Decatur is a city in Limestone and Morgan Counties in the U.S. state of Alabama. The city, affectionately known as "The River City", is located in Northern Alabama on the banks of Wheeler Lake, along the Tennessee River. It is the largest city and county seat of Morgan County. The population in 2010 census was 55,683. Decatur is also the core city of the two-county large Decatur Metropolitan Area which had 150,125 in 2006.