Why is the Missouri considered a tributary to the Mississippi?

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.

The people who were naming the rivers on the historic maps, didn’t know where the headwaters of the missouri were yet before they made the maps and wrote the names in.

The key reason, is that the Missouri joins at an acute angle to the Mississipi, so it really just looks more like a tributary. Imagine yourself on a road that goes straight, another road of the same size joins it from the side at about 90 degrees. Generally the name of the road you are on doesn’t change after the side road enters.

I think the volume and width of the Mississppi is larger at the confluence too, but I’m not sure of that.

As a rule, whoever “discovers” a river not only gets naming rights, but gets to decide which river is tributary to the other. They typically do so with little information (whoever first saw the mouth of the Missouri had no idea it is a damn sight longer than the upper Mississippi). In this case (and also the confluence of the Mississippi and the Ohio) I suspect the Mississippi was declared the principal river because it pretty much passes straight through these confluences; the Big Muddy and the Ohio are obliged to turn at these points.

In short, these matters are judgment calls, and not always correct.
William Least Heat-Moon (a born-and-bred Missouri River Rat) has some interesting comments on this matter in River Horse, his account of boating across the country on the Ohio, Mississippi and Missouri, among others.

Yes - the source is considered to be Lake Itasca

It acquires its name at Three Forks Montana, where the Madison, Gallatin and Jefferson Rivers join. The headwaters are considered to be those of the Jefferson River in southern Montana. Interestingly, for the first 100km the water flows WNW - directly away from the Mississippi-Missouri confluence.

There’s also the point that at the confluence, the flow of the Mississippi is often larger than that of the Missouri (Googling suggests the average ratio is 55-45). Prior to exploration it would have been natural to view the Mississippi as the more important.

The main reason for the nomenclature has to do with the fact that the Mississippi River was originally explored from north to south, so the major rivers leading off to the “sides” were considered tributaries of the river that was explored. The first European explorers were Joliet and Marquette, who got to the confluence of the Mississippi and Arkansas rivers before turning back north. Then, later, the Sieur de la Salle made it down the whole river to the mouth. The word Mississippi is an approximation of one of the names for the river used by the Amerinds of the Minnesota area, Misi-Ziibi, “great river.”

While the Missouri River at times exceeds the flow of the Mississippi at their confluence, the normal state is for the Missouri to be slightly less. The Ohio River manages a significantly higher discharge, I believe, than the Mississippi, and it also currently forms a confluence that, if you were travelling up the Mississippi, you might naturally assume the Ohio was the main river. And, of course, the Ohio was independently discovered by Europeans (de la Salle, I believe), but wasn’t traversed East to West until later, by which time the Mississippi was already set as being the “main” river by the French explorers of the time.

As a general rule, if you know the drainage areas of the two, the stream draining the larger drainage area is the “main stem” and that of the smaller drainage area is the “tributary”.

Of course, in this discussion, that was unknown at the time. The best I can gather is that the Missouri has over twice the drainage area of the Mississippi at that point, supporting the OP’s position.

That’s also my impression, from the picture I linked to.

But it has become clear from this thread that this (IMO rather odd) nomenclature is mostly due to the history of the exploration of the two rivers, not to any inherent logic. Thanks anybody for their input!

I’ve always found it weird that the Missouri is considered to “end” at the Mississippi. The name is kind of immaterial - the river continues to the sea whatever you call it.

To my mind, the length of a river is the length from the most distant headwater to the mouth. But of course, if you’re surveying a river “upwards” then you don’t know which branch is the longer unless you go up each branch and then back down and up the other one. That would quickly get pretty tedious I imagine!

(The Jefferson is 298 miles long before it even joins the other two rivers that create the Missouri by name.)
As for the OP, this picture gives a good idea. The Missouri is coming in from the upper left, the Mississippi comes in from the right. The Mississippi clearly looks like the “main channel” - it is wider and straighter, while the narrower Missouri comes in from an angle.

Colophon, the trouble with that is that it’s likely that the course of the Mississippi at the confluence is affected by the work done to channelize the river. It’s not clear that the confluence would have been the same in the pre-engineered state of either river. Interestingly, one excerpt I read from the report by Joliet of their expedition says that the inflowing Missouri River waters overwhelmed the Mississippi River waters with turbulent flow and substantial mud. Possibly, at the time he arrived, the Missouri was in one of its periods of higher flow.

Is there a practical reason why one would have to be designated the mainstem and the others designated tributaries? Granted they form a huge frickin’ watershed and drain a giant hunk of the country, but why can’t they simply be separate rivers and not lesser tributaries of some greater river? Really, I’m asking, is there an actual, practical reason for such nomenclature?

In hydrology, yes there is a reason.

When figuring the discharge at a certain point, one of the things we use is the stream length. If we have a point at which two converge, which length should we use? Answer: the length of the main stem. Which is the main stem, then? The one with the greater drainage area.

In other words, it helps to have a ‘convention’ so that everyone does the calculation the same way.

Intersting aside: the opposite of tributary is distributary, which is a channel which takes water away from the main stem.

Well, you can see roughly how it looked in Lewis and Clark’s day on the map here - different, yes, but the Missouri still came in from a near 90-degree angle.

Clark’s map, and a later much more detailed map are on this page. (Incidentally, that’s a really interesting website for anyone who likes maps, the history of exploration and geography as much as I do.)

Oooh I like that site!!! Thank you!!! :slight_smile:

If you ever come to Charlotte, be sure to leave this notion behind. :smiley:

From Wikipedia, on the nomenclature of the Upper Mississippi:

Since they were probably not trying to do hydrological calculations, I wonder why the white explorers in the early 1800s were so interested in finding “the headwaters” of the Mississippi.

The Treaty of Paris, drawn up at the end of the Revolutionary War, stated that the border between the United States and Great Britain’s North American possessions would proceed to the northernmost point of Lake of the Woods and then “due west to the Mississippi River” — this despite the fact that the course of the Mississippi wasn’t known that far north. It therefore became important to know exactly where the boundary lay. When it was discovered that the headwaters of the Mississippi were in fact well south of Lake of the Woods, things had to be rejiggered, resulting in the creation of the Northwest Angle.

It defined the boundary between the United States and Spanish Louisiana, and also played a role in the boundary between the US and Canada.

:smack: …And for the first time in my life I just realized the relationship of “Tribute” and “Distribute” as related and antonymish words.

The three rivers don’t join all at exactly the same point, either: The Madison and Jefferson join first, with the Gallatin getting in on the action a few hundred meters downstream. There’s actually some debate between the State of Montana and the federal government over where, exactly, the Missouri River starts: I don’t remember the details, but it has to do with who has to pay how much for maintenance of the parklands in the vicinity.