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. . . . . . . . . . . W (Xl o o processes are considered to be the principle source of salinity within the waters of the Colorado River. In many instances these processes . cannot be controlled or significantly altered by the activities_ of man. In contrast to the Colorado, rivers which drain humid areas may actually improve in quality from a salinity standpoint as they flow down- stream. For example, the Platte River which rises in the high mountains of Colorado and Wyoming substantially increase in salinity concentrations as it flows eastward across the arid lands of the western plains. However, as the river enters the more humid region east of the lOOth meridian, its waters are diluted by higher quality tributary inflows from well leached watersheds, and, even though the total salt load increases, net salinity levels, or concentrations, actually fall. Unlike the Platte, down- stream from the headwaters of the Colorado River tributary inflows from the arid drainages tend to degrade rather than improve water salinity levels. Hence, it is almost axiomatic that rivers entirely within arid land basins will increase in salinity from source to mouth. Effect of ManIs Activity on Salinity. ManIs development of the water resources of the basin has brought about extensive changes in water quality. His intervention in the natural hydrologic pattern affects both the quantity and quality of surface and underground waters. Evapo- ration and transpiration processes in irrigated agriculture decrease the quantity of water available to carry a particular salt load. This in turn 9