Horsetooth Reservoir is a reservoir on the northern end of Horsetooth Mountain, one of the prominent landmarks in Fort Collins, Colorado. The purpose of the reservoir is to provide hydroelectric power and recreation for Fort Collins residents.
The Northern Water Utility Board (a consortium made up primarily of Poudre Valley Rural Electric Association members) owns and operates the reservoir. It was built between 1960 and 1970 at a cost of roughly $10 million.
In addition to providing drinking water for Fort Collins residents, the reservoir serves as an emergency source if other reservoirs are unable to produce adequate amounts of electricity during peak demand periods.
The basic design of Horsetooth Dam and Reservoir was modeled after another Northern Water Utility project: Fox Canyon Dam and Reservoir, built in the early 1960s and located two miles east of Poudre Canyon.
Horsetooth Dam is a concrete structure high, long, containing material. It was designed by Merritt-Chapman & Scott Corporation to be an impervious core with perforated upper sections (i.e., what engineers call “through-type”).
Its design cross-section follows closely that of another through-type structure—the Narrows Dam on Coal Creek near Decker in northwestern Colorado —but Horsetooth’s impervious core is slightly larger than Narrows’.
Such structures are considered recently developed dams; older dams typically used impervious sections as foundations for water passageways called spillways. The term “through-type” refers to the fact that water normally passes through the core section of the dam.
The design of Horsetooth Dam consists of a through-type concrete core with a silt trap on the downstream face, originally designed to handle around 10 years of sedimentation before replacement.
Sedimentation in the river since 1970 is averaging about per year. The highest recorded level was in 1997 at above sea level; levels that high would have required immediate remedial action under original design parameters.
Because of increased sedimentation loads from two large upstream reservoirs—Glen Canyon and Flaming Gorge —the Northern Water Utility Board has undertaken a $1 million bank stabilization project, complete as of this writing.
The Horsetooth Dam Reservoir Storage area is (1,786 surface acres) at full pool. Normal surface elevations range from above sea level. When the reservoir was filled to capacity in 1973, it contained enough water for 56 days of consumption. At present levels, it has about 30 days of reserve supply.
Although the dam stores water primarily for use during periods of peak demand on Fort Collins’s electrical distribution system, recreational opportunities are frequently available due to low stream inflows into the reservoir when demands for electricity are low as well (such as during summer), resulting in sufficient stored water to provide recreation.
Horsetooth Dam and Reservoir was built to provide irrigation water for Fort Collins’s agricultural industry, but this service has been largely discontinued.
Irrigation served only a small area of land on either side of the Poudre River where it flows through Fort Collins, and after completion of large upstream reservoirs—Glen Canyon in 1956 and Flaming Gorge in 1963—which capture virtually all river water entering Wyoming from Colorado, there is almost no flow left to divert for irrigation purposes at Horsetooth.