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<br />suggests the decline of salinity concentrations may have been in part caused <br />by the transition from a relatively drier period (1955-65) with an annual <br />virgin (undepleted) flow of 13.23 million acre-feet at Lees Ferry, to a <br />relatively wetter period (1965-75) with an annual virgin flow of 14.76 million <br />~ acre-feet. This is an increase in the flow of approximately 10 percent and may <br />~ be responsible for a portion of the decrease in the salinity concentration <br />~ observed at Imperial Dam. <br /> <br />e> <br /> <br />The more recent period (1983-86) was even more extreme with some <br />flows well above those ever recorded. This was probably the major cause of <br />the near 200 mg!L drop in salinity over the last few years. A statistical <br />analysis of the CRSS projections for salinity indicates that salinity will <br />usually range between 635 and 1,035 mg/L, with an average of 820 mg!L under <br />the present level of development; however, about 5 percent of the time <br />salinity could vary outside this range, as it did in 1985 with a salinity of <br />607 mg/L. <br /> <br />2. Reservoir Effects <br /> <br />One of the most significant changes '~ich has occurred to the <br />salinity of the Colorado River is due to the regulation of the natural flow of <br />the river basin. One study [3J shows that storage in take powell reduced the <br />month to month variation from the mean salinity below Glen Canyon Dam from 299 <br />mg/L to 72 mg!L. This is readily apparent in a plot of the data, Figure V-3. <br /> <br />__ 1 400 <br />~ 1200 <br />a 1000 <br />-- <br />800 <br />600 <br />400 <br />200 <br /> <br />t' <br />~ <br /> <br />I7.l <br /> <br />, <br />.. . ,. . . <br />1. . I.. <br />I':... ... 1......,. \' . <br />. '." ," 0.. o. <br />. 0 .... __ <br />.. -:.-0: o. ... ._.. .. .. <br />,.,... ~...:.:"""... .~ , .~ <br />.0 . ~_. ... _.. .. <br />.. .0 ., : o. . .,. .1 <br />. .. .- .,C Tl <br />. .. .. ." <br />- .. -. ._ ...:0. . ..,,&~;~~, <br />.. .. 0 _. .... . <br />.. ..-... . <br />. .. o. . ..... 7 <br />.-. . . ..- <br />'. . 1 _.~. 0 -. <br />, . <br /> <br />1940 <br /> <br />1950 <br /> <br />1960 <br /> <br />1970 <br /> <br />1980 <br /> <br />1990 <br /> <br />Year <br /> <br />riqure v-3. Monthly salinity below Lake Powell (1941-85). <br /> <br />The period of 1963-80 represents the most significant period of <br />reservoir storage in the history of water development on the Colorado River. <br />Storage in Flaming Gorge Reservoir, Lake Powell, and Lake Mead increased from <br />less than 20 million acre-feet in 1963 to over 50 million acre-feet by 1980. <br />The spill of Glen Canyon Dam in 1980 ended the initial filling of the major <br />reservoirs on the Colorado River. Water ~nich was being used to fill the <br />mainstem reservoirs is now being released. This has increased the flow to the <br /> <br />- <br /> <br />V-J <br />