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8 DESCRIPTION OF BASIN o Q':l ~ Reservoirs constructed above Lee Ferry (Lake Powell, Flaming Gorge, 01:;:. Fontenelle, Navajo, Morrow Point, and Blue Mesa), together with Lake Mead downstream, have caused some major changes in stream :"egimen: (1) The stream ~hannels inundated by these reservoirs will no 10llger be sQbjected to natural stream erosio~, (2) the accww~lation of sediment and water within the reservoirs slows the growth and flooding of the Colorado River delta, (3) flooding has diminished in many areas, and (4) sections of sediment-laden streams have given -,r-ay to clear water streams and lakes. The mineral concentration in runoff increases from 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 mineral con~entra- tio~s in natural run::Jff are evaporites of Paleozoic age, shale of Cre- taceoJ.s age, a'li salt and gypsum of Tertiary age. B. Soils Tile soils of the Colorado River Basin closely resem'::lle the geologic fornations of their origin. O~ly in limited areas at the higher eleva- tions has the precipitation leached the soil mass of its soluble con- stituents. Over most of the area both residual and transported soils are basic in reaction and well supplied with carbonates with normal or m~ture soils exhibiting a distinct horizo~ of carbo~ate accumulation. The impress of soil-forming factors has resulted in the widespread de- velopment of soils classified as members of the Gray-Desert Great Soil Group. In areas with higher rainfall, soils of -the Brown and Chestnut Great Soil Groups have developed. Saline and alkali (sodic) soils occur in ~ny parts of the basin. The residual soils comprise the larger area and are usually shallow in depth over shale a~d sa~dstone of various ages. Many of the shales are saline but contain rrDlcLl gy:psum as well as other chloride and_ sul- phate salts. Some formations are high in sodium ,~hloride and some have sodium carbonate or bicarbonate strata. Very few residual soil areas are suitable for irrigatio:l ievelopment. The alluvial materials are extremely variable ani range from allu- vial fans and terraces, o~twash plains, to la~Qstrine sediments. Some areas have soils from )~~terial transported only short distances and re- semble the original m~terials. other areas have soils which have been transp::Jrted and mixed extremely well. Most of the agricultural areas are on these well-mixed.alluviu~s and, therefore, the soils are quite variable. Extensive areas of Eolian deposits occur in parts of the basin, principally in southwestern Colorado. The uniformly textured soils