Anglers and river floaters have noticed a strong bloom of Didymo, or "rock snot" this year in the Boise River.   Didymo is native to the mountain west, and periodically has large blooms, with cause unknown.  This year appears to be one of those. The Idaho Statesman has a report on this summer development.

Increased ‘gunk’ in Boise River led to DEQ testing. It’s ‘rock snot.’ Huh?

Read more at: https://www.idahostatesman.com/news/local/community/boise/article316619141.html#storylink=cpy