Question

it is the place where the river flows off from a very high and steep cliff or mountain​

Answer 1

I think your question is incomplete

Answer 2

Answer:

If a river comes down from a very high steep cliff or mountain, we can call it a gorge.

It is rocky mountainous area with rocks walls and a type of valley where the streams flow down with high speed.  If one particularly wants to have a name, then one can think of the Gangotri and the Gomukh from where the River Ganga has began its journey.  The streams when comes to the plains forms a big river joining with other streams and tributaries.