Henrys Lake, Reservoir in Fremont County, Idaho, US.

Henrys Lake is a shallow alpine lake in Fremont County, Idaho, about 4 miles long and 2 miles wide, with a surface of roughly 8 square miles at 6,472 feet. A dam built at its outlet in 1923 enlarged the lake. Springs feed it at Staley Springs, Pintail Point, Kelly Springs, the Cliffs and along the north shore; inlets include Timber, Targhee, Howard, Duck, Hope and Kenny creeks.

The lake was named after Andrew Henry, an early American fur trapper and explorer from the early 1800s. The region later saw military movement when troops passed through during conflicts with the Nez Perce.

The lake takes its name from Andrew Henry, who entered the Snake River plateau as a trapper in 1810 and built Fort Henry on the upper Snake before abandoning it the following spring. The Nez Perce passed the lake on their eastward route toward Yellowstone during the Nez Perce War. Trumpeter swans nest here, and the lake's marshes draw birders in spring.

Henrys Lake State Park on the south shore, off U.S. 20 near Goose Bay and the outlet, has hiking trails, camping and a boat ramp; Frome Park on the west shore, run by Fremont County, offers a free boat ramp and free campsites. The South Shore Boat Access adds a ramp, primitive camping and a wildlife-viewing platform. The Continental Divide Trail skirts the lake, though many northbound hikers skip this section after leaving Yellowstone.

The lake lies just 16 miles west of Yellowstone National Park and serves as a crucial breeding ground for white pelicans, mallards, and trumpeter swans. These bird populations rely on the lake annually for nesting and rearing young.