Box Canyon below Island Park Dam
The Henry's Fork leaves Island Park Reservoir at the dam, picks up the Buffalo River just below it and runs through Box Canyon, a lava-boulder gorge of fast water and heavy rainbow numbers. The boat ramp is just below the dam on river left and the Box Canyon to Last Chance float is 5.5 miles; a trail along river left off Box Canyon Road serves wading anglers. TroutHunter's access guide puts the wading cutoff at about 1,000 cfs, above which the canyon is hard to wade, and below about 400 cfs it becomes difficult to row. Studded or felt soles help on the basalt.
Last Chance and Harriman State Park
Below the canyon the river flattens through Last Chance, the strip of fly shops and lodges on US 20 with pull-off access, and then enters Harriman State Park, the Railroad Ranch: miles of slow, clear meadow water where the river fishes like a spring creek and big rainbows sip mayflies in swirling currents. This is wade fishing, and the standard approach is to spot a rising fish, learn its rhythm and make one good cast from downstream. Osborne Bridge, at the lower end of the park, has a ramp inside the park boundary and marks the start of the Wood Road 16 and Riverside Campground water below.
Rainbow trout
Wild rainbows dominate the river from Island Park Dam down through the Ranch, sustained by spring flow and dense insect life, and decades of catch-and-release have made the Ranch fish famous for refusing anything less than a perfect drift. Salmonflies work up from Ashton to Box Canyon in late May and June, green drakes bring the Ranch alive from its June 15 opener into early July, brown drakes and golden stones follow, then PMDs, flavs, tricos, callibaetis on the backwaters and honey ant flights and hoppers through late summer, with mahogany duns and BWOs into fall. Carry emergers, cripples and spinners for every hatch; Ranch fish change stages mid-rise.
Brown trout
Brown trout are a lower river fish. They appeared in the 1970s near the mouth of Warm River, below Mesa Falls, and spread downstream, so the Warm River to Ashton and Ashton to St. Anthony water is where you meet them; Box Canyon and the Ranch are rainbow water. Lower river browns hold tight to rock and snag edges in bright light and come out for the salmonfly, caddis and drake hatches of late May through early July, and for streamers on overcast days and in fall.
Season and regulations
This page reads USGS 13042500 Henrys Fork near Island Park, the gauge directly below Island Park Dam, so the number is the Box Canyon release. The 400 to 1,000 cfs range is the wading window: TroutHunter calls the canyon hard to wade above 1,000 cfs and hard to float below 400, and Henry's Fork Anglers treats releases in the 400s as prime wading water. Floating fishes well higher; RiverReports puts the all-around float range at 800 to 1,200 cfs. The Bureau of Reclamation sets the release for irrigation storage, with a winter target near 380 cfs, and in drought years the late-summer release can drop well below the wading range, as it has in 2026.
IDFG rules for 2025 to 2027: from Island Park Dam down to the upper Harriman boundary, no bait, barbless hooks, catch-and-release for trout except brook trout, open all year. The Harriman Ranch, upper to lower park boundary, is fly fishing only, barbless, catch-and-release, open June 15 through November 30 and closed the rest of the year. From the lower park boundary to the posted line above Riverside Campground it is catch-and-release with barbless hooks and no bait; below that to the Highway 20 bridge and from Ashton Dam down the trout limit is two, with no harvest of cutthroat, and the Vernon Bridge to Ashton Dam reach is catch-and-release from December 1 until Memorial Day weekend. Confirm everything in the current Idaho Fish and Game rules before you go.