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6 o W 1-' Ul DESCRIPTION OF BASIN The new reservoirs recently constructed or under construction above Lees Ferry (Lake Powell, Flaming Gorge, Fontenelle, Navajo, Morrow Point, and Blue Mesa), together with Lake Mead downstream, have resulted in some major changes in the stream regimen: (1) The stream channels sub- ject to inundation by these reservoirs will no longer be subjected to natural stream erosion, (2) The accumulation 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 way 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 concen- trations in natural runoff are evaporites of Paleozoic age, shale of Cretaceous age, and salt and gypsum of Tertiary age. B. Soils The soils of the Colorado River Basin closely resemble the geologic formations of their origin. Only 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 mature soils exhibiting a distinct horizon of carbonate 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 many parts of the basin. The residual soils comprise the larger area and are usually shallow in depth over shale and sandstone of various ages. Many of the shales are saline but contain much gypsum as well as other chloride and sul- phate salts. Some formations are high in sodium chloride and some have sodium carbonate or bicarbonate strata. Very few residual soil areas are suitable for irrigation development. The alluvial materials are extremely variable and range from allu- vial fans and terraces, outwash plains, to lacustrine sediments. Some areas have soils from material transported only short distances and resemble the original materials. Other areas have soils which have been transported and mixed extremely well. Most of the agricultural areas are on these well-mixed alluviums and, therefore, the soils are quite variable. Extensive areas of Eolian deposits occur in parts of the basin, principally southwestern Colorado. These uniformly textured soils are