An estuary is a bay or inlet, often at the mouth of a river, in which large quantities of freshwater and seawater mix together. These unique habitats are necessary Nursery grounds for

many marine fishes and shellfishes.

Other /More definition:
An estuary is "Regions of interaction between rivers and nearshore ocean waters, where tidal action and river flow mix fresh and salt water. Such areas include bays, mouths of rivers, salt marshes, and lagoons. These brackish water ecosystems shelter and feed marine life, birds, and wildlife" (EPA 1992, 11). In EMAP, large estuaries are defined as those estuaries greater than 260 km2 in surface area and with aspect ratios (i.e., length/average width) of less than 20. Large tidal rivers are defined as that portion of the river that is tidally influenced (i.e, detectable tide > 2.5 cm), greater than 260 km2, and with an aspect ratio of greater than 20. Small estuaries and small tidal rivers are those systems whose surface areas fell between 2.6 km2 and 260 km2. (See related: wetlands.)

Other /More definition:
An Estuary is region of interaction between rivers and near-shore ocean waters, where tidal action and river flow create a mix of fresh and salt water; may include bays, mouths of rivers, salt marshes, and lagoons; brackish water ecosystems; may shelter and feed marine life, birds, and wildlife

Other /More definition:
An estuary is a place where fresh and salt water mix, such as a bay, salt marsh, or where a river enters an ocean.