It's definitely cottonwood seed. I live a mile from the Cowlitz between Toledo and CR. The river is lined with huge cottonwoods. It looks like a blizzard from about mid May until early July. The crap stays in the river into August. I hooked and caught a 15lb summer run last year just above Toledo I was fighting 15lbs of fish and about the same weight in cotton seed. It sucked but well worth the effort. This stuff may look like algae as many have stated, it does have algae that sticks to it as it rolls along and bounces off rocks. You can tear it apart and see the tiny black seeds in the middle of those doughy balls. I see they've upped the flow out of Mayfield to over 5000 cfs today. It should help root that junk out and flush up some more steelies.