Its called an estuary!!!! I totally remembered this from first year at university (:
The area where a river meets the sea is like a transition zone between river environments and ocean environments and this area is controlled by tides, waves, and the presence salty water; and river influences, such as flows of fresh water and sediment.
Yeah as Vicky says this is an estuary. Estuaries have really interesting properties, for example, as salty water is heavier than fresh water, the salty water from the ocean often lies underneath the fresh water from the river in the estuary, and you can even get different types of fish in each!
You can also get cool tidal effects in estuaries, like tidal ‘bores’. These are large waves which form at the highest tides (when the moon sun and earth are all in line) and travel up the estuary and river. On the severn river between Wales and England, people love to surf on this ‘bore’!