Looking at a map of North American rivers, I assume the source of the Mississippi is located somewhere in Minnesota, and the river is named Mississippi at that point already. Near St Louis, it is joined by the Missouri, which has its headwaters somewhere in Montana.
The Missouri, measured from its source to their confluence, is, however, far longer than the Upper Mississippi from its source to the confluence. This makes me wonder why the Missouri is, in geographic nomenclature, considered a tributary of the Mississippi. Wouldn’t it be more logical to say that the shorter river joins the longer one, defining the river below St Louis as the “Lower Missouri” and the Mississippi as the Missouri’s tributary? I guess there is no “natural” of inherent way of telling which river joins which one; it’s just two streams flowing together, resulting in one new stream, so it’s a matter of human nomenclature to say which river is the other one’s tributary.
I assume one might also take water volume, not only length, into account in defining which is the main river and which the tributary. This picture of the confluence of the Mississippi and Ohio rivers from Wiki, OTOH, seems to show that at the confluence of these rivers, the Ohio is at least wider and presumably of larger volume than the Upper Mississippi at this point. This would argue in favor of the Ohio as the “dominant” river in the system, and the Mississippi as its tributary.