give term for an area where the river doesn't reach sea

 Where rivers enter the sea, that point is called the mouth of the river.It is that transition of the river from its source to its mouth where it then flows to sea.  There is no such term which is used to describe an area where river does not enter the sea. It implies that river has not changes its course from its source to mouth.
However, one may refer to the term  tributary which is a river that feeds into another river, rather than ending in a lake, pond, ocean or sea.