One river, two rulebooks
Fall River starts on the Madison and Pitchstone plateaus in the southwest corner of Yellowstone and runs about 64 miles to the Henry's Fork; Idaho Fish and Game's planner puts it at 64.2 miles in Fremont County and has it joining at Chester. Three Rivers Ranch notes a small piece of history that still confuses maps: trappers called it the Falls River, the Hayden survey fixed that name in 1872, and the state asked for it to be changed to Fall River in 1997. You will see both on signs and in agency databases.
The top of the river is Yellowstone water, and that changes everything about what you need in your pocket. The park requires its own fishing permit for anyone 16 or older, and an Idaho license is not valid inside the boundary. The park season runs from the Saturday of Memorial Day weekend through October 31, and cutthroat, whitefish and other native fish must go back. Once the river crosses into Idaho, Fish and Game rules take over and the water is open all year.
Cave Falls and the long walk above it
Most people meet the Fall at Cave Falls, and Three Rivers Ranch says plainly that this area gets most of the pressure because it is the easy access point. The trout there are modest, around 10 inches on average by the ranch's account, but the action is steady. Henry's Fork Anglers points to Cave Falls and the stepped Terraced Falls as the landmarks of the upper river, a run of cascades that gives the headwaters its character.
The rest of the upper river is largely roadless. Three Rivers Ranch describes it as reachable mostly by long hikes and off-trail travel, with fish getting larger the farther up you go and pressure dropping off accordingly. Brook trout have moved into these upper waters in recent decades, so do not be surprised to see them mixed in with the rainbows and cutthroat.
The canal problem, and what 500 to 700 cfs at Ashton means
Think of the lower Fall River as a supply line for farms as much as a trout stream. The Henry's Fork Foundation's water reports, republished by Three Rivers Ranch, spell out how hard it gets worked: in the dry summer of 2016 all of the natural flow in both Fall River and Teton River was being diverted by late July, and the rest of the demand was met with storage from Island Park Reservoir. In wetter years Grassy Lake storage is sent down the Fall as well, which is why a late-summer bump on this page is not always rain.
USGS 13047600, near Ashton, sits above the worst of the lower diversions and is the most useful number for the wadeable Idaho water. No shop publishes a wading band for it, so the 500 to 700 cfs range used here comes straight from the gauge's own record since 1993: the median monthly flow is about 700 cfs in July, then roughly 520 to 570 cfs from August through November. May and June medians run above 2,000 cfs, which is runoff and not wading water. Compare the Chester gauge downstream if you are curious how much is being pulled out; its August median is under 250 cfs.
Cutbows, a few browns, and rocks that bite
Henry's Fork Anglers describes the Idaho reaches as county-road water with cutbows and bigger trout, plus a few browns down toward the Henry's Fork confluence. Fish and Game's planner calls it a good river for wade anglers and suitable for small float boats. Take that second part literally: this is not drift boat water, and the flows swing too much with diversions to plan a float far in advance.
Budget for the footing. Henry's Fork Anglers warns that the bottom is slippery and rocky in many places, and the river is a freestone that overflows its banks in runoff and shrinks hard in late summer. A wading staff and studded soles are not overkill here.
Two trout in Idaho, none of them cutthroat
Idaho's 2025 to 2027 Upper Snake Region rules list Fall River and its tributaries as a special rule water: the limit is two trout and no cutthroat may be kept. The Upper Snake Region is open all year, so there is no closed season on the Idaho side. Inside Yellowstone the park's own regulations apply instead, with native fish released and a park permit required.
The fishing itself follows the snow. Henry's Fork Anglers says the river generally peaks from mid-July, after runoff, and stays productive well into fall. That lines up with the Ashton record, which shows the river falling out of its June peak through July.
Borrowing the Henry's Fork calendar
No shop publishes a hatch chart for the Fall itself, so the table below borrows Henry's Fork Anglers' regional timing and trims it to what matters once runoff clears. Spring baetis and grannom come off while the Fall is often high and cold, so the practical season starts with golden stones and PMDs in early summer, then mahogany duns from late August and fall baetis into October. Midges carry the winter, when the Idaho water is open but rarely fished.