Cottonwood Creek, a third-order
tributary, flows 16 miles south through a confined, high-gradient, and well-armored
channel before entering the Blackfoot River at river mile 43. As the stream exits its
canyon, sinuosity increases and a pool-riffle sequence develops. As the stream enters
an outwash plain it loses water; an intermittent section occurs between stream mile 9 and
11, near the mountain-valley interface. The lack of water in this section is exacerbated
by irrigation withdraws. In order to prioritize restoration resources, we developed a
fisheries-based restoration priority scorecard, based on biological, social and financial
considerations, for 83 impaired tributaries of the Blackfoot River. Cottonwood Creek
ranked 5 of 83 streams surveyed.
Cottonwood Creek gains groundwater
beginning at stream mile 9.0, and appears to be a gaining stream to the mouth due largely
to three spring creeks located between river mile 6.3 and 7.5. Beaver activity
greatly influence channel characteristics through most of this reach.
|Two major irrigation diversions occur
at stream miles 5 and 12. Below the upper diversion, the stream dries in low flow years
for 2.5 miles. Management of the headgate has isolated westslope cutthroat trout and
bull trout attempting to escape the dewatered section, resulting in fish
kills. Additionally, both diversion structures were barriers to upstream movement of
fish and entrained out-migrants. The upper diversion serves an 8,000 foot irrigation
canal that irrigates approx. 400 acres of hay land. This canal was inefficient, carrying
only 28% of its volume to the end-point. At the time this information was recorded,
the canal took the entire stream flow (8 cfs) to an irrigation pump with a 1 cfs capacity.
Restoration projects in the
Cottonwood Creek drainage began in 1991. Denil fish ladders were fitted to both of
the headgates. Self-powered, self-cleaning fish screens with 1/8 diameter screen size
were installed in both canals immediately below the point of diversion.
To improve the efficiency of the canal
at the upper diversion, an impervious 12 mil rubber/fiber liner was placed 12 inches
underground over the entire length of the canal.
||During and after installation of a
An annual 8,663 acre
feet of salvage water, ranging from 6 cfs at base flow to 37 cfs during runoff periods, is
also being leased for instream flows during the irrigation season.
This project is designed to benefit
bull trout and cutthroat trout by improving connectivity between the upper and middle
stream reaches, improving flows for two miles of dewatered stream, reducing loss of fish
to irrigation canals, improving riparian plant communities previously affected by
dewatering during the growing season.
In September of 2001, we resampled fish
populations at mile 12.0, downstream of the diversion. We recorded significant changes in
westslope cutthroat trout and minor changes in bull trout. In 1997, prior to restoration
efforts, the Catch Per Unit Effort (CPUE) for young-of-the-year (YOY) westslope cutthroat
trout was 0.7 fish/100 feet, compared to 7.7 fish/100 feet in 2001. CPUE for age 1+
westslope cutthroat trout was 2.5 fish/100 feet in 1997 and 14.3 fish/100 feet in
2001. In 1997, we recorded no YOY bull trout in this section; in 2000 we picked up
our first YOY bull trout in this section. The 2001 surveys indicate that both native
species continue to respond to increase flows, fish passage and ditch screening.
Other current projects to improve the
fishery in the Cottonwood Creek watershed include implementing rotational grazing systems,
removal of stream-side livestock corrals, removal of under-sized culverts and replaced
with open-bottom box culvert, purchase of 4,000 acres of Conservation Easements along
Cottonwood Creek and a graduate study on whirling disease as it relates to habitat types.