South Fork

Fly Fishing Report

SOUTH FORK

Report
AUG 19 — 26, 2026
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Flow
10300CFS
SNAKE RIVER NR IRWIN ID
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Water Temp
48.9°F
Updated 2026-08-18
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Weather
56–92°F
Mostly Clear
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Reported By
Bruce Staples
On the water
The South Fork is pushing around 10,300 cfs at Irwin (USGS), with water temps at 48.9°F — strong flows are shoving trout tight to banks and into soft water where terrestrials rule the top. Sunny skies and highs near 85°F Wednesday set up ideal afternoon hopper conditions.
From the ShopBruce Staples · August 19, 2026

The South Fork Road from Heise upstream is open for travel. The surroundings are closed in places because of mud slides coming from burn areas resulting from rainfall. Mop-up work on the Big Rock fire area is ongoing and uses the road. The road is closed downstream from the Heise bridge.

Look for continued increased boating use of the river from Palisades Dam downstream to Spring Creek and on the lower river. Flow is strong enough in the main river to push fish toward banks and soft water. Here terrestrial insects become their main top-water food source. Dry-dropper rigs and streamer patterns presented close by also interest trout.

Early in the day, mutant golden stone patterns are very effective, especially when skittered or twitched on the surface. Flow varies around 10,500 cfs at Irwin, about 11,500 cfs at Heise, and about 7,000 cfs at Lorenzo.

Hatch Chart
Insect Size Activity Prime Time
Golden Stonefly (adult) #8–#12 High — skitter or twitch on surface Early morning
Grasshoppers / Terrestrials #10–#14 High — main top-water food source along banks Late morning through afternoon
Ants #14–#18 Moderate — banks and soft-water edges Midday through afternoon
Best Time Window
  • EARLY MORNING (first light to 9 AM): Skitter or twitch mutant golden stone / stonefly patterns on the surface — this is the prime window for that specific bite
  • LATE MORNING THROUGH AFTERNOON (10 AM to evening): Work dry-dropper rigs and hopper/ant patterns tight to banks and soft-water seams; throw streamers close to the bank for trout pushed out of the main current
Guide's Tip
From the benchThe field report specifically calls out 'mutant golden stone' patterns as the top early-morning producer — we don't carry that exact fly by name, so ask at the shop counter for the closest match or pick up the Olsen's Straggle Stone as your stand-in. Whatever you throw, the key technique is to skitter or twitch it across the surface rather than dead-drifting it. Access note: the South Fork Road is open from Heise upstream, but closed downstream from the Heise bridge — plan your put-in and take-out accordingly and expect road traffic from Big Rock fire mop-up crews.
Main Species
Yellowstone Cutthroat Trout
Brown Trout
Rainbow Trout
Report generated August 19, 2026 — Next update: August 26, 2026