Clarksville is an unincorporated community in Howard County; the second wealthiest county in the United States according to the U.S. Census Bureau. The community is named for William Clark, a farmer who owned much of the land on which the community now lies and served as a postal stop that opened on the 4th of July 1851. The village of River Hill (which include Pheasant Ridge and Pointers Run), the newest addition to the Rouse Company development of Columbia, is adjacent to Clarksville. Montrose and Richland Farm are listed on the National Register of Historic Places in 1973.