7041

• UCREFRP Catalog Number: 7041
• Author: Upper Colorado River Commission.
• Title: Forty-Third Annual Report of the Upper Colorado River Commission
• USFW Year: 1991.
• USFW - Doc Type: Colorado River Storage Project and Participating Projects.
• Copyright Material: NO

Several important hydrologic facts are apparent from these two charts
on pages 26 and 27.

(1) A vast majority of the high flows occurred prior to 1929.

(2) Since the 1924-1933 decade, the progressive ten-year average
virgin flow has not exceeded the average virgin flow except in the 1941-
1950 and the exceptionally wet 1975-1984 and 1981-1991 decades.

(3) For the period 1896-1921, which is prior to the Colorado River
Compact ofl922, the average virgin flow was estimated to be 16.8 million
acre-feet per year, which is considerably greater than for any other period
selected, including the long-term average. A stream-gaging station at
Lees Ferry, Arizona was not installed until 1921. Thus, the virgin flow at
Lees Ferry prior to the 1922 eompact is estimated based upon records
obtained at other stations, e.g. the stream gage on the eolorado River at
Yuma, Arizona for the period 1902-1921.

( 4) For the longest period shown, 1896-1991, 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, 1906-1991, the estimated average
annual virgin flow is 15.0 miIIion 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 1896-1991 period, are based upon the estimates
offlows made at other gaging stations, as mentioned in (3) above. This
averageis 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 eentral Arizona Project authorized in 1968.

( 6) The estimated average annual virgin flow during the 1914-1991
period is 14.7 million acre-feet. This period is an extension of the 1914-
1965 period used in the Upper Colorado Region Comprehensive Frame-
work Studies of 1971. 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 negotiators
of the Upper eolorado River Basin Compact of 1948.

(8) For the period 1922 -1991, which is the period of record since the
signing of the Colorado River Compact, the average annual virgin flow is

28