Gibson Dam, Concrete arch dam in Teton County, Montana.

Gibson Dam is a concrete arch dam spanning the Sun River, characterized by its thick concrete structure that tapers from a broad base to a narrower crest. Behind the structure lies a large reservoir where water accumulates for storage and distribution downstream.

The dam was built between 1926 and 1929 by the United States Bureau of Reclamation. This project marked an important advancement in engineering methods, as it relied on new mathematical techniques for analyzing structural behavior.

The dam is named after Gibson, though the sources do not record which engineer or official this was. Its water feeds the Greenfields Irrigation District north of the Sun River and the Fort Shaw Irrigation District south of it, irrigating about 81,000 and 10,150 acres respectively. The reservoir behind it, Gibson Reservoir, is the water body the dam holds back.

The dam is accessible by a local road that provides several vantage points along its length. Water levels change significantly throughout the year depending on snowmelt and irrigation demands, so what you see varies by season.

Gibson Dam was among the first built using the trial-load method, which relied on mathematical calculations of water pressure to set the final design. The Bureau of Reclamation estimated this saved about 41,000 cubic yards of concrete. Two penstocks were installed on the south side for planned hydropower, but were sealed when the reservoir proved too small to sustain power production.