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Fontenelle Dam and Reservoir

October 12, 2018
by John Weisheit

The small volume reservoirs that immediately capture the run-off from the melting snow fields of the Rocky Mountains are earthen dams with inadequate spillway capacities. The advancements made in the study of paleoflood hydrology in the last 40-years indicate that our understanding of flood events, in every watershed on this planet, are grossly misunderstood. For the Colorado River Basin we now understand that maximum flood volumes are typically 3 times greater than the original assessments made in the 20th century, and developed to provide a determination of the magnitude and frequency for an assummed probable maximum flood event, and to then design a functional dam that could safely bypass such volumes.

In 1983, the integrity of Fontenelle Dam was compromised by a large snow melt that had a volume below the presummed yield of an 100-year flood event. Even the concrete dams embedded in the bedrock canyons of the Colorado River had difficulties bypassing this snowmelt. So what happens when a real 100-year flood event occurs? Will these earthern dams placed immediatly below the perched snow fields of the Rockies function safely, or not? What about the assumed 500-year flood event, which is layered with five 100-year flood events in between. When does the five-hundred year flood arrive? Can this event even be forecasted so that dam managers can prepare? To find out please visit the section called Floods, in the Resource Section of this website.

Some history from John Shields, USBR
From: John Fleck's Ink Stain Blog
June 26, 2015 at 11:57 am

HR 2273. Hearing held yesterday. Testimony by the Director of the Wyoming Water Development Commission is available at:


The need to add riprap is to simply allow the reservoir to be drawn down  there are existing contracts for water supply out of Fontenelle that Wyoming would be unable to service because the reservoir can not be drawn down. It was built as a diversion dam rather than as a water supply reservoir. More details below:

The water right for Fontenelle Reservoir indicates its primary purposes are irrigation, domestic, industrial, municipal, stockwatering, fish and wildlife and recreation; and when not required for the primary purposes, storage water can be used for power generation, the secondary purpose. However, the major existing benefits of Fontenelle Reservoir relate to industry.

The construction of Fontenelle Dam was completed in December, 1967, under water right Permit No. 6629 Res. Fontenelle Reservoir has a total capacity of 345,397 acre feet. Originally, only 190,250 acre feet was designated as active capacity for the above listed purposes and 154,584 acre feet was designated as inactive capacity. The remaining 563 acre feet was the designated dead storage, as it could not be physically released. In 1962, the State of Wyoming contracted with the Bureau of Reclamation for 60,000 acre feet of the active capacity for a price of $900,000. This amount was loaned to the State of Wyoming at an interest rate of 2.632% at a term of 50 years, plus the state was obligated to share in the actual operation, maintenance and replacement costs for the facilities.

Originally, the purpose of the inactive capacity (154,584 acre feet) was to raise the reservoir surface to an elevation high enough to release water to the proposed East Side and West Side Canals. In 1973, it was apparent that the two canals would not be constructed. Therefore, an enlargement to the original permit was granted (Permit No. 9502). The enlargement served to move the previously designated inactive capacity to active capacity; thereby increasing the active capacity from 190,250 acre feet to 344,834 acre feet, which could be used for the permitted purposes, previously discussed. In 1974, the State of Wyoming again contracted with the Bureau of Reclamation for 60,000 additional acre feet of active capacity; thereby increasing its total interest in Fontenelle Reservoir to 120,000 acre feet. The price was $11,410,000 for the additional 60,000 of active capacity, which was loaned to the State of Wyoming at an interest rate of 2.632% at a term of 40 years, plus the state was obligated to share in the actual operation, maintenance and replacement costs for the facilities.

In the 1974 contract, 5,000 acre feet was designated for the Seedskadee Wildlife Refuge. The United States reserved 65,000 acre feet of capacity for its uses, subject to provisions that the Bureau of Reclamation would not compete with the State of Wyoming in the water market. This contract also required the United States and State of Wyoming to ensure operations that would provide for the maintenance of 50 cfs in the Green River at the USGS streamgage near Green River, Wyoming.

Presently, the State of Wyoming, through the Wyoming Water Development Commission, has allocated 46,550 acre feet of its entitlements to Fontenelle water through the following water supply or readiness to serve contracts: Jim Bridger Power Plant (35,000 acre feet per year), FS Industries (10,000 acre feet per year), Church and Dwight (1,250 acre feet per year, and Exxon, USA (300 acre feet per year). The fact that there is unused and unallocated water in Fontenelle Reservoir has caused some to question its value. Fontenelle Reservoir provides hydropower, recreational, and environmental benefits. Further, its present operation supplements natural flow in the Green River. In addition, it can be surmised that the availability of water in Fontenelle Reservoir, and Wyomings entitlements in the reservoir, were key considerations in the siting of the Jim Bridger Power Plant and the chemical fertilizer plant presently owned and operated by FS Industries. In addition, and perhaps most importantly, it offers future economic development opportunities for the Green River Basin.

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