Upper river, Palisades Dam to Conant
The South Fork begins at Palisades Dam on the Idaho-Wyoming line and runs 66 miles to the Henrys Fork confluence near Menan. The upper section is not the canyon: from the dam ramp past Palisades Creek (Husky), Irwin and Spring Creek to Conant the river runs through the open Swan Valley in braided channels along cottonwood banks, and this is the most wadeable water on the river once flows drop in late summer. The Irwin gauge this page uses sits in this reach. Reservoir drawdown can muddy the water above Conant in late summer.
The canyon, Conant to Byington
Conant is the last ramp before the canyon, a roadless run of about 25 miles past Cottonwood (Fulmer), Wolverine and Wolf Flat to Byington, with cliffs, side channels and the overnight camps the outfitters run. This is float-only water in practice; wading is limited to gravel bars reached by boat. Byington marks the end of the canyon and the start of the lower river, and the Devils Kitchen hydraulics just above it deserve respect.
Lower river, Byington to Lorenzo and Menan
Below Byington the river braids through farmland and one of the largest cottonwood forests in the West, past Twin Bridges to Lorenzo and on to Menan. Local guides call it brown town: browns are the target here, the channels close out and change yearly, and it is best fished from May into early July before the water warms. Lorenzo is about 20 minutes from Idaho Falls and the usual take-out for the lower floats.
Yellowstone cutthroat
The South Fork holds the largest river population of native Yellowstone cutthroat in Idaho, including the fine-spotted Snake River form, and all cutthroat must be released. They are willing dry fly fish: salmonflies and golden stones from late June through July, PMDs in July and August, hoppers and mutant stones from August, mahoganies and baetis in fall. Three Rivers Ranch quotes IDFG counts of more than 4,000 trout per mile at Conant.
Brown and rainbow trout
Browns are strongest in the lower river and are the fall streamer fish; the limit is 2. Rainbows and rainbow-cutthroat hybrids are the problem: IDFG asks anglers to keep every one they catch, there is no limit on them, and the department runs a South Fork Angler Incentive Program around tagged rainbows. Whitefish are abundant everywhere.
Season and regulations
The South Fork is open all year and the Bureau of Reclamation runs it for irrigation: winter releases around 1,000 to 1,500 cfs, a June-July peak often above 12,000 cfs, then a drop through late summer to fall flows. Guides put the best wade fishing at 6,000 to 8,000 cfs as flows come down in fall, and this page compares the Irwin gauge to that range; floats work at almost any release, and the winter fishery runs near 1,000 cfs.
South Fork Snake River, Palisades Dam to the Henrys Fork: no harvest of cutthroat trout, brown trout limit 2, no limit on rainbow trout or hybrids; tributaries are closed in June. A BLM day use or season pass is required at the Palisades Dam and Palisades Creek access sites. Check the current Idaho Fish and Game Upper Snake Region rules before you go.