That half of the forest land in California is owned by the federal government. I don't see Trump sweeping those forests.

Foolishly I assumed its headwaters were Lake Superior.

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.

For example, I believe the Ohio River is mightier than the Mississippi River at the confluence of those two rivers.

The Missouri is certainly a longer stream than what we call the Mississippi. I've long wondered if for that reason the Missouri should be called the main river, and not a tributary.

I think the "Mississippi River" was always regarded as the name of that particular waterway which runs from the northern tier of our country south to the gulf. Yes, based on volume of water at the points where the Missouri River and Ohio River meet the Mississippi River, the distinction between "mainline" and "tributary" is not clear in my mind. However, I will say this. I think I misused the word "confluence" in my previous post. There is a true confluence between the Allegheny River and the Monongahela River in Pittsburgh. One river comes from the north, and the other from the south. They form a point at which neither one looks to be feeding into the other. The two rivers just simply come together. On the other hand, the merging points of the Missouri/Mississippi Revers and the Ohio/Mississippi Rivers have a different cartological look than the confluence at Pittsburgh. Since the Mississippi River is the central receiver in the watershed which bears its name, I can see where it is considered the mainline, rather than a tributary to the Missouri River and the Ohio River.

Length vs. drainage basin vs. discharge. Can't they all get along?

Knowing the volume or length of a river 200+ years ago was an inexact science. Once a name is established it is very hard to change it.

I was taught that the Missouri River AND the Mississippi together made the longest flow. The Missouri flows into the Mississippi so perhaps the combined should be called something else.....