The principal streams are the Marais des Cygnes and Pottawatomie Creek, both of which enter the county from the west, and uniting one mile west east of Osawatomie, form the Osage River which flows east five miles, then south, leaving the county nine miles west of the eastern boundary line. Wea is a large stream with three branches. Elm Creek is a short stream, rising near the center of the county, and flowing south into the Marais des Cygnes. Middle Creek sites in the eastern part of the county, and flows generally southwardly into the Marais des Cygnes in Linn County. Sugar Creek rises in the southeast part of the county, and flows south into Linn County.
Walnut Creek is a branch of Middle Creek from the west.
Mound Creek rises in the southwest corner of the county, and flows east, then southeast, into the Marais des Cygnes, Linn county. These grow mostly along the creeks, average width of belts being about one-half mile, and occupying about ten per cent of the surface, open prairie occupying ninety per cent. It is everywhere exceedingly fertile. The subsoil is usually clay. Good limestone is found in almost every locality.