? How is The Gambia different from other river names like The Bronx, The Congo, The Niger, etc.?
I may not be Pushkin, but I am pretty sure there are no definite articles in Russian. Not that one can’t play games with prepositions and such.
I’m talking about the country. Zambia became independent in 1964. When the colony of Gambia gained independence in 1965, they added “the” to prevent confusion, since the two words sounded much the same.
Here’s what Wikipedia says:
In 1964, shortly prior to the country’s independence, then-Prime Minister Dawda Jawara wrote to the Permanent Committee on Geographