American Falls Reservoir

American Falls Dam
Aerial view of the dam
Official name	American Falls Dam
Location	Power County, Idaho, U.S.
Coordinates	42°46′51″N 112°52′32″W / 42.78083°N 112.87556°W / 42.78083; -112.87556Coordinates: 42°46′51″N 112°52′32″W / 42.78083°N 112.87556°W / 42.78083; -112.87556
Construction began	1976
Opening date	1978
Operator(s)	United States Bureau of Reclamation
Dam and spillways
Impounds	Snake River
Height	94 ft (29 m)
Length	5,277 ft (1,608 m)
Width (base)	42.5 ft (13.0 m)
Reservoir
Creates	American Falls Reservoir
Total capacity	1,671,300 acre·ft (2.0615 km3)
Normal elevation	4,354 ft (1,327 m) AMSL
Power station
Operator(s)	Idaho Power Company
Turbines	3
Installed capacity	112 MW

The American Falls Dam is a concrete gravity-type dam located near the town of American Falls, Idaho, on river mile 714.7 of the Snake River. The dam and reservoir are a part of the Minidoka Project on the Snake River Plain and are used primarily for flood control, irrigation, and recreation. When the original dam was built by the Bureau of Reclamation, the residents of American Falls were forced to relocate three-quarters of their town to make room for the reservoir. A second dam was completed in 1978 and the original structure was demolished. Although the dam itself is located in Power County, its reservoir also stretches northeastward into both Bingham County and Bannock County.

A lava dam created a broad shallow lake in the area of the Raft River during late Pliocene time, over one million years ago. Much of the basin filled with fine sand, silt, and gravel; then the dam was breached and the lake drained. These sediments (called the Raft Formation) lie beneath most of the present-day American Falls Reservoir. At other times the Snake River was dammed completely by basalt flows extruded from vents. One lava dam a few miles downstream from the present American Falls Dam formed a reservoir in which more than 80 feet (24 m) of sediments (clay, silt, and sand) were deposited. This series of basalt flows and original sediments were covered by the new lake bed sediments and are named the Snake River Group and the American Falls Lake Beds. These events occurred up until the late , less than one million years ago. The Snake River has continued to erode its channel in the basalt and modify the lake bed sediments until the present time.