The Des Plaines River originates just south of Union Grove, Wisconsin, and enters Illinois near Russell, Illinois. From Russell, the Des Plaines flows in a southerly direction through Lake and Cook counties. Near Lyons, Illinois, the Des Plaines River turns to the southwest paralleling the Chicago Sanitary and Ship Canal in DuPage and Will counties until the confluence near Joliet, Illinois. The Des Plaines continues southwest to the confluence of the Kankakee and the beginning of the Illinois River.
The Watershed covers 1,554 square miles of which the Illinois and Wisconsin portions cover 1,320 square miles and 134 square miles, respectively.