Williams Fork Near Parshall
Williams Fork · Rockies · USGS gage 09037500
Coldwater (summer mean 14.8°C — prime coldwater).
Water
66°F
Flow
13 cfs
Air
70°F
Sky
0%
Predicted hatches · 2026-06-16
PredictedSulphurEphemerella dorothea / invaria1.00
Predicted by conditions; no reports yet. water 19.0°C (66°F) sits in the prime band (15-19°C); peak season; 8 days sustained ≥13°C; watch evening, the last two hours of light.
flies: Sulphur Comparadun #16-18, Sulphur Spinner, Pheasant Tail
PredictedGolden StoneHesperoperla / Calineuria0.67
Predicted by conditions; no reports yet. water 19.0°C (66°F) is cooling past the prime band (14-18°C); peak season; 8 days sustained ≥13°C; watch afternoon and evening.
flies: Chubby Chernobyl #8-10, Yellow Stimulator, Pat's Rubber Legs
PredictedEastern Green DrakeEphemera guttulata0.67
Predicted by conditions; no reports yet. water 19.0°C (66°F) is cooling past the prime band (14-18°C); peak season; 8 days sustained ≥12°C; watch evening; the spinner (Coffin Fly) falls at dusk.
flies: Green Drake #8-10, Coffin Fly Spinner, Drake Emerger
PredictedLight CahillStenacron / Stenonema0.60
Predicted by conditions; no reports yet. water 19.0°C (66°F) sits in the prime band (16-20°C); peak season; watch dusk.
flies: Light Cahill #14-16, Cream Cahill, Cahill Spinner
PredictedHexagenia (Hex)Hexagenia limbata0.60
Predicted by conditions; no reports yet. water 19.0°C (66°F) sits in the prime band (19-23°C); peak season; watch after dark — the famous night hatch.
flies: Hex Dun #4-8, Hex Nymph, Hex Spinner
PredictedSpotted / Summer CaddisHydropsyche spp.0.50
Predicted by conditions; no reports yet. water 19.0°C (66°F) sits in the prime band (14-19°C); shoulder of the season; 8 days sustained ≥12°C; watch evening, last hour of light.
flies: Elk Hair Caddis #14-16, Iris Caddis, X-Caddis
PredictedSlate Drake / IsonychiaIsonychia spp.0.50
Predicted by conditions; no reports yet. water 19.0°C (66°F) sits in the prime band (16-21°C); shoulder of the season; 3 days sustained ≥14°C; watch afternoon/evening; nymphs crawl to shore to emerge.
flies: Isonychia #10-12, Slate Drake, Leadwing Coachman
PredictedPale Morning DunEphemerella spp.0.33
water 19.0°C (66°F) is cooling past the prime band (13-18°C); shoulder of the season; 8 days sustained ≥12°C; watch late morning to early afternoon.
flies: PMD Comparadun #16-18, Sparkle Dun, Pheasant Tail #16
PredictedMarch Brown (Eastern)Maccaffertium vicarium0.33
water 19.0°C (66°F) is cooling past the prime band (14-18°C); shoulder of the season; 8 days sustained ≥12°C; watch sporadic through the day, midday into afternoon.
flies: March Brown #10-12, Hare's Ear, Spinner
PREDICTED from the first-principles model only. Report-confirmed signal (fly-shop & forum reports) is the next milestone.