The slide that made the lake
On June 23, 1925 roughly 50 million cubic yards of the north face of Sheep Mountain slid into the Gros Ventre valley, rode 300 feet up the far slope, and left a natural dam more than 200 feet high. The river backed up behind it into Lower Slide Lake. Two years later, on May 18, 1927, part of that dam failed, and the flood ran six feet deep for at least 25 miles downstream and wiped out the town of Kelly, six miles below, killing six people. The slide area is seven miles east of the Jackson Hole valley. Lower Slide Lake is still there, and the river below it is the one that flood rearranged.
The river runs 74.6 miles in all, from the Gros Ventre Range past the slide and Kelly to the Snake. Its native trout is the Snake River fine-spotted cutthroat, which Wikipedia's account puts at 12 to 16 inches with some to 20.
Park water, refuge water and state water
Downstream of the slide the river reaches Jackson Hole, and the part inside Grand Teton National Park fishes under Wyoming law: you need a Wyoming license, sold in the park at Flagg Ranch, Colter Bay Marina and Signal Mountain Lodge. Two park rules that catch visitors do not apply here. Most park streams are closed December 1 through July 31, but the Gros Ventre is one of the listed exceptions and is open all year, and the artificial-flies-and-lures rule on park streams also excludes it. The park stream limit is 3 trout, only 1 over 16 inches and only 1 cutthroat over 12, plus 16 brook trout and 25 whitefish; you may fish two rods in attendance with up to three hooks per line. Gros Ventre Campground, the park's largest, is reservation-only.
Downstream, WGFD names a separate reach: from the eastern boundary of the National Elk Refuge down to the U.S. 26/89/191 bridge, refuge rules prohibit fishing from December 1 through March 31. Outside the park and refuge the Gros Ventre is Area 1 water under the 2026 WGFD regulation, with the same 3 trout stream limit, no live baitfish anywhere in Area 1, and fishing allowed year-round, day and night. In the drainage, Soda Lake is its own case: 1 trout of 20 inches or more, flies and lures only. A nonresident license costs $14 a day, $56 for five days or $102 a year plus the $21.50 conservation stamp.
Kelly reads the river; Zenith reads what is left
Two gauges report on the lower river. The one on this page is at Kelly, draining 622 square miles, and it describes the water through the park. The Zenith gauge is farther downstream toward the Snake, and in late summer it tells a very different story. In August 2025 Kelly averaged 170 cfs and Zenith 5.18; in September 2025, 158 against 9.12. Zenith's record includes August and September monthly means that round to zero. None of the sources we checked explains the loss, so the useful rule is simple: in late summer, fish at or above Kelly, not down toward the Snake.
No Jackson shop publishes a flow range for the Gros Ventre. The 230 to 310 cfs band on this page is the USGS Kelly record for the settled months, 307 cfs in August, 232 in September and 229 in October. Snowmelt comes late here: the long-term means are 1,254 cfs in May, 1,827 in June and still 817 in July. The Kelly gauge ran between 198 and 291 cfs in the two weeks to September 24, 2026, bumping up around September 20.
Walk and wade after runoff
WorldCast Anglers has long pointed people to the Snake's tributaries, the Gros Ventre and the Hoback, as prime walk-and-wade water, and its late July and mid-August reports from 2009 said both were fishing very well while the Snake ran higher. That matches the flow record: the Gros Ventre becomes a wading river from late July. The park lists six concessioners allowed to guide rivers, among them Snake River Angler, WorldCast Anglers and Grand Teton Fly Fishing, if you want a guide.
Borrowing the Jackson Hole chart
Grand Teton Fly Fishing publishes a Jackson Hole hatch chart for the valley rather than for this river, so use it as a regional guide. Salmonflies come in June and July and golden stones June through August, but on the Gros Ventre both mostly land in the high water. What matters once the river drops is PMDs from July through September, Little Yellow Stones from July into September, caddis through August, gray drakes in July and August, and Tricos in August and September. Hoppers and other terrestrials run July through September, and WorldCast's August notes lean on hoppers, ants and beetles. Baetis and midges return from September into November, with Mahogany Duns in September and October. The cutthroat flies WorldCast names for the nearby Snake, the PMX and the Stimulator, are a sound start here too.