NEPA Scoping Meetings | Physical
Characteristics | Additional Project
Project Design | Recreation
| Fish | Funding |
Canyon Dam (furthest north of the east-facing dams) begins construction
1946. An empty Horsetooth Valley is on the right. Dixon Reservoir
is to the east on the left.
Construction to bring Horsetooth Reservoir's four dams to state-of-the-art
standards is to begin in early 2001.
The Horsetooth Reservoir Modernization Project will take three to
five years and cost an estimated $105 million.
The 50-year-old dams pose no immediate danger, emphasize officials
from the Northern Colorado Water Conservancy District and the U.S.
Bureau of Reclamation.
Horsetooth Reservoir is west of Fort Collins, Colo. It is one of
12 reservoirs in the Colorado - Big Thompson Project. The C-BT Project's
purpose is to provide a supplemental water supply to northeastern
Colorado. The C-BT was built between 1937 and '57.
The U.S. Bureau of Reclamation built and owns the reservoir, while
the Northern Colorado Water Conservancy District operates and maintains
it. Under a contract with Reclamation, Larimer County Parks and
Open Lands manages the recreational areas surrounding the reservoir.
The county maintains the roads around the reservoir.
NEPA Scoping Meetings
NCWCD and Reclamation, in January 2000, began the process toward
construction by conducting two environmental scoping meetings.
The meetings, in La Porte and Fort Collins, were conducted to fulfill
a National Environmental Policy Act requirement and to seek public
input. NEPA mandates consideration of the environmental impacts
associated with the project or alternatives to it. NEPA compliance
is required as plans are finalized, construction schedules are developed
and funding is procured.
District General Manager Eric Wilkinson and Beth Boaz, Reclamation's
engineering and construction liaison, emphasized the project was
given high priority because Fort Collins' population has increased
from 15,000 to 100,000 since Horsetooth was built.
During the scoping meetings, Wilkinson, Boaz and staff members from
each entity used photos, maps and diagrams to explain how the downstream
faces of the dams will be stripped off, a filter layer of sand and
gravel will be placed and then new downstream shells put in place.
trucks from valley "borrow areas" with material to be used in
the dams. Circa '46-48.
Horsetooth Reservoir is located just west of the City of Fort Collins
in the foothills of the Rocky Mountains, approximately 70 miles
north of Denver. The reservoir is 6.5 miles long, averages about
1/2 mile in width, and occupies a narrow north-south valley between
two hogback ridges.
Four earthen dams and one dike impound the reservoir's waters --
Horsetooth Dam and Satanka Dike enclose the north end of the reservoir.
A hogback ridge and the remaining three dams --Soldier Canyon, Dixon
Canyon, and Spring Canyon dams -- border the eastern side of the
The maximum active capacity of Horsetooth Reservoir is 156,735 acre-feet,
at a high water elevation of 5,430 feet above sea level. The reservoir
provides water for irrigation, recreation, municipal, domestic and
industrial use. Horsetooth Reservoir is filled primarily with water
from the Colorado River headwaters on the Western Slope. It is delivered
through the Charles Hansen Feeder Canal. Due to its relatively small
drainage area -- 17.5 square miles -- Horsetooth Reservoir can retain
inflows from a major storm event, and therefore has no spillway.
The wisdom of the decision was borne out in July of 1997 during
the Fort Collins Spring Creek Flood. A portion of Horsetooth Reservoir's
drainage area received nearly 10 inches of rain in around 5 hours.
Storm runoff raised the reservoir's surface level by 2 feet, or
by approximately 4,000 acre-feet, which would otherwise have increased
flooding in Fort Collins.
Additional Project Information
Reclamation officials monitor the dams at Horsetooth Reservoir
as part of the agency's mandate under the Safety Evaluation of Existing
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In 1989, after a particularly pronounced reservoir drawdown by Reclamation
and District officials, five sinkholes were discovered at the southern
end of Horsetooth Reservoir. Four additional sinkholes appeared
in the same area over the next several years. Further investigation
revealed the sinkholes might be associated with the geology of the
underlying Lykins Formation beneath the reservoir and the foundation
of Horsetooth Dam.
As a result, Reclamation officials organized an exploration program
to observe subsurface conditions and install additional monitoring
wells at Horsetooth Dam. Shortly thereafter seepage increased in
the dam's left toe drain and along the dam's downstream left side.
Seepage downstream of the dam has been present and monitored since
completion of the reservoir and dams in 1949.
Since the early 1990s, though, water pressures within the Lykins
formation beneath Horsetooth Dam have increased, as have seepage
flows. This increased seepage during the past decade directly coincides
with higher than average precipitation and water storage levels
during this same period. All earthen dams seep water, and all at
Horsetooth Reservoir are being examined thoroughly. Frequent, extensive
monitoring indicates that all four dams are performing safely.
Foundation seepage is a concern only at Horsetooth Dam and no unusual
seepage has been detected through any of the reservoir's four dams.
The modifications being proposed at Horsetooth Reservoir are to
ensure the continued safe operations of the dams, and will provide
defensive measures against future conditions that could threaten
the large population living immediately below the reservoir. Reclamation
and District officials believe these proactive measures are warranted
to ensure continued public trust and confidence in the facility.
