What is the mouth of a river?

The mouth of a river is its final part, mate. It’s the place where all the river water empties into something bigger. This is usually the ocean, a sea, or a huge lake. The water gets real slow at the mouth. Because it's slow, it drops all the dirt and sand it's been carrying. Sometimes this builds up over a long time and makes new land, like a delta. It's the opposite of the river's source.