9596

• UCREFRP Catalog Number: 9596
• Author: Ayers Associates.
• Title: Yampa River Basin Research Final Synthesis Report.
• USFW Year: 1999.
• USFW - Doc Type: Fort Collins, CO.
• Copyright Material: NO

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3.1.3. Modde and Smith (1995)

Modde and Smith developed flow recommendations for the Yampa River by ranking estimated
daily "virgin" flows at the Maybell gage for each of the non-runoff months (August-March) for
water-years 1950-82 (Hydrosphere 1995). Based on this ranking, they categorized estimated
virgin flows typical of wet conditions (flows exceeded 20% of the time), normal conditions (flows
exceeded 50% of the time) and dry conditions (flows exceeded 80% of the time).

The 80% exceedance flows were the basis for the dry-year flow recommendations during non-
runoff months (see Table 3 in Section 5). However, the Service did not recommend that flows
be held constant at these monthly exceedance levels. These flows represent monthly means,
rather than flow constants. To mimic the natural diurnal variability of the hydrograph, daily flows
should vary about this monthly mean, exceeding the mean during some portions of any month,
while falling below it at other times. During runoff periods, recommended flows also vary, based
on the variable natural flows minus a fixed depletion base line.

3.1.4. Colorado Division of Wildlife (1995)

The CDOW considered the September flow recommendation of Modde and Smith (45 cfs) to be
inadequate to protect the endangered fishes. CDOW adopted a flow of 88 cfs in September for
Colorado's instream flow application, based on its best professional judgement. It was apparent
that low-flow recommendations were needed based on the needs of fishes rather than statistical
analyses of virgin conditions.

3.1.5. Modde, Miller and Anderson (1999)

The RIP initiated a study of the potential impacts of low flows on the riverine environment to
determine the low-flow requirements of endangered fishes and if some form of stream flow
management is necessary or desirable during the low-flow period. The study also investigated
biological components of riverine habitat used by both endangered species and nonnative game
fish (channel catfish and northern pike). Endangered species studied were the humpback chub
in Dinosaur National Monument and Colorado pikeminnow from the Green River upstream to
Craig, Colorado.

Conducted over two years (1996-97), the study included measurements of channel width, depth,
velocity and other physical attributes of the river as a function of flow for each of three
"mesohabitat" types: pools, riffles, and runs. In their 1999 report entitled Determination of Habitat
Availability, Habitat Use, and Flow Needs of Endangered Fishes in the Yampa River Between
August and October, Modde et al. found that riffle habitats were particularly important for
Colorado pikeminnow due to the abundance of macroinvertebrates in riffles and the relative
sensitivity of riffles to changes in stream flow. They estimated that the rate of loss of riffle
habitat was greatest at flows below 93 cfs in the Yampa River between its confluence with the
Green River and Craig, Colorado. The authors acknowledged that low flows may have occurred
during drought conditions prior to the advent of irrigated agriculture. They also found that viable
populations of two endangered fish species continue to exist in the Yampa River even though
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daily average flows have fallen below 93 cfs to some extent in 34 out of 81 years of the gage
record at Maybell. However, they concluded that depletions from the river could adversely
impact endangered fish during droughts by extending or deepening these low-flow periods.
Severe droughts or frequent, prolonged periods of below-normal base flows may adversely affect
even native fish that are adapted to occasional droughts. This research led to the following
conclusions and recommendations: