Gross Reservoir Dam, Colorado

The Gross Reservoir is the main reservoir for water crossing over the Continental Divide through the Moffat tunnel. Denver Water, as the city’s main water supply company is called, operates four collection systems to capture water for the city, three of which collect water from the western slope of the Continental Divide, using the watersheds of the Fraser, Williams Fork, and Blue Rivers. Denver Water’s Moffat Collection System has one lobe on the east side of the Divide, and another on the west side, with the Moffat Tunnel between them. The western side collects water from Fraser River tributaries through a number of aqueducts, tunnels, and existing stream channels. Each of the two underground tunnels, the Vasquez Tunnel and the Gumlick Tunnel, cross the Continental Divide, bringing water from the Williams Fork River watershed to the Moffat Collection System. Water emerges from the Moffat Tunnel as a canal that joins Boulder Creek, which flows into the Gross Reservoir, six miles southwest of Boulder. Efforts are underway to enlarge this reservoir, by raising the dam by more than 100 feet.