetic Survey; University of California, Scripps Institution of Oceanography D. DRAINAGE BASIN TRIBUTARY TO LAKE MEAD

By H. E. THOMAS, U.S. Geological Survey

Lake Mead is in the Lower Colorado River Basin, peaks of the Rocky Mountains as well as high moun- as defined in the Colorado River Compact of 1922. 1 tain valleys and broad upland plains, extensive pla- Hoover Dam is about 354 miles downstream from Lee teaus and spectacular canyons, broad alluvial valleys, Ferry, \vhich is the Compact point marking the lower and desert ranges. Climatologically, the basin pre- limit of the Upper Colorado River Basin. The Lower sents the extremes of year-round snow cover and heavy Basin, however, provides only a very small propor- precipitation on the higher peaks of the Rockies, and tion of the water that flows into Lake Mead; these desert conditions in which annual precipitation is com- contributions come from the Little Colorado River, the monly less than 5 inches. The temperature range is Virgin River, small perennial or ephemeral creeks, and from the temperate—as in the mountain meadows of some fairly large springs in northern Arizona. In an Colorado and Wyoming, where there is a scanty 90-day average year about 95 percent of the inflow to Lake growing season—to the semitropical—as near Lake Mead comes from the Upper Basin. Thus the water Mead, where only occasional winter frosts occur. Geo- that accumulates in Lake Mead comes from an area logically, the rocks in the basin are of a wide variety of that extends far beyond the limits of the area encom- types, ranging in age from Precambrian to Recent. passed by the reservoir survey of 1948-49. Basic water As might be expected in a region of such diverse data collected by the Geological Survey in coopera- physical characteristics, there are some areas with very tion with the States of Arizona, Colorado, Nevada, high rates of runoff, and other areas that yield prac- New Mexico, Utah, and Wyoming constitute a broad tically no water to the Colorado River; also certain general outline of the hydrologic conditions in that areas in which the streams are generally clear, and large tributary area, but they are far from adequate other areas that are major contributors of the sedi- for comprehensive planning of water-resource develop- ment that enters Lake Mead. ment above Lake Mead. This section summarizes these existing data and provides a background of informa- PHYSIOGRAPHY tion as to the source of the water and sediment in Lake Mead. The drainage basin tributary to Lake Mead includes parts of five physiographic provinces as outlined by The part of the Colorado River drainage basin tribu- Fenneman (1930). The easternmost part of that basin tary to Lake Mead comprises about 168,000 square is within the Southern Rocky Mountains province, miles, or 5 percent of the continental area of the United States. If this vast area has one characteris- which comprises high mountain ranges and intermon- tane valleys. Characteristically, this part of the basin tic in common, it is perhaps the lack of hospitality receives heavy precipitation, and most of it is covered to mankind. Very little of the area is desirable for habitation, and the lands occupied by the basin are by forest and alpine flora. Many of the largest streams accordingly among the least populated parts of Colo- of the Colorado River system—the Yampa, White, Col- rado, Wyoming, Utah, New Mexico, and Arizona. orado, Gunnison, Dolores, and San Juan Rivers—rise The basin has a greater range in physical charac- in this mountainous province. acteristics than is to be found in most other areas of The northwestern part of the Colorado River basin comparable size on the North American continent. lies within the Middle Rocky Mountains province. Like Physiographically, it includes several of the highest the Southern Rockies, this province is composed of high forested mountain ranges and intermontane valleys. 1 The agreement concerning the apportionment of the use of the waters Because of its generally higher latitude, the growing of the Colorado River System dated November 24, 1922, executed at season in these valleys is shorter than in valleys of the Santa Fe, N. Mex., by Commissioners for the States of Arizona, Cali- fornia, Colorado, Nevada, New Mexico, Utah, and Wyoming, approved Southern Rockies, and the annual evaporation rate is by Herbert Hoover as representative of the United States of America, less. In this province are the headwaters of the Green and proclaimed effective toy the President of the United States of America, June 25, 1929. River and many of its important tributaries. 496918 O—60———3 21 22 COMPREHENSIVE SURVEY OF SEDIMENTATION IN LAKE MEAD, 1948-49

