The dam diverts water from the Blue River Basin through the 23.3 mile (37.5 km) Harold D. Roberts Tunnel under the Continental Divide into the South Platte River Basin. Tunneling began in 1942 and the tunnel was holed-through in early 1960, with construction completed 1962. Curiously, alternative routes were still under consideration as late as the mid 1950s. Tunneling began from the east portal, just west of Grant on the North Fork South Platte River, but was eventually done from both ends as well as from a 916-foot (279m) deep access shaft 1.7 miles (2.7 km) northwest of Montezuma. At its deepest, the tunnel passes 4,465 feet (1361m) below the surface near Santa Fe Peak on the continental divide. The tunnel is fully lined, with an inside diameter of 10.25 feet (3.1 m), and it permits a flow of 1020 cubic feet per second (29 m3/sec). In a typical year, about 60,000 acre-feet (74 million m3) of water pass through the tunnel.