Migrating Hatches

[fulano]

Any of you bug experts out there know anything about what triggers some of the progressive hatches to occur. Many (if not all, I'm not that familar) of the stonefly hatches progress upstream along a river. Last summer I learned that the Green Drakes do the same, at least on the Middle Provo. Is it water temperature that causes the nymphs to finally reach "hatching" maturity. Is it just an incubation period so that they hatch X number of days from the time their mommy green drake laid her eggs on the water, and since each gereation progressively hatches from downstream to the upstream end of the river, that's what order the eggs got laid in so that's the order they hatch the next year?

I picked up the Drake hatch on the middle last summer when theyhit the River Road area and fished it several times from there up to the dam. I didn't fish any of the upstream stretches. How has construction affected the progression of the drake hatch on the middle? Does the hatch progress steadily from deer creek on upstream, or is it all messed up because of construction in the individual stretches?

[Cary]

Its been a bit funky. Some days you'll hit it, and some days you wont.

When you hit it hot, chances are it wont be the next day. So by the time you hear reports, its too late.

[RAW]

Not sure what makes them migrate upstream.

Over the years I have noticed that the stones (pteronarcys) will hatch on the South Fork of the Ogden before they hatch on the lower Ogden below Pineview. I would think they would hatch down lower (in canyon) and then up higher on South Fork, but most years it is the opposite. Maybe the dams and narrowness of the canyon change the temperature of the water. Maybe the sun doesn't hit the narrow canyon water enough to get the hatch going as early??

[Utah DaveII]

the overriding factor water temperature?

Take the Madison for example the stonefly hatch in the park (gets hot water from themral springs) is a full two weeks ahead of the hatch on the river below Hebgen.

What is nice about the middle is that the reservoir has a selective withdrawal structure and a mechanism to oxygenate the water that lets the river maintain it's thermal and dissolved oxygen requirements. Certain species need temperatures to get into the sixties before they can complete their life cycle. that's why the only bug in the Colorado at Lee's Ferry are midges. The river is too cold for everthing else.

[Jason]

Yes, water temperature is the major factor that influences most aquatic insects for when the hatch starts and how fast it moves up stream. (some mayflies are influenced more by the weather patterns but it varies between species) Typically the water downstream is warmer than the upper sections, thus an emergence from bottom to top. I believe 50 degrees is the magic temp or more for the giant salmonfly. The giant stonefly usually comes out in full force in the month of May but again it all depends on the water temp. You could have a late snowmelt, higher spring flows, colder water temps; thus a later emergence will take place. This past year on the Henry's Fork the salmonflies came out 2-3 weeks earlier than expected because of low snowpack and warmer water temps.

I believe Green Drakes have a slightly different emergent factors; water temp, geographic location, stream flows, character of the stream combined with weather conditions. The GD hatch is a hit and miss hatch; it's sort of a mystery. YOu can have two identical days of weather and water temp, but the hatch only occurs during one of those days. Go figure. I guess they just hatch when they feel like it, usually starting in the early afternoon. I've even seen them hatch in the late morning. It might have to do with their biological clock where they hatch at the same time each year within a few days of each other no matter what. Who knows.