Lake Hefner is a reservoir in northwestern Oklahoma City, Oklahoma. It was built in the 1940s to expand the water supply for the city of Oklahoma City, in part through the labor of German POW's. It is named after Robert A. Hefner, who served as mayor of Oklahoma City from April 11, 1939 to April 8, 1947, but was originally named the "Bluff Creek Reservoir." It also serves as a major recreational destination, as it is surrounded by 9.5 miles (15.3 km) of multi-use (bicycle, skating and pedestrian) trails.