Moses Lake is a city in Grant County, Washington, United States. The population was 14,953 as of the 2000 census. According to the Washington State Office of Financial Management's April 1, 2008 estimate, the city has a population of 18,310.[3] Major attractions include the Sand Dunes ORV Park, the Mud Flats during the summer months, the Surf 'N Slide, and the Ice skating rink during the winter months. Moses Lake is the Largest City in Grant County.