[slharder]

Two weeks ago I was fishing the St.Vrain in WildBasin in RMNP - just below the bridge near the horse trailer lot. I'm a flatlander from Iowa who only gets into the mountains for a week a year.I'd like someone to educate me on the 'hatch' activity that went on for about 2hrs that afternoon.There was a rectangular window of sunlight reaching from the bridge downstream for 75ft or so, down the middle of the river. There was a 'conveyor belt' of small grey insects that were flying upstream at an altitude of 1ft above the water. When they reached the shadow of the bridge they pulled up, did a nice tight 180 turn and then flew downstream at an altitude of 1inch, sometimes touching down - and when they got to the other end of the sunlight they turned and came back up stream. There were thousands and thousands. It was a constant stream that just went on and on.And lots of surface trout activity. When I finally got the right fly on (small, grey 'mosquito') I had 4 good takes in 30 minutes.The most fun I've had on a mountain stream for a couple of years.

[BigCliff]

It sounds like what you saw were freshly hatched/emerged mayflies that were using the suns rays to dry their wings out. Based on this hatch chart http://www.frontrangeanglers.com/Fis...vrainhatch.htm the only mayflies that could have been hatching then are red quills or green drakes. I'm guessing if it was either of those two it was the red quills, as they're not all that red, and green drakes are not small.

That hatch also could have been another bug altogether, another mayfly or possibly even a caddis. Mayflies fly much more gracefully than caddis do. Caddisflies fly much like moths, which reminds me of an old joke.

q: How can you tell when a moth/caddisfly farts?

a: it flies in a straight line

The hatch possibly could have even been a midge hatch. I have found that many times hatches of large midges can look like small mayflies.

The "sometimes touching down " you saw was actually them dropping their eggs into the water, which then sink to the bottom to grow for a year before they start crawling around as nymphs.