Horsetooth Modernization Project

NEPA Scoping Meetings | Physical Characteristics | Additional Project Info
Project Design | Recreation | Fish | Funding |

Soldier 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.


Loading trucks from valley "borrow areas" with material to be used in the dams. Circa '46-48.

Physical Characteristics
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 reservoir.

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 Dams Program.

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.


This is an excavation shovel from the mid-40s.

Project Design
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 assessment.

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 foundation.

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 volume.

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.


Construction 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.

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.

Question: 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 translocations.

In 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.

Translocation 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.

Not knowing the exact plans for Horsetooth, I spoke with Ken Kehmeier, a fisheries biologist for the DOW.

"People 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."

"The north end is still about 40 feet deep, which is a significant amount of water and plenty adequate to keep fish alive."

This 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."

"We haven't stocked much in the last two to three years. We did stock a few trout this last summer."

"When 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."

Managing 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 complications.

Kehmeier is eagerly anticipating the opportunity to build a top-quality fishery from an almost clean slate.



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