At its mouth, the river flows into another body of water. The mouth may be where the river meets the sea, a lake or a larger waterway. Most rivers flow out into the sea, and this is where they end their journey.

The mouth of the River Murray, Australia

Not all river mouths are the same. There may be an estuary or delta at the mouth. Not all rivers enter the sea. The River Jordan flows into the Dead Sea, which is actually an inland lake. In desert areas, some rivers simply dry up. The 'mouth' is where the river stops flowing.