One of the top ten inland sailing places in the world, Lake Hefner is a reservoir in northwestern Oklahoma City, Oklahoma. It was built in 1947 to expand the water supply for the city of Oklahoma City. It also serves as a major recreational destination, as it is surrounded by 9.5 miles (15.3 km) of bicycle trails. Lake Hefner covers 2,500 acres (10 km²) and averages 29 feet (9 m) deep. It is named after Robert A.