Boiling River, Yellowstone National Park
Near the North entrance to Yellowstone park is a small parking lot. A trail from this lot leads to hot spring flow from Mammoth Hot Springs. Swimming is permitted here (as of summer 2007 at least). The water temperature ranges from hotter than one can stand to cold. Move around until you find the temperature right for you. Montana mountains can be seen in the distance.
Camera and Lens Settings
70mm f/11.0 1/100s ISO 100 4992 x 3328px