If I had $38 to spare ...

The upper Mississippi River — the part of the Mississippi north of the mouth of the Ohio at Cairo — is really a tributary of a larger river that includes the Ohio River and the lower Mississippi. Everyone knows that except people who assign place names and zealous Mississippi River fans.

So one scholar posted an article in the Journal of the American Water Resources Association using several measures to determine whether the Mississippi or the Ohio is the largest river in this part of the U.S.A. From the abstract of his article:

The Mississippi, Missouri, and Ohio River Basins were evaluated using data from nine selected U.S. Geological Survey gaging stations and ranked using the six metrics. Using an average for the rankings across the three rivers, the Ohio is ranked highest for three individual metrics (discharge, fish richness, and fish endemism), and highest across the average for all six metrics, and for an average of five metrics, including hydrology and biodiversity metrics. Thus, our results suggest that the Ohio River could be considered the most prominent river in the U.S. and that the river itself should have the same name (Ohio or Mississippi) from New Orleans to at least Pittsburgh, Pennsylvania.
I could buy the entire article for $38, but money in the Ross household is kind of tight right now and will be for the foreseeable future. Let's just say, though, that the conclusion of this scholarly article was no surprise to people who know the river.

It was surprising to me at least to read that the Ohio was ahead of the others in biodiversity.

P.S. So you don't have to go to the dictionary, endemism is "natural to or characteristic of a specific people or place; native; indigenous" or "belonging exclusively or confined to a particular place", according to dictionary.com.