This isn't a hatch in the bug sense - it's a feeding window that opens when rain raises the river and flushes live earthworms off the banks and out of the soil. Trout don't pass up free protein. As the water comes up and colors off, fish slide to the bottom of runs and seams and start eating drifting worms.
The window runs March through October, with the peak in May when spring rains hit hardest. Water temps in the 45-65F band keep the trout active. It's an all-day bite - cloud cover and stained water mean you don't have to wait for a particular hour.
Tie on a San Juan Worm or a Squirmy Wormy in red, pink, or wine and get it deep with split-shot or a heavy bead. A Mop Fly works the same job with a chunkier profile. The classic move when the river's blown out is to trail the worm behind a stonefly nymph or a Pat's Rubber Legs - the stonefly anchors the rig and reads as the big meal, the worm does the quiet eating on the dropper.