9383

• UCREFRP Catalog Number: 9383
• Author: SWCA, I.
• Title: Recovery Goals for the four Colorado River Endangered Fish Species.
• USFW Year: 2000.
• USFW - Doc Type: Logan.
• Copyright Material: NO

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much of the upper basin still have a heightened spring runoff and low base flows (Van Steeter
k 1998). Water temperatures are similar to historic ranges and means (Kaeding and
son 1989), except for dam tailwaters that are released from deep, cold hypolimnetic
regions of reservoirs. Sediment transport has been modified in some regions of the upper basin,
particularly below dams (Andrews 1991). Invasion of alien plants and fish have also impacted
the historic ecosystem of the upper basin, resulting in extensive groves of nonnative tamarisk that
line and stabilize most of the river banks, sometimes at the exclusion of native willows and
cottonwoods. App ately 42 species of nonnative fishes compete with 13 maintem native
species in the uppe (Tyus et al. 1982), resulting in competitive and predatory forces that
ecline of some of the native species. These changes in hydrology,
temperature, sediment, and animal and plant diversity have resulted in subtle to extreme changes
in the riverine ecosystem. Extreme changes in macroinvertebrate and fish assemblages have
. taken place in reservoir basins and dam tailwaters (Holden and Crist 1979, Holden and Selby
1979), but further downstream, where flows are seasonally warmed and variable, much of the
historic macro invertebrate community i act (Haden 1998), although reduced or modified by
changes in flow regime and sediment smundson et al. 2000). Energy flow through the
ecosystem has also been altered by flow regulation and the presence of large nwnbers and
biomass of nonnative fishes (Osmundson et al. 2000). Despite these changes, there still remain
large, reproducing, and self-sustaining populations of many of the native species in the upper
basin.

In summary, the upper basin has about 1,355 km of ater riverine habitat suitable
for native warmwater fishes, and sustains reproducing, self-s 'ng populations of all native
fishes, except for bonytail and razorback sucker (some repro uction and possibly recruitment is
taking place with the razorback sucker). Despite reductions in flow volume and modifications to
flow regimes from dam operations and diversions, much of the historic structure and function of
the riverine ecosystem remain intact for several reaches of the upper basin. Because the upper
basin is now physically separated from the lower basin, the ecological features unique to the
upper reaches of the river continuwn have become isolated and these exert unique ec
forces on the evolutionary legacy of the native fishes, independent of the lower basin.

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