From Wikipedia (your named source):
The Missouri River is the longest river in North America. Rising in the Rocky Mountains of western Montana, the Missouri flows east and south for 2,341 miles (3,767 km) before entering the Mississippi River north of St. Louis, Missouri. The river takes drainage from a sparsely populated, semi-arid watershed of more than half a million square miles (1,300,000 km2), which includes parts of ten U.S. states and two Canadian provinces. When combined with the lower Mississippi River, it forms the world's fourth longest river system.
Also - Wikipedia now lists the Amazon as longer than the Nile (6992 km for Amazon, 6853 km for Nile).
In Wikipedia, it said 1. Nile River - 6,650 km, 2. Amazon River - 6,400 km.
Great quiz, btw.
Shatt Al Arab is listed for Euphrates and that's also just the mouth section after it merges with the Tigris. So that's equivalent.
I would have got all of them in my youth - many years ago - although never sure where the San Francisco is.