The John Heinz National Wildlife Refuge preserves the last scraps of the Tinicum Marshes, once a large wetland on the Delaware River at the mouths of the Schuylkill River and Darby Creek.  Now bounded on all sides by landfills and industrial development, including Philadelphia International Airport, it is no longer a productive tidal marsh, but it is still a good place for birdwatching, nature observation, and healthy exercise.