Image via Wikipedia

I feel pretty certain that the answer is the Mississippi River but apparently it is not as simple as that, as some very reliable sources disagree and say it is the Missouri River.

According to Encyclopædia Britannica I am correct and it is the Mississippi (see here & here), but…

…the US Geological Survey (USGS) say I am wrong and hand it to the Missouri by a clear 200 miles! I feel you have to believe the USGS but Britannica is also a very reputable and normally reliable source.

The figures quoted are as follows:

Encyclopædia Britannica list the Mississippi at 2350 miles whereas the USGS says 2340 miles and Britannica has the Missouri at 2315 miles while the USGS say 2540 miles.

So, bearing in mind the simplicity of the question – i.e. “What is the longest river in the U.S.A.?” – what is the definitive answer?