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BASIC STUDIES o W irrigation canals and to past pUlllping of the ground water. The vertical ~ variation appears to be dependent partly on the depth of the water table W but mostly on the nature, the stratification, and the degree of cementa- tion of the alluvial sediments. An estimate of the water quality at proposed well sites can be made by considering water from three zones of the alluvial sediments--the upper, gravel, and deep zones. The upper zone extends from the water table to an extensive gravel zone whose upper surface generally is about 100 feet below the flood plain. Although Some gravel layers are present locally in the upper zone, it is primarily made up of layers of sand, silt, and clay. As evaporation causes local accumulation of soluble salts in areas where the water table is within a few feet of the land surface, the water in the very top of the upper zone may be more saline than in the rest of the zone (analyses 4, 7). In irrigated areas, the shallow water is generally similar chem- ically to Colorado River water but is more saline. Water in the lower part of the zone is, in general, slightly more saline than that in the gravel zone beneath (analyses 2, 8). The gravel zone consists of several layers of permeable coarse gravel, and its thickness ranges from about 50 to more than 150 feet. Most of the water pUlllped from irrigation wells comes from this zone. Water pumped from the gravel zone is generally somewhat more saline and contains rela- tively more sodium chloride than Colorado River water but generally is of better quality than that from the upper zone. However, in some areas, water of lower salinity than Colorado River water (analyses 11, 22) is found in this zone. All fresh-water-bearing material beneath the gravel zone is desig- nated as the deep zone. Increases in concentration with increasing depth have been found at several, but not at all, locations for which data are available. Electric logs of oil-test wells indicate that brackish water may be widespread at depths of two thousand to three thousand feet. How- ever, the limited amount of information about the variation of the salin- ity with depth in the deep zone does not permit generalization.. Whatever the salinity, which ranges from less than that of Colorado River water (analysis 19) to quite saline, water in the deep zone (analysis 9) gener- ally contains more chloride in proportion to sulfate than Colorado River water. At present, only a small number of wells pump water from the deep zone but the pumpage from the zone is increasing. Many of the wells tap- ping the deep zone yield water very high in fluoride (analyses 5, 9). 5. Flaming Gorge Reservoir The effects of impoundment on water quality are important in evaluat- ing present 'later-quality conditions and in predicting future conditions in the Colorado River system. In order to appraise effects of impoundment on river-water quality, knowledge of prereservoir conditions is imperative. 34