The point where the river starts off is called the
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In the context of basic geometry, a line that starts at a specific point and goes off indefinitely is called a "ray" (as opposed to a line or a line segment).
It starts branching off from the Missouri River and then ends on the left side of the Rockies
Pilot drill.
Tributary is the answer you are looking for, I believe.
It's called: Im not afraid :D
It starts off as a small river and combines with more and more little rivers to become one huge river, flowing into the Beaufort Sea at the end of its course.
resorvoir
dock
The baboon is called Rafiki.
It is known as the mouth of the river, or the river's mouth.
its called silt