I had several 4 to 6 hour fly fishing trips last year with consistent 12 big trout/day (catch and release).
Some of the best fishing is to find one of these dredge holes, get up stream and cast your fly upstream and let it drift over the edge. A 24 inch native rainbow or steelhead? that shoots out of the bottom of the dredge hole and strikes your fly is great. I will fish the dredge holes over riffles any day.
The only problem is that the water in the Yuba is so clear that you can't read how deep the water is in these holes. A member of our fly club misread one and slipped into one that was about 12' deep. He went down and got caught in the river's flow and barely escaped from drowning. He remember my personal tale and what to do and not to do and saved himself. He lost his rod, reel, line and some other gear. He is up fishing today, and I would have been with him except for an appointment to sell off some of my old gear.