I found the same thing on the Snoqualmie two summers ago. I asked the biologist . He said they spawn in the mud/silt of the slow parts of the rivers. I had never seen one as a kid and I grew up on the Snohomish and Sky. They are good food for several spiecies. The favorite food fo the Sturgeon in the Columbia and Snake systems. Seems like a pop boom the last few years however. Maybe just a cycle or the primary predator is in decline.