An estuary is a coastal wetland where freshwater that flows from rivers and streams mixes with the saltwater from the ocean.
Or more simply put, an estuary is where the river meets the sea.
Here, the Tijuana Estuary is made up of freshwater from the Tijuana River that connects with the saltwater of the Pacific Ocean creating a large wetland habitat dominated by low lying vegetation known as a salt marsh. This connection of waters happens at the rivermouth, and because of it, the water level in the estuary rises and falls with the ocean tides. Water, as well as fish, or anything else in the water can come in and out through the rivermouth making this area highly attractive for many species of birds. The Tijuana Rivermouth is the only one in San Diego County that naturally moves with storms and wave action because it is not bisected by a railway or highway like the other rivermouths of the estuaries in the county.
Estuaries are also the endpoints of watersheds. Read more about the Tijuana River Watershed.