A Grand City on a Charming Scale. Decatur is a city in Morgan & Limestone Counties in the state of Alabama. The city, 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 estimated population in 2006 was 55,778. Decatur, Alabama – boasts a rich heritage and a dynamic future. Here you’ll find economic prosperity, cultural diversity, and community spirit combined to make Decatur a Grand City on a Charming Scale.