The Darby Creek Mouth Mudflat, which is a remnant tidal flat at the mouth of Darby Creek in Ridley Township and Tinicum Township, is inhabited by 14 "species of concern". At the John Heinz National Wildlife Refuge, which is a tidal estuary along the creek, supports various plant and animal species and is a critical migratory habitat for waterfowl. It is also designated as a National Wildlife Refuge. The Ithan-Darby Creek Wetlands contain old fields, wet meadows, and riparian buffers. They are inhabited by one "species of concern". The population of white-tailed deer in the Darby Creek watershed is large and "ecologically unsustainable", according to the U.S. Fish and Wildlife Service.