Flies

@nexttogone, these flies were quite small, so I don't think robber flies is not likely. The cuprinus group of long-legged flies (mentioned below) is still more likely. Your opinion is welcome. And Lauren, it appeared to me that they were simply doing "field exercises".

Thanks, ReikoS. I was walking along the stream and noticed the leaf against the black water. Then suddenly the flies landed, like a squadron; quite an interesting event. Appreciate your comments, Aaron and J.

@Small Wonders: I believe you are correct, but with about 200 genera... The position of the wings and the red eyes are different than most long-legged flies I've seen. The closest I've been able to get is the cuprinus group, http://bugguide.net/node/view/611221/bgimage?from=0. Opinion?