I was collecting some bugs today, and found this among many other finds. It the larvae of the rhyacophila caddis encased in its cocoon. When the transformation to a pupae is finished the caddis will chew out of this almost clear cocoon and the pupae will rise to the surface and hatch into the adult. You can make out the legs of the larvae near the right end of the green body. While photographing this I could see the larvae moving withing the cocoon.
A major caddis hatch is expected on this river any time now.