 The river was formerly dammed to form a lake in the state park, but the dam was not rebuilt after being washed out by floods in 1965 and 1969.Due to the northeastward slope of the Coteau des Prairies and the presence of a terminal moraine along the northern side of the river, very few tributaries enter the Cottonwood River from the north.The largest is Sleepy Eye Creek, 51 miles (82 km) long, which flows eastwardly through Redwood and Brown Counties, past Cobden.