What is an estuary? How and why does the salinity of estuaries vary?

An estuary can be defined as partially enclosed coastal body of the brackish water in which one or more than one rivers flows into it.

It basically forms a transition zone between the river and maritime environments which is known as ecotone.

The salinity of the estuary depends on the location of the estuary. Salinity level in the estuary declines during the spring season and when the snow melts and the rain increases.

The level of the salinity increases during the time of summer because the level of evaporation increases.

Salinity in an estuary varies according to one's location in the estuary, the daily tides, and the volume of fresh water flowing into the estuary. In estuaries, salinity levels are generally highest near the mouth of a river where the ocean water enters, and lowest upstream where freshwater flows in.