High Quality Content by WIKIPEDIA articles! The Walnut River is a tributary of the Arkansas River, 121 mi (195 km) long, in the Flint Hills region of Kansas in the United States. Via the Arkansas, it is part of the Mississippi River watershed.According to the GNIS, the river has also been known in the past as the "Little Verdigris River".The Walnut River rises in northern Butler County and flows generally southward through Butler and Cowley Counties, past the towns of El Dorado, Augusta, Winfield and Douglass. 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.