I've always wondered why a city never sprung up where the Ohio River meets the Mississippi, where Cairo, Illinois, is today. It seems intuitive that this would be a good spot for a city to have arisen, what with this being the junction of two of the longest rivers in the eastern United States. There is St. Louis at the confluence of the Mississippi and Missouri (or near it, anyway), and New York at the mouth of the Hudson. And of course, further upstream along the Missouri Ohio there are Louisville, Cincinnati, and Pittsburgh.
It seems like a logical place for a city to have emerged; if not a metropolis, then surely one substantially larger than Cairo which isn't even 3,000 people. But there isn't a big city for 150 miles in either direction along the Mississippi, and twice that to Louisville. Is it simply related to the terrain and the potential for flooding? Or was the Ohio just not that important for navigation and commerce? Or was it something else?
Merely being at a confluence of rivers that can handle the same kind of boats is not a great advantage over other river ports. Any cargo just sails past. As a port, having easily accessible bluffs out of danger in spring floods would give places like St. Louis the advantage. Places where cargo must be shifted from ship to barge or railroad (like Chicago) are classic entrepôts, and have natural advantages as places to set up factories.
This exact question was part of my senior thesis for my BA, so most of this is from memory, I'll come back with some sources.
There are two major differences between the location of Cairo and the location of St. Louis, as /u/MrDowntown pointed out. Cairo (pronounced KAY-roh, as opposed to the Egyptian city KYE-roh) lies in a flood prone area. It and the surrounding farmland is protected by a large network of levees. St. Louis, on the other hand, was protected by natural bluffs before lock-and-dam flood control was in place, and was therefore a safer place for people to settle and set up shop.
The other major difference between the two locations has to do with the volume and depth of the river. Currently, the river is dredged (essentially a deep path is dug down the middle) to keep the river open to barge and tugboat traffic. Before dredging was commonplace, in many places the river was shallow and prone to sandbars, slowing steamboat traffic. Large steamboats coming north from New Orleans could only go so far upstream before they had to offload onto smaller, shallower-draft vessels that could go farther up the shallow river. Both the Ohio and the Mississippi were deep enough upstream from Cairo that the large boats didn't have to stop there to offload. However, the Missouri, Mississippi, and Illinois rivers were all too shallow to take on these larger boats, so they had to stop at St. Louis to unload their goods onto smaller boats. This essentially made Cairo a rest-stop, and St. Louis became a major hub.
As far as sources go, I couldn't find my paper (I'm pretty sure it's somewhere in my mom's basement), but I recall using The Economic Rivalry Between St. Louis and Chicago by W.W. Belcher extensively as a jumping off point, and this article from Illinois History: A Magazine for Young People corroborates some of my statements about Cairo being passed over by steamboat traffic.
It isn't right where the Mississippi and Ohio come together, but the city of Paducah, KY is just a few miles up the Ohio River where the Ohio and Tennessee meet. As of 2010, Paducah itself has a population of 25,024 and the county it is in has 65,565 residents. Having been a former resident of Paducah for 15 years, I can tell you that the area is susceptible to flooding. The most famous being the Ohio River flood of 1937. There are also several low laying farmlands in the area that are constantly being flooded near the river as well.
While Paducah's location at the Ohio and Tennessee was beneficial for riverboating and as a port, I would say it's largest claim to fame was the uranium enrichment plant located there. The plant opened in the early 50s giving Paducah the nickname "The Atomic City."
And just as a random fun fact, there technically is a "metropolis" within 150 miles of Cairo on the Ohio. Metropolis, Il is a small town in southern Illinois that was officially declared by DC Comics as the home of Superman. They've got a whole section of their downtown dedicated to him including "The Super Museum" and a rather large statue of Superman. Here's a random picture of Obama with it from 2008.
I guess all of this is to say, while there isn't a major city exactly where the Mississippi and Ohio meet, the area isn't as empty as you think it is.
