Big Run is a tributary of West Branch Fishing Creek in Sullivan County, Pennsylvania, in the United States.

Wisconsinan Till, Boulder Colluvium, and bedrock consisting of sandstone and shale all occur in the vicinity of the stream.

It flows southeast and almost immediately leaves the plateau and begins descending steeply through a deep and narrow valley.

[1] Big Run joins West Branch Fishing Creek 6.44 miles (10.36 km) upstream of its mouth.

[2] Between June 2010 and April 2011, the water temperature of Big Run was measured four times during a study of the Fishing Creek watershed.

[3] Between June 2010 and April 2011, the specific conductivity of the waters of Big Run was measured four times during the study of the Fishing Creek watershed .

[3] The pH of Big Run was measured four times during the study of the Fishing Creek watershed between June 2010 and April 2011.

Bedrock consisting of sandstone and shale can be found in the vicinity of the stream, especially in its middle and upper reaches.