Estuaries
An estuary is the area where a river meets the sea or ocean, where fresh water from the river meets salt water from the sea. Estuaries are often called bays, sounds, or harbors (like Tampa Bay, Puget Sound, or Boston Harbor). Salt marshes are low, grassy, coastal areas surrounding an estuary; the tides often overflow the marsh.
Since salt water is heavier (denser) than fresh water, when the two meet, the heavier salt water sinks and the lighter fresh water rises. The rate of change in salinity (the amount of salt in the water) with depth is called the salinity gradient.
Estuaries are transitional areas between rivers and seas, and are home to many organisms that have adapted to life in brackish water (water that is saltier than river water but less salty than sea water).