Radnor Lake in Nashville,TN is within the city limits and the water is not blue. At this late afternoon as the ducks were feeding, the blue sky and bright fall colors reflected in the lake to give the lake a blue cast. Equipment was a Nikon D200 with a Nikkor 18-200 f/3.5-5.8 lens. Setting was f/9.0, ISO 250, 1/250.