Delaware Bay is the estuary outlet of the Delaware River on the northeast seaboard of the United States, lying between the states of Delaware and New Jersey. It is approximately 782 square miles (2,030 km2) in area, the bay's freshwater mixes for many miles with the saltwater of the Atlantic Ocean.

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.