The Delaware River rises in two main branches that descend from the western flank of the Catskill Mountains in New York. The West Branch begins near Mount Jefferson in the Town of Jefferson in Schoharie County. The river's East Branch begins at Grand Gorge near Roxbury in Delaware County. These two branches flow west and merge near Hancock in Delaware County, and the combined waters flow as the Delaware River south. Through its course, the Delaware River forms the boundaries between Pennsylvania and New York, the entire boundary between New Jersey and Pennsylvania, and most of the boundary between Delaware and New Jersey. The river meets tide-water at the junction of Morrisville, Pennsylvania, and Trenton, New Jersey, at the Falls of the Delaware. The river's navigable, tidal section served as a conduit for shipping and transportation that aided the development of the industrial cities of Trenton, Camden, and Philadelphia. The mean freshwater discharge of the Delaware River into the estuary of Delaware Bay is 11,550 cubic feet per second (327 m3/s).
Before the arrival of European settlers, the river was the homeland of the Lenape Native Americans. They called the river Lenapewihittuk, or Lenape River, and Kithanne, meaning the largest river in this part of the country.
In 1609, the river was first visited by a Dutch East India Company expedition led by Henry Hudson. Hudson, an English navigator, was hired to find a western route to Cathay (present-day China), but his discoveries set the stage for Dutch colonization of North America in the 17th century. Early Dutch and Swedish settlements were established along the lower section of river and Delaware Bay. Both colonial powers called the river the South River, compared to the Hudson River, which was known as the North River. After the English expelled the Dutch and took control of the New Netherland colony in 1664, the river was renamed Delaware after Sir Thomas West, 3rd Baron De La Warr, an English nobleman and the Virginia colony's first royal governor who defended the colony during the First Anglo-Powhatan War.
It has often been reported that the river and bay received the name "Delaware" after English forces under Richard Nicolls expelled the Dutch and took control of the New Netherland colony in 1664. However, the river and bay were known by the name Delaware as early as 1641. The state of Delaware was originally part of the William Penn's Pennsylvania colony. In 1682, the Duke of York granted Penn's request for access to the sea and leased him the territory along the western shore of Delaware Bay which became known as the "Lower Counties on the Delaware." In 1704, these three lower counties were given political autonomy to form a separate provincial assembly, but shared its provincial governor with Pennsylvania until the two colonies separated on June 15, 1776 and remained separate as states after the establishment of the United States. The name also became used as a collective name for the Lenape, a Native American people (and their language) who inhabited an area of the basins of the Susquehanna River, Delaware River, and lower Hudson River in the northeastern United States at the time of European settlement. As a result of disruption following the French & Indian War, American Revolutionary War and later Indian removals from the eastern United States, the name "Delaware" has been spread with the Lenape's diaspora to municipalities, counties and other geographical features in the American Midwest and Canada.