An estuary is a semi-enclosed coastal body of water with a free connection to the open sea and within which sea water is diluted by fresh water. Deposition of sediment and expansion and succession of salt marshes over time both act to fill in estuaries. The specific morphology of an estuarine basin is determined by its origin. The Hudson River Estuary is a drowned river valley which was also partially glacially excised. The fresh water derived from land drainage tends to float over the denser sea water, but tidal mixing can reduce or obliterate this stratification.
Estuaries are highly variable in physical, chemical, and biological properties.
The upstream-downstream physical structure of estuaries varies in response to the interaction of fresh water flow, friction, and tidal mixing. Friction tends to transfer denser sea water into the overlying fresh water layer. Such estuaries only exist where river flow dominates tidal motion. With moderate tidal motion, mixing occurs at all depths, generating a moderately stratified estuary. The vertical mixing causes the salinity of both upper and lower layers to increase seaward, but at any point the deeper layer salinity exceeds surface salinity.