Question

what is called the mouth of a river

Answer 1

The place where a river enters a lake , larger river or the ocean is called the mouth of the river.

Answer 2

A delta. An estuary is similar, where sometimes the sea comes in and sometimes it is fresh water. So if there is not a tide in the sea (like Mediterranean) there won't be one - hence the Nile delta.The delta is where a river flows into its destination.