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Comprehensive Survey of Sedimentation in Lake Mead, 1948-49

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Comprehensive Survey of Sedimentation in Lake Mead, 1948-49 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.
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