West of the Continental Divide, the input of the tunnel is water pumped from Windy Gap Reservoir, Willow Creek Reservoir and other North Fork of the Colorado River drainages to Lake Granby. From Lake Granby, the water is pumped into Shadow Mountain reservoir and then flows by gravity to Grand Lake, Colorado's largest natural body of water, from where it pours through the tunnel. Lake Granby is the second largest reservoir in the state of Colorado, with a nominal capacity of . Once the water reaches the Front Range on the eastern side of the Rocky Mountains, it is used to generate electricity as it falls almost half a mile (0.8 km) through five power plants on its way down the Front Range. To get the power from the Front Range power plants back to the pumps on the western slope, a diameter watertight nitrogen-filled conduit carrying a 69,000 volt electric power transmission line is mounted on the roof of the tunnel. This power is used to run the pumps on the western side of the tunnel, while surplus power is sold. West slope water is stored on the Front Range in Mary's Lake, Lake Estes, Carter Lake (all reservoirs), Flatiron Reservoir, Horsetooth Reservoir and Boulder Reservoir for power generation or release after the peak snow melt months of May to July.