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FACTORS IN SAUNITY TRENDS 21 . .. . . .:. .:e. :.... !..~..... ;..... ,. ... . .:.. . , .. . ., .. .. ..... ....- .. .. :.. . . ... . .. ... . . .... . . .. . .e:....... , ..... . \.. . . .. ~ .. e. ...... . . . . .. .~...: ... .... .1- ...... ."I..f . .. . ,. \ . ,. ) ~. · ... ",.. . ..C:r.:-1(tit\\,\\"t\~~. 1.~ · .. . · · ... V.. , p · 'Y ~ 'L-~ . .. . . .. .. '. , , '..,~- . .. ..... . ,. .. e.. .. .... ~ ,-: ..'.... .::. , .. . statistical analysis of the CRSS projections for salinity indicates that salinity will usually range between 678 and 1,070 mgIL, with an average of 873 mgIL under the present level of development; however, about 5 percent of the time salinity could vary outside this range, as it did in 1986 with a salinity of 577 mgIL. Reservoir Effects One of the most significant changes which has occurred to the salinity of the Colorado River is due to the regulation of the natural flow of the river basin. One study [3] shows that storage in Lake Powell reduced the month to month variation from the mean salinity below Glen Canyon Dam from 299 mgIL to 72 mgIL. This is readily apparent in a plot of the data, figure 3. The period of 1963-80 represents the most significant period ofreservoir storage in the history of water development on the Colorado River.. Storage in Flaming Gorge Reservoir, Lake Powell, and Lake Mead increased from less than 20 million acre-feet in 1963 to over 50 million acre-feet by 1980. The spill of Glen Canyon Dam in 1980 ended the initial filling of the major reservoirs on the Colorado River. Water which was being used to fill the main stem reservoirs is now being released. This has 1400 1300 ~ 1200 e 1100 '-' 1 000 >. :::: 900 s:l ;:: 800 ClI t/J 700 ~ 600 .t: ~ 500 o ::e 400 300 200 . . 1940 1950 r increased the flow to the Lower Basin over the flows during the filling period (1963-80). Salinity would have been much higher without these additional releases. Reservoir salinity in 1985 was greatly reduced by the high inflows of the early 1980's. Figure 4 shows salinity at the Hite monitoring station significantly below normal in 1985. Normal to below normal flows in 1987 and 1988 returned the salinity in the upper end of the reservoir to more typical levels in 1988. The same is true of the Wahweap monitoring station near Glen Canyon Dam on Lake Powell, although there remains some high quality water near the surface of the reservoir. The bottom half of the water column at this site has shown a dramatic increase in salinity in just 3 years. This increase is attributed to the return to more normal inflows. Still, the quality of the stored water in the Colorado River Basin remains above average. During the initial filling, significant leaching of gypsum (calcium sulfate) was documented at Flaming Gorge [9] and Ruedi Reservoirs[10] and at Lake Mead[ll], but gypsum leaching at Lake Mead and Ruedi Reservoir has diminished. Final documentation of the long-term salt leaching at Flaming Gorge Reservoir is part of the ongoing reservoir studies. - I 1960 1970 1980 1990 Year Figure 3.-Monthly salinity below Lake Powell (1941-89).