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.