Orange, rusty colored particulates were in the water, particularly along the banks. If you put your hand in the water, you would get tiny particulates on your hand. Not sure if this is pollen or something else.
Robin Mulvey, Southeast Alaska Forest Pathologist with the U.S. Forest Service, writes:
I feel very confident that that is spruce needle rust, given the appearance, location and timing (sporulation on spruce generally occurs starting the 3rd week of July through late August)! 9/13/2018