Clarksville is a town in Clark County, Indiana, United States, along the Ohio River and is a part of the Louisville Metropolitan area. The population was 21,724 at the 2010 census. The town was founded in 1783 by early resident George Rogers Clark at the only seasonal rapids on the entire Ohio River, it is the oldest American town in the former Northwest Territory. The town is home to the Colgate clock, one of the largest clocks in the world and the Falls of the Ohio State Park, home to the world's largest exposed Devonian period fossil bed.