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<br />Co..) <br />00 <br />l\:) <br />CO <br /> <br />. <br /> <br />rocks of marine and continental origin of the Cretaceous and Tertiary <br /> <br />Ages. Here the residuum is high in soluble material! and the arid climate <br /> <br />. <br /> <br />produces soils which tend to be shallow and immature. <br /> <br />River alluvium reflects the geology of the area. ln the interior <br /> <br />. <br /> <br />valleys the alluvium is a complex mixture of weathered and eroded ma- <br /> <br />terial which has been transported and deposited from nearby and upstream <br /> <br />sources. When underlying or nearby formations contain soluble minerals <br /> <br />. <br /> <br />or evaporites (for example, mancos shale), the river alluvium contains <br /> <br />an abundance of soluble minerals which neces sarily affect the chemical <br /> <br />quality of the stream. <br /> <br />. <br /> <br />Soils. The great diversity in climate, topography, geology, and <br /> <br />vegetation in the Colorado River Basin is reflected in the large number of <br /> <br />. <br /> <br />soil types found in the basin. The soil types, however, can be classified <br /> <br />into 3 basic soil orders, namely, Aridisols, Entisols, and Millisols. <br /> <br />About 60 percent of the soils belong to the Aridisol order, with the Enti- <br /> <br />. <br /> <br />sol order comprising about 25 percent, and the remaining 15 percent <br /> <br />classified as Mollisols. The Upper Basin contains a total area of approxi- <br /> <br />. <br /> <br />mately 75 million acres and He Lower Basin contains about 90 million <br /> <br />acres giving a basin a total of roughly l62 million acres. <br /> <br />Aridisols form in an arid to Semi- arid climatic regime where po- <br /> <br />. <br /> <br />tential evapotranspiration exceeds precipitation; thus, no water drains <br /> <br />through the soil. The soils are usually light-colored with a low organic <br /> <br />matter content. In the Lower Basin the occurrence of an argillic (clay) <br /> <br />. <br /> <br />to <br /> <br />. <br />