Between the Middle and Southern Rocky Mountains of the plateau country attract the hobbyist without is the Wyoming Basin, a physiographic province char- formal training in geology; yet the complex structures acterized by elevated plains in various stages of ero- of many of the mountain ranges are brain teasers for sion, and isolated low mountains. Precipitation is the ablest geologists and geophysicists. generally less than 20 inches a year, and the basin is The geologic formations of the tributary drainage classed as semiarid. Most of the streams draining this basin are significantly related to the water and the dis- area are intermittent; they contribute only small quan- solved and suspended materials that enter Lake Mead. tities of water to the Colorado River system, chiefly A negligible proportion of the runoff is derived by during annual freshets. direct precipitation upon the stream channels. Some- The central part of the drainage basin tributary to what larger quantities may result from overland run- Lake Mead is within the Colorado Plateaus province. off, particularly from bare rock or impervious mantle Some of these plateaus are high and forested, and they rock. Most of the runoff is derived from melting of receive a moderate amount of precipitation; others are winter accumulations of snow in the higher mountains, lower canyoned plateaus of high relief; and large areas but nearly all this water moves for at least a short dis- are rather thoroughly dissected. The precipitation tance through the soil or underlying ground-water over most of the plateau country is scant. Sonoran zones before reaching the streams. The quantity of types of vegetation—including pinon, sagebrush, and runoff generally is only a minor fraction of the precipi- grasses—provide sparse cover, leaving large areas of tation upon the region and represents the excess of bare or thinly mantled rock. The streams rising in the water beyond the requirements for transpiration by plateau country make relatively small annual contribu- plants and for direct evaporation. Pervious rock mate- tions to the flow of the Colorado River; the Little rials that absorb a large proportion of the precipitation Colorado River is the largest of these contributors. may subsequently be dewatered by evapotranspiration In drainage basins where steep, barren or sparsely draft or by discharge of ground water to streams. vegetated slopes are prevalent, rainstorms may produce Thus the geology is a significant factor in the precipita- high runoff for short periods. This is particularly true tion-runoff relations throughout the tributary basin. of the southern part of the drainage basin, where in- The chemical constituents carried by the river are tense summer rainstorms may produce flash floods that derived in part by solution of rock materials over constitute a substantial proportion of the annual which the river and its tributaries flow. Thus the runoff. Colorado River in its annual flood stages laps against The lowest part of the drainage basin tributary to beds of gypsum in Cataract Canyon in southeastern Lake Mead is in the Basin and Range province; the Utah, and some of that rock is dissolved and carried lake itself is almost entirely within this province. The away in the river. Some tributaries likewise flow di- climate here is arid, and the desert mountains and rectly over soluble bedrock, and as a result their waters valleys contribute practically no water to the lake ex- have a high mineral content—a fact that is suggested cept after exceptional rainstorms. Two perennial by the names given to several small streams in the streams enter Lake Mead within the province: Muddy basin, such as Onion Creek, Salt Wash, Saleratus Wash, Creek, which has a sustained flow from springs, and and Gypsum Canyon. However, outcrops of soluble the Virgin River, which derives most of its water from bedrock in stream channels occupy a very small propor- high plateaus and from the Pine Valley Mountains in tion of the total channel area, and the quantity of dis- southwestern Utah. solved materials derived from them is also a small GEOLOGY fraction of the total dissolved solids carried into Lake The geology of the drainage basin tributary to Lake Mead. Mead is sufficiently diverse that its problems attract Most of the dissolved material in the river is derived specialists in all phases of the earth sciences. Rocks in from ground water. The most obvious sources are the area range in age from Precambrian to Recent and saline springs along the main stem and tributary chan- in type through a wide assortment of igneous, sedi- nels, springs rising from limestones or igneous rocks, mentary, and metamorphic. Deposits of economic fault springs, and return flow of irrigation water. value in the basin include gold, silver, lead, zinc, cop- Ground water seeping into the river and its tributaries per, molybdenum, antimony, vanadium, and uranium; carries a large proportion of the total dissolved solids coal, petroleum, natural gas, oil, shale, and rare hydro- of the river. Types of rock that are important con- carbons ; phosphate rock, potash, salt, gypsum, helium, tributors of dissolved materials include evaporites, and carbon dioxide; and construction materials. The shales, and sandstones. These rocks crop out princi- broad, simple structures and easily recognized strata pally in the Colorado Plateaus and the Wyoming Basin. DRAINAGE BASIN TRIBUTARY TO LAKE MEAD 23

