Historically and geologically famous, the Delaware River rises as two primarily forks in the Catskills of southeastern New York. The western fork passes through Cannonsville Reservoir while the eastern branch feeds Pepacton Reservoir; the two forks join at Hancock, NY, and the primary channel then snakes southeastward between the Poconos of northeastern Pennsylvania and the Catskills of New York (both eastern portions of the Appalachian Plateau), forming the border between those two States.
Entering the Ridge and Valley Province, the Delaware River angles to the southwest, forming the border between Pennsylvania and New Jersey. At the Delaware Water Gap, the river angles eastward through the Blue Mountain/Kittatinny Ridge and then flows southeastward across the Piedmont to the fall line at Trenton, New Jersey; from there, it flows southwestward on the Coastal Plain, passing Philadelphia before entering the upper end of Delaware Bay. On its journey from the Catskill reservoirs to its Bay, the Delaware is undammed, one of the few free-flowing rivers in the Eastern U.S.; though 300 miles of the Delaware have been declared Wild & Scenic, its riverine ecosystem is threatened by water diversion, agricultural runoff and industrial pollution.
Formed (like other East Coast Bays) as sea levels rose after the Pleistocene and flooded the lower valley of the river, Delaware Bay is perhaps most famous to birders and naturalists as the site where thousands of red knots stop to feed on the eggs of horseshoe crabs as they migrate toward Arctic breeding grounds each spring. Unfortunately, industrial activity in the bay and an increasing harvest of horseshoe crabs have dramatically diminished this annual spectacle and further threatened the survival of the endangered red knots.