South Fork

 

SOUTH FORK

Report
SEP 30 — OCT 7, 2026
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Flow
4190CFS
SNAKE RIVER NR IRWIN ID
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Water Temp
48.9°F
Updated 2026-09-30
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Weather
47–72°F
Sunny
🎣
Reported By
Bruce Staples
On the water
The South Fork is running clear and cold at 48.9°F with flows around 4,000 cfs at Irwin, offering excellent wading access as trout push toward cut and vegetated banks. Sunny skies dominate the next two days with highs near 71–72°F, though any unsettled breaks will fire the best mayfly action.
From the ShopBruce Staples · September 30, 2026

The river is in prime early autumn condition. Water coming out of Palisades Reservoir (about 10% full) remains clear and cold and will continue to drop. Minor turbidity remains in water just below the dam.

Flow in the river has decreased enough to offer numerous wading locations, and trout are moving toward cut and vegetated banks. We at Fly Fish Food Jimmy's can recommend some of the best wading locations.

Terrestrial insects remain the main top-water food source, with the best mayfly hatches — BWOs, mahogany duns, pink alberts — during unsettled weather. Caddis hatches take over top-water fishing as evening approaches. During low-light conditions, stripped streamer patterns also interest trout.

Boat traffic on the river is decreasing, and because of low flow rates, float trips take longer for any distance compared to the same during summer. Flow varies around 4,000 cfs at Irwin, about 4,700 cfs at Heise, and about 1,800 cfs at Lorenzo.

Hatch Chart
Insect Size Activity Prime Time
Terrestrials (Ants, Hoppers) #10–16 All day along cut and vegetated banks Midday–Afternoon
Blue-Winged Olive (BWO) #20–22 Best during overcast or unsettled weather Midday
Mahogany Dun #14–16 Sporadic; peaks with unsettled conditions Afternoon
Pink Albert (Mayfly) #14–16 Sporadic; unsettled weather triggers best activity Afternoon
Caddis #18–20 Evening top-water takeover Evening
Streamers (Baitfish/Sculpin) #6 Stripped near banks in low light Dawn & Dusk
Best Time Window
  • MIDDAY–AFTERNOON: Work terrestrials and, during any cloud cover, BWO and mahogany dun dries tight to cut and vegetated banks
  • EVENING: Switch to caddis dry flies as the hatch takes over the top-water bite in the last hours of light
  • DAWN & DUSK (LOW LIGHT): Strip sculpin and minnow streamer patterns along deeper cut banks for aggressive trout
Guide's Tip
From the benchMadison notes that mahogany duns and pink alberts are also producing during unsettled weather — we don't have exact catalog matches for those specific patterns, so ask us in the shop and we'll sort you out. With flows dropping and wading access opening up, position yourself along cut and vegetated banks early; switch to caddis dries as the light fades, then pick up a stripped streamer for the last hour of shooting light.
Main Species

Yellowstone Cutthroat Trout

Brown Trout

Rainbow Trout
Report generated September 30, 2026 — Next update: October 7, 2026