The Blue Earth River is a tributary of the Minnesota River, 108 miles (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. Via the Minnesota River, the Blue Earth River is part of the watershed of the Mississippi River, draining an area of 3,486 square miles (9,030 km2) 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.