WSP08192

• File Number: 8240.200.38.A
• Description: Coordinated Reservoir Operations
• State: CO
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 1/1/1997
• Author: CWCB
• Title: 1997 Coordinated Reservoir Operations Bypasses for Endangered Fish - Annual Summary of Operations for 1997 to Benefit the Endangered Fishes of the Upper Colorado River Basin
• Water Supply Pro - Doc Type: Publication

I
I
I
I
I
I
It
I
m
I
,
I
I
I
I
t
I
I
I

Analysis during the initial study was conducted to gain an understanding of the potential
magnitude of contribution to peak flows that might be attainable through CROS efforts and to
identify the types of water years where contributions by coordinated releases might be
significant. Characteristic years were identifIed in the period of 1984 to 1994. It was found that
during dry years, reservoirs would be operating strictly to meet existing demands and would
have little, if any, discretionary water available.

Further analyses were conducted for water years 1993 and 1994, which bracket the 'average' year
of 199 I and represent the types of years in which discretionary releases may be able to be made
and provide some benefit to peak flows. The analyses indicate that even the hypothetical rernoval
of the total effects of existing trans mountain diversions and reservoirs, which is not contemplated
by the RIP, would not produce major increases in the peak spring flows. This is of particular
concern as the operational modifications considered under this study are only a small fraction of
the total effects of present reservoir operations. Additionally, since discretionary releases
represent a shifting of timing of releases from a reservoir and not an increase in overall volume
released, there are seasonal tradeoffs to making the releases.

The study team requested that the Program extend previous research into the flow effects upon
habitat formation. Dr. John Pitlick of the University of Colorado found that certain velocities
must be surpassed and sustained to cause channel formation in the gravel beds of the Reach.
There are several non-fluvial factors that impede channel formation, particularly levees, riprap,
and vegetative growth. If the flow increases due to coordinated reservoir operations are only a
small percentage of the overall Colorado River flow during peak runoff, critical flow velocities
for channel formation may not be obtained, or there may only be a marginal impact on channel
formation. Dr. Pitlick is conducting further research into the flow effects upon silt movement,
and it is possible that greater benefits may be attainable by targeting discretionary releases
toward silt movement.

The team analyzed historic climatological trends due to a concern that the IS-Mile Reach flow
recommendations may have included data from a wetter period of record than is currently being
experienced in the Colorado River Basin. The results indicate that up to half of the decreases in
some mean monthly flows may be due to climatological changes during the past 90 years. Some
of the flow recommendations are greater than 'virgin' flows, with all effects of development
removed, for the recent drier period of 1954 to 1989.

The study team simulated potential discretionary releases that might have been made in past
years had the reservoirs been operated to enhance peaks. This analysis was being conducted with
very broad, simplifying assumptions. Initial indications were that there is some ability to enhance
peak flows through coordinated discretionary releases though it is possible they should be aimed
at cleaning out silts to improve habitat in the Reach. The U.S. Fish & Wildlife Service has also
identifIed a range of peak flows outside of which it may be more benefIcial to target releases at
augmenting low flows during non-peak times ("Relationships Between Flow and Rare Fish
Habitat in the '15 Mile Reach' of the Colorado River", U.S. Fish & Wildlife Service 1995). The
conclusion of the feasibility study was that reservoirs should seek to bypass inflows during the
peak, but only if bypasses would result in peaks greater than 12,900 cfs without exceeding the
National Weather Service flood level of 26,600 cfs. This range is based on the the need for
sediment flushing from the interstitial voids in the river channel for better survival of fIsh eggs
and invertehrates (food source for the fIsh).

4