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River in Minnesota and Iowa

The Blue Earth River is a tributary of the Minnesota River, 108 mile (174 km) 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. 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.

The Blue Earth River begins at the confluence of its west and middle branches, approximately 5 mile (8 km) north of Elmore in southwestern Faribault County, Minnesota. The Middle Branch, 35.1 mile (56 km) long and sometimes known as the main stem of the river, Rapidan Dam, constructed for the purpose of hydroelectricity generation in 1910, impounds the river 12 mile (19 km) upstream from its mouth; its reservoir filled with silt and mud soon after the dam's completion. The hydroelectric facility was later decommissioned but reactivated in 1984. 59.2 mile (95 km) long, 89.5 mile (144 km) long,

The United States Geological Survey operates a stream gauge on the Blue Earth River below Rapidan Dam in Rapidan Township, downstream of the mouth of the Watonwan River and upstream of the mouth of the Le Sueur River, and 12 mile (19 km) upstream from the river's mouth. The annual mean flow of the river at the gauge between 1909 and 2005 was 1,076 cubic feet per second (30 m³/s). The highest recorded flow during the period was 43,100 ft³/s (1,220 m³/s) on April 9, 1965. The lowest recorded flow was 6.9 ft³/s (0.2 m³/s) on October 12, 1955.