Choose your water
USGS station locations — not fishing access pointsSource: USGS monitoring station. Temperature is measured at the selected gauge only. No direct hatch counts are available.
Hatch window watch
Days 1–3 combine measured river-temperature trends with NWS hourly weather to show cautious scenario ranges. Days 4–7 show weather and normal species timing only. No hatch sightings or exact emergence hours are claimed.
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Water-temperature momentum
Measured USGS observations versus experimental next-three-day bands.
River temperature is important, but mayfly emergence also depends on biological development, stream habitat, light, and weather. The bands are an uncalibrated scenario, not a temperature forecast. Wider bands mean more uncertainty.
When sensors, history, or hourly forecasts fail, this tool falls back to a seasonal watch. It never replaces missing water measurements with guessed data.
Mayfly outlook
Loading observations…Favorable means environmental conditions match a broad field-guide association; it does not mean insects have been seen.
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Au Sable mayfly season calendar
Typical regional windows. Actual timing changes with weather, water temperatures and local habitat.
Why branches matter
The North Branch, South Branch, and mainstem can experience different thermal conditions and hatch timing. Downstream dam-influenced stations are not a substitute for Grayling or the Holy Waters. The historic Grayling USGS gauge is no longer a source of live water temperature.
For the Holy Waters, we deliberately do not publish a made-up live temperature from a downstream site. Exact-reach monitoring can be added when a verified measurement source is available.
Great Lakes expansion
This is the Au Sable watershed launch of a broader Great Lakes mayfly system. Other Michigan trout rivers are natural next locations. Nearshore Hexagenia emergences on lakes and bays require a separate habitat-and-surface-water model rather than reusing trout-stream rules.
Reference material: Grayling hatch chart · USGS Water Data APIs · NWS API.
Great Lakes mayflies: a different emergence model
Looking for lake mayflies around Saginaw Bay or western Lake Erie? Follow the lake Hexagenia tracker with live NOAA buoy temperature, the lake-bottom 68°F emergence reference, and a seven-evening NWS wind watch. This is not the same model as the trout-river hatch forecast. Open Great Lakes Mayfly Hatch Tracker →