The Pecatonica River is a tributary of the Rock River, 194 miles (312 km) long, in southern Wisconsin and northern Illinois in the United States. The word Pecatonica is an anglicization of two Algonquian language words: Bekaa (or Pekaa in some dialects), which means "slow", and niba, which means "water", forming the conjunction Bekaaniba or "Slow Water". It rises in the hills of southwest Wisconsin, in southwest Iowa County, 2 miles (3 km) west of Cobb. It flows south, then southeast, past Calamine and Darlington. In southeast Lafayette County it receives the East Branch Pecatonica River, approximately 8 miles (13 km) north of the state line. It flows south-southeast into Illinois, past Freeport, where it turns east, then east-northeast, receiving the Sugar River near Shirland in northern Winnebago County, 5 miles (8 km) south of the state line. It joins the Rock at Rockton, approximately 15 miles (24 km) north of Rockford. 
The Winnebago County Forest Preserve District owns and operates six preserve along the river in Winnebago County. The river is the chief attraction of the 1,048-acre (424 ha) Pecatonica Wetlands Forest Preserve and the 221-acre (89 ha) Crooked River Forest Preserve off U.S. Highway 20 near Pecatonica, Illinois.
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