Well...

Deciding between these two turns out to be pretty tough. I had originally indicated that the Missouri River was the correct (and somewhat surprising) answer. However, Mark wrote me with the solidly researched but conflicting answer of the Mississippi. I've included his letter and my response. Together they indicate that this is not as trivial a question (and the answer by no means as clear) as you might expect.

Mark wrote:

 >

 > Question number one asks for the longest river in the United States.
> Your answer the Missouri is incorrect. The Mississippi R. from it's
> source at Lake Itasca, Minn. to it's mouth at the Southwest Pass is
> 2,340 miles. The Missouri R. from it's source, the junction of the
> Jefferson, Madison, and Gallatin rivers in Madison County Mon. to it's
> junction with the Mississippi river is only 2315 miles. If you were to
> add the Red river to the Missouri or just measure the Mississipi from
> it's junction with Missouri you would be correct. One of the many
> sources that will verify my claim include The World Almanac 1997 and a
> whole slew of Geography textbooks.
>

> Cheetah

Hi Mark,

 I used the Times Atlas ((8th Ed.) as my source. It claims the Mississippi is 2,348 miles long, and the Missouri is 2,466 miles long.

Since the Times Atlas doesn't list the source of the Missouri, it might not be starting its count from Three Forks (as your source is). My guess is that the Times has tacked the Jefferson River on to the beginning of the Missouri to give it that additional 140 miles. I base this on their map of the region, which names the other two rivers which meet at Three Forks but does not label the Jefferson.

Why'd they do that? Perhaps they used different sources and/or different definitions.

This raises the interesting question of how rivers are measured. I noticed that our sources differ slightly on the length of the Mississippi, too. I wonder if the difference has to do with simple numeric rounding, or whether the definition of the river channel was not the same? Alternatively, the length of a complex, wiggly thing like a river will vary depending on the scale at which you measure it.

Thanks for your letter. Unless you have objections, I'd like to post this discussion as the answer to the trivia question. I guess that's the thing about good questions - the answers lead to more questions!

Ashton