Annual Report of the Chief of Engineers
on Civil Works Activities
Fiscal Year 1972
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. 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 provide 38,000 acre-feet additional storage capacity for flood control. Modifications would permit operation of reservoir for flood control to a maximum water surface elevation of 6126. 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 52 feet to 94 feet. The spillway would be equipped with new gates. Outlet works would be modernized and rehabilitated for flood control operation. China Hat Dam would be raised 10 feet, and Idaho state Highway No. 34 would be raised in grade for approximately ½ mile along with grade raising of two existing bridges. Recommended modification was authorized by Flood Control Act of 1962. Construction to be accomplished by the Corps, with continued operation of project by Bureau of Indian Affairs. Estimated Federal cost of proposed modified operation (July 19723) is $3,320,000.
Local cooperation. None required.
Operations and results during fiscal year. Real estate design memorandum submitted December 1971. An Environmental Impact Statement is being prepared.
Condition at end of fiscal year. 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.
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. The District has indicated to the Idaho Congressional Delegation that a public meeting to explain and answer questions on land acquisition would be arranged following approval of the real estate design memorandum by Office, Chief of Engineers.