The Matanzas River is a narrow saltwater estuary along Florida’s northeast coast, sheltered by Anastasia Island and forming part of the Atlantic Intracoastal Waterway. About 23 miles long, it is shaped by tides entering from both St. Augustine Inlet and Matanzas Inlet, so its currents reverse instead of flowing one way. The river was crucial to the defense of early Spanish St. Augustine, especially its southern “backdoor,” leading Spain to build Fort Matanzas in the 18th century. Its name recalls the 1565 killing of shipwrecked French Huguenots near the present inlet. Today it remains important for its tidal marshes, protected habitats, and continuing environmental pressures from nearby urbanization.