Had to think a bit to get Yukon. Adding that the length included portions in other countries helped. I was surprised to see that the Ohio was at the bottom of the list.
The Ohio is somewhat rare in that it starts where two other rivers end (in my city, Pittsburgh). So the length all the way to some small field is not included in the Ohio river like it is in most other rivers.
The 2,341 mi does not include any part of the Mississippi River, so it is technically the longest. The Mississippi is the largest North American River system, though, which includes the Missouri (as well as the Arkansas, Red, and Ohio on this list) so that may be a big part of why it's more famous.
The Mississippi is probably more well known because its north-south course was the second natural western "boundary" (after the Appalachian Mountains) in the early days of the US. Also, the Mississippi was thought to be the longest river, before the Missouri was fully mapped - which happened later in US history.
What is the source for the lengths? Wikipedia says the Missouri is 2,341 miles, but gives 2,320 miles for the Mississippi - just slightly shorter than the Mississippi.
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