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suggests the decline of salinity concentrations may have been in part caused by the transition from a relatively drier period (1955-65) with an annual virgin (undepleted) flow of 13.23 million acre-feet at Lees Ferry, to a relatively wetter period (1965-75) with an annual virgin flow of 14.76 million acre-feet. This is an increase in the flow of approximately 10 percent and may be responsible for a portion of the decrease in the salinity concentration observed at Imperial Dam. The more recent period (1983-86) was even more extreme with some flows well above those ever recorded. This was probably the major cause of the near 200 mg/L drop in salinity over the last few years. A statistical analysis of the CRSS projections for salinity indicates that salinity will usually range between 635 and 1,035 mg/L, with an average of 820 mg/L under the present level of development; however, about 5 percent of the time salinity could vary outside this range, as it did in 1985 with a salinity of 607 mg/L. 2. 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 mg/L to 72 mg/L. This is readily apparent in a plot of the data, Figure V-3. ~ 1400 ~ 1200 r 1000 "'"' .t' 800 ~ 600 rtJ 400 200 . . . . . . . . ). .,. . I,: · ... .-:........ .. . ..., 1"-.. .,... . . e. ... .... .. .:--: . . ... ..... ... .. ;J'... " .. ......... ..... . . : '"' . oS.....", · ~.. , .... · A. . . . .,., - ..... .,e I .. .. ." · .. . .. ~.Q4Aa"lLJ~ · ... ,.. · .~';r"'~ · .. .... ~. . . ~:.. .... . .. ... . e.. .. .... . . . . ... , , . \. ... 1940 1950 1960 1970 1980 1990 Year Figure V-3. Monthly salinity below Lake Powell (1941-85). The period of 1963-80 represents the most significant period of reservoir 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 mcillion 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 mainstem reservoirs is now being released. This has increased the flow to the v-3