9406

• UCREFRP Catalog Number: 9406
• Author: Osmundson, D. B.
• Title: Importance of the '15-Mile Reach' to Colorado River Populations of Endangered Colorado Pikeminnow and Razorback Sucker.
• USFW Year: 2000.
• USFW - Doc Type: Grand Junction.
• Copyright Material: NO

EXECUTIVE SUMMARY
The U. S. Fish and Wildlife Service
(USFWS) takes the position that the quality
and quantity of habitat in a 15-mile reach of
the upper Colorado River must be protected
and enhanced if Colorado River populations
of two endangered species are to be main-
tained and recovered. The reach extends
upstream from the Gunnison River conflu-
ence in Grand Junction, Colorado to the
Grand Valley Diversion dam at Palisade,
Colorado. The flow regime of the reach has
been altered in two primary ways: (1) up-
stream diversion dams have significantly
reduced summer flows since the turn of the
century; and (2) headwater dams have re-
duced spring runoff flows since the middle of
the century. Despite these and other impacts,
the 15-mile reach and the 18-mile reach
immediately downstream continue to be
concentration areas for a remnant Colorado
River population of the endangered Colorado
pikeminnow, and until recent years, for the
endangered razorback sucker, now almost
extirpated. This report attempts to summa-
rize the biological basis for why the 15-mile
reach, and the Grand Valley in general, are so
critically important to the maintenance and
recovery of these populations.
Floodplains contain habitats critical to
the reproductive strategy of razorback suck-
ers. Alluvial valleys containing such habitats
appear to have been centers of distribution
for this species in the upper Colorado River.
These areas include the Grand Valley as well
as the more upstream De Beque-to-Rifle
reach. Both areas should therefore be the
focus of recovery efforts for this species in
the upper Colorado River.
For Colorado pikeminnow, movements
are an integral part of the life history strategy:
adults migrate to spawn, larvae drift long
distances down river, and sub- and young
adults disperse from nursery areas to reaches
containing more,suitable adult habitat. As a
consequence, an individual fish is likely to
use many sections of river during the course
of its life. Each part of the river is therefore
important; however, based on the distribution
of these life stages, some areas are clearly
more important than others. For young, the
reach downstream of Moab, Utah evidently
contains the most important rearing habitat in
the river. Conversely, the reach upstream of
Westwater, Utah evidently contains the most
important adult habitat.
Upstream of Westwater, the 33-mile
long Grand Valley consistently supports the
greatest concentrations of adult Colorado
pikeminnow. The relatively high gradient,
clear water, broad floodplain and a diversity
of habitat types make these upstream sections
of river more productive than more down-
stream sections. Higher standing stocks of
periphyton and invertebrates support rela-
tively high numbers of forage fish, in turn
providing greater food availability for the
river's top piscivore, the Colorado pikemin-
now. However, main-channel temperatures
decline with distance upstream and upstream
movements therefore take these warmwater
fish away from a more preferred thermal
regime. The Grand Valley, including the 15-
mile reach, may represent the best balance
between food supply and preferred tempera-
tures. It is not known how much farther
upstream of the Grand Valley these fish
would establish home ranges were it not for
the diversion dams that currently block their
movement. However, results of an analysis
of pikeminnow distribution in the Yampa
River (where adults have unlimited upstream
access), and a comparison between the ther-
mal regime there and that in the Colorado
River, suggests that suitable temperatures in
the Colorado River may extend approxi-
mately 14 miles into canyon habitat upstream
of the Grand Valley.
Alluvial valleys with floodplain habitats,
high productivity and relatively warm main-
channel temperatures are in short supply in
the upper Colorado River. The Grand Val-
ley, including the 15-mile reach, is the center
of distribution for adult Colorado pikemin-
now in the upper Colorado River and may
have historically been so for the razorback
sucker. It is therefore clear that this area will
be critical in recovering populations of these
endangered fish.