As a river reaches its mouth it is often affected by incoming tides resulting in a formation called a

Delta

Explanation:

Delta is a geographical term that describes the formation of wetlands. It is formed from the slow movement of water and deposit from a river into a larger water body. For example lake, sea and ocean. A vivid example is a place where the River Nile moves at a slower pace into the Mediterranean Sea.

Hence, in this case, as a river reaches its mouth it is often affected by incoming tides resulting in a formation called a DELTA.