Annual Report of the Chief of Engineers
on Civil Works Activities
Fiscal Year 1977
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 1977) is $4,320,000.

Local cooperation. None required.

Operations and results during fiscal year. Supplement 2 to the General Design Memorandum, Maximum Operating Pool, was returned for review of economic viability using the current interest rate.

Condition as of September 30. Hydrology design memorandum has been approved. The general design memorandum has been conditionally approved subject to comments of the Office of Management and Budget on post-authorization changes. 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.

Opposition to the project has been voiced to the Idaho Congressional Delegation by owners of summer homes adjacent to the reservoir which have been or are being constructed below the existing authorized reservoir storage elevation.