The geologic formations cropping out within the the river and its tributaries are cutting both downward drainage basin are also the ultimate sources of the sedi- and laterally into the alluvial deposits of past centuries. ment carried by the Colorado River and its tributaries. These alluvial deposits are believed to constitute the The formations include a great variety of rock types, immediate source of much of the sediment now moving ranging from highly indurated to loosely cemented or toward Lake Mead. unconsolidated rocks. Individual sedimentary forma- WATER tions commonly include a wide range in textures, both The Colorado River is most impressive in its canyons horizontally and vertically. A thorough appraisal of below the mouth of the San Juan River—in Glen Can- the sediment-producing characteristics of any part of yon, or in Marble Canyon or Grand Canyon where the basin would require detailed geologic mapping of tourists most commonly view it. Viewed from the rim those formations and, in addition, mapping in cor- of one of these canyons, the river is a thin ribbon of responding detail of the soil or mantle conditions, the water that is dwarfed by the massiveness of the setting. vegetative cover, and the topography. Such mapping The great plateau country of northern Arizona and has not yet been completed for any major part of the southeastern Utah provides a scenic environment and Colorado River basin. much sediment for the river, but little more. Of the A rough comparison of the sediment-producing capa- water that enters Lake Mead, only slightly more than bilities of various parts of the basin may be made on the 5 percent is contributed by the small streams of Ari- basis of the generalized outcrop areas as shown on the zona, Nevada, New Mexico, and southern Utah. All geologic map of the United States. Extensive areas, erosion. principal tributaries. Indeed, more than three-fourths For the most part these rocks form the mountain ranges of the annual inflow to Lake Mead is derived from the and are of Paleozoic age or older. However, there are high headwater areas of streams that rise in the numer- also extensive areas of volcanic rocks, including some ous ranges of the Rocky Mountain chain. that are remote from mountainous areas, which are for For the most part the arid plateau land contributes the most part of Tertiary or Quaternary age. Gener- only enough water to the river to offset the rather ally the streams rising in these areas are fairly free of sizable losses from the river due to evaporation and sediment at all seasons. The rocks most common to the transpiration. Along extended segments of the river areas that produce relatively clear water are crystalline there is practically no inflow during most of the year, and metamorphic types, lavas, limestones, and well- and because of the evapotranspiration draft the river cemented sandstones and conglomerates. passes through those reaches with a net loss during Much of the drainage basin in the Colorado Plateau, most of the year. and nearly all of the Wyoming Basin, is underlain by The sources of the inflow to Lake Mead during the shale, mudstone, and clay—rocks which upon weather- 1948 water year are shown graphically in figure 5. It ing yield predominantly silt- and clay-sized particles. is evident from this diagram that the main stem of the The silt-producing formations are chiefly of Mesozoic Colorado in Utah and Arizona, as well as the channel and Tertiary age. The Little Colorado, San Rafael, of the Green River below the Uinta Basin and the and Dirty Devil Rivers drain basins in which fine tex- channel of the San Juan River below the mouth of the tured materials predominate. Animas River, serve principally as conduits for water A classification of the drainage basin according to that has originated in headwater areas to the north and capabilities of producing sediments, as inferred from east. geologic maps, is presented in figure 4. Although the The concentration and chemical character of dis- rock outcrops and the mantle rock formed from them solved solids in the streams vary considerably through- constitute the ultimate source of the sediment load car- out the year. As a rule the concentration is greatest ried by the river, it is likely that only an insignificant during periods of low flow and least during flood part of the load comes directly from those sources. periods. Howard (1948) has shown that there is a Indeed, the main stream and tributaries rarely come progressive downstream increase in concentration of in contact with bedrock except in their headwaters, and dissolved solids both in the Colorado River and in its instead the bed and banks of the channels are formed principal tributaries. These increases are caused of stream-borne sediment, or alluvium. Geologic re- chiefly by natural ground-water inflow and by return connaissances show that the area covered by alluvium water from agricultural and industrial developments. is only a very small proportion of the total area of the Large increases are recorded, for example, in a short basin, but that, rather generally throughout the basin, reach of the river near Grand Junction, Colo., where 26 COMPREHENSIVE SURVEY OF SEDIMENTATION IN LAKE MEAD, 1948-49 there is considerable return flow of irrigation water to August, September, and