All those little scallops are fish redds, so they're everywhere.
Every time a fish digs out gravel, it digs a hole, drops its eggs in, and spreads a bunch of gravel on top of that, and then there's a hump behind that. Hydraulically, the hump forces water through the embryos and the alevins and makes sure that the oxygenated water percolates across the fish. They're very easy to see once you're out there.