8249

• UCREFRP Catalog Number: 8249
• Author: Modde, T., W. J. Miller and R. Anderson.
• Title: Determination of Habitat Availability, Habitat Use, and Flow Needs of Endangered Fished in the Yampa River Between August and October.
• USFW Year: 1999.
• USFW - Doc Type: Project #CAP-9,
• Copyright Material: NO

signata, Simuliidae, and other species during certain seasons. White et al. (1981) indicated that an 85%
reduction from normal flow during the fall would cause the genus Baetis to increase drift to the point that
would significantly decrease their densities in riffles. Poff and Ward (1991) reported that several species
representing four different orders responded to high flows with increased drift. Most of the species found
to be influenced by a change in flow in the upper Colorado River are represented by members of the same
taxonomic families in the Yampa River. These include: Ephemerella sp., Baetis sp., Paraleptophlebia
sp., IsoperZa sp., Brachycentrus sp., Lepidostoma sp., and Hydropsyche sp.

White et al. (1981) found no significant change in total abundance of benthic invertebrates as a result of
reductions in discharge (up to 95%), however, certain species were significantly depleted. This research
indicates that the density of invertebrates in the wetted area of the stream channel before dewatering was
not significantly different from the density of invertebrates in the stream channel after flow reductions.
However, if the wetted area is reduced by 50%, halfofthe invertebrate community and biomass is lost.
White et al. (1981) suggested that a reduction in discharge results in a change in the hydraulic
characteristics of a stream thus affecting the factors responsible for microhabitat selection and
invertebrate distribution. The increased drift observed as a response to flow reductions may be an
evolutionary adaptation to insure survival in response to natural flow fluctuations. For many species this
may be an attempt to relocate to an area of more suitable habitat (White et al. 1981).
Within a week or two after flow reductions, White et al. (1981) found that drift rates returned to normal
densities (at flow reductions between 50 and 85%). Even when drift densities were within the range
considered "normal", the rate of drift was far less (about 50% less when flow was reduced by 50%) than
that observed in the control channel (because of the reduction in wetted area in the test stream). White et
al. (1981) found that during 95% flow reductions in the fall, invertebrate drift densities did not increase
and were reduced by 50% or more and remained below normal throughout the study period. This
information is important because many fish species rely on invertebrate drift as a food supply. Assuming
no mortality, flow reductions will cause fish densities to increase. Chapman (1966) suggests that the
availability offood is an important factor in determining the density offish. The increase in fish density
combined with the observed decrease in invertebrate drift rate should result in a negative impact to some
fish species at low flows. White et al. (1981) suggested that fish may be adversely affected by the
decrease in drift density at flows less than 85% of normal.

Ames (1977) provided a species list for macroinvertebrates occurring in the Yampa River. Most of the
species on this list have physical or behavioral adaptations specific to the habitat (and velocity) where
they reside. The macroinvertebrate communities in the Yampa River exhibit longitudinal changes in

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