October, following intense the stream. rainstorms, and may carry heavy loads of sediment to SEDIMENT the Colorado River at that time. That river ordinarily Historically the sediment has always been a con- is at relatively low stage during those months and is spicuous feature of the Colorado River, as indicated by unable to carry the heavy load; consequently, much of the name "colored river", first applied by the early the sediment may be dropped along its channel. Trans- Spanish explorers. At all times the observer is likely portation of such debris may not be resumed until the to be impressed by the murkiness of the water of the high main-stem flows of the succeeding year. The me- main stem and its principal tributaries—generally chanics of sediment transportation are complex, and tawny brown, perhaps reddish or grayish, but character- there is not necessarily a close correlation between pre- istically muddy. cipitation over the drainage basin, runoff, and sediment The rock materials which the streams have trans- load in a designated period. ported and deposited during the latest geologic epoch In its relation to the stream system, the alluvium may are classified by geologists as Recent alluvium. This be classified as (1) sediment "in transit"—sediment alluvium forms the flood plains that flank the main moving either as suspended load or as bed load moving stems and tributaries along many miles of their courses; along the channel bottom; (2) sediment in "live stor- it constitutes the islands, bars, and shoals in the river age"—sediment that is temporarily at rest in bars, and underlies the river channel. In places where the islands, stream banks, and bottoms, where the annual channel is constricted, the alluvium may be as much as fluctuations in streamflow may readily place it in transit 200 feet thick, as shown by test borings at prospective again; and (3) sediment in "dead storage"—alluvium dam sites. And in the broad valley plains the Recent that has remained in place for a long time beneath flood alluvium may form the surficial material in a belt as plains or far beneath the channel bed, where reexcava- much as 3 miles wide. tion and further transportation are unlikely unless there The ultimate source of the Recent alluvium is the are appreciable changes in runoff, gradient, or other older rock formations of the drainage basin, and the controlling factors. fragments that result from their weathering. Where SUSPENDED LOAD gradients are steepest—chiefly in the small tributary channels—a stream may flow directly over bedrock for Data concerning the sediment load during the water some distance; and in other reaches the channel may be year 1948, based on samples collected daily or oftener, bordered by bedrock walls. In these places the stream have been obtained by the Geological Survey at eight itself becomes the eroding agent, and acquires some sedi- stations in the drainage basin tributary to Lake Mead. ment directly from bedrock. Overland flow over bare During the year records were begun at half a dozen or impermeable rock material may also carry sediment other localities, which permit a more thorough analysis into the streams. of the transportation of sediment by the river and its It is likely that only a negligible proportion of the tributaries during subsequent years. sediment in Lake Mead has moved continuously from The suspended load during 1948 is shown graphically its original source. Generally the process of sediment on figure 6. Although the data are inadequate to show transportation is a series of short travels, with inter- how much sediment is carried by several of the tribu- vening deposition and reexcavation. Typically a stream taries that are suspected of being important contrib- that is eroding along a certain reach will carry some of utors, comparison with figure 5 shows that the streams that sediment only to a reach having lesser gradient, and that yield most of the water do not carry a proportionate the sediment remains there until a greater volume of share of sediment. In 1948 the flow of the Colorado water can carry it farther. The tributaries quite gen- River at Cisco, Utah, plus that of the Green River at erally have higher gradients than do the main stems, Green River, Utah, was more than three-fourths of the and boulders and blocks that can be moved by those trib- total that entered Lake Mead (fig. 5); but the sediment utaries may come to rest opposite their mouths, in the passing those two points was less than a quarter of the main stems. Practically all of the rapids in the Colo- amount entering the lake. On the other hand, the rec- rado and Green Rivers in Utah are caused by this accu- ords for three tributaries in the southern part of the mulation of coarse detritus at the mouths of tributaries. basin—the Little Colorado, Paria, and San Juan Riv- Climatologic conditions are a factor contributing to ers—show that these streams yield a far greater propor- the discontinuity and irregularity of sediment transpor- tion of the suspended sediment than of the water that tation. Many of the minor tributaries that drain the reaches Lake Mead. It is inferred from the sediment Colorado Plateaus have high peak discharges during "pickup" along the main stem in southern Utah and DRAINAGE BASIN TRIBUTARY TO LAKE MEAD 23

