Monday River Trivia

An argument ensued... what is the longest river North America? I of course said "The Rio Grande" (not really), ... I mean it even has "Grande" in it's name. - I should have upped the Canadian Mackensie, but I didn't. Yukon potatoes, great, but rivers... no way.
So everyone settled on the Mississippi. And this brings me to my point of departure.
The longest river in North America is the Missouri River, a tributary of the mighty Miss. Now how good is that? (for God's sake it just empties into the Mississippi and never even makes it to the gulf).
Final results:
1. Missouri 2,500 mi.
2. Mississippi 2,339 mi.
3. Rio Grand 1,885
4. Mackenzie 1,200 mi.