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<br /> <br /> <br />EFFECT OF RIVER BASIN DEVELOPMENT UPON USABILITY OF WATER <br /> <br />Carl G. Paulsen, Chief Hydraulic Engineer, U. S. Geological Survey <br /> <br />The Colorado River drainage system, like that of any river, is Nature's <br />method of disposal not only of water but of all material that can be dissolved <br />or moved downstream by that water. Through the ages it has carried enough <br />solid matter to form a vast delta in the Gulf of California. Beyond the <br />delta is the water of the ocean whose salinity is evidence of the effective- <br />ness of rivers in carrying soluble material from the continents. <br /> <br />The total runoff to the Gulf is only a small fraction of the total <br />precipitation in the basin. It is likely that much more than half the water <br />that falls as precipitation returns to the atmosphere from the soil zone <br />through plants or by evaporation, without entering a stream or ground-water <br />reservoir. Of the water that contributes to the surface and ground-water <br />supplies, a substantial part also returns to the atmosphere within the <br />drainage basin. <br /> <br />The water that returns to the atmosphere leaves behind any mineral <br />matter that it had carried in solution. Thus, we can expect a gradual accumu- <br />lation of soluble materials in various parts of the basin, and generally there <br />is progressive deterioration in quality of surface and ground-water with in- <br />creasing distance from the area of precipitation which was the ultimate source <br />of the water. <br /> <br />Man's development and utilization of water affects these natural <br />processes in several ways. Many'of the uses to which water is put are <br />consumptive uses--that is, water is evaporated and returned to the atmosphere <br />in the process. The user then has the problem of disposing of the mineral <br />matter that had been dissolved in and is left by the water. <br /> <br />Evaporation from reservoir surfaces and diversion canals is recognized <br />as a legitimate consumptive use of water, and is generally charged to the water <br />users who benefit from the storage as represented by this evaporation. The <br />evaporation loss may vary considerably from one reservoir to another, depend- <br />ing upon the latitude, altitude, climate, and reservoir characteristics. In <br />a fully developed river basin this cost in loss of water is an important item. <br />There may be alternatives of storing water in reservoirs where the area of the <br />water surface per unit of storage is less, or in reservoirs at higher altitudes. <br />Ground-water reservoirs may be suitable for storage of flood flows, without <br />loss by evaporation. The Gila River Basin, for example, offers opportunities <br />for storing vast quantities of water underground. <br /> <br />The term "consumptive use" has also been applied to the nonbeneficial <br />consumption of water by native vegetation that has little or no practical <br />value, or by evaporation from marshes, ponds and barren ground. The water <br />consumed in these ways might better be called "consumptive waste." Any water <br />that can be salvaged from these natural forms of "consumptive use" and put to <br />beneficial use is a net gain for the basin. <br /> <br />-5- <br />