Fall River

The Fall River is the biggest tributary of the Henry's Fork, and it's the one most Henry's Fork anglers drive past without stopping. It comes off the Pitchstone Plateau in the far southwest corner of Yellowstone, drops through a string of waterfalls — Cave Falls, Terraced, Rainbow, then Sheep Falls once it's in Idaho — and by the time it reaches the farmland east of Ashton it's a wide, cold freestone river running over a slick lava-rock bed. The thing to understand up front is that it fishes as a runoff river, not a spring creek: high and off-color into June, then it drops and clears and turns on around mid-July. Show up in May and you'll find a brown torrent. Show up the third week of July and you've got dry-fly water most people forgot about.

Character changes a lot by reach. The canyon above the Yellowstone Canal diversion is steep, rocky, and pushy — pocket water where the salmonflies come off in July and a size 4 Chubby drifted tight to the bank does damage. Below the diversion, through Marysville and down toward Ashton, it flattens into meadow and farmland runs where PMDs, Green Drakes, and caddis matter and the wading gets genuinely treacherous (the lava-rock bottom is felt-and-studs slick). The lower river toward the Chester confluence is bigger and slower, and that's where you start bumping into brown trout that have run up out of the Henry's Fork. Fish average 12 to 14 inches across the system, but 20-inch trout are real, especially in the lower meadows and near the confluence.

The catch worth naming is irrigation. The Yellowstone Canal diversion pulls a huge slug of water out just below Squirrel — the gauge above the canal will read two or three times what the one a couple miles below reads, so in the heat of summer roughly two-thirds of the river can be sitting in a ditch. In a dry year the reach right below the diversion gets thin and warm by August; return flows and small tributaries build it back up by Ashton. Access is a patchwork of BLM/Forest ground and county-road bridges (Kirkham Bridge, the 4500E bridge off Marysville Road), plus the long Cave Falls Road up into the roadless meadows below Terraced Falls — a hike-in cutthroat proposition, not a drive-up. It sees a fraction of the Henry's Fork mainstem's traffic, and that's most of the appeal.