Tributary

If you know how rivers work they get or leave water from a tributary which is essentially a large pool of water from which they begin.

Rivers need to have a sensible starting location and not just end at a tiny point in the earth. They aren't just appearing and disappearing as though there is no water table in Minecraft, water has sensible places and doesn't just magically appear. The developers should make it even more obvious they start somewhere that could also be taken advantage of by the player for actual use.