Only three of the 92 rivers on this site list a green drake in their hatch chart: the Henry's Fork, the Provo, and the Middle Provo. That scarcity is the first honest thing to say about this insect. The western green drake, Drunella grandis, is the biggest mayfly most trout anglers will ever match, a true size 10 or 12 with a dark olive body and slate wings, and it brings the largest fish in a river to the surface in daylight. But it is a short, localized, weather-dependent hatch that many good rivers simply do not have in fishable numbers, and it is easy to miss by a week.
Drunella grandis and its relatives
The green drake belongs to the Ephemerellidae, the same family as the pale morning dun, and the genus Drunella. Drunella grandis is the classic western green drake. A second species, Drunella doddsii, hatches on some of the same rivers and looks nearly identical on the water. A third, Drunella flavilinea, is the "flav" or small western green drake, a size 14 to 16 that hatches later in the summer and evenings and is really its own event. When a river page on this site says "Green Drake" in a size 10 to 12, it means grandis.
The nymph is a crawler, stocky and armored, with a habit of clinging to the undersides of rocks in moderate riffles and runs. It does not swim well. Trout eat plenty of them, especially in the days before the hatch when the mature nymphs move toward slower water, but there is no listed green drake nymph on any of the three river pages here. A large dark Hare's Ear or a Pheasant Tail in size 10 to 12 covers it.
Why the emergence is so good for dry-fly fishing
A green drake dun takes a long time to get off the water. The nymph rises, splits its shuck in or just under the film, and the big dun sits there drying its wings while the current carries it downstream, sometimes for a yard or more. Many never make it, stuck half in the shuck, and drift as cripples. Trout that would never rise to a size 18 baetis in bright sun will come up for a drake because the meal is worth the trip. And because the insect is large and slow, the fish get a long look, which is why the three green drake patterns listed for the Henry's Fork are a Lawson's Green Drake (an extended-body dun), a Parachute Green Drake, and a Green Drake Cripple, all in 10 to 12. On the Provo and Middle Provo the list is a Parachute Green Drake and an Olive Sparkle Dun in the same sizes, and the Sparkle Dun, with its trailing shuck, is doing the cripple's job.
The hatch usually comes off in the afternoon, and the heaviest emergence tends to happen on cool, overcast days. Bright, hot afternoons produce a trickle. The spinner fall is minor compared to the dun hatch on most water, and the dun is the stage to fish.
The Henry's Fork in late June and July
The Henry's Fork page lists green drakes as "on" in June, "peak" in July, and "on" again in August. The hatch on the Island Park flats builds toward the end of June and carries into July, which is what the matrix says. This is the drake hatch that made the river's reputation: broad, slow, weedy meadow water where big rainbows hold in the open and take drakes with a slow, deliberate rise. The fish are very selective and see many flies. The extended-body Lawson's pattern gets fished by people who have been refused on the parachute, and the cripple when both fail. A 12-foot leader to 4X or 5X, a downstream reach cast, and a drag-free drift the length of the fish's window are the whole method. You may cast to one fish for half an hour.
The Provo and Middle Provo in June
Both Provo pages list the drake as "on" in May, "peak" in June, and "on" in July. The Provo below Jordanelle and the Middle Provo through the Heber Valley are tailwaters, and their drake hatch tends to arrive as the spring runoff from the tributaries settles and the water clears. Because the Provo is a tailwater with a steady summer temperature, the hatch can spread over several weeks rather than blowing through in days. Look for drakes in the afternoons on the riffle-to-run transitions rather than the flat water, and expect the browns to take them harder and less cautiously than Henry's Fork rainbows. The Olive Sparkle Dun in 10-12 is the fly to start with, and a Parachute Green Drake when the light is bad and you need to see the fly.
Fishing the hatch when you find it
- Fish the days before. When you see the first drakes of the year, the nymphs have been migrating for days. A size 10 dark nymph fished deep through the riffles above the flat water catches fish stuffed with them.
- Don't leave at the first lull. Green drake emergence is sporadic and comes in pulses through the afternoon. A ten-minute gap does not mean it is over.
- Carry a cripple. A dun that lands perfectly may be refused by a fish that is eating only the stuck ones. The Green Drake Cripple listed for the Henry's Fork exists for exactly this fish.
- Use heavier tippet than you think. A size 10 fly on 5X twists the leader, and a big trout on 6X in weed beds is a lost trout. 4X is fine; the fly is big enough to hide it.
- Check the water temperature and the gauge. A cold front or a sudden bump in flow can shut the hatch down for a day.
If you are planning a trip around it
Plan loosely. The three river matrices agree on a June-to-July window, but the exact week shifts with snowpack and weather, and the hatch on any one stretch lasts a couple of weeks at most. Watch the river page and its gauge as June approaches, be ready to move to find the emergence, and carry a box of PMDs alongside the drakes, because on both rivers the pale morning dun overlaps the drake and is what the fish switch to when the big bugs stop. Harriman State Park on the Henry's Fork and the Provo both have their own access and season rules, so check Idaho Fish and Game or the Utah Division of Wildlife Resources before you go.