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<br />/&). <br />f$X4 <br /> <br />f\) <br />-.J <br />1\;.. <br />-.J <br /> <br />;. .~-: ". <br />'.: (:...~" <br /> <br />Present Hydrologic Conditions <br /> <br />Given the depletions which occur under present (1976) conditions <br />of development, as discussed previously, and the effects of the <br />numerous reservoirs which have been built in the Upper Basin,l the an- <br />nual amount and monthly distribution of stream flows has been substan- <br />tially altered from what it naturally would have been. Typically, the <br />resulting hydrologic regime is characterized by estimating the dis- <br />tribution of flows which would occur over the long run if present con- <br />ditions of development and reservoir regulation criteria were to remain <br />unchanged (recall, however, that depletions would vary from year to <br />year, even if the level of development were to remain constant over <br />time). Put another way, the hydrologic regime can be depicted in the <br />"steady state," or equilibrium, condition to which it would come over <br />the long run. <br /> <br />Pursuant to this definition, the distribution of flows which would <br />prevail undEir "steady state" conditions is given in table 3.4 in the <br />form of estimated percentile exceedEince flows at the outflow point of <br />each WAU.2 These estimates are basEid upon the estimated natural flows <br />for the period 1906-1974. With respect to reservoir content in the <br />"steady state" condition, it is estimated that the mean active content <br />of the six largest reservoirs (Blue Mesa, Morrow Point, Flaming Gorge, <br />Lake Powell, Fontenelle, and Navajo) would be about 27.5 maf, as com- <br />pared to a total capacity of 31.4 maf. It is estimated that the mini- <br />mum active content of these reservoirs would be about 21-22 maf, which <br />levels would occur only 1-2 percent of the time. Annual evaporation <br />from these reservoirs would average better than 700,000 acrEi-feet. <br /> <br />Even if the period of record 1930-1974 is used, with estimated nat- <br />ural inflows of 13.8 maf, minimum reservoir content for the specified <br />reservoirs would still be about 21 maf and would still occur only about <br />2 percent of the time. Mean flows would, however, be lower than those <br />shown in table 3.4. For example, the mean annual flow at Lees Ferry <br />drops from 11.2 maf to about 9.8 maf if the estimated natural flows for <br />the period 1930-1974 are used as the basis of analysis. <br /> <br />1. There are over 30 reservoirs in the Upper Basin, the maximum live <br />storage of which totals 34.5 maf. Of this, the Colorado River Storage <br />Project (CRSP) reservoirs (Blue Mesa, Morrow Point, Flaming Gorge, and <br />Lake Powell) account for 29.7 maf (U.S. .Bureau of Reclamation, 1977a, <br />p. 24). The operation of the CRSP reservoirs obviously has a major im- <br />pact on the hydrology of the Upper Basin. The most important operating <br />criteria for the CRSP reservoirs are briefly set forth in chapter 8. <br /> <br />2. A percentile exceedence flow is defined as that flow which will be <br />equaled or exceeded a specified percentage of the time. <br /> <br />3-13 <br />