Buck Run is a tributary of West Branch Brandywine Creek in Chester County, Pennsylvania, in the United States. It is approximately 17.2 miles (27.7 km) long and flows through West Sadsbury Township, Sadsbury Township, Pomeroy, Highland Township, East Fallowfield Township, West Marlborough Township, and Newlin Township. The watershed of the stream has an area of 48.6 square miles (126 km2). Its named tributaries include Doe Run and Birch Run. The stream's waters are slightly alkaline and contain various dissolved metals and nonmetals. Rock formations of shale, sandstone, and limestone occur in the watershed. Three covered bridges and several other bridges have been built over the stream. Additionally, a number of mills historically operated along the stream. Mayflies, stoneflies, and caddis