7848

• UCREFRP Catalog Number: 7848
• Author: Burdick, B. D.
• Title: Minimum Flow Recommendation For Passage Of Colorado Squawfish And Razorback Sucker In The 2.3-Mile Reach Of The Lower Gunnison River
• USFW Year: 1997.
• USFW - Doc Type: Redlands Diversion Dam To The Colorado River Confluence.
• Copyright Material: NO

smaller water projects occupy a portion of the Gunnison River upstream from
historic habitat of the four endangered fish, alterations in water quality and
quantity have had significant adverse affects on downstream warmwater reaches.
The Dolores Project (McPhee Reservoir on the Dolores River) and Dallas Creek
underwent Section 7 consultation, and were built with the stipulation that water
from the Aspi nal l Uni t (148, 000 acre- feet [AF]) woul d be used to parti al l y offset
the impacts of these two projects on endangered fishes in downstream, warmwater
reaches of the Gunnison and Colorado rivers (see section on "Interim Cooperative
Water Agreement").
Historically, the Gunnison River was typical of Colorado River basin
tributaries with high, turbid spring flows and clearer low flows from late summer
through winter. However, the timing of water delivery has been significantly
altered by water development projects. The greatest change caused by the
Aspinall Unit is the reduced magnitude of spring runoff. The mean-monthly flows
measured at the U. S. Geological Survey (USGS) gage at Whitewater, Colorado, have
declined about 25% in May and 32% in June since construction of the Aspinall
Unit. In contrast, flow during the remainder of the year has increased,
particularly during the winter when average monthly flows are more than 100%
greater than pre-Aspinall flows. In general, spring and early summer flows have
declined, and fall and winter flows have increased (Figure 1).
Flows over 15,000 cfs are considered high flows for the Gunnison River.
Flows of this magnitude were common in the earliest periods of record (prior to
the Aspinall Units), and were frequent in the 1950's, but only four years, 1983,
1984, 1993, and 1995, have flows of this magnitude occurred since 1965 (Figure
1). Peak flow during May 1993 reached 20,500 cfs, the second highest annual flow
since 1957. Flows of 20,000 cfs occur 10% prior to construction of the Aspinall
Unit. The number of days with flows greater than 10,000 cfs, on average, prior
to construction of the Aspinall Unit, was 3 weeks. The frequency of flows of
10 , 000 cfs following construction of the Aspi nal 1 Unit is now only 1 week (Cooper
and Severn 1994).
A secondary effect is that wate
and increased during winter. Summer
historic temperatures by a maximum of
squawfish and razorback sucker (McAda
typical of the ecological changes in
of large dams (Vanicek et al. 1970).
temperatures have decreased during summer
water temperatures have been reduced from
about 1.8°C in occupied habitat of Colorado
and Kaeding 1991). These modifications are
upper basin rivers after the construction
Large dams and diversion structures are also effective instream barriers
that fragment stream reaches which reduces the range of native species by
precluding both young and adult fish from returning upstream after they have
migrated downstream. Barriers are particularly harmful to species that migrate
long distances to fulfill life history requirements such as the potamodromous
Colorado squawfish (Tyus 1984; Tyus 1990). Instream barriers can divert water
from main channel rivers for irrigation and power generation and reduce the range
of these species by rendering downstream reaches uninhabitable due to low flow.
In upper basin rivers, this situation usually occurs downstream of diversion
structures during low-flow years in mid-summer months when irrigation demands are
high. In some reaches, reduced water volume has led to water quality degradation
by increasing the water temperature and elevating concentrations of human-pro-
2