WSP02261

• File Number: 8282.600.10
• Description: Colorado River Operating Annual Reports
• Basin: Colorado Mainstem
• Date: 1/1/1990
• Author: USDOI/BOR
• Title: Operation of the Colorado River Basin 1990 Projected Operations 1991
• Water Supply Pro - Doc Type: Publication

River Regulation

The runoff into Lake Powell for water year 1990 continued to
be well below normal for the third straight year. Unregulated
inflow into Lake Powell was 46 percent of normal for watcr
year 1990. Unregulated infloV05 for water years 1988 and 1989
were 70 percent of normal and 54 percent of normal
respectively. Upper Basin reservoirs, Flaming Gorge, Blue
Mesa, and NaVoljo, experienced modest rebounds in storage
in" water year 1990. These increases in storage occured
because rc lea.ses from these reservoirs were primarily
constrained to minimum levels. Storage in Lake Powell and
Lake Mead, howe",r, is down considerably. The Octobet 1,
1990, sr;tem vacant space was approximately 17.6 million
acre.feet. This vacant space has resulted from three
successive years of low inflow and the requirement to maintain
the minimum deliveries to meet obligations pursuant to 'The
Lawofthe Ri"'r."

Dailyreleases are provided from the storage reservoirs in the
Lower Basin to meet the needs of water user agencies, for
river regulation, and as needed for flood control. .When
possible, all water passes through the powerplant units. The
daily releases are regulated on an hourly basis to meet, as
nearlyas possible, the power needs of the hydroelectric pOVv'Cr
customers. At appropriate locations. minimum instream flow
objectives have been established, which preserve the present
aquatic resources downstream of certain Colorado River
dams. In many cases. these resources were poor or
nonexistent prior to the time of dam construction. and the
subsequent minimum, cool water releases have provided an
improved environment for aquatic resources and sport

fisheries. In general, controlled releases allow for an
extended recreation season, and reduce the high flow periods
in Mayand June. Water years 1988, 1989and 1990 ha", been
drier than normal and a return to more normal runoff levels
in future years will provide increased benefits to fish, wildlife
and recreation uses.

Operational objectj",s at and below Laguna Dam are to
conserve water, control sediment, and maintain the river
channel. Storage of water above Laguna Dam in the reservoir,
in surcharge, and in bankstorage provides for controlled flows
in the river at Yuma, which combined with seepage and
drainage, alloVolS a continuous li~ stream serving recreational
and fish and wildlife purposes. On a few occasions each year,
higher dailyflo'WS below Laguna, caused by rainstorms or user
rejected water orders, are used to maintain sufficient river
channel capacity. This occasional practice reduces channel
maintenance expense without impairment to water
conservation o~ power production.

Because of existing resermir conditions and river regulalion
operations below Hoover Dam, the total water year 1990
delivery to Mexico WJS approximately 23,OC>O acre-feel in
excess of the scheduled treaty delivery of 1,500,000 acre-feet
for the calendar year. In addition, 136,000 acre-feel of
drainage 'Waters were b)'passed to the Gulf of California via
the Bypass Drain during water year 1990. This bypass channel
was constructed pursuant to provisions of Minute No. 242 of
the International Boundary and Water Commission.

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