On April 10, 2026, the lower Madison below Ennis Lake was already running at 52°F. The Yellowstone at Corwin Springs was at 46, the Big Hole near Melrose at 46.8. All three rivers were within or within a degree of the ranges anglers traditionally watch for caddis activity — yet none of them would produce a Mother’s Day Caddis hatch for another three to five weeks.

That gap matters, because it reframes the most common piece of advice you will hear about this hatch. Brachycentrus emergence is not gated by a single water temperature. It is gated by degree-days — the accumulated warming time the insect needs to complete its pre-emergence development. The river doesn’t have to cross 50. It has to live in the 40s and low 50s long enough for the larvae to be ready. Once they are, the event is less a hatch than a meteorological phenomenon — a blizzard of size 14–16 adults thick enough, on the peak day, to crowd the air above the water and coat a parked drift boat.