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<br />. <br /> <br />PART II. DESCRIPTION OF BASIN <br /> <br />l-& <br />o <br />A <br />C.U <br /> <br />A. Geology <br /> <br />Rocks of all ages from those of the Archean age (the oldest known <br />geological period) to the recent alluvial depos its, including igneous, <br />sedimentary, and metamorphic types, are found in the Colorado River Basin. <br />The high Rocky Mountains which dominate the topography of the upper regions <br />are composed of granites, schists, gneisses l lava, and sharply folded <br />sedimentary rocks of limestone, sandstone, and shale. Many periods of <br />deposition, erosion, and upheaval have played a part in the present struc- <br />ture of these mountains. <br /> <br />In contrast to the folded rocks of the mountains which fringe the <br />basin, the plateau country of southwestern Wyoming, eastern Utah, and <br />northern Arizona is composed principally of horizontal strata of sedi- <br />mentary rocks. Slow but constant elevation of the land area has allowed <br />the Colorado River and its tributaries to cut narrow, deep canyons into the <br />flat-topped mesas. This type of erosion reaches its culmination in the <br />Grand Canyon where the Colorado River has cut through all of the sedi- <br />mentary rocks down to the oldest Archean granites. <br /> <br />.' <br />.!r <br /> <br />The Lower Basin <br />low mountain ranges. <br />alluvial deposits. <br /> <br />is characterized by broad, flat valleys separated by <br />These valleys are filled by large accumulations of <br /> <br />Sediment removed by <br />in Arizona, Cali foroia, <br />Colorado River. <br /> <br />constant erosion of the upper areas was deposited <br />and Medco and no<, forms the great delta of the <br /> <br />Reservoirs constructed above Lee Ferry such as Lake Powell, Flaming <br />Gorge, Fontenelle, Navajo, Wayne N. Aspinall Unit (Blue Mesa, Morrow <br />Point, and Crystal), together with Lake Mead downstream, have caused some <br />major changes in stream regimen: (l) the stream channels inundated by <br />these reservoirs are no longer subject to natural stream erosion; (2) the <br />accumulation of sediment and water within the reservoir slows the growth <br />and floodin~ of the Colorado River delta; (3) flooding has diminished in <br />many areas; and (4) sections of sediment-laden streams have given way to <br />clear water streams and lakes. <br /> <br />The salt concentration in runoff generally increases from the head- <br />water areas downstream and occurs in relation to the geologic character of <br />the terrain across which the Colorado River and its tributaries flow. The <br />geologic formations that largely contribute to the salinity concentrations <br />in natural runoff are evaporites of Paleozoic age, shale of Cretaceous age, <br />and salt and gypsum of Tertiary age. <br /> <br />. <br /> <br />7 <br />