Montana ·
Yellowstone River Fly Fishing Report - September 20, 2026

YELLOWSTONE RIVER
ReportSEP 20 — 27, 2026
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Flow
—CFS
🌡️
Water Temp
—
☀️
Weather
40–73°F
Patchy Fog
💧
Clarity
Clear
Check post-storm
Live USGS gauge data is unavailable, but recent web intel puts flow near 1,460–1,710 CFS at gin-clear clarity with water temps in the 55–62°F range — textbook fall conditions. Expect patchy morning fog burning off to partly sunny skies with a high of 65°F and calm winds, setting up a productive all-day window.
What's Working — Hot Flies

Bionic Hopper - Tan #12
#12

Bionic Hopper - Purple #12
#12

Bionic Ant 2.0 - Black #16
#16

Parachute - Blue Wing Olive #22
#22

Top Secret Baetis #16
#16

Juju Baetis Tungsten #22
#22

Egan's Frenchie #12
#12

Egan's Thread Frenchie Jig - Olive #12
#12

Coffey's CH Sparkle Minnow Sculpin #6
#6

Sculpzilla - Olive #4
#4

Egan's Silver Bullet - Baetis #14
#14

Corn-fed Caddis (CDC) Tan #20
#20
Hatch Chart
| Insect | Size | Activity | Prime Time |
|---|---|---|---|
| Blue-Winged Olive (Baetis) | 20-22 | Moderate | 11 AM - 2 PM |
| Grasshopper | 10-14 | Moderate | 12 PM - 4 PM |
| Terrestrial Ants & Beetles | 14-18 | Light | 10 AM - 3 PM |
| Caddis | 16-20 | Light | 5 PM - 7 PM |
| Sculpin / Baitfish (Streamer) | 4-8 | Moderate | 7 AM - 10 AM |
Best Time Window
- 6:30 AM - 9:30 AM: Fog-shrouded streamer hour — swing sculpin and minnow patterns through deep pools and undercut banks before the sun penetrates.
- 11 AM - 2 PM: BWO hatch window — watch tailouts for rising fish and match with small dries or emergers in the surface film.
- 12 PM - 4 PM: Prime hopper time — work grassy cutbanks and willow-lined edges with a tan or purple hopper as temperatures peak near 65°F.
Guide's Tip
From the benchWith patchy fog this morning, start with streamers — swing a Sculpzilla or Sparkle Minnow through the deep pools and undercut banks before the sun hits the water. Once the fog lifts around 10 AM and temps climb toward 65°F, switch to a hopper fished tight to grassy cutbanks in Paradise Valley; the calm WNW wind keeps presentations clean. If you see noses during the midday BWO window, downsize to a Parachute BWO or Juju Baetis and target the slow tailouts where fish are sipping emergers just under the film. Keep your indicator depth set at 1.5 to 2 times the water column when nymphing — in this gin-clear water, fish are holding right on the bottom and will reject anything that doesn't track naturally.
Main Species
Yellowstone Cutthroat Trout
Brown Trout
Rainbow Trout
