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Thus, the virgin flow at Lees Ferry prior to the 1922 Compact is estimated based upon records obtained at other stations, e.g, the stream gage on the Colorado River at Yuma, Arizona for the period 1902-1921. (4) For the longest period shown, 1896-1990, the estimated average annual virgin flow is 14,9 million acre-feet and the average annual historic flow is 12,3 million acre-feet, (5) For the next longest period, 1906-1990, the estimated average annual virgin flow is 15.0 million acre-feet and the average annual historic flow is 12.2 million acre-feet, Many of the early records for this series of years, as well as for the 1896-1990 period, are based upon the estimates of flows made at other gaging stations, as mentioned in (3) above, This average is about equal to the 15.0 million acre-feet estimated for the 1906-1967 period which was used as the basis for justification of a water supply for the Central Arizona Project au- thorized in 1968. ( 6) The estimated average annual virgin flow during the 1914-1990 period is 14.7 million acre-feet. This period is an extension of the 1914-1965 period used in the Upper Colorado Region Compre- hensive Framework Studies ofl971. The average annual virgin flow for the 1914-1965 time period is 14.6 million acrefeet. (7) The average annual virgin flow for the period 1914-1945 is 15.6 million acre-feet. This was the period of record used by the nego- tiators of the Upper Colorado River Basin Compact of 1948. (8) For the period 1922-1990, which is the period of record since the signing of the Colorado River Compact, the average annual virgin flow is 14.2 million acre-feet and the average annual historic flow is 11.2 million acre-feet. Records for this series of years are based upon actual measurements of flows at Lees Ferry. The ten-year moving average flow since 1922 is considerably less than the ten-year moving average flow prior to 1922. (9) Two completely unrelated ten-year periods of minimum flows have occurred since 1930. During these periods, 1931-1940 and 1954- 1963, the average annual virgin flow amounts to only 11.8 million acre-feet. (10) For a 12-year period, 1953-1964, the average annual virgin flow amounts to only 11.6 million acre-feet. 28