A river flows downhill from a mountain, a spring, or from a lake. The beginning of a river is called its “headwater,” and its ending is called its “mouth.” Along the way, it meets up with another river, flowing downhill from another source. The two rivers combine at that point, called a “confluence,” and thereafter the downflow is in a single river.
Sometimes, both contributing rivers end, and the resulting river is given a new name. For instance, at Pittsburgh, Pennsylvania, the Monongahela River and the Allegheney River meet, and their confluence is the origin of the Ohio River. Thus, Pittsburgh is the mouth of the Monongahela, the mouth of the Allegheney, and the headwater of the Ohio.
But sometimes, one river is called a “tributary” of another river. For instance, at Cairo, Illinois, the Ohio River meets the Mississipi River, and the river that flows downhill from the confluence at Cairo is called the Mississippi. Cairo is the mouth of the Ohio. But it is merely another spot along the course of the Mississippi. Go north-by-northwest about 140 miles from Cairo, Illinois to just north of St. Louis, Missouri, and you will find the spot where the Missouri River becomes a tributary of the Mississippi. Some geographers refer to that portion of the Mississippi north of these confluences as the “upper Mississippi” and the rest of it as the “lower Mississippi” but that does not seem to be a universal appellation.
So why is it that the confluence of the Ohio and the Mississippi produces the Mississippi River, while the confluence of the Monongahela and the Allegehney produce the Ohio? How does a geographer or a hydrologist distinguish a situation in which one river “takes over” a tributary from a situation in which two rivers unite to create a third? Or is it just arbitrary? Inquiring minds want to know.
For that matter, how do you determine which river flows into the other? I've read that at Cairo, Illinois, the Ohio discharges more water than the Mississippi at that point. IF that's the case, shouldn't the upper Mississippi be considered a tributary of the Ohio?It's probably because people want rivers to be straight lines, and on a map it looks like the Ohio should be a tributary of the Mississippi. But if you go by total flow, who's the main stem and who's the tributary?
As far as I know, it's generally the order in which rivers are discovered and then named by Europeans. Although both the Mississippi and Missouri Rivers retain (more or less) Native American names, the Mississippi was "discovered" before the Missouri. The Missouri is much longer upstream of its confluence with the Mississippi than the the Mississippi River, yet below that point it becomes the Mississippi. At many times, the flow coming out of the Missouri is greater as well. But because the Mississippi was known both up and downstream of this confluence first, it retains the name.