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. .~ N o '~j . . . PART II. DESCRIPTIO~ OF BASI~ A. Geolo~v Rocks of all ages from those of the Archean age (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. Many periods of deposition. erosion. and upheaval have played a part in the present structure of these mountains. In contrast to the folded rocks of the mountains which fringe the basin. the plateau country of southwestern Wyoming, eastern Utah, and northern Arizona is composed principally of horizontal strata of sedi- mentary rocks. Slo~ but constant elevation of the land area has allo~ed the Colorado River and its tributaries to CUL narrow. deep canyons into the flat-topped mesas. This type of erosion reaches its culmination in the Grand Canyon ~here the Colorado River has cut through all of the sedimentary rocks down to the oldest Archean granites. The Lower Basin is by lo~ 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 areas ~as depos- ited in Arizona, California, and ~exico and noy forms the great delta of the Colorado RiVer. Reservoirs constructed above Lee Ferry (Lake Powell. Flaming Gorge, Fontenelle, ~avajo. Morrow Point. and Blue Mesa). together ~ith Lake ~ead downstream, have caused some major changes in stream regimen: (1) The stream channels inundated by these reservoirs are no longer subject to natural stream erosion; (2) the accumulation of sediment and water within the reservoir slows the gro~th and flooding of the Colorado River delta; (3) flooding has diminished in many areas; and (4) sections of sediment- laden streams have given ~ay to clear water streams and lakes. The salt concentration in runoff increases froc the headwater areas downstream and occurs in relation to the geologic character of the terrain across which the Colorado River and its tributaries flow. The geologic formations that largely contribute to the salinity concentra- tions in natural runoff are evaporites of Paleozoic age, shale of Cre- taceous age, and salt and gypsum of ,Tertiary age. 5