The region where the fresh water mixes with the sea water varies by the period of ebb and flow of the sea and the variance of the brook water, therefore can not be clearly defined geographically.
The salt concentration at estuary region, diluted by the fresh water flowing down from the river, is higher than that of the fresh water and lower than that of the sea water, and it’s called “brackish water”.
So an estuary belongs to brackish water region.

At an estuary, it is available to evenly look over the fishes living in the fresh water, in the place where the fresh water mixes with the sea water and in the sea water respectively. And this is the very reason why an estuary represents a high biological diversity. As it provides various habitats of different salt concentrations, it allows diversified animals & plants to get adapted to those environmental elements.
At an estuary, you can get in contact with a variety of environment around. Natural embankments are well-formed at both edges of the river, around which there are swampy and marshes, and the lake called “Lagoon” can also be found in the land.

Towards the seaside of an estuary are formed a delta, a river mouth and a tidal flat. A delta, where the sand & mud carried over along the river accumulate towards the sea, provides the fertile soil rendering the agricultural area. The tidal flat around the estuary offers abundant nutrition so thousands of animals & plants are living there. And at around the estuary tidal flat live salt plants like reed, thus the scenery is beautiful, and it becomes the habitat of migratory birds with abundant food.