American Falls Dam


American Falls Dam is a 94-foot-high composite concrete and earth structure on the Snake River near American Falls, Idaho. Project storage was increased by 1,700,000 acre-feet in 1927 with the completion of the dam. A core-drilling program in the early 1960s revealed that the concrete in portions of the dam was in a relatively advanced stage of deterioration due to a chemical reaction between alkalis in the cement and the aggregate. This type of reaction, unknown at the time of construction, resulted in a significant loss in strength and durability, threatening the competence of the dam. In the early 1970s, storage was limited to 11.3 feet below full pool, which reduced the reservoir storage capacity to 1,125,000 acre-feet, about 66 percent of maximum design capacity.

By congressional act of December 28, 1973, the American Falls Reservoir District, acting as the constructing agency representing the storage spaceholders, was authorized to finance and contract for the replacement of American Falls Dam. The new dam, completed in 1978, replaced the concrete portion of the original structure and was built immediately downstream from the old dam. During reconstruction, the reservoir area was surveyed and the total storage capacity is now 1,672,600 acre-feet (active 1,672,600 acre-feet). After Congress passed the 1978 Safety of Dams Act, the American Falls Reservoir District was repaid for their construction costs.


Little Creek basalt extends from high on both abutments to well below streambed.  Basalt is highly jointed with some faulting. 

RegionFacilities in Pacific Northwest | Pacific Northwest Home Page
Dam TypeConcrete Gravity and Zoned Earthfill
ReservoirAmerican Falls
WatercourseSnake River
LocationAmerican Falls, Idaho
Original Construction1925-1928
Modified ConstructionReconstructed 1976-1978
National ID NumberID00274
Hydrologic Unit Code

Structural Height104.0 ft
Hydraulic Height (Normal Operating Depth at Dam)78.0 ft
Top Parapet (Elevation)
Spillway Crest Elevation4354.5 ft
Crest Elevation4366.5 ft
Crest Length5277.0 ft
Crest Width42.5 ft
Base Width59.0 ft
Volume of Dam Construction Materials65,000 cubic yard
Streambed at Dam Axis
Top of Exclusive Flood Control Pool (Elevation)
Top of Joint Use Pool (Elevation)
Top of Active Conservation Pool (Elevation)4354.5 ft
Top of Inactive Conservation Pool (Elevation)
Top of Dead Storage Pool (Elevation)

Hydraulics & Hydrology
Total Water Storage at Elevation1,672,590 acre-ft at 4354.5
Maximum Water Surface Elevation4356.2 ft
Normal Water Surface Elevation 4354.5 ft
Spillway Type Radial Gates
Auxiliary SpillwayNo
Spillway Capacity at Elevation87,000 cfs at 4354.5 ft
Auxiliary Spillway Capacity at Elevation
Uncontrolled Spillway Capacity at Elevation
Outlet Works Capacity at Elevation19,400 cfs; 32,900 cfs including Power Plant releases
Diversion Capacity at Elevation
Drainage Area13580.0 sq mi
Hydrometeorological Report (HMR)
Probable Maximum Flood (PMF) Report1981 General Storm

Contact Information
OrganizationUpper Snake Field Office 
Address470 22nd Street
CityHeyburn, ID 83336
Phone (Fax)208 678-4321 
Phone (Office)208 678-0461
OrganizationUpper Snake Field Office
Address470 22nd Street
CityHeyburn, ID 83336
Phone (Fax)208 678-4321 
Phone (Office)208 678-0461

Last updated: Feb 12, 2015