March Brown

Rhithrogena morrisoni · species

A classic larger mayfly that brings trout up with drifting nymphs, duns, and spent spinners.

Overview

About March Brown

March Brown is the angler name for Rhithrogena morrisoni, a western March brown mayfly in the family Heptageniidae. Like other flatheaded clinger mayflies, it is tied to moving water and is an important hatch for match-the-hatch anglers.

Life Cycle

Eggs

Female mayflies deposit hundreds to thousands of eggs either in the water or nearby, which then settle to the stream bottom.

Nymph

Nymphs reside on the stream bottom, where they forage and develop, undergoing several molts as they grow.

Emerger

The nymph ascends to the water's surface, where it begins its transformation into an adult mayfly.

Adult / Dun

A freshly emerged adult, or dun, rests on or just above the water's surface, often drying its wings before its first flight.

Spinner

Mated adult mayflies, referred to as spinners, take to the air to complete their reproductive cycle.

Egg Laying Spinner

Female spinners return to the water's surface specifically to deposit their eggs.

Spent Spinner

After successfully laying their eggs, adult mayflies typically die and are often found floating on the water's surface.

Stages: Anglers commonly fish this bug as a nymph, emerger, dun, and spinner. Nymphs develop on stream bottom, then rise or drift before emerging into duns; adults later molt into spinners that return to mate and fall spent to the water.
Emergence cues: Most important during spring to early summer conditions, especially as water warms and insects become active through the day; spinner activity often builds in softer light.
Behavior: Nymphs are clingers that stay tight to rocks in current but can become vulnerable when dislodged. During emergence they drift in or just under the film, duns ride the surface, and spent spinners lie flush and helpless.

Where It Hatches

March Brown

One species, multiple rivers.

River

Fly Patterns

March Brown Dry#12-16
Pheasant Tail Nymph#12-22

Angler Notes

Why it matters: March Browns are substantial mayflies, so trout often key hard on them when nymphs begin drifting and when adults collect on the surface. Because they are larger than many spring mayflies, they can pull fish into clear feeding lanes and reward precise matching.
Imitation tips: Fish slim clinger-style nymphs in brown or olive-brown tones, then switch to a comparadun, cripple, or soft-hackle emerger when fish start showing. Keep adult patterns low riding rather than overly bushy, and carry a spent spinner for flat water or late-surface feeders.
Key life stages: nymphemergerdunspinner
Presentation: Dead-drift nymphs near riffles and current seams, adding occasional lift at the end of the drift to suggest emergence. For dries, target foam lines, slick edges, and transition water with drag-free floats; for spinners, a flat, flush presentation matters most.

Identification

Key features: A sturdy, flat-bodied clinger mayfly with broad head and wing pads as a nymph; adults are larger, upright-winged mayflies with a robust profile.
Size: Generally a medium to large mayfly for anglers, commonly imitated around hook sizes 10–14.
Coloration: Usually mottled brown to dark brown overall, with nymphs often showing earthy olive-brown tones and adults tending brown-gray with darker markings.
Similar bugs: Gray DrakeMahogany DunBlue-Winged OliveLight Cahill

Habitat

Water types: freestone riversriffled runsmedium to large streams
Substrate: Rocky bottoms, especially cobble and gravel in riffles and broken runs.
Conditions: Clean, well-oxygenated moving water with steady current and enough rock structure for clinging nymphs.