Minidoka Powerplant and Dam is a combined diversion, storage, and power structure located just south of Minidoka, Idaho. A key structure in the initial development of the project, the zoned earthfill dam is 86 feet high. The reservoir, Lake Walcott, has a total storage capacity of 95,200 acre-feet. During the irrigation season, water is diverted at the dam into a canal on each side of the river.
The original powerplant was constructed within the concrete buttress section of the dam, located at the right end of the rockfill section, was completed in 1909 and had five generating units. Unit 6 was added in 1927 and unit 7 in 1942 to provide a total capacity of 13,400 kilowatts.
During 1989 - 1990, the spillway radial gates were replaced. The original gates were installed in 1913 and were in need of complete replacement.
Construction was completed in 1997 at the Allen E. Inman Powerplant. Intended to replace the retired units (1-5) and utilize more of the excess flows previously spilled, this powerplant houses two 10-megawatt horizontal shaft Kaplan Units.
The combined generation capacity of all four units is 28.5 megawatts, with a combined flow of 8670 cubic feet per second.
Minidoka Dam was originally designed and constructed without a powerplant and was completed in 1907. The powerplant (and three pumping plants) was added later (1909-1910) to allow water to be delivered to lands which could not be fed by gravity irrigation. The concrete buttress section of the dam forms the upstream structure of the original powerplant. The powerplant could only use a fraction of the water that passed through the dam, so Unit 6 was added in 1927 to allow commercial sales of power in excess of project needs. Unit 7 was added in 1942 to help feed the public`s growing thirst for electric power. In September 1995, the five original units were retired and Units 6 and 7 were rebuilt. Units 8 and 9 were added in 1997 at Minidoka Dam with the building of the Minidoka Replacement (now Allen E. Inman) Powerplant. The original powerplant building continues to house units 6 and 7.
Full automation and remote control was completed in 2000. Control is carried out from the Black Canyon Control Center near Emmett, Idaho.
The geology of the area of Minidoka Dam is complex. The mountain masses have been extensively folded and faulted. They are made up of crystalline, metamorphic, volcanic, and consolidated sedimentary rock ranging in age from Precambrian to Quaternary. The valleys are structurally depressed areas containing large deposits of volcanic rock, lake sediments, alluvial and fluvioglacial deposits, and windblown silts. The valley deposits have been collecting from early or middle Tertiary time to present.