7750

• UCREFRP Catalog Number: 7750
• Author: U.S. Bureau of Reclamation.
• Title: Study of Alternative Water Supplies for Endangered Fishes in the "15-Mile Reach" of the Colorado River.
• USFW Year: 1992.
• USFW - Doc Type: \
• Copyright Material: NO

CHAPTER I INTRODUCTION
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A corresponding 75,776 AF supply would be necessary to ensure that there would be at least
' 600 cubic feet/second (cfs) in the river during a dry water year. With this supply the flow in
the Reach should never fall below 600 cfs. The 47,102 AF and 75,776 volumes are
exclusive, in that either volume is necessary to achieve either flow.

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It is important to point out that since the above flow recommendations and the estimated
shortages were developed, the river has experienced dramatic changes in river operation and
use. Four factors that will impact this analysis are; 1) changes in the operation of the
Orchard Mesa Check, 2) changes in the operation of Green Mountain Reservoir (GMR) have
developed, 3) perfection of new water rights, and 4) the availability of water from Ruedi
Reservoir. A more thorough water availability study should be conducted to evaluate the
impacts of these changes to the delivery requirements.
The above flow recommendations are based upon mean monthly discharge. Instantaneous
discharge can vary markedly from mean-monthly values within months. The frequency of
extremely low flows, which can have important limiting effects on both habitat availability
and fish populations, is not evident when such data is used.'
An inherent weakness in studying the 15-Mile Reach was a lack of stream measurement
devices. As a feature of the Recovery Program, the U.S. Geological Survey (USGS) has
installed a river gage approximately 1/4 mile downstream from the upstream end of the
Reach. This gage became operational at the beginning of the 1991 water year. Another
gage near the downstream end of the Reach would provide a means to measure the flow
gains in the Reach. Additionally, measurement of the discharge from the radial gates of the
Orchard Mesa Power and Pumping Plant would provide a reliable means for ensuring the
Ruedi Reservoir releases are delivered to the Reach. There is no reason to doubt that this is
not currently the case, but, with present means of measurement, it is difficult to verify.
In lieu of a gage on the downstream end of the Reach, the Recovery Program hired a
consultant to measure the flow near the downstream end from August 1 through
November 30, 1991. With the inherent variability of hydrological data, the usefulness of 4
months of data will be limited. For example, during the first 3 months of September
precipitation was'aq above normal. Consequently, the irrigation requirement for alfalfa was
2/3 of the average of the last 5 years over the same period. Under normal conditions, when
crop irrigation requirements decrease, return flows increase.
With a demand for over 700 cfs, the study presents alternative supplies of water in varying
amounts. It is possible to select portions of an alternative, full alternatives, or combinations
of alternatives to meet the required flow.
' Ibid. p. 25.
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