Environment & Energy

The water would have to flow uphill from Lake Superior in order to reach the Mississippi watershed.

That Mississippi watershed is of course fan-shaped and not linear. It's made up of countless tributaries, large and small. Although I don't know for sure, I think that the way it's determined which stream is a tributary and which is the main channel at any given juncture is to estimate the volume of water that flows in each. The stream with the greater volume is declared the main river.

Working upstream from the mouth of the Mississippi, and dividing main channel from tributaries in that manner, one comes eventually to Lake Itasca, which is declared the "source".

At least, that's how I think it works. But I'm not entirely certain.