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. . ..... -J A t;.J, The Sad Juan division comprises the entire San Juan River drainage basin, which e~pties into Lake Powell. The major tributary is the Animas River. Chaco Wash and Chinle Wash drain large areas of arid New Mexico and Arizona but contribute little flow. The San Juan division contains about 1/4 of the total area of the basin, and contributes 15 percent of the total flow and 12 percent of the total load. The remainder of the basin is comprised of the Colorado River drainage below the confluence of the Green River. Tributaries include the San Rafael,' Dirty Devil, and Paria Rivers. This area is warm and arid, aad contributes only a small amount of saline flow. Long-term mean flow and concentration at several stations are shown in Figure 2. Effects Of Reservoirs The construction of Glen Canyon Dam has greatly altered the seasonal cycle of flow and concentration of the river at Lees Ferry (Figure 3). The period of record was separated into a pre-reservoir period (1926 to 1962), a filling period (1966 to 1980), and a post-reservoir period (1981 to 1983). The pre~reservoir period was characterized by a snowmelt runoff cycle June flows averaged 3.0 Maf, 1.25 Mtons, and 303 mg/l; January flows averaged .32 Maf, .48 Mtons, and 1098 mg/l. The annual mean was 11.8 Maf, 8.]5 Mtons,and 544 mg/l. An increasing trend in dissolved-solids concentration was noted, at the rate of 39 mg/l per year. When linearly extrapolated, this corresponds to a rise from 4]3 mg/l in 1926 to 616 mg/1 in 1962. The filling period was characterized by overall reduced flow and a strong decrease in seasonal variability. Mean monthly flows ranged from .35 to.9 Maf, and concentration from 510 to 639 mg/l. Monthly discharge was mainly a function of electrical power demand, with the highest releases occurriag in late summer and winter. Dissolved-solids concentration was greatest when the lake was at its lowest level. The annual mean was 8.]5 Maf, 6.7 Mtons, and 564 mg/l. Although this represents a significant decrease of 3 Maf compared to the pre-reservoir period, mean concentration was not significantly different. Preliminary analysis indicates a slight reduction in the concentration of calcium and carbonate equivalent during the IS-year filling period, of about l'mg/1 per year. The post-reservoir period is not long enough to show any significant trends, and more time will be required before any definitive conclusions may be drawn. Although Flaming Gorge Reservoir has altered the seasonal flows of th~ Green River, the annual flow and salt load have changed only slightly. (This a~nual change may be an artifact of the short record available for the pre~reservoir period). For the 6-year pre-reservoir period, monthly , "^ :,/ it i.. , ""' '",,"'" 0/.' t ,":}: ",1, i;'!i",""~--~~ ,,~~.:3~ij