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.'\ DESCRIPTION OF BASIN (continued) B. Soils ,'. The soils of the Colorado River Basin closely resemble the geologic formations of their origin. Only in limited areas at the higher elevations has the precipitation leached the soil mass of its soluble constituents. Over most of the area both residual and transported soils are basic in reaction and supplied with carbonates with normal or mature soils exhibit- ing a distinct horizon of carbonate accumulation. The impress of 50i1- forming factors has resulted in a wide range of soil development. Soils formed in areas with low precipitation are classified in the orders Entisols and Aridisols. Those formed in areas with high precipitation are classified in orders Mollisols and Alfisols. Saline and alkali (sodic) soils occur in many parts of the basin. o II::> *,. 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 sulphate 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. A large part of the salt pickup occurs in areas where the natural runoff contacts the saline shales before entering the streams. .' / The alluvial materials are extremely variable and range from alluvial 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 in southwestern Colorado. The uniformly textured soils are reddish brown in color and have no resemblance to either the underlying formations or adjacent areas. These are excellent agricultural soils, but in many areas topography makes agriculture difficult. C. Climate . The Colorado River Basin has climatic extremes, ranging between year-round snow cOVer and heavy precipitation on the high peaks of the Rocky Mountains to desert conditions with very little rain in the southern part of the basin. This wide range of climate is caused by differences in altitude, latitude, and by the configuration of the high mountain ranges. The encircling mountain ranges obstruct and deflect the air masses to such an extent that storm patterns are more erratic than in most other parts of the United States. Most of the moisture for precipitation on the Upper Basin is derived from the Pacific Ocean and the Gulf of Mexico. The Pacific source predominates generally from October through April and the Gulf source during the late spring and early summer. 8