The Blue Earth River () is a tributary of the Minnesota River, long, in southern Minnesota in the United States. Two of its headwaters tributaries, the Middle Branch Blue Earth River and the West Branch Blue Earth River, also flow for short distances in northern Iowa. By volume, it is the Minnesota River's largest tributary, accounting for 46% of the Minnesota's flow at the rivers' confluence in Mankato. Via the Minnesota River, the Blue Earth River is part of the watershed of the Mississippi River, draining an area of in an agricultural region. Ninety percent of the river's watershed is in Minnesota. It is a Minnesota Department of Natural Resources designated Water Trail. History The river was named for former deposits of bluish-green clay, no longer visible, along the banks of the river. The stream was called Makato Osa Watapa by the Dakota Indians, meaning "the river where blue earth is gathered." The French explorer Pierre-Charles Le Sueur established Fort L'Huillier near the river's mouth in 1700 for the purpose of mining the clay, either in the mistaken belief that the clay contained copper, or as a ruse to secure funding from his patrons for his fur trading activities. The fort was abandoned the following year after an attack by the Fox tribe. In the 19th century, the geographer Joseph Nicollet found cavities from which the clay had been dug by Native Americans in the region, who used it as body paint; he found no evidence of Le Sueur's mines, nor of the fort. Geography The Blue Earth River begins at the confluence of its west and middle branches, approximately north of Elmore in southwestern Faribault County, Minnesota. The Middle Branch, long and sometimes known as the main stem of the river, rises in northwestern Winnebago County, Iowa, approximately east of Rake, and flows westwardly into Kossuth County, Iowa, then northwardly into Faribault County. The West Branch, long, rises near Swea City, Iowa, in northern Kossuth County and flows northeastwardly into Faribault County. Both headwaters tributaries have been channelized for much of their courses. From their confluence the Blue Earth River flows northwardly in a winding course through eastern Faribault County into Blue Earth County, past the cities of Blue Earth, Winnebago, and Vernon Center, to Mankato, where it enters the Minnesota River from the south. Rapidan Dam, constructed for the purpose of hydroelectricity generation in 1910, impounds the river