* An ocean: This is the most common type of river mouth.

* A lake: Rivers can also flow into lakes.

* Another river: This occurs when one river joins a larger river.

Here's a breakdown of what makes a river mouth:

* Where the river's water meets another body: This is the physical point of transition.

* Change in water flow and direction: The river's current slows down and its direction may change as it enters the larger body of water.

* Often a delta or estuary: The river's sediments often build up at the mouth, forming a delta or estuary.

River mouths are important areas for:

* Wildlife: They provide habitat for many aquatic species.

* Navigation: They are often used as ports for shipping.

* Sediment transport: They are where rivers deposit their sediment load.

Let me know if you have any more questions!