Annual Report of the Chief of Engineers
on Civil Works Activities
Fiscal Year 1979
Department of the Army Corps of Engineers
Extract Report of Walla Walla District

Blackfoot Reservoir, Idaho

Location. On Blackfoot River in Caribou County, Idaho, about 40 miles southeast of city of Blackfoot. (See Geological Survey quadrangle sheets Paradise Valley, Cranes Flat, Portneuf, and Henrys Fork location and surroundings in immediate vicinity of project.)

Existing project. See table 39-B for authorizing legislation. Blackfoot Reservoir is owned and operated by Fort Hall Agency of Bureau of Indian Affairs, Department of Interior. Main dam, which is about 55 feet high above streambed, is a rockfill structure with a concrete core. China Hat Dam, an earthfill structure, closes an arm of reservoir near its upstream end to minimize storage loss due to seepage in pervious formation of that area. Existing main dam structure includes a 3,800-cubic-foot-per-second spillway on the right abutment and an outlet tunnel in left abutment with a capacity of 2,000 cubic feet per second. Reservoir has a usable storage capacity of 413,000 acre-feet at design maximum water surface Elevation 6124. However, due to experience with seepage losses from southern end of reservoir, operating level has been limited to about Elevation 6120, or about 340,000 acre-feet storage capacity. Water stored in reservoir is used to irrigate lands in Fort Hall Indian Reservation in vicinity of Blackfoot and south. Recommended modifications to the project would maintain the present maximum operating pool level at Elevation 6120.5. The spillway capacity would be increased to 21,700 cubic feet per second by lowering the crest elevation from 6118.5 to 6112 and increasing the spillway width from 51 feet to 86 feet. The spillway would be equipped with new gates and would contain the outlet works. The existing outlet would be abandoned. China Hat Dam would be raised from Elevation 6122 to 6124. The project modification will require only limited additional lands (110 acres) in the construction areas, no mitigation of fish and wildlife resources, and no relocations. Construction to be accomplished by the Corps, with continued operation of project by Bureau of Indian Affairs. Estimated Federal cost of proposed modified operation (October 1978) is $4,600,000.

Local cooperation. None required.

Operations and results during fiscal year. A public information meeting was held in Blackfoot, Idaho, on 29 March 1978. Full public support for the project was voiced at this meeting. Supplement 2 to the General Design Memorandum, Maximum Operating Pool, was disapproved by higher authority because the modification as now proposed is essentially a correction for dam safety rather than flood control as authorized. Recommendation has been made that the project be considered for deauthorization.

Condition at end of fiscal year. Hydrology design memorandum has been approved. The Real Estate Design Memorandum was approved 8 April 1972. The post-authorization report was approved by Office of Management and Budget 9 January 1973. A letter dated 2 July 1979 was forwarded to the Bureau of Indian Affairs indicating that the Corps of Engineers does not intent to pursue additional funding for the modification.