I believe that "A solution to pollution is dilution", but it is not THE solution. Best solution, of course is to not do it in the first place. Since, while living, that is an impossibility, we move on to Plan B in between damned quickly and instantly. Clean it up.
I don't drink water from rivers or lakes unfiltered. Used to, was lucky, don't do it anymore. If we're talking a white water river, then aeration will clean up a bit of human urine in less than a mile. Sunlight will shorten that distance. A slow, lazy river will carry it along, nurture it, and help it grow.
I guess it comes down to respect. If a paddler shows disrespect to the group or to Mother Nature, it isn't a mark in the Atta Boy column. I've likely wracked up a few marks for myself in the Sphincter of the Week column, but generally try not to.
Pee belongs on the trees surrounding your camp site. This is written in the language of animals that "I live here. Stay away!" Larger animals pee higher and send even more pronounced signals. Best if you can sprinkle it about chest high. Some guys can do that au-naturalle; I have to use a cup.
"Chuckie, can I borrow your cup again, please?"