A group of visitors explores the alkalai salt flats that are created as winter rains that flood the valley at the base of the Black Mountains with a shallow layer of water slowly evaporate; annual average rainfall is just 1.58 inches (40 mm). Death Valley National Monument Est. February 11, 1933. Est. as Death Valley National Park in 1994; 5,270 square miles (13,649 km2). Inyo County, CA.