What defines a river?

[installLSC]

I looked in an atlas recently that stated the “Mississippi-Missouri River System” is the third longest river in the world. It reminded me of an old question I’ve had. We all assume that the Mississippi extends from Minnesota to the Gulf of Mexico and that the Missouri flows into it near St. Louis. How did we decide that it wasn’t the Mississippi flowing into the Missouri? Is it just an old notion that geographers grandfathered in, or is there an actual geographic rule that decided it?

[Antonius_Block]

At their confluence, the Mississippi provides a greater volume of water than the Missouri (in an average year), so the river downstream of the confluence carries the former name rather than the latter. . From Wikipedia:

The mean flow of the Missouri at it’s mouth is approximately 35,000 cfs, compared to 47,000 cfs of the Mississippi below the Illinois River, which joins about 17 miles north of the confluence

@installLSC

I looked in an atlas recently that stated the “Mississippi-Missouri River System” is the third longest river in the world.

Some lists put the Mississippi-Missouri at 4th longest…

[Aside]
Last week, the USCIS (US Citizenship and Immigration Services) unveiled a new test to be administered to those who wish to become US citizens. One of the questions was "What is the longest river in the United States?‘’, and, according to the San Jose Mercury News of December 2:

The only acceptable answer, the government says, is "The Mississippi.‘’ Unfortunately, the government is wrong. The correct answer is "The Missouri’‘-- which several readers helpfully pointed out to the Mercury News after the paper published a story on the new test Friday.
…
Sharon Rummery, a spokeswoman for the immigration agency, was taken aback by the gaffe and wasn’t sure how it happened. "You’re kidding!’’ she said Friday. "Thanks for the feedback. I’m going to get word back to our people immediately.‘’

I’m happy to report that the latest version of the test on the USCIS website now has the correct answer to question #125 as “The Missouri River”.

I’m hoping that they would accept “Mississippi-Missouri River System” as an alternative…
[/Aside]

[N9IWP]

Nowadays the “main river” is defined by the branch that drains the largest area.
In the olden days, the “main river” was defined by flow and geomorphology.

(at least that was the answer I got when I asked on sci.geo.hydrology, asking about finding the source of teh Nile)

For the MM system, it also helped that the upper Mississippi was explored sooner than the upper Missouri. Perhaps if the US was explored from the west it would be different.

Brian

[FatBaldGuy]

We just did this question a few weeks ago.

Which is the tributary and which is the river?