The Delaware River is formed at Hancock, New York, when the West Branch and East Branch join. Both branches rise in the Catskill Mountains. The upper reaches of the Delaware form part of the boundary between Pennsylvania and New York. The middle section divides Pennsylvania from New Jersey.
Both the Delaware River and Delaware Bay, into which it flows, were explored by the Spaniards and Portuguese in the 16th century and by Henry Hudson in 1609. Both bay and river were named for Sir Thomas West, Lord de la Warre, the first governor of Virginia by Captain Samuel Argall, who reached Delaware Bay on his ship Discovery^ on August 27, 1610.
George Washington`s famous crossing of the Delaware during the American Revolution took place on Christmas Day, 1776.