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".

Were they drinking water or having a pleasure cruise? :)

These look more like a type of robber fly to me.... You may have already checked for this. http://bugguide.net/index.php?q=search&a... Great find!

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/bgi.... Opinion?

It's like an entire squadron of flies. :-)

Wow! An interesting phenomenon!

Very interesting series.