From what I've been reading here, all those gravel extractions are below the Vedder Crossing bridge. In all my years of fishing the Chedder system I've pretty much (repeat - pretty much) only seen Chum and Pinks spawn in that part of the river in the Fall. I'm in no way attempting to justify the digging, but I don't see redd loss as being a big factor. We as rec fisher cause far more damage to redds by stomping into this river, than the few that may be lost in these new pools.
God seems to yearly bring along a blowout right after the Chum finish spawning which cleans the bulk of the rotting carcasses out of the system. That's when these holes will resettle and in most cases fill back in. Many of these digs are totally filled in after the first pre-winter blowout - before steelhead arrive in any numbers. I've seen these holes become above-water bars again after the early November blowout. I believe the steelhead which spawn in the lower river are usually the Feb, Mar, April returnees. River's re-established by then.