South Fork
SOUTH FORK
ReportSEP 1 — 8, 2026
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Flow
—CFS
SNAKE RIVER NR IRWIN ID
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Water Temp
48.9°F
Updated 2015-11-04
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Weather
51–77°F
Isolated Showers And Thunderstorms
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Reported By
Bruce Staples
On the water
The South Fork is running strong and clear out of Palisades Reservoir, with flows around 6,900 cfs at Irwin pushing trout into banks and soft water. Water temperature is a cold 48.9°F, keeping fish active and looking up for terrestrials and a mix of mayflies, caddis, and BWOs throughout the day.
From the ShopBruce Staples · September 1, 2026
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, with some PMDs and pink albert mayflies adding to the mix until around midday. Caddis and BWO activity increase later in the day. Dry-dropper rigs and streamer patterns presented close to the banks also interest trout. Early in the day, mutant golden stoneflies are still around.
Water coming out of Palisades Reservoir remains clear and cold. Flow varies around 6,900 cfs at Irwin.
What's Working — Hot Flies
Hatch Chart
| Insect | Size | Activity | Prime Time |
|---|---|---|---|
| Terrestrials (Hoppers & Ants) | #10–16 | High — primary top-water food source | All day, peak midday–afternoon |
| PMD (Pale Morning Dun) | #18–20 | Moderate | Morning through midday |
| Pink Albert Mayfly | #16–18 | Moderate | Morning through midday |
| Mutant Golden Stonefly | #10–12 | Tapering — still present | Early morning |
| Caddis | #18–20 | Increasing | Afternoon into evening |
| Blue-Winged Olive (BWO) | #20–22 | Increasing | Afternoon into evening |
Best Time Window
- EARLY MORNING (first light to 9 AM): Work mutant golden stonefly patterns and early PMD/pink albert nymphs on dry-dropper rigs along bank seams before switching to terrestrials.
- MIDMORNING TO MIDDAY (9 AM to noon): Prime terrestrial window — hoppers and ants on the surface with a PMD or pink albert dropper. Target soft water and eddies where strong flows concentrate fish.
- AFTERNOON TO EVENING (noon onward): Caddis and BWO activity increases. Transition to smaller dries or dry-dropper rigs, and consider swinging streamers tight to the banks as light fades.
Guide's Tip
From the benchPink albert mayflies are producing alongside PMDs through midday but don't have a direct catalog match — ask in the shop for our current pink albert dry-fly recommendations. With flows this strong, focus all your energy on the soft water: eddies, bank seams, and any slack behind structure. Life jackets are non-negotiable on the South Fork right now — two anglers lost their lives below Palisades Dam this season after their boat capsized.
Main Species
Yellowstone Cutthroat Trout
Brown Trout
Rainbow Trout
