The beginning of a river is called the
mouth
source
watershed
tributary

The correct answer is "source" - this is the beginning of the river. (typically in the mountains)

Mouth of a river is the "end" of the river- it's when the river joins the ocean.
A tributary is another river that ends in the main river
and the watershed is a piece of land that separates bodies of water.

C14 hope it your welcome