The geologic formations cropping out within the the river and its tributaries are cutting both downward drainage basin are also the ultimate sources of the sedi- and laterally into the alluvial deposits of past centuries. ment carried by the Colorado River and its tributaries. These alluvial deposits are believed to constitute the The formations include a great variety of rock types, immediate source of much of the sediment now moving ranging from highly indurated to loosely cemented or toward Lake Mead. unconsolidated rocks. Individual sedimentary forma- WATER tions commonly include a wide range in textures, both The Colorado River is most impressive in its canyons horizontally and vertically. A thorough appraisal of below the mouth of the San Juan River—in Glen Can- the sediment-producing characteristics of any part of yon, or in Marble Canyon or Grand Canyon where the basin would require detailed geologic mapping of tourists most commonly view it. Viewed from the rim those formations and, in addition, mapping in cor- of one of these canyons, the river is a thin ribbon of responding detail of the soil or mantle conditions, the water that is dwarfed by the massiveness of the setting. vegetative cover, and the topography. Such mapping The great plateau country of northern Arizona and has not yet been completed for any major part of the southeastern Utah provides a scenic environment and Colorado River basin. much sediment for the river, but little more. Of the A rough comparison of the sediment-producing capa- water that enters Lake Mead, only slightly more than bilities of various parts of the basin may be made on the 5 percent is contributed by the small streams of Ari- basis of the generalized outcrop areas as shown on the zona, Nevada, New Mexico, and southern Utah. All geologic map of the United States. Extensive areas, the rest comes from Colorado, Wyoming, and northern especially near the boundaries of the basin, are under- Utah by way of the main stem of the river or its lain by indurated rocks that are resistant to erosion. principal tributaries. Indeed, more than three-fourths For the most part these rocks form the mountain ranges of the annual inflow to Lake Mead is derived from the and are of Paleozoic age or older. However, there are high headwater areas of streams that rise in the numer- also extensive areas of volcanic rocks, including some ous ranges of the Rocky Mountain chain. that are remote from mountainous areas, which are for For the most part the arid plateau land contributes the most part of Tertiary or Quaternary age. Gener- only enough water to the river to offset the rather ally the streams rising in these areas are fairly free of sizable losses from the river due to evaporation and sediment at all seasons. The rocks most common to the transpiration. Along extended segments of the river areas that produce relatively clear water are crystalline there is practically no inflow during most of the year, and metamorphic types, lavas, limestones, and well- and because of the evapotranspiration draft the river cemented sandstones and conglomerates. passes through those reaches with a net loss during Much of the drainage basin in the Colorado Plateau, most of the year. and nearly all of the Wyoming Basin, is underlain by The sources of the inflow to Lake Mead during the shale, mudstone, and clay—rocks which upon weather- 1948 water year are shown graphically in figure 5. It ing yield predominantly silt- and clay-sized particles. is evident from this diagram that the main stem of the The silt-producing formations are chiefly of Mesozoic Colorado in Utah and Arizona, as well as the channel and Tertiary age. The Little Colorado, San Rafael, of the Green River below the Uinta Basin and the and Dirty Devil Rivers drain basins in which fine tex- channel of the San Juan River below the mouth of the tured materials predominate. Animas River, serve principally as conduits for water A classification of the drainage basin according to that has originated in headwater areas to the north and capabilities of producing sediments, as inferred from east. geologic maps, is presented in figure 4. Although the The concentration and chemical character of dis- rock outcrops and the mantle rock formed from them solved solids in the streams vary considerably through- constitute the ultimate source of the sediment load car- out the year. As a rule the concentration is greatest ried by the river, it is likely that only an insignificant during periods of low flow and least during flood part of the load comes directly from those sources. periods. Howard (1948) has shown that there is a Indeed, the main stream and tributaries rarely come progressive downstream increase in concentration of in contact with bedrock except in their headwaters, and dissolved solids both in the Colorado River and in its instead the bed and banks of the channels are formed principal tributaries. These increases are caused of stream-borne sediment, or alluvium. Geologic re- chiefly by natural ground-water inflow and by return connaissances show that the area covered by alluvium water from agricultural and industrial developments. is only a very small proportion of the total area of the Large increases are recorded, for example, in a short basin, but that, rather generally throughout the basin, reach of the river near Grand Junction, Colo., where 24 COMPREHENSIVE SURVEY OF SEDIMENTATION IN LAKE MEAD, 1948-49

