Fontenelle Dam was built between 1961 and 1964 on the Green River in southwestern Wyoming. The high zoned earthfill dam impounds the Fontenelle Reservoir. The dam and reservoir are the central features of the Seedskadee Project of the U.S. Bureau of Reclamation, which manages the Fontenelle impoundment primarily as a storage reservoir for the Colorado River Storage Project. The dam suffered a significant failure in 1965, when the dam's right abutment developed a leak. Emergency releases from the dam flooded downstream properties, but repairs to the dam were successful. However, in 1983 the dam was rated "poor" under Safety Evaluation of Existing Dams (SEED) criteria, due to continuing seepage, leading to an emergency drawdown. A concrete diaphragm wall was built through the core of the dam to stop leakage.
Use
Initially conceived as a storage reservoir for irrigation water, the project was suspended for a time in 1962 to pursue a study of high-altitude irrigation methods. The results of the studies caused the cancellation of many irrigation features for the project. The primary project rationale evolved to support Wyoming water rights in the Colorado River basin, retaining water that would otherwise go downstream to states in the lower Colorado basin. The dam provides power generation as a secondary feature. One motivation for the deferral of irrigation activities was the discovery of trona in the southern portion of the proposed irrigated region. As the extraction of trona was a higher-value activity, the removal of those lands from the project made the irrigation project nonviable.
Construction
Development of the upper Green River basin was included in a 1946 Bureau of Reclamation report on the Colorado River basin. A 1950 supplementary report dealt with details of the proposed Seedskadee Project, followed by another addendum in 1953. The Colorado River Storage Project, including the Seedskadee Project, was authorized by Congress in 1956. Initial planning was completed in 1959, and amended in 1961 to increase the capacity of Fontenelle Reservoir to , making the construction of a powerplant feasible. Work was stopped on irrigation infrastructure in 1962. The dam featured unusually large outlet works, capable of discharging (compared with a spillway capacity of ) because the outlet works could be increased in size at lower cost than the spillway.
The first element of construction at Fontenelle Dam was the building of the Fortenelle community, the base camp for construction, with work starting in 1961 to build prefabricated relocatable houses for dam workers. The contract for the base camp was awarded to and completed by the D.H. Butcher & Co. construction company. The construction contract for the dam was awarded on June 13, 1961 to Foley Brothers, Inc. and the Holland Construction Company of St. Paul, Minnesota, with construction starting on June 30, at a cost of $7.9 million. The surface of the bedrock at the dam's base was found to be fragmented so the cut-off trench intended to preclude water infiltrating under the dam was deepened by . The exposed rock was not prepared or smoothed with concrete. The foundation grouting required an unusually large amount of material to fill cracks in the upper , with additional grouting required at the right abutment and outlet works. Work on the dam was completed by the end on 1963, with final completion on April 24, 1964. Work on the powerplant started in 1963, with completion in 1965.
Partial failure
Problems with the embankment became apparent in May 1964, when part of the backfill slid into the stilling basin downstream, attributed to too-fast lowering of the reservoir level. When the reservoir had filled to 10% of capacity, seepage became apparent at the dam's base, with further seepage observed about below the dam from shale outcroppings. In July 1965 another slide occurred in the area of the stilling basin.
A significant leak appeared on September 3, 1965 at the west abutment, starting as a wet spot that grew with time. The weakened downstream face lost of material that slid into the stilling basin, accompanied by continuing water flow, a condition known as "hydraulic piping." Work began the next day to lower the reservoir, with a 24-hour watch on the dam. The wet spot became a waterspout, flowing at a rate of between 10 and 12 million gallons per day.