The Green Drake hatch can span the early summer and into August on the upper Bow. Late in comparison to southern hatches, but important just the same. The big May flies are targeted by even the largest of trout, some that mainly feed on small fish, will include this giant fly in their diets. They also have been around a long time, so they are familiar with the hatch, when it happens. The nymph, emerger and adult imitations are carried by those fly fishers that are intent on fish this annual hatch.
I have fished the Western Green Drake Grandis and the Flavilinea which vary in size, but the Flav’s are smaller but the hatch is more spread out over a month some times. Both hatches are members of the Drunella family of may flies. The giants will hatch over a week or so and late morning is what I have found to be the best time. Jake Gotta and I found a good hatch on in Canmore area a few years ago. The trout were small, but they were plentiful and eager to mangle my dry flies. The drying salts I carry were in heavy use that day, as the patterns dwindled away, in my fly box.
The river is still flowing high, so the hatch will be later on for the upper Bow River. Now is a good time to stock up on some important imitations for the Green Drake. The Flav’s are a size 14 dry fly and the larger Grandis are a size 10. I tie different variations of the giant Western Green Drake and they all seem to work just fine. Fined some choppy riffle and start casting this pattern, when the hatch begins.
If you would like to learn more about Mayfly hatches, try a few books like: Hatches 1 & 2; by Caucci & Nastasi, or Mayflies; by Malcolm Knopp & Robert Cormier, I also have books by other authors, but the first two are more detailed. Dick Pobst handy little hatch book is a real good one that I have, and I would recommend this one for beginners. It is small enough that you can carry it in your fly vest or pack.