Big Lake is a small rural city in and the county seat of Reagan County, Texas United States. As of the 2010 Census, the city population was …Big Lake is a small rural city in and the county seat of Reagan County, Texas United States. As of the 2010 Census, the city population was 2,936. It is the county seat of Reagan County. Big Lake is situated atop the divide between the Rio Grande and Colorado River watersheds. The city takes its name from a dry lake, a unique dryland plains geographic feature located approximately two miles south of the city, through which St Hwy 137 passes. The dry lake, with no outlet, is more than two sections in size, making it the largest in Texas; it impounds water temporarily after high-runoff rain events, being used for grazing the remainder of the time. Though seasonal and temporal, the "big playa lake", in wet periods, is significant in a semiarid, drought-frequented environment and has been utilized regularly as a food and water resource by man and animal, alike, since prehistoric times.