The Rock River is a tributary of the Mississippi River, approximately 299 miles (481 km) 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 northernmost 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 ... Source: Wikipedia