March Brown is a common name for a popular Mayfly hatch that most often happens in the spring on most rivers, although these patterns can be used to imitate brown mayflies throughout the year on all waters. A true March Brown typically lives in riffles and free drifts to to the surface where it then hatches into a Dun and attempts to fly away. During this process trout will forage on free drifting nymphs, emergers near/on the surface, and of course the adult insects attempting to dry their wings and fly away.