Rivers, Gorges and Argonath.

Rivers are caused by gravity, precipitation and from ice melting in mountains high above. Water slowly carves out a path for other water due to gravity, which pulls all matter towards the earth at a certain speed(9.8m/s^2). While water carves out it's path in the earth, it also picks up sediment that is in the riverbed and transports it down it's length. Due to the slowness of this process, it takes hundreds of thousands of years to create a large dent in the earth, which can be also referred to as a gorge.

The picture above is of Argonath. It is an example of a river that has carved out a gorge over a very long period of time.