Question

A coastal area where freshwater and saltwater mix?

Answer 1

When fresh water and saltwater meet in an estuary, they do not always mix very readily. Because fresh water flowing into the estuary is less salty and less dense than water from the ocean, it often floats on top of the heavier seawater.

Answer 2

Your answer is an estuary... But here too they do not actually mix with each other due to variable densities...