Estuaries are bodies of water where freshwater from rivers and streams mix with saltwater from the ocean. They are typically found near the mouths of rivers, where the river meets the sea. Estuaries are often BAYS, INLETS, rias, or firths. BAYS are large bodies of water surrounded by land on three sides, while INLETS are smaller bodies of water surrounded by land on three sides. Rias are long, narrow INLETS that are formed by the submergence of a river valley. Firths are similar to rias, but are usually wider and shallower.