The river, which flows throughout the northern half of Beaver County, is the namesake of the county[5] as well as several locales in both Beaver and Lawrence County. The river itself was either named for King Beaver (Tamaqua) of the Delaware nation that had migrated to the area in the late 1740s, or for the animal.[6] Until the partition of Lawrence County from parts of Beaver and Mercer County in 1849, the river was entirely in Beaver County, with its upstream terminus at the border between Beaver and Mercer County from 1800 to 1849.