there are some ponds on prospect
that are pretty clean. head east from college, go past timberline, and look for the last right turn before going over the bridge over the poudre. i can't remember the name of the street. head south past a couple of ponds. go to the end of the road, and the pond you want is on the left.
i took my wife to the one out by the drive in earlier this summer, and i didn't like it. it didn't seem very clean to me.
you could also head up to horsetooth reservoir....