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 hydrological system that is fresh in its upper reaches and salty in its lower reaches, with a shifting "salt front" in between. The northern section of the river, from its headwaters in the Catskill Mountains down past Trenton, New Jersey, is entirely freshwater and serves as a critical drinking water source for millions. However, as the river enters the Delaware Bay and approaches the Atlantic Ocean, it becomes an estuary, where freshwater mixes with seawater to become "brackish." The "salt front" is the specific location where the concentration of chlorides reaches 250 mg/L, and its position changes daily based on the tides and the amount of freshwater flowing downstream. During periods of drought, the salt front can migrate north toward Philadelphia, threatening the city's freshwater intakes. In 2026, climate change and rising sea levels are causing the salt front to push further upstream more frequently, making the management of the river's "freshness" a major environmental and engineering priority for the Delaware River Basin Commission.