Henrys Fork (Snake River) - River Ecology

River Ecology

The Henrys Lake outlet is subject to substantial draw-downs from irrigation diversions during the summer. Late in the season, as the draw-downs decrease with the cooler weather, more water is released into the stream, allowing fish to move up from the lower section of the river. The Nature Conservancy sponsors a learning station near the outlet stream.

South of the lake at Big Springs, nearly 500,000 US gallons (1,900 m3) of constant 52 °F (11 °C) water flow into the river each day. The river flows south through a high plateau in northern Fremont County, through the Caribou-Targhee National Forest, and passes through Island Park Reservoir. It emerges from the reservoir and flows through a canyon that opens up into a broad, flat meadow in the Island Park Caldera in central Fremont County. The river flows slowly past the town of Island Park, through the Harriman State Park, otherwise known as the "Railroad Ranch", and then descends swiftly as it approaches the wall of the caldera, flowing over both Upper Mesa and Lower Mesa Falls, and emerges from the mountains onto the Snake River Plain near Ashton. It flows southwest across the plain, past St. Anthony, and splits into multiple channels into a broad inland delta north of Rexburg. It receives the Teton River from the east approximately 5 miles (8.0 km) west of Rexburg. It joins the Snake from the northeast approximately 10 miles (16 km) southwest of Rexburg, just below 4,800 feet (1,463 m).

Island Park Reservoir, a component of the Minidoka Project, is used for irrigation in the Snake River Plain. Its drainage provides one of the most important rainbow trout fisheries in Idaho in terms of habitat, fish populations, and use by anglers. The section of the river between Henrys Lake and Big Springs is a major spawning area for trout and is closed to fishing.

Henrys Fork has long been noted for its superb fishing, especially its dry fly fishing. Bing Lempke, a pipefitter from nearby Idaho Falls, was considered the local dean of the fishery, until he died in 1990.