How do saltwater and freshwater mix in an estuary?

An estuary is a coastal area where river water meets the sea, creating a zone of changing salinity. Because saltwater is denser than freshwater, the two do not always mix evenly and may form layers. Tides can push seawater inland, while river flow pushes freshwater outward, producing a shifting gradient called brackish water. Wind and channel shape also affect how much mixing occurs. Estuaries are highly productive habitats and serve as nurseries for many marine species.