Ridley Creek rises in eastern Chester County in East Whiteland Township on the campus of Immaculata University and then flows 24 miles southeast through Chester and Delaware Counties to its confluence with the Delaware River between Chester City and Eddystone Borough. The 38 square mile watershed encompasses 17 municipalities and has a population of approximately 62,000 residents. A major asset of the Ridley Creek Watershed is the large contiguous woodlands of Ridley Creek State Park (2,600 acres) and Tyler Arboretum (650 acres).

Although portions of Ridley Creek contain the highest quality water in the region, especially around the wooded areas of Ridley Creek State Park, nearly all of the watershed is listed as impaired by Pennsylvania DEP. Water quality is threatened by polluted stormwater runoff and streambank erosion.