——r ——————— ______I K j I 'li:

EXPLANATION

Areas contributing few sediments

Boundary of drainage basin NEVADA

!*• NEW MEXICO

FIGURE 4.—Map of drainage basin tributary to Lake Mead, showing source areas of sediments as inferred from surface geology. DRAINAGE BASIN TRIBUTARY TO LAKE MEAD 25

EXPLANATION Of 14,000,000 acre-feet of water entering Lake Mead-

Each solid line represents 5 percent ^WYOMING Each dashed line represents 2 to 4 percent

Each dotted line represents 1 percent

Stream-gaging station

Outline of Colorado River drainage basin

FIGURE 5.—Streamflow entering Lake Mead during water year 1948. 26 COMPREHENSIVE SURVEY OF SEDIMENTATION IN LAKE MEAD, 1948-49 there is considerable return flow of irrigation water to August, September, and October, following intense the stream. rainstorms, and may carry heavy loads of sediment to SEDIMENT the Colorado River at that time. That river ordinarily Historically the sediment has always been a con- is at relatively low stage during those months and is spicuous feature of the Colorado River, as indicated by unable to carry the heavy load; consequently, much of the name "colored river", first applied by the early the sediment may be dropped along its channel. Trans- Spanish explorers. At all times the observer is likely portation of such debris may not be resumed until the to be impressed by the murkiness of the water of the high main-stem flows of the succeeding year. The me- main stem and its principal tributaries—generally chanics of sediment transportation are complex, and tawny brown, perhaps reddish or grayish, but character- there is not necessarily a close correlation between pre- istically muddy. cipitation over the drainage basin, runoff, and sediment The rock materials which the streams have trans- load in a designated period. ported and deposited during the latest geologic epoch In its relation to the stream system, the alluvium may are classified by geologists as Recent alluvium. This be classified as (1) sediment "in transit"—sediment alluvium forms the flood plains that flank the main moving either as suspended load or as bed load moving stems and tributaries along many miles of their courses; along the channel bottom; (2) sediment in "live stor- it constitutes the islands, bars, and shoals in the river age"—sediment that is temporarily at rest in bars, and underlies the river channel. In places where the islands, stream banks, and bottoms, where the annual channel is constricted, the alluvium may be as much as fluctuations in streamflow may readily place it in transit 200 feet thick, as shown by test borings at prospective again; and (3) sediment in "dead storage"—alluvium dam sites. And in the broad valley plains the Recent that has remained in place for a long time beneath flood alluvium may form the surficial material in a belt as plains or far beneath the channel bed, where reexcava- much as 3 miles wide. tion and further transportation are unlikely unless there The ultimate source of the Recent alluvium is the are appreciable changes in runoff, gradient, or other older rock formations of the drainage basin, and the controlling factors. fragments that result from their weathering. Where SUSPENDED LOAD gradients are steepest—chiefly in the small tributary channels—a stream may flow directly over bedrock for Data concerning the sediment load during the water some distance; and in other reaches the channel may be year 1948, based on samples collected daily or oftener, bordered by bedrock walls. In these places the stream have been obtained by the Geological Survey at eight itself becomes the eroding agent, and acquires some sedi- stations in the drainage basin tributary to Lake Mead. ment directly from bedrock. Overland flow over bare During the year records were begun at half a dozen or impermeable rock material may also carry sediment other localities, which permit a more thorough analysis into the streams. of the transportation of sediment by the river and its It is likely that only a negligible proportion of the tributaries during subsequent years. sediment in Lake Mead has moved continuously from The suspended load during 1948 is shown graphically its original source. Generally the process of sediment on figure 6. Although the data are inadequate to show transportation is a series of short travels, with inter- how much sediment is carried by several of the tribu- vening deposition and reexcavation. Typically a stream taries that are suspected of being important contrib- that is eroding along a certain reach will carry some of utors, comparison with figure 5 shows that the streams that sediment only to a reach having lesser gradient, and that yield most of the water do not carry a proportionate the sediment remains there until a greater volume of share of sediment. In 1948 the flow of the Colorado water can carry it farther. The tributaries quite gen- River at Cisco, Utah, plus that of the Green River at erally have higher gradients than do the main stems, Green River, Utah, was more than three-fourths of the and boulders and blocks that can be moved by those trib- total that entered Lake Mead (fig. 5); but the sediment utaries may come to rest opposite their mouths, in the passing those two points was less than a quarter of the main stems. Practically all of the rapids in the Colo- amount entering the lake. On the other hand, the rec- rado and Green Rivers in Utah are caused by this accu- ords for three tributaries in the southern part of the mulation of coarse detritus at the mouths of tributaries. basin—the Little Colorado, Paria, and San Juan Riv- Climatologic conditions are a factor contributing to ers—show that these streams yield a far greater propor- the discontinuity and irregularity of sediment transpor- tion of the suspended sediment than of the water that tation. Many of the minor tributaries that drain the reaches Lake Mead. It is inferred from the sediment Colorado Plateaus have high peak discharges during "pickup" along the main stem in southern Utah and DRAINAGE BASIN TRIBUTARY TO LAKE MEAD 27

