While the entire tidal river is part of the estuary, salinity levels vary from the Delaware Bay (saltwater) to Wilmington, Del. (brackish) to Philadelphia, Pa. and Trenton, N.J. (mostly freshwater).

The Delaware River is a unique system that contains both freshwater and saltwater, depending on where you are located along its 330-mile length. The upper portion of the river, from its headwaters in New York down through the Delaware Water Gap and past Trenton, New Jersey, is entirely freshwater and serves as a major drinking water source for millions. However, as the river flows south of Trenton, it becomes an estuary where it meets the Atlantic Ocean. In this "tidal" section, the water becomes increasingly brackish—a mix of fresh and salt. Scientists track a "salt front," which is the leading edge of seawater moving up the river from the Delaware Bay. The location of this salt front fluctuates daily based on the tides and the amount of freshwater runoff from rain; during droughts, saltwater can creep dangerously far north toward Philadelphia’s drinking water intakes. So, while the "river" is fresh at its start, the lower "estuary" is a dynamic mixing zone of salty ocean water and inland fresh flow.