The western March brown, Rhithrogena morrisoni, is the first big mayfly of the year on the rivers west of the Cascades, and it hatches under conditions that keep most people home: gray skies, cold rain, water that is barely above winter temperature. On the McKenzie it is the hatch the river's dry-fly tradition was built on. On the Yakima it shares the early spring with the skwala stonefly. And it is worth saying up front that none of the 92 river hatch charts on this site carry a separate March brown line. The charts fold it into the early-season mayfly picture, which means you need to know where to look on your own. This page is that.
Rhithrogena, not the eastern insect
The name gets used for two different bugs. The eastern March brown is Maccaffertium vicarium, a Heptageniid that hatches on Appalachian and Catskill rivers in May. The western March brown is Rhithrogena morrisoni, also a Heptageniid, and it hatches earlier, in March and April, on the coastal and Cascade rivers of Oregon and Washington, and later in spring on some Rocky Mountain freestones. Both are clinger mayflies: flat, broad-headed nymphs built to hold on in fast water, with gill plates that work like suction cups. The adult is a size 12 to 14 dun, tan to reddish brown, with mottled brownish wings that are easy to spot on the water because nothing else that size is hatching yet.
How the nymph gets to the surface
Clinger nymphs live in the fastest, most oxygenated riffles, and they do not swim well. In the weeks before the hatch they move to the slower edges and the tailouts below the riffles, and that migration is when nymphing with a size 12 to 14 Hare's Ear or a flat-bodied clinger nymph is at its best. During the emergence itself, the nymph rises through the current and the dun splits out at or just under the surface, often in the riffle, and then rides the current down into the run below. Duns on a cold day take a long time to dry their wings, and the drift from the riffle into the run is where trout line up. The emergence is also the reason a swung soft hackle is the traditional March brown method: a Partridge and Hare's Ear or a March Brown Spider, swung and lifted through the riffle, imitates the rising nymph and the emerger better than any dead drift.
The Pacific Northwest rivers on this site
The McKenzie is the river to name first. Its spring March brown hatch is well known, the drift-boat fishing on the McKenzie grew up around it, and the classic McKenzie fly patterns were tied for it. The McKenzie page lists blue-winged olives peaking from October through April and caddis peaking April through August, and the March brown sits right between them, in March and April, on the same water. The Deschutes gets a March brown hatch in spring on the lower river before the salmonflies arrive, and the Yakima's is well established enough that a March brown dun in size 12 to 14 belongs in any Yakima spring box. The other Northwest freestones on the site, the North Umpqua, Rogue, Sandy, Klickitat, Wenatchee, and Methow, are the same kind of water in the same climate, and they are the rivers to watch for it in March and April, but treat that as a place to look rather than a promise.
East of the Cascades, in western Montana, Rhithrogena hatches later, in late April and May, on freestones like the Bitterroot and Clark Fork, overlapping the skwala and the first blue-winged olives and running right up to the start of runoff. Those hatches are sparser than the Oregon ones but the fish take the duns hard because it is the first big mayfly they have seen since fall.
Timing within the day
This is an early-afternoon hatch. On most days the first duns show around midday and the heaviest emergence runs for an hour or two after that, when the water has warmed as much as it is going to. Cold, overcast, drizzly days produce the best hatches and the longest drifts; a bright warm afternoon shortens both. If the morning is dead, do not leave. Fish nymphs through the riffles until early afternoon, then swing a soft hackle when the first duns show, then switch to the dry once fish are rising steadily in the runs.
Patterns that work
- March Brown Parachute or Comparadun #12-14, tan to brown body, mottled or dun wing. Fish it on a 9-foot 4X leader on the McKenzie and Yakima; the fish are not tippet-shy in cold water and the fly is big enough to need it.
- Hare's Ear Nymph #12-14, or any flat-bodied clinger nymph in brown, fished dead-drift through the riffles and the tailouts below them in the days before and during the hatch.
- Partridge and Hare's Ear or March Brown Spider #12-14, swung down and across through the riffle and allowed to rise at the end of the swing.
- A cripple or emerger with a trailing shuck in the same size for the fish that are sitting in the run picking off stuck duns.
Practical notes for a March trip
Spring flows on the Northwest freestones swing with the rain, and a river that was fishing on Tuesday can be unfishable brown water on Thursday. Check the USGS gauge on the river page before driving, and prefer the days when the flow is dropping after a rain rather than rising. Water is cold; wade carefully and expect fish to hold in the slower seams rather than the heavy riffle. Many of these Oregon and Washington rivers have seasonal closures and gear rules that change in spring, so check the Oregon Department of Fish and Wildlife or the Washington Department of Fish and Wildlife for the river you plan to fish. And keep a blue-winged olive box in the vest: on the same gray afternoons that bring March browns off, the baetis will be hatching too, and a fish that refuses your size 12 may be taking size 18s.