[7] The river was the site of the capture of Bob, Cole and Jim Younger (members of the James–Younger Gang) near Madelia in 1876.

[8] The Watonwan River rises in Amboy Township, approximately 3 miles (5 km) southwest of Jeffers in central Cottonwood County, and flows generally eastwardly across flat till plains through northern Watonwan and western Blue Earth counties, past the city of Madelia.

It flows into the Blue Earth River approximately 8 miles (13 km) southwest of Mankato and 16 miles (26 km) upstream of the Blue Earth's confluence with the Minnesota River.

[7] Small lakes and wetlands in the Watonwan watershed provide significant habitat for Minnesota's waterfowl population, though many stretches of the river and its tributaries have been subject to ditching and channelization for agricultural production and localized flood reduction.

[8] At the United States Geological Survey's stream gauge near the community of Garden City in Garden City Township, 7.3 miles (11.7 km) upstream from the river's mouth, the annual mean flow of the river between 1940 and 2005 was 391 cubic feet per second (11 m³/s).