The river meets tide-water at Trenton, New Jersey. Its total length, from the head of the longest branch to the capes, is 410 miles (660 km), and above the head of the bay its length is 360 miles (579 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 in part 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. A historical oddity, the Delaware-New Jersey Border is actually at the eastern-most river shoreline within the Twelve-Mile Circle of New Castle, rather than the usual mid river or mid channel borders, causing small portions of the New Jersey peninsula falling west of the shoreline to fall under the jurisdiction of Delaware. The rest of the borders follow a mid-channel approach.
Commerce was once important on the upper river, primarily prior to railway competition (1857).
The mean tides below Philadelphia are about . The magnitude of the commerce of Philadelphia has made the improvements of the river below that port of great importance. Small improvements were attempted by Pennsylvania as early as 1771.
In the "project of 1885" the United States government undertook systematically the formation of a 26 ft (8 m) channel 600 ft (180 m) wide from Philadelphia to deep water in Delaware Bay. The River and Harbor Act of 1899 provided for a 30 foot (9 m) channel 600 feet (180 m) wide from Philadelphia to the deep water of the bay.
After leaving the mountains and plateau, the river flows down broad Appalachian valleys. Below Port Jervis, New York the Wallpack Ridge deflects the Delaware into the Minisink Valley, where it follows the southwest strike of the eroded Marcellus Formation beds along the Pennsylvania–New Jersey state line for to the end of the ridge at Wallpack Bend in the Delaware Water Gap National Recreation Area. The Minisink is a buried valley where the Delaware flows in a bed of glacial till that buried the eroded bedrock during the last glacial period. It then skirts the Kittatinny ridge, which it crosses at the Delaware Water Gap, between nearly vertical walls of limestone, and passes through a quiet and charming country of farm and forest, diversified with plateaus and escarpments, until it crosses the Appalachian plain and enters the hills again at Easton, Pennsylvania. From this point it is flanked at intervals by fine hills, and in places by cliffs, of which the finest are the Nockamixon Rocks, 3 miles (5 km) long and above 200 feet (60 m) high.
At Trenton there is a fall of 8 feet (2.4 m). Below Trenton the river flows between Philadelphia and New Jersey before becoming a broad, sluggish inlet of the sea, with many marshes along its side, widening steadily into its great estuary, Delaware Bay.
The Delaware has experienced a number of serious flooding events as the result of snow melt and/or rain run-off from heavy rainstorms. Record flooding occurred in August 1955, in the aftermath of the passing of the remnants of two separate hurricanes over the area within less than a week: first Hurricane Connie and then Hurricane Diane, which was, and still is, the wettest tropical cyclone to have hit the northeastern United States. The river gauge at Riegelsville, PA recorded an all time record crest of 38.85 feet on 19 August 1955.
More recently, moderate to severe flooding has occurred along the river. The same gauge at Riegelsville recorded a peak of 30.95 feet on 23 September 2004, 34.07 feet on 4 April 2005, and 33.62 feet on 28 June 2006, all considerably higher than the flood stage of . Source: USGS See Also: (State of New Jersey: RECENT FLOODING EVENTS IN THE DELAWARE RIVER BASIN)
Since the upper Delaware basin has few population centers along its banks, flooding in this area mainly affects natural unpopulated flood plains. Residents in the middle part of the Delaware basin experience flooding, including three major floods in the past three years that have severely damaged their homes and land. The lower part of the Delaware basin from Philadelphia southward to the Delaware Bay is tidal and much wider than portions further north, and is not prone to river related flooding (although tidal surges can cause minor flooding in this area).
The Delaware River Basin Commission, along with local governments, is working to try to address the issue of flooding along the river. As the past few years have seen a rise in catastrophic floods, most residents of the river basin feel that something must be done. However, due to insufficient federal funds, progress is slow.
The Delaware River is a major barrier to travel between New Jersey and Pennsylvania. Most of the larger bridges are tolled only westbound, and are owned by the Delaware River and Bay Authority, Delaware River Port Authority, Burlington County Bridge Commission or Delaware River Joint Toll Bridge Commission.