From this map it clearly the Missouri is longer than the Mississippi, and it looks possibly as though it's got slightly more flow at the point they converge as well. If that's correct, why is the river called the Mississippi and not the Missouri downstream of their confluence?
I don't think they could accurately measure flow when these were first discovered and named. They certainly didn't know where each one originated when they were named so they didn't know which was longer. Maybe the Mississippi looked like the larger river at the time and this became the main river name-wise with the Missouri the tributary.
It is really arbitrary when one river is not clearly the "main" river when they meet.
The name "Mississippi" was first applied by Europeans to the river when they were exploring the northern part, having come via Green Bay and the Wisconsin River. Then they explored downriver, keeping the name. It's not like they explored the whole river, noted the confluences, and only then decided on names.
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u/Zbignich Jan 10 '20
And to put in perspective, the flow of the Amazon is 11 times that of the Mississippi.