Statement by Bureau of Reclamation on Hydrologic

Document: Statement by Bureau of Reclamation on Hydrologic Basis for Colorado River Water Supply Projections, July? 1965 (carbon copy), page 2that excess must be stored for future use. The resrvoirs must be filled so that the critical period yield can be realized. It is also unavoidable that there will be occasional years of high runoff in which the reservoirs will already have been filled and thus be incapable of storing all the available flows. This "spillwater" is lost water that can make no contribution to the usable yield. The important basic concept is that although the critical period is the period by which the hydrologists measure the safe water yield of a river system with its storage reservoirs, the runoff over the entire period of record is basic to determining the safe yield during that critical period. Turning now to the Colorado River, the long-term average virgin flow of the river at Lee Ferry (1896-1965) is about 15,000,000 acre-feet. The attached bar chart illustrates the annual variations over the period. It also shows clearly the Colorado's "critical period" at Lee Ferry which is 1930-1964. Article III(d) of the Colorado River Compact prohibits the Upper Basin from causing the flow of the river at Lee Ferry to be depleted below 75,000,000 acre-feet in any period of ten consecutive years. For comparison purposes, this is an average of 7,500,000 acre-feet annually.However, the average annual virgin flow of the Colorado River at Lee Ferry has dropped to about 11,800,000 acre-feet over two specific periods of ten consecutive years, 1931 through 1940 and 1954 through 1963. Over the entire critical period, the average annual virgin flow was about 13,000,000 acre-feet. Without Upper Basin reservoir regulation, and, for purposes of illustration setting to one side the question of responsibility for meeting Mexican Treaty deliveries, because of the prohibition against depletions imposed by Article III(d) of the Compact, the Upper Basin would be limited to an average annual consumptive use of only 4,300,000 acre-feet over a ten-year period similar to those of 1931-1940 and 1954-1963. While over successive ten-year periods within the 35-year critical period would have enabled higher depletions in the Upper Basin, nevertheless, because Article III(d) of the Compact speaks in terms of consecutive ten-year periods, the decade 1954-1963 would have established, absent basin storage, 4,300,000 acre-feet as the maximum assured consumptive use figure for the Upper Basin. A major function of the Upper Basin reservoirs is to store water during periods of plentiful runoff for release in dry periods in order that the flow at Lee Ferry during dry periods can be increased. This, in turn, permits the Upper Basin to utilize a greater portion of the Colorado River water apportioned to it by the Colorado River Compact and still meet its compact commitments at Lee Ferry. 2Epson Perfection 4870 Photo, 400 dpi, 24 bit, 3,223,628 byte