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. PART II. DESCRIPTIO}; OF BASIN ~ A. Geologv o o Rocks of all ages from those of the Archean a~e (the oldest known ~ geological period) to the recent alluvial deposits, including igneous. sedimentary, and metamorphic types. are found in the Colorado River Basin. The high Rocky Mountains which dominate the topography of the upper regions are composed of granites. schists. gneisses, lava, and sharply folded sedimentary rocks of limestone. sandstone, and shale. ~Iany periods of deposi tion, erosion. and upheaval have played a part in the present structure of these mountains. In contrast to the folded rocks of the mountains which frin~e the basin, the plateau country of south\.lestern "'yarning, eastern Utah, and northern Arizona is composed principally of horizontal strata of sedi- mentary rocks. Slo~' but constant elevation of the land are<l has allowed the Colorado River and its tributaries to cut narrow, deep canyons into the flat-topped mesas. This type of erosion reaches its culmination in the Grand Canyon M,nere the Colorado River has cut throu~h all of the sedimentary rocks down to the oldest Archean granites. . The Lower Basin is by low mountain ranges. of alluvial deposits. characterized by broad. flat valleys separated These valleys are filled by large accumulations Sediment removed by constant erosion of the upper <lreas IoCS depos- ited in Arizona, California, and l'fexico and now forms the ~reat delta of the Colorado RiVer. Reservoirs constructed above Lee Ferry (Lake Powell, Flaming Gorge, Fontenelle, Navajo, M:orrow Point, and Blue l'lesa) , together with Lake Mead downstream, have caused some m3jor changes in stream regimen: (1) The stream channels inundated by these reservoirs are no lon~er subject to natural stream erosion; (2) the accumulation of sediment and wter within the reservoirs slows the grOlo'th and floodin~ of the Colorado River delta; (3) flooding ha.s diminished in many areas; and (4) sections of sedirnent- laden streams have given \..ny to clear wter streams and lakes. The :;alt concentration in runoff increases from the head\olater areas downstream and occurs in relation to the geologic character of the terrain across ",,+aich the Colorado River and its tributaries flow. The geologic formations that lar~ely contribute to the &1.linity concentra- tions in natural runoff are evaporites of Paleozoic a~e, shale of Cre- taceous age, and salt and gypsum of Tertiary age. . 10