Dam safety activities during the past two years have focused on
rising foundation pressures and increased seepage at Horsetooth
Dam, as well as potential dam safety risks at the reservoir's other
three dams. Reclamation consultants and engineers conducted two
primary studies; (1) a screening of potential corrective actions
to mitigate seepage concerns, and (2) a risk analysis of both the
seepage and seismic risks posed at all of the dams.
Both of these studies were incorporated into an environmental assessment
of the proposed modernization work at Horsetooth Reservoir.
The modernization project also falls under the 1984 Safety of Dams
Act, which necessitates congressional approval of a dam modification
report. Once Congress endorses the project, the federal government
will provide 85 percent of the project's cost. NCWCD and Western
Area Power Administration will each pay 7.5 percent.
is an excavation shovel from the mid-40s.
Officials from Reclamation and the District are preparing to replace
the downstream face of each dam at Horsetooth Reservoir. The decision
to modernize the four dams comes after nearly two years of investigation,
data analysis, and Reclamation's completion of an environmental
The design alternative for modernizing all four dams is the construction
of a filter buttress and berm. The repair entails stripping the
downstream shell of each dam, placing a filter buttress of graded
sand and a gravel drain on the downstream slope, and finally replacing
the downstream shell. A berm is also proposed for the base of each
dam, necessitating excavation of each dam base down to the rock
Installation of a filter buttress enhances an earthen dam's ability
to collect and properly drain seepage, and to prevent the migration
of core particles through the dam. Reclamation incorporated filter
buttresses as a standard design feature in dam construction in the
1960s. By then, the dams at Horsetooth were nearly 20 years old.
In addition to the filter buttress and repairs, a cutoff wall also
will be constructed at Horsetooth Dam. This feature will be designed
to control seepage through the Lykins formation beneath the foundation
of the dam, and will reduce safety concerns at this structure.
Modernization work will require lowering of the water level in Horsetooth
Reservoir by approximately two thirds of the reservoir's normal
Horsetooth Reservoir was built to supplement local water needs,
including irrigation, municipal, and industrial uses. The planned
water level restriction during construction is the result of investigation
and analysis by Reclamation engineers and consultants, and has been
reviewed by a team of independent technical experts. All parties
agree that maintaining the reservoir at or below the proposed level
will make it possible to supply municipal, agricultural and industrial
water users while safely modifying the dams. At this lower level,
the reservoir will continue to fluctuate as it always has, while
water flows in and out of the facility.
Congress received the Horsetooth Dams Modification Report Nov. 3,
where it will remain for 30 calendar days before authorization.
Approval of construction funds for the Horsetooth work is tied directly
to the report.
President Clinton signed an appropriations bill Oct. 22 for Reclamation.
The bill provided agency operating budgets for fiscal year 2001,
but not money specifically for Horsetooth.
The bill did include an amendment to the 1984 Safety of Dams Act,
which sets Reclamation's spending limits on dam safety improvements.
The bill raised the ceiling set in the 1984 act.
Now that the limits were raised, enough money is available for construction
at Horsetooth. All that is needed is for Congress to approve the
Modification Report, which formally allocates money from the SOD
budget to Horsetooth. Without the report's approval, SOD money could
go to other Reclamation projects, instead of Horsetooth.
on the Horsetooth Dam outlet works. Almost finished, here, workers
are begining to bury all but the intake section, which needs
trash gates still, and other finishing touches.
Reclamation and District officials are working closely with the
Larimer County Parks and Open Space, the City of Fort Collins, and
other entities to help mitigate as much as possible the project's
effects on recreation, water quality and public access during construction.
of a new boat ramp is underway by Larimer County. The ramp will
allow boat access to the reservoir during the construction period.
Editor's note: Kevin J. Cook, a Fort Collins-based naturalist,
initially published this information in his column, "On the Outdoors,"
for the Fort Collins Coloradoan. It's reprinted here by permission.
What's going to happen to the fish in Horsetooth Reservoir once
the water level is lowered?
Answer: When the Division of Wildlife and water companies
cooperate to develop publicly accessible fisheries in reservoirs,
contingency plans for drastic drawdown must be in place and ready
to implement. Typically, such plans include salvage fishing and
a salvage fishing situation, anyone with a valid Colorado fishing
license can keep as many fish as he or she cares to catch by the
usual legal methods of fishing. The double emphasis here is valid
license and legal methods.
involves physically capturing animals -- in this case fishes --
and moving them to a different place. When the water level drops
far enough, fishery workers can net the concentrated fishes and
move them to other lakes or reservoirs.
knowing the exact plans for Horsetooth, I spoke with Ken Kehmeier,
a fisheries biologist for the DOW.
need to realize that the reservoir is not going to go completely
dry," he explained. "The reservoir is already about as low as it's
going to get."
north end is still about 40 feet deep, which is a significant amount
of water and plenty adequate to keep fish alive."
being the case, Kehmeier and other fisheries biologists do not expect
much to happen. "We don't anticipate any dieoffs or fishkills, winter
or summer," he said. "There's enough water up there to keep the
fish going, and the water will continue to run through the reservoir."
haven't stocked much in the last two to three years. We did stock
a few trout this last summer."
the reservoir refills in a few years, people will probably see what
we saw in the 1980s after the drawdown of 1977. That was an explosive
growth and a few years of a really good fishery."
a man-made reservoir as a recreational fishery presents different
challenges than managing a natural lake, but during the last decade,
fisheries biologists have made great strides in understanding the
is eagerly anticipating the opportunity to build a top-quality fishery
from an almost clean slate.
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