S6_11-15-04

• Basin: Statewide
• Title: SWSI Phase 1 Report - Section 6 Water Needs Assessment
• Date: 11/15/2004
• Author: CWCB
• SWSI - Doc Type: Final Report

Section 6
Water Needs Assessment
Flow recommendations to benefit endangered fish have
been developed for the Colorado and Gunnison Rivers
(USFWS 2003). It is emphasized that flow
recommendations are not monolithic absolute values,
and they may be revised from time to time to include the
results of research. Flow recommendations may not
inhibit the development of Colorado's compact
entitlements. The goal of the recommendations is to
provide the annual and seasonal patterns of flow in the
Gunnison River for the Colorado pikeminnow and
razorback sucker (see Section 6.2.4.2 for Gunnison
River flow recommendations), and in the Colorado River
downstream from the confluence to enhance populations
of the four endangered fishes (Colorado pikeminnow,
razorback sucker, bonytail chub, and humpback chub).
Base flow and peak flow recommendations are provided.
The objectives are to allow Colorado the full ability to
develop its compact entitlements, while creating and
maintaining the variety of habitats used by all life stages
of the four endangered fishes:
^ Provide habitats and conditions that enhance gonad
maturation and provide environmental cues for
spawning movements and reproduction
^ Form low-velocity habitats for adult staging, feeding,
and resting areas during snowmelt runoff
^ Inundate floodplains and other off-channel habitats at
the appropriate time and for an adequate duration to
provide warm, food-rich environments for fish growth
and conditioning, and to provide river-floodplain
connections for restoration of ecosystem processes
^ Restore and maintain in-channel habitats used by all
life stages: (1) spawning areas for adults; (2) spring,
summer, autumn, and winter habitats used by
subadults and adults; and (3) nursery areas used by
larvae, young-of-the-year, and juveniles
^ Provide base flows that promote growth and survival
of young fish during summer, autumn, and winter
exceedance); Moderately Dry (70 to 90 percent
exceedance); Average Dry (50 to 70 percent
exceedance); Average Wet (30 to 50 percent
exceedance); Moderately Wet (10 to 30 percent
exceedance); and Wet (0 to 10 percent exceedance).
Flow recommendations are for the Colorado River near
the Colorado-Utah state line (USGS 09163500). Peak-
flow recommendations include two components:
(1) threshold levels corresponding to 1/2 bankfull
discharge and bankfull discharge, and (2) the number of
days (duration) that flows should equal or exceed these
levels. In addition, recommended durations are
presented as a range of days. In general, spring flows
recommended for the dry categories provide small peaks
used as spawning cues by endangered fish, but
contribute little to habitat maintenance; spring flows
recommended for average categories promote scouring
of cobble and gravel bars and provide localized flooding
of short duration; and spring flows for the wet categories
promote widespread scouring of cobble and gravel bars,
flushing of side channels, removal of encroaching
vegetation, and inundation of floodplain habitats.
Base flow recommendations also vary with hydrologic
category and are designed to allow fish movement
among river segments and to provide maximum amounts
of warm, quiet-water habitats to enhance growth and
survival of young endangered fish.
The flow recommendations were developed using
information currently available; however, it is recognized
that uncertainties exist. Biological and physical
uncertainties are described in the recommendations,
(USFWS 2003) and additional studies are proposed. The
recommendations will be implemented using adaptive
management. Modifications will be made as more
information is gained.
Because historical river flows were dependent on water
availability, peak flow recommendations were developed
for six hydrologic categories that correspond to
unregulated April to July inflow based on the 1937 to
1997 period of record: Dry (90 to 100 percent
The peak flow recommendations for the Colorado River
near the Colorado State Line are shown in Table 6-10
and are one way of achieving the objectives of the
program.
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