9542

• UCREFRP Catalog Number: 9542
• Author: Upper Colorado River Commission.
• Title: Fifty-Sixth Annual Report of the Upper Colorado River Commission.
• USFW Year: 2004.
• USFW - Doc Type: Salt Lake City, UT.
• Copyright Material: NO

The graphs on pages 27 and 28 illustrate some of the pertinent historical facts
related to the amounts of water produced by the Colorado River System above Lee
Ferry, Arizona, the compact division point between the Upper and Lower Colorado
River Basins. The first graph, on page 27, is entitled Colorado River Flow at Lee
Ferry, Arizona. The top of each vertical bar represents the estimated virgin flow of
the river, Le., the flow of the river in millions of acre-feet past Lee Ferry for a given
year had it not been depleted by activities of man. Each vertical bar has two
components: The lower shaded part represents the estimated or measured historic
flow at Lee Ferry, and the difference between the two sections of the bar in any
given year represents the stream depletion, or the amount of water estimated to
have been removed by man from the virgin supply upstream from Lee Ferry. It is
worth noting that in 1977, and again in 1981, the historic flow at Lee Ferry
exceeded the virgin flow. Beginning in 1962, part of this depletion at Lee Ferry was
caused by the retention and storage of water in storage units of the Colorado River
Storage Project. The horizontal line (at approximately 14.8 million acre-feet) shows
the long-term average virgin flow from 1896 through 2004. Because the Colorado
River Compact is administered on the basis of running averages covering periods
of ten years, the progressive ten-year average historic and virgin flows are
displayed on this graph.

The second graph on page 28, entitled Lee Ferry Average Annual Flow for
Selected Periods, is a graphical representation of historic and virgin flow averages
for several periods of record. The periods of water years selected were those to
which reference is usually made for various purposes in documents pertaining to
the Colorado River System.

Several important hydrologic facts are apparent from these two graphs on pages
27 and 28.

(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 through 1984-1993 decades.

(3) For the period 1896-1921, which is prior to the Colorado River Compact of
1922, 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 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, 1896-2004, the estimated average annual
virgin flow is 14.8 million acre-feet and the average annual historic flow is
12.0 million acre-feet.

(5) For the next longest period, 1906-2004, the estimated average annual
virgin flow is 14.8 million acre-feet and the average annual historic flow is
11.9 million acre-feet. Many of the early records for this series of years,
as well as for the 1896-2004 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 authorized in 1968.

(6) The estimated average annual virgin flow during the 1914-2004 period is

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