We encountered these guys in a small inland (freshwater) creek in southern WA. There were hundreds, covering low flat rocks in the water; every one carrying a shell made out of either tiny rocks, or a piece of a twig. They're probably 1/8" in diameter, maybe 3/4" long with the shell. Some were moving at a slow crawl along the rocks (below and above the surface), others were burrowing in and out of the dirt on the creek bottom.
Anyone know what these might've been? Are they native to the area or invasive?