As promised, let's talk about Cities.
In searching for examples, whenever possible I will try to produce both an American and a European example, as I have readers from both areas. If I think of one in Australia, I'll jump on it. I have very few readers in Asia, so I'll restrain myself from offering examples from there. However, I would like to point out that the examples I'll be discussing appear everywhere, not just in any one part of the world.
Having started this article, I may intentionally set out to write a dozen posts, one for each major kind of settlement (the remaining 10 I mentioned yesterday are sub-categories of these 12). That should give me something to do when I need something to write about.
This is Pittsburgh. If you look it up on Wikipedia, you may notice that it doesn't say That Europeans founded the city, only that it was named Pittsburgh by General John Forbes. In fact, the value of where the Monongahela & Allegheny rivers met (forming the Ohio River) was well known to native Americans centuries before, as well as to French trappers in the 17th century. Of course, Wikipedia history doesn't begin until the French and British begin fighting over it, but then it isn't as if trappers were writing stuff down.
The smaller river on the map, the Allegheny, is the important one. It flows southwards through the west Pennsylvania plateau it created, with tributaries reaching a mere 8 miles from Lake Erie. Those 8 miles are critical. It allowed explorers and trappers to portage from the river system of the Great Lakes and St. Lawrence River, through the heart of North America into the Ohio-Mississippi river basin. Where it joined with the Monongahela allowed access into the mountains of West Virginia.
In the 18th century, Pittsburgh would grow big and rich on farm products, minerals, coal and the production of steel. In the early 19th century, both before and after the opening of the Erie Canal, it would become a transportation hub and supply center for the development of Ohio, Kentucky and Indiana.
Duisburg. It's what Pittsburgh would look like if it had been founded by the Romans (there's a myth that Duisburg was founded by the first German in 2395 BC).
Note the elaborate terraforming of the Ruhr River, entering on the right of the picture before it joins with the Rhine. The Ruhr has long been the richest coal-and-iron district in the world, forever competing with Silesia in Poland, Krivoy Rog in Russia and Liaoyang in China. There is really nothing like it in America.
The Ruhr river is short, only 135 miles - but then the Monongahela is only 130 miles, so there's a definite comparison there. It doesn't matter that these rivers do not extend very far. They are deep and they lead into important heartlands for industry and agriculture. The connection where the rivers meet provides fabulous opportunities for trade and exploitation, enabling the masters of these towns to grow rich through the influence they had over where the rivers reached. Duisburg, like Pittsburgh, is set up to take advantage of trade moving through the middle of the continent. The Danube and Rhine river systems were connected by the Ludwig Canal, completed in 1846 (and since vastly improved upon). Of course, long before the actual completion of a joint waterway, goods were moving up the Danube since the late Roman period and being tranferred into the Main river valley. The Main, like the Ruhr, also flows into the Rhine.
Mainz. 164 miles upstream from Duisburg. It too has been around since the Romans. The Main river, again entering from the right, is a bit longer - 327 miles.
Incidentally, my apologies to some for not listing these numbers in kilometers. It isn't that I'm not comfortable with kilometers; I can envision them fine and I wouldn't hesitate to use metric for technical work. It is only that for my D&D world I have gotten into the habit of using miles because metric did not exist as a measurement system in the 17th century. As such, I torture the hell out of my players by insisting on the use of pounds, gills, drams, firkins and pottles, etcetera. I think it is fun. I trust most gentle readers are able to deal in both miles and kilometers - there are plenty of calculators around online if it is getting frustrating.
This means the Main river reaches deep into central Germany. In the time of the Holy Roman Empire, the Catholic Church in Rome owned the city of Mainz - they knew what they were doing. The profits from river taxes alone served to keep the church fathers in velvet for many hundreds of years.
I have one more example.
Khartoum. Recently it has grown to the size of Chicago. That is not a lake at the bottom left of the picture, that is the White Nile River (often thought of as just the Nile). The Blue Nile, entering from the right, is the stream full of muddy water. Yes, that is correct: the White Nile is blue and the Blue Nile is brown.
Khartoum's history is even shorter than Pittsburgh's. At the time Pittburgh was founded, this confluence was populated by just a few small villages. It took British engineering to make practical use of the fork's potential access to southern Sudan and high Ethiopia.
That is because without modernization, Khartoum is just too remote. It may be perfectly located on two rivers that both reach a thousand miles into the hinterland, but this is eleven hundred miles by river from Aswan in Egypt, through some of the driest desert on earth. Without railroads and modern shipping, it must rely on local trade - in a part of the world without industry, where agriculture is thoroughly homogeneous.
Large towns and cities are not magically made important by the confluence of rivers. It's important to consider which rivers are involved. Granted, any two significantly sized rivers will probably result in some sort of town anywhere in Europe, where the population is very high, but other parts of the world, not so much. Where the Murray and Darling rivers meet in Australia (two very large river basins), there is only the small town of Wentworth. There's no need for more infrastructure than this - and besides, it floods like crazy. That is something to keep in mind where river towns exist. Two rivers - twice the chance of flooding.
Consider, too, that where the Ohio and Mississippi rivers meet, the only significant town is Cairo, Illinois - not a big city and somewhat removed from the confluence. It also floods. Worse, it does not serve a significantly populated hinterland - therefore, minimal trade.
I hope this has been helpful. Perhaps it has not been as rigorous a measurement for a 'city type' that readers had been hoping for . . . but towns and cities grow and develop for very specific, individualized reasons. A particular fork between two rivers is made important by more than the fact that there are two rivers. We must also consider the portages those rivers offer in their upper courses, the eventual destination of downstream, the resources surrounding the river valleys, the predilection for the area to flood, difficulties in creating weirs to defend against flooding, the length of time the city has had to terraform, the balance of agricultural hinterlands vs. mountains, etcetera, etcetera. There are many things to consider in deciding what makes a big, important city and what makes a small transshipment dock.