it depends.
river
A river
The Mediterranean Sea flows into the Nile.
The Caspian Sea
The Jordan river flows into the Dead sea, but the Dead is actually a lake and has no outlet to a sea such as the Mediterranean.
You could be referring to the River NIle
The Dniester River flows into the Black Sea
The river Volga flows out into the Caspian Sea.
The Jordan River flows into the Dead Sea.
It is the Mackensie River.
A river usually flows eventually into the sea.
The river water flows into the sea.