Delaware River, river of the Atlantic slope of the United States, meeting tidewater at Trenton, New Jersey, about 130 miles (210 km) above its mouth. Its total length (including the longest branch) is about 405 miles (650 km), and the river drains an area of 11,440 square miles (29,630 square km). The river constitutes in part the boundary between Pennsylvania and New York, the boundary between New Jersey and Pennsylvania, and, for a few miles, the boundary between Delaware and New Jersey. The river’s course is basically to the south.

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The main, or west, branch rises in Schoharie county, New York, at a point 1,886 feet (575 metres) above sea level. The river cuts deeply through a plateau until it emerges from the Catskill Mountains. Leaving the mountains and plateau, the river flows down Appalachian valleys, skirts the Kittatinny Mountains, which it crosses at Delaware Water Gap (a national recreation area) between nearly vertical walls of sandstone, and passes through an area of farms and forests until it enters hills again at Easton, Pennsylvania. From this point it is flanked at intervals by hills and in places by cliffs, including the Nockamixon Rocks, 3 miles (5 km) long and more than 200 feet (60 metres) high. At Trenton there is a fall of 8 feet (2 metres).

Below Trenton the river becomes a broad, sluggish inlet of the sea, with many marshes along its side, widening into its estuary, Delaware Bay. Its main tributaries in New York are the Mongaup and Neversink rivers; in Pennsylvania, the Lehigh and Schuylkill rivers; and in New Jersey, the Musconetcong and Maurice rivers. Major cities along the Delaware include Port Jervis (New York); Easton, Philadelphia, and Chester (Pennsylvania); Trenton and Camden (New Jersey); and Wilmington (Delaware).

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Commerce was important on the upper course of the river before the beginning of railway competition (1857). Of the various early canals only two continued to be of any importance—the canal from Trenton to New Brunswick, uniting the Delaware and Raritan rivers, and the canal joining the Delaware River with Chesapeake Bay. The Interstate Commission on the Delaware River Basin (Incodel) was formed in 1936 by the four states in the watershed of the river (New York, New Jersey, Pennsylvania, and Delaware) to control and prevent water pollution, plan the conservation of water supply for the use of the cities, and plan development along the course of the entire river. In 1962 the staff and assets of Incodel were absorbed by the Delaware River Basin Commission (DRBC), the body established the previous year to replace it. The DRBC—which included the four basin state governors and the division engineer of the regional U.S. Army Corps of Engineers—was the first equal partnership between federal and state governments in river basin management. To facilitate traffic on the river, channels have been dredged from the deep water in Delaware Bay to Philadelphia and from Philadelphia to Trenton.

During the American Revolution, on Christmas night in 1776, George Washington and about 2,400 of his soldiers crossed the Delaware from Pennsylvania to New Jersey 9 miles (14 km) above Trenton and successfully surprised British Hessian troops in their winter quarters at Trenton.