The March Brown (Rhithrogena to entomologists) is a sturdy mayfly that hatches from April through June, peaking in May, when water sits in the 48-60F range. Despite the name, the bulk of the hatch fires later than March on most American streams, so treat it as a mid-spring bug. It runs bigger than the early-season BWO, usually sized 12-14, with mottled tan-brown wings and a brown-amber body.
The defining trait is how it comes off: sparse and stretched across the middle of the day rather than in a tight blanket. You see ones and twos drifting for long stretches, and the duns are slow to leave the water, so trout hold and pick them off without rushing. Watch the slower edges of riffles and the heads of pools around midday.
Match the dun with a March Brown dry, or a size 12-14 Parachute Adams when you want a cleaner profile on the surface. The nymph is a clinger that lives in fast riffles and is a weak swimmer, so before the hatch shows, dead-drift a Hare's Ear deep along the riffle and its slower edges rather than swinging it. Once duns are riding, switch to the dry.