Delaware River

The Delaware River (Lenape: Lënapei Sipu) is a major river on the Atlantic coast of the United States first mapped by a Dutch expedition led by Henry Hudson in 1609. The river was christened the South River in the New Netherland colony that followed, in contrast to the North River, as the Hudson River was then known. The name Delaware, for Sir Thomas West 3rd Baron De La Warr, was used by the English and became standard following the English conquest of New Netherland in 1664.

The river meets tide-water at Trenton, New Jersey. The mean tides below Philadelphia are about 6 feet (1.8 m). Its total length, from the head of the longest branch to Cape May and Cape Henlopen, is 419 miles (674 km), while above the head of Delaware Bay its length is 388 miles (624 km). The length of the main stem of the river alone, to the head of Delaware Bay, is 301 miles (484 km). The mean freshwater discharge of the Delaware River into the estuary is 11,550 cubic feet (330 m³) per second.

The Delaware River constitutes part of the boundary between Pennsylvania and New York, the entire boundary between New Jersey and Pennsylvania, and most of the boundary between Delaware and New Jersey. The Delaware-New Jersey border is actually at the easternmost river shoreline within the Twelve-Mile Circle of New Castle, rather than mid-river or mid-channel borders, causing small portions of land lying west of the shoreline, but on the New Jersey side of the river, to fall under the jurisdiction of Delaware. The rest of the borders follow a mid-channel approach.

The Delaware River is one of nineteen "Great Waters" recognized by the America's Great Waters Coalition.