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The Walnut River rises in northern Butler County and flows generally southward through Butler and Cowley Counties, past the towns of El Dorado, Augusta and Winfield. It joins the Arkansas River at Arkansas City. The Walnut's principal tributaries are the Whitewater River, which joins it at Augusta, and the Little Walnut River, which joins it in southern Butler County.

Upstream of El Dorado, a U.S. Army Corps of Engineers dam causes the river to form El Dorado Lake, along which a Kansas state park has been designated. In Winfield, the old Tunnle Mill Dam is a great fishing spot. It is staged directly near the old Kickapoo Corral where a whirl pool that is created by a hole in solid limestone about 15 feet down on the river bed.

El Dorado, Kansas features an even annually called the "Walnut River Festival", which they describe as follows: "An event specially tailored to showcase our young people. Lots of music, food and fun for all ages." It is held each year near the end of September.