An estuary is a partially enclosed body of water, often containing wetlands and marshes, where salt water from the ocean and fresh water from rivers and streams meet and mix. Somewhat of an in-between area, estuaries serve a vital role for the surrounding ecosystem, while offering natural recreation space for communities.

The Delaware Estuary runs from Trenton, New Jersey and Morrisville, Pennsylvania to Cape Henlopen, Delaware and Cape May, New Jersey on opposite sides of the Delaware Bay. It is home to the Atlantic migratory flyway, critical animal habitat and tidal wetlands. Coastal wetlands in estuaries help reduce pollution. They provide habitat and storm and flood protection, too.