The reservoir collects water from a 3.5 square mile watershed in the Rocky Mountains of northcentral Colorado. The basin is roughly rectangular in shape, with its major axis oriented in a west to east direction. The tunnel delivers water from the Pole Hill Powerplant Afterbay. Elevations in the basin range from 6,577.0 feet at the normal water surface of the reservoir to 7,760 feet at the extreme south tip of the watershed. The mean basin elevation is 6,800 feet.
Vegetation consists of native grasses and some scrub pines.
Slopes steepen slightly in the upper reaches of the basin, and some evergreens were noted at peak elevations. The area is well grassed with no heavy undergrowth. The dam forming the reservoir is an earthfill-type structure with appurtenance spillway and outlet works, both of which discharge into a small intermittent tributary of Cottonwood Creek. The structure is specifically designed to withstand conditions resulting from rapid reservoir drawdown required to supply the daily peakload generation at Flatiron Powerplant. The outlet works of Rattlesnake Dam , located to the right of the centerline of the dam.