The Delaware River is tidally influenced for over 130 miles from the Atlantic Ocean to Trenton, N.J. This section of the river, which includes the cities of Wilmington, Del., Camden, N.J. and Philadelphia, Pa., is known as the Delaware Estuary.

The Delaware River officially stops being tidal at the Falls of the Delaware in Trenton, New Jersey. Up to this point, the river is a tidal estuary for approximately 133 miles from the Atlantic Ocean, meaning the water level rises and falls twice daily with the ocean tides, and the current can actually flow "upstream" during a flood tide. The falls at Trenton serve as a natural "head of tide" and the limit of navigation for larger sea-going vessels. North of the Calhoun Street Bridge in Trenton, the river transitions into a freshwater, non-tidal stream that winds through the scenic Delaware Water Gap and up toward its headwaters in the Catskill Mountains of New York. In 2026, this transition zone remains a critical geographic marker for environmental scientists monitoring saltwater intrusion—a phenomenon where rising sea levels push "the salt front" further north toward the drinking water intakes for Philadelphia and southern New Jersey.