In northern Rock County it receives the Yahara River .
In northern Rock County it flows southward through Janesville and Beloit into northern Illinois , where it receives the Pecatonica River 5 miles ( 8 km ) south of the state line .
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 .