By the 1930s, the water requirements for Los Angeles continued to increase. LADWP started buying water rights in the Mono Basin (the next basin to the north of the Owens Valley). An extension to the aqueduct was built, which included such engineering feats as tunneling through the Mono Craters (an active volcanic field). By 1941, the extension was finished, and water in various creeks (such as Rush Creek) were diverted into the aqueduct. To satisfy California water law, LADWP set up a fish hatchery on Hot Creek, near Mammoth Lakes, California.