Of 144,100,000 tons of sediment measured at Grand Canyon-

Each solid line represents 5 percent

Each dashed line represents 2 to 4 percent

Each dotted line represents 1 percent

Sediment-gaging station

Outline of Colorado River drainage basin

N E] V)A\D A I]

NEW .MEXICO

FIGURE 6.—Suspended sediment Inflow to Lake Mead during water year 1948. 28 COMPREHENSIVE SURVEY OF SEDIMENTATION IN LAKE MEAD, 1948-49

Arizona that other small tributaries in this area are Grand Canyon and the proportions of that load that likewise important contributors of sediment. pass the three upstream stations are depicted in figure 7. The areas from which the bulk of Lake Mead sedi- In the period of record at Grand Canyon, the year of ment is derived are in the Colorado Plateaus province highest runoff (1929) was also the year of greatest sedi- in Arizona, southeastern Utah and New Mexico. These ment contribution, and the year of least runoff (1934) areas are generally remote from the highest parts of was also the year of least sediment transportation. the basin and include the areas having least precipita- There are striking variations in the proportions of the tion and least runoff in the drainage basin. If the res- load that pass the three upstream stations from year to ervoir sedimentation is to be controlled, it would appear year, though it is generally true that the San Juan River that the control must be in these areas. carries less water and more sediment than either the Long-term records of suspended sediment are avail- Green River or the Colorado River above its junction able at four stations in the tributary drainage basin. with the Green. There is a noteworthy contrast between These records indicate that the sediment transported 1939 and 1943, two years of approximately equivalent during the water year 1948 was close to the average sediment load at Grand Canyon. In 1943 the load meas- annual load in the period 1930-48. Probably more than ured at the three upstream stations amounted to one- 95 percent of the sediment entering Lake Mead passes third, but in 1939 to more than two-thirds, of the through the Grand Canyon, where records of the sus- quantity measured at Grand Canyon. pended load have been obtained since 1925. Beginning with the water year 1930, similar records have been ob- BED LOAD tained for the Colorado Kiver at Cisco, the Green Kiver In many of the canyons of the Colorado and its prin- at Green River, and the San Juan River at Bluff, all cipal tributaries, the stream bed is composed predomi- in Utah. The annual variations in suspended load at nantly of sand and gravel, boulders, and huge blocks

Entife. column represents suspended load SUSPENDED LOAD IN measured in Colorado River ot Grand MAJOR TRIBUTARIES Canyon , Arizona (1943 estimated) 300 Part of column below dashed line is Upper Basin contribution as measured at Lees Little Colorado River at Cameron Ferry, Arizona (I93I-33-, 1948-50) Arizona (beginning 1949)

250 San Juan River near Bluff, Utah

200- Green River at Green River, Utah

Colorado River near Cisco, Utah Data from U. S. Geological Survey water- supply papers____ 150

