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
Table 6-10 Spring Peak-Flow Recommendations for the Colorado River Near the Colorado-Utah State Line (USGS 09163500)a:
Number of Days per Year the Flows Should Exceed 1/2 Bankfull Discharge (Q~ =18,500 cfs) and Bankfull Discharge
(Qn = 35,000 cfs)

VVet

10% . -
~~
80-100 ~ "r"
~
30-35

39,300-69,800d
Moderatel Wet 20% 50-65 15-18 35,000-37,500e
Avera e Wet 20% 30-40 6-10 ? 35,OOOf
Avera e D 20% 0-10 0 18,500-26,600e
Moderatel D 20% 0-10 0 9,970-27,3009
D 10% 0 0 5,000-12,1009
Lon -Term Wei hted Avera e 28-39 7.2-9.1
a This table represents one possible way of achieving the long-term weighted average for sediment transport.
b Lower value in each range is for maintenance, higher (bold) value in each range is for improvement.
~ Weighted values equal days/year x expected occurrence (the sum of all weighted average values equals the long-term weighted
average in days/year).
d Instantaneous peak flows within this range have occurred in these hydrological categories since Blue Mesa Reservoir was closed.
These observed instantaneous peaks are desired in the future in conjunction with meeting the flow targets. No specific peak flow is
recommended to ensure continued variability among years.
e Lower number reflects the expected minimum peak flow when recommendations are met and the upper number reflects peak flows that
have occurred since Blue Mesa Reservoir was closed. Peak flow is expected to occur within this range, but no specific value is provided
to ensure variabiliiy among years.
f Expected peak flow when recommendations are met. Actual peak may exceed this level, ensuring continued variability among years.
g Range of peak flows that have occurred since Blue Mesa Reservoir was closed. Peak flows are expected to continue to fall within this
range when Q~ is not reached. No specific recommendation within this range is made to ensure variability among years.
Summer through winter base flow recommendations for
the Colorado River, measured at the USGS gage near
the Colorado Utah state line (09163500), for the different
hydrologic conditions are as follows:
^ Wet (10 percent exceedance): 3,000 to 6,000 cfs
^ Moderately Wet (10 to 30 percent exceedance) and
Average Wet (30 to 50 percent exceedance): 3,000 to
4,800 cfs
^ Average Dry (50 to 70 percent exceedance) and
Moderately Dry (70 to 90 percent exceedance): 2,500
to 4,000 cfs
The Colorado River immediately upstream from the
confluence with the Gunnison River (15-mile reach) is
currently operating under a PBO that provides ESA
compliance for 1,000,000 AFY of existing depletions, and
up to 120,000 AFY of new depletions additional water
development in the upper subbasin in compliance with
the ESA, provided that sufficient progress is made
toward recovery of the four endangered fishes. This
reach of the Colorado River can be seen on Figure 6-4.
The PBO provides for coordinated operation of upstream
reservoirs to assist in meeting flow recommendations
made for the 15-mile reach.
^ Dry (90-100 percent exceedance): >_ 1,800 cfs Ultimately, flows in the lower reaches of the upper
The base flow period begins after spring runoff is
completed and continues through initiation of spring
runoff the following year, depending on inflow to the
upper Colorado River subbasin. Flows should remain
within the bounds specified, but the upper and lower
limits are not intended to be targets. The onset of the
base flow period will vary considerably - beginning as
early as late June in dry years and as late as October in
wet years. No specific recommendations are presented
for the transition between recommended peak flows and
the recommended base flows.
Colorado River will depend on the combination of
(1) flows provided in the Gunnison River following re-
operation of the Aspinall Unit, and (2) flows provided in
the Colorado River under the PBO. Recommendations at
the Colorado-Utah state line do not override
recommendations for the upstream reaches and
agreements already in place for the upper Colorado
River. Therefore, the actual flows at the state line gage
for endangered fish are the combination of the flows
recommended for the Gunnison (USFWS 2003) and the
flows recommended for the 15-mile reach (USFWS
r~
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