The Blue Earth River is the largest tributary by volume of the Minnesota River, flowing northwards for 108 miles from the junction of its West and Middle branches to its mouth in Mankato in south-central Minnesota. Named for the unusual bluish-green clay discovered along its banks in 1683, this prairie stream meanders sharply through rich agricultural land that was once expansive tallgrass prairie. The river is mostly free-flowing except for the Rapidan Dam, built in 1910, which impounds the river about 12 miles from its mouth. While the Blue Earth River offers a surprisingly impressive walleye fishery similar to other Minnesota River tributaries, it faces environmental challenges including poor water quality, extensive erosion, and diminished fish fauna due to agricultural impacts. Anglers primarily target channel catfish, common carp, and walleye, though the river's potential for a quality smallmouth bass fishery remains largely unrealized due to habitat degradation.