2 100-

50-

1931 1935 1940 1945 1950 FIGURE 7.— Sediment carried by the Colorado River in water years 1931-50. DRAINAGE BASIN TRIBUTARY TO LAKE MEAD 29 and one might conclude that if the river moves such intervening period: meanders may have enlarged in material it must have truly magnificent power. Ac- some places and the channel straightened elsewhere, tually, the water moves around the larger obstacles, islands and bars shown on earlier photographs may have and if they are displaced at all it is for a negligible disappeared or changed in shape, and new ones may distance. Individual boulders can be identified in the have appeared on the later photographs. Marked same positions as when they were photographed more changes are noted in comparisons between the topo- than 40 years ago. Other evidence that gravel or larger graphic maps of 30 or more years ago and aerial photos size fragments are not carried far by the main stem taken in the 1930's. Channel sections measured during may be deduced from the distribution of rocks of cer- stream-gaging operations showed marked differences tain distinctive types. As an example, pebbles of from season to season, and sometimes from week to trachyte or of basalt in Glen Canyon may be identified week, particularly in certain reaches of the streams. in the deposits of the Colorado River, but only a few All these are evidences of the marked changes in live miles below the lowest tributary that drains the out- storage of sediment in the stream channel. crop areas of those rocks, which are, respectively, in It appears that streams on a rising stage carry in- the Henry Mountains and on the plateau near Navajo creased sediment in transit, and on a falling stage they Mountain. Finally, gravel is rare among the river drop the sediment from suspension, thus increasing the deposits in the lower parts of reaches of low gradient, live storage. At some places, as for instance along the of which Glen Canyon and the Uinta Basin are good San Juan River at Shiprock, N. Mex., the channel scour examples. It is concluded that for the most part the is so great at peak discharge that the gage height may gradients along the river are insufficient for large-scale be very little above, or even lower than, the height movement of gravel in the bed load. recorded when discharge of the stream is at minimum. Although the movement of gravel in the larger No quantitative surveys or even detailed reconnais- streams, expressed in ton-miles, is considered to be sances have been made to determine the volumes that small, the total transportation of sediment along the may be involved in this type of "live" storage along the bed may be significant. This bed-load transportation channel. It is probable, however, that the total volume is not included in determinations of suspended load. along the main-stem channels may be many times the It is likely that it represents a higher proportion of average annual deposition of sediment in Lake Mead. the total sediment in transit at some gaging stations than at others. For example, along the main stem of DEAD STORAGE the Colorado River suspended load is being measured By far the greater part of the Recent alluvium along in the Grand Canyon, where turbulent flow, rapids, the Colorado River and its tributaries above Lake and high gradients occur throughout a long reach of Mead is not readily accessible for further movement the river; at Lees Ferry, where smooth flow and low downstream. This alluvium underlies the channel at gradients are characteristic of the river upstream in depths that are not reached by the normal scouring Glen Canyon, and steeper gradients and turbulent flow action of the river but that have been penetrated in are the rule downstream in Marble Canyon; and at test drilling; and it forms the flood plains, which are Hite, in the upper part of Glen Canyon, where stream known to antedate the first visits of white men by many gradients are gentle and where the river might be ex- centuries, and which in many places are covered by pected to drop some of its suspended load after a tur- profuse vegetation. Even this alluvium has by no bulent course through Cataract Canyon. Comparing means reached a permanent resting place, however. these broad physical characteristics at the three sta- Under natural conditions the only resting place for tions, it might appear that the proportion of bed load sediment was at the mouth of the river in the Gulf of should be least in the turbulent flow of Grand Canyon, California; but the creation of Lake Mead above and greatest at Lees Ferry where the velocity is in- Hoover Dam has afforded a temporary substitute for creasing near the head of Marble Canyon. It is likely, this resting place. The alluvium upstream from Lake however, that the proportion of bed load is determined Mead is merely interrupted in its movement down- partly by the changes in gradient and velocity in the stream, but much of it will not be dislodged until some immediate vicinity of the sampling points, and data major change occurs in the basin: a change in climate are not available as to these factors at the established or vegetative cover resulting in markedly greater sea- gaging stations. sonal or annual runoff; increased stream gradients due LIVE STORAGE either to diastrophic movements or to further down- Aerial photographs taken in different years com- ward cutting of the channels; or modification of the monly show many changes in the channel during the natural conditions by development projects. In con- 30 COMPREHENSIVE SURVEY OF SEDIMENTATION IN LAKE MEAD, 1948-49 sequence of water-resources development, for example, major changes in climate, vegetative cover, or physi- the release of clear water below Hoover Dam caused ography are involved. Detailed analysis of the areas erosion of the river channel, which had been stable of renewed erosion and their recorded history may under natural conditions when the river carried a heavy result in establishing the date when renewed erosion sediment load. began, and the causes of that erosion. At some places in the Colorado River basin, grada- REFERENCES CITED tional processes are now at work on sediment that has long been in "dead" storage. Streams are eroding Fennernan, N. M., 1930, Physical divisions of the United States: U.S. Geol. Survey map, scale 1: 7,500,000. some flood plains that had become fairly stable fea- Howard, C. S. 1948, Quality of water in the Upper Colorado tures of the landscape, but it is not known whether River Basin, Trans. Am. Geophys. Union, v. 29, p. 375-378.