Masses of insect larvae near streams

[juniper]

We hiked on the (very very dusty) Tevis trail in Granite Chief Wilderness this weekend. Near a few of the streams, we saw huge masses--like thousands of individuals--of what looked like small caterpillars. They were maybe 1/2" long and dark colored. They were on the muddy ground near the water and crawling around on rocks, too. Any idea what these might have been? I don't think they were tent caterpillars--not fuzzy.

Maybe related: the Tevis Cup event, a big horse riding thing, was on this trail just a couple of weeks earlier, so there would have been loads of horses passing by.

[Jimr]

Look up Stone Fly larvae and see if the pictures match what you saw.