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BWO BWO nymph March Brown March Brown nymph Pale Morning Dun Pale Morning Dun nymphs Mahogany Dun Mahogany Dun Green Drake Nymph Green Drake nymph bottom March Brown nymph

Yakima River Hatches




Blue Winged Olive is a generic name for mayflies from the Baetidae family and includes multiple genera and species. These prolific Mayflies can be seen on the Yakima all seasons of the year. The best hatches occur in March and April and again in October and November, ranging in size from #24 to #14. A smaller emergence occurs during the summer months in sizes 22 and 20.  BWOs nymphs are swimmers and live in a variety of moving waters, but prefer slow to moderate runs and riffles.

Size: 14-22 / Colors: nymphs: mottled brown. Adults: (underside) light to dark olive.

Rhithrogena morrisoni

Western March Brown nymphs are clingers found in moderate to fast flowing runs and riffles.  Emergence generally begins in late March and continues through early May.  These are large Mayflies which can produce some of the Yakima's  best dry fly fishing for large fish.     

Size: 12-14 / Colors: nymphs: mottled brown. Mature nymphs have a reddish tinge in the gills. Adults: (underside) tan.

Ephemerella excrucians, Ephemerella dorothea infrequens

Pale Morning Dun activity begins in May and continues through July.  Infrequens emerges first, followed by and overlapping excrucians.  This can be an important late spring, early summer hatch, but varies from year to year.  PMD nymphs are classified as crawlers and inhabit slow to medium sections of the river.

Size: 18-16 / Color: nymphs: muddy olive. duns: (underside) pale yellow to olive yellow to rusty yellow. Spinner: rust.


Mahogany Dun
nymphs are crawlers found in moderate to fast runs and riffles that migrate to slower water as they mature. Mahogany Duns Emerge in the spring in April, May and June, and again in the fall in September, October and November, generally in slower water.

Size: 16 / Color: nymphs: Dark dun to Black. duns: (underside) rusty brown. Spinner: dark rusty brown.

Hatch Charts

Yakima River Mayflies

Drunella grandis, Drunella doddsii

Green Drake
nymphs are crawlers that are found in moderate to fast riffles and runs. Emergence occurs in late April through early July. The strongest hatches are in June in the upper river.