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000792 in forecasts of a more severe shortage that will occur sooner than the 2020 Bureau projection. Most decisions concerning the need for and type of Federal projects and programs have been baseu on the Bureau's water supply estimates. Studies to measure the Colorado River water supply generally start with an estimate of the "virgin" or undepleted flow at Lee Ferry, Arizona, which is the most universally used lndicator of the river's water yield. Estimates for the years 1896 to 1922 were made from records of the flow at other points on the mainstream and its major tributaries and estimates of the depletions in the Upper Basin. Since 1922 the flow has been measured by USGS gauges located upstream from Lee Ferry. According to the Bureau, the virgin flow at Lee Ferry has ranged from a maximum flow of 24 maf in 1917 to a low of 5.47 maf in 1977; the long-term average has been about 14.8 maf. Many decisions involving river development have been based on varying estimates of annual virgin flows, primarily because of conditions present when decisions were made. The following are examples of estimates used when important decisions were made. Event Period on which estimates were based Average annual virgin flow Years Lee Ferry (maf) !' , !' Colorado River Compact negotiations 1903-1921 19 18.0 Studies for the Boulder Canyon project 1897-1928 32 16.9 Upper Colorado River Compact 1914-1945 32 15.6 1956 Colorado River Storage Project Act 1914-1947 34 15.5 1968 Colorado River Basin Project Act 1906-1967 62 15.0 Long-term average 1906-1977 72 14.8 The Colorado River water was divided among the States based on flows in excess of the long-term average. The Bureau's records indicate that the highest flows occurred prior to 1929. It is interesting to note that the 1922 7