Luke I see what you're saying but there are area's like that upstream of the hatchery (the tailout above ethel bar comes to mind) that meet the criteria but yet are void of said "cow snot"
Cohoangler's post is what I was thinking as well....yeah its algae but something seems to be feeding it and the hatchery seems to be the obvious culprit.

RL
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They can have my eggs when they pry em from cold dead hands