WSP10742

• File Number: 8282.600.10
• Description: Colorado River Operating Annual Reports
• Basin: Colorado Mainstem
• Date: 1/1/1991
• Author: USDOI/BOR
• Title: Operation of the Colorado River Basin 1991 Projected Operations 1992
• Water Supply Pro - Doc Type: Publication

River Regulation

The runoff into Lake Powell for ""-tcr ~ar 1991 continued to
be well below normal for the fourth straight ~ar. Unregulated
inflow into Lake Powell was 71 percent of normal for water
~ar 1991. Unregulated intlow.;for water~ars 1988,l989and
1990 were 70 percent of normal. 54 percent of normal and 46
percent of normal respectively. Upper Basin reservoirs,
Flaming Gorge, Blue Mesa, and Navajo, c,,!,crienccd modest
rebounds in storage in water year 1991. These increases in
storage occurred because releases from these rc-servoirs were
primarily constrained to minimum levels. Storage in Lake
Powell and Lake Mead, howc'-"r, is down significantly. The
October 1. 1991. 'Weant space in these two reservoirs was
approximately 17.8 milJion 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 Lawofrhc River",

Daily releases are provided from the storage reservoirs in the
Lower Basin to meet the needsofwater user agencies, for river
regulation, and as needed for flood conlrol. When possible,
all water' passes through the powerplant units. The daily
releases are regulated on an hourly basis to balance the power
needs of the hydroelectric power customers, the now needs of
endangered species of fish and recreational needs. At
appropriate locations, minimum instream flow objectives
have been established, which preserve the present aquatic
resources dO\lll1stream 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 \Wter 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 May and June. Water
~ars 1988, 1989, 1990 and 1991 ha'-" been dr~r than normal
and a return to more normal reservoir inflow levels in future
~ars will provide increased benefits to fish, wiidlife and
recreation uses.

Operational objectives at and below Laguna Dam are to
. conserve water, conlrol 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. When combined with seepage and
drainage, this storage allo'WS a continuous live stream serving
recreational and fish and wildlife purposes. On a few
occasions each year, higher daily flows below Laguna, caused
by rainstorms or user rejected water orders, arc used to
maintain sufficient ri\er channel capacity through flushing of
sediment. This occasional practice reduces channel
maintenance expense without impairment to waler
conserv.l.tion or power production.

Based on existing reservoir conditions and river regulation
operations below Hoover Dam, the total 1992 delivery to
Mexico is scheduled to be a treaty dcli,-"ry of 1,500,000
acre.feel for the calendar year. In addition, approximately
140,000 acre-feet of drainage waters are expected to bypass to
the Gulf of California via the Bypass Drain during calendar
year 1991. This bypass channel was constructed pursuant to
provisions of Minute No. 242 of the International Boundary
and Water Commission.

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