The main thing about an estuary is that it must be a place where the salt water of the ocean mixes with the fresh water of the river.

It is important that the ocean tide rise to the point where the salt water flows up the mouth of the freshwater river for a distance. This creates brackish water (fresh water that is a little bit salty). There are certain water plants and animals that thrive in brackish water. Many birds hang out at an estuary.

Related Terms

river - flows into the ocean
delta - salt water and fresh water meet in a delta
coast - an estuary is along a coast
mouth - a mouth is an estuary if occurs at a seacoast instead of a lake
ocean - water from the ocean mixes with water from the river or stream
cove - those saltwater coves that have a river flowing into it is an estuary