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average annual flow at Lees Ferry drops from 11.2 (see table 3.4) to 9.8 maf. Similarly, the average annual flows at the outflow point of each WAU are about 10 to 15 percent less than those shown in table 3.4 if the period 1930-1974, rather than 1906-1974, 'is assumed. ::./t~ "... "~~' I\:) 4X> ~ toO Surface.Water Quality Dissolved solids (salt) concentrations and loadings have long been recognized as the most prevalent water quality problems in the Colorado River Basin. In general, salts are contributed primarily by the Upper Basin and become a problem as they are concentrated downstream in the Lower Basin, although there are also some significant sources of salt loading in the Lower Basin. Of the salt load in the Colorado River at Hoover Dam, approximately 60 percent comes from natural sources and the balance from man's activities. Of ali man's activities, irrigation is responsible for the the largest increase in salinity. Water is removed through evaporation and consumption by plants. but nearly all of the salt is returned to the river, concen- trating the salts in a smaller volume of water. At the same time the return flows leach salts from the soils and rocks which add to the River's salt load. Reservoir evaporation also contributes to increasing sa- linity because evaporation removes water but leaves the salts to be concentrated in a smaller volume of water. Out-of-basin exports and in-basin uses that do not return salt to the system ordinarily cause higher salt concentra- tions down stream. Salt loads contributed by municipal and industrial use are of minor significance . . . . (U.S. Water Resources Council, '75 Water, Assessment, Upper Colorado Region Tech. Memo. No.2, Aug., 1976, p. 28). ;f~f,\?) , ., In the subbasins most likely to experience the development of oil shale or coal gasification industries, there are other water quality parameters which are also of concern. These have been summerized in table ).8. Ground Water The ground water resources of the Upper Basin are largely untapped at the.present time. As a consequence, there is a general lack of knowledge concerning the extent and quality of ground water aquifers and ground/sur- face water relationships. Nonetheless, it appears that ground water re- serves could be a significant source of supply for oil shale and coal gasification developments in some instances. ::.::-.:, xxii