All 92 rivers on this site list the blue-winged olive, and on 28 of them, nearly all tailwaters, it is marked as peaking for seven months of the year, October through April. No other mayfly comes close. Baetis is the most important mayfly in Western trout fishing not because it is big (it is a size 18 to 22) or because its hatches are spectacular (they are not) but because it is there, in fishable numbers, on every river, in both shoulder seasons, on exactly the gray, cold days when nothing else is happening. If you fish through March or November anywhere in the West, this is the hatch you are fishing.
Baetis: a swimmer with two broods
Blue-winged olive is a common name covering several genera of small olive mayflies in the family Baetidae, but on Western rivers it almost always means Baetis, most often Baetis tricaudatus. The nymph is slim and a strong swimmer, darting between weed and gravel in moderate current. Unlike most mayflies, Baetis produces at least two generations a year, one in spring and one in fall, and on cold, stable tailwaters the broods blur into a hatch that runs on and off all winter. The spring bugs are usually a little larger (size 16 to 18); the fall bugs are 18 to 22. The dun is olive to olive-brown with slate-gray wings, and the spinner is rusty olive.
One behavior matters more than any other: the emergence is fast and messy. The nymph swims to the surface, often several times before it commits, and the dun pops out and sits on the water drying its wings for a long time in cold air. On a raw November afternoon a Baetis dun may drift thirty feet before it flies. That long drift is what makes trout rise to a bug this small.
Why bad weather is good weather
Baetis hatch heaviest on overcast, cool, drizzly days and thinnest on bright, warm, still ones. The duns dry slower in damp air and stay on the water longer, but the practical point is simple: the day that looks worst from the truck is the day to go. Snow flurries in March or a steady cold rain in October are the classic conditions. The hatch comes in the warmest part of the day, midday to mid-afternoon in the cold months, and can be over in an hour or run for three. Wind scatters the duns and kills the rise.
Where and when: the river charts sorted by water type
The peak months in the 92 matrices follow the water, not the state line:
| Peak months | What that water is | Rivers on this site |
|---|---|---|
| October through April | Tailwaters and mild-climate rivers where Baetis hatch all winter | Missouri, Bighorn, South Platte, Frying Pan, Blue, Taylor, San Juan, Salt, Verde, Upper Sacramento, Hat Creek, Owens, Truckee, East Walker, and the Oregon and Washington rivers (Deschutes, McKenzie, Metolius, Yakima) |
| February through April and October, November | Colorado and Utah tailwaters and lower freestones | Arkansas, Colorado, Gunnison, Eagle, Roaring Fork, Provo, Middle Provo, Strawberry, Boise, Rio Grande |
| March, April and October, November | Mid-elevation rivers and spring creeks | Henry's Fork, Silver Creek, Green, Weber, Logan, Big Wood, Teton, North Platte, Poudre |
| March, April and October | Cold freestones with a runoff gap in between | Madison, Yellowstone, Big Hole, Bitterroot, Clark Fork, Gallatin, Rock Creek, South Fork Snake, Snake (WY), and the Idaho, Washington, and New Mexico freestones |
On a freestone the spring Baetis hatch ends when runoff hits and the fall hatch starts when the water cools in October. On a tailwater there is no runoff and no freeze, so the "spring" and "fall" hatches join into a winter-long one. The Firehole, a thermal spring creek, is the oddity: it peaks in June and again in October and November, because its geyser-warmed water is too hot for Baetis in midsummer.
Patterns from the river pages
The fly lists are consistent across the site: you do not need many Baetis patterns, you need the right few in the right sizes.
- RS2 #18-22, listed on 72 rivers. An emerger with a split tail and a tuft of wing that fishes in the film or a few inches under it. The single most useful Baetis fly on this site. The Mercury RS2 adds a glass bead.
- Parachute Adams #16-20, listed on 75 rivers. Gray, not olive, and it does not matter; the post makes it visible and the profile is right.
- CDC Baetis Emerger #18-20 and CDC Baetis: on the spring creeks and flat tailwaters (Henry's Fork, Silver Creek, Missouri, Frying Pan) where fish inspect the fly.
- Sparkle Dun #16-20: a Comparadun with a trailing shuck, for fish taking emergers in the surface.
- Vis-A-Dun #18-20, a high-visibility dun for gray light, plus the Parachute BWO, Thorax Baetis, and Barr Emerger on individual rivers.
- Pheasant Tail #18-20 for the nymph, the fly for the two hours before the first rise.
How to fish a Baetis afternoon
Start at eleven with a Pheasant Tail and an RS2 under a small indicator, drifting the seams and soft edges of riffles where nymphs collect before they emerge. When the first duns show, clip off the indicator, grease the leader to a foot from the RS2, and fish it in the film to the first risers. When fish are clearly taking duns (the fly disappears, rather than a bulge under it), go to the Parachute Adams or Sparkle Dun on 6X. Set on any rise near the fly. Fish the slicks and foam lines, not the fast water. A dun in a heavy riffle is gone in seconds; the same dun in the slow eddy below drifts for a minute, and that is where the trout are looking up.
Before you go
Check the gauge on the river page. On tailwaters a flow change can shut the hatch off or move the fish; on freestones a spring rain can end the season for a week. Most of the winter tailwaters above have special regulations below the dam, so check the state fish and game agency for the section you plan to fish. And on those same overcast days, midges will be hatching alongside the Baetis, and a fish that refuses a size 20 olive may be eating size 24 midges.