Spot on Rivrguy. We do tend to be so salmon-centric as if that was the only fish out there. Sculpins, at least some species here in the PNW, migrate downstream to spawn, often in the estuary, and then head back up. A fall/blockage of a couple of inches can block the return of juveniles. There are more than a few records were permanent fish traps, like I worked on, essentially wiped out the stream's sculpins in a few years.

Working on traps, especially those that catch the small beasties, is a real eye-opener as to what is moving up and down a stream and just how many there are.