The people of the Colorado actually take really good care of it. 40 million people get their drinking water from that river, and I'm one of them. So you are right. It's WAY more clean than most.
I think Sam and I are going to the river today. It's cool enough that I won't be standing in the river to repeat that photo :) I'll only get wet today from the sprinkles I get from Sam. And yes, there is lots of swimming around here. These pics were taken fairly early in the morning so the crowd was slim.