WSPC03077

• File Number: 8062
• Description: Federal Water Rights - Colorado Litigation - National Forest ISF Claims - Division 2
• State: CO
• Basin: Statewide
• Date: 1/1/1996
• Author: CWCB
• Title: Materials of Interest - Development of Instream Flow Recommendations in Colorado Using R2CROSS - CWCB
• Water Supply Pro - Doc Type: Report/Study

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Table 2, Criteria used to determine minimum flow requirements using the R2CROSS
single transect method (Nehring 1979)

Stream Top Average Percent Wetted Average
Width (ft)' Denth (ft) Perimeter (%)1 Velocity (ft/sec)
1-20 0,2 50 1.0
21-40 0.2-0.4 50 1.0
41-60 0.4-0,6 50-60 1.0
61-100 0,6-1.0 ~70 1.0
I At bankfull discharge,

based upon biologic considerations such as
stream conditions, species composition, and
aquatic habitat quality.
These hydraulic criteria can be applied
to the R2CROSS staging table from the Iron
Creek example (Figure I) to develop an initial
biologic instream flow recommendation. In this
example, the grassline top width of Iron Creek
is 9,97 ft. Therefore, the DOW criteria for an xd
of 0,2 feet would be satisfied at a flow of
approximately 0,6 cfs, The %WP criterion of
50% would be met at a flow of around 1.75 cfs
and an x of 1 ft/sec at a flow of 2.25 cfs,
v
Based upon this analysis, a winter flow
recommendation of 1.75 cfs would meet the xd
and % WP criteria and a summer flow
recommendation of 2,25 cfs would satisfy all
three criteria, These initial recommendations
may be adjusted up or down based upon
biologic judgment and expertise.

Water Availability Requirements

Once an initial biologic instream flow
recommendation has been developed, the
CWCB staff must determine whether water is
physically available to satisfy the biologic
recommendation, The staff uses stream gaging

records to analyze physical water availability
whenever possible. In the absence of a gage
record, the staff may use standardized
hydrologic techniques, such as areal
apportionment or synthetic streamflow
modeling (Kircher et al. 1985), to estimate
physical water availability, The staff may also
conduct a review of the State Engineer's water
rights tabulation and consult with Division
Engineers and District Water Commissioners to
determine the effect of senior diversions on a
stream reach.
The water availability analyses may lead
the CWCB staff to conclude that sufficient
water is not available to meet the biologic
recommendation, In that situation, the CWCB
staff may request that the cooperating agency
reconsider its biologic recommendation and
determine whether the natural environment can
be preserved with the amount of water
available, If the natural environment can be
preserved with the available water, the instream
flow recommendation may be revised to reflect
the lower available flow amounts, If the
statutory water availability requirement cannot
be satisfied, the CWCB must reject the instream
flow recommendation,

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