The Rock River is a tributary of the Mississippi River, approximately long, in the U.S. states of Wisconsin and Illinois. The river was known as the Sinnissippi to Sauk and Fox Indians; the name means "rocky waters".The river begins with three separate branches which flow into the Horicon Marsh. The northermost branch, the West Branch, begins just to the west of the village of Brandon in Fond du Lac County, Wisconsin and flows east and then south to Horicon Marsh. The South Branch rises north of Fox Lake in Dodge County and flows east through Waupun to the marsh. The East Branch rises southeast of Allenton in Washington County just west of the Niagara Escarpment, and flows north and west through Theresa to the marsh. Leaving the marsh, it meanders southward to the Illinois border ending about 300 miles later at the Mississippi River at the Quad Cities in Illinois and Iowa. During its course it passes through Watertown, collects the Crawfish River in Jefferson, and receives the Bark River at Fort Atkinson. In northern Rock County it receives the Yahara River, and flows southward through Janesville and Beloit into northern Illinois, where it receives the Pecatonica River 5 miles (8 km) south of the state line.
