San Mateo Creek below Lo Crystal Sp Reservoir near San Mateo
San Mateo Creek · Sierra / West · USGS gage 11162753
Coldwater (summer mean 15.4°C — prime coldwater).
Water
64°F
Flow
3 cfs
Air
58°F
Sky
100%
Pressure
29.87 in
Wind
11 mph
Predicted hatches · 2026-09-15
PredictedSlate Drake / IsonychiaIsonychia spp.1.00
Predicted by conditions; no reports yet. water 17.6°C (64°F) sits in the prime band (16-21°C); peak season; 8 days sustained ≥14°C; watch afternoon/evening; nymphs crawl to shore to emerge.
flies: Isonychia #10-12, Slate Drake, Leadwing Coachman
PredictedHopper / TerrestrialsAcrididae (grasshoppers)0.56
Predicted by conditions; no reports yet. water 17.6°C (64°F) sits in the prime band (17-24°C); shoulder of the season; 8 days sustained ≥14°C; wind puts terrestrials in play; watch warm, breezy afternoons.
flies: Morrish Hopper #8-12, Chubby Chernobyl, Dave's Hopper
PredictedSpotted / Summer CaddisHydropsyche spp.0.50
Predicted by conditions; no reports yet. water 17.6°C (64°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
PredictedTricoTricorythodes spp.0.50
Predicted by conditions; no reports yet. water 17.6°C (64°F) sits in the prime band (16-21°C); shoulder of the season; 8 days sustained ≥15°C; watch early morning spinner falls.
flies: Trico Spinner #20-24, CDC Trico, Griffith's Gnat
PREDICTED from the first-principles model only. Report-confirmed signal (fly-shop & forum reports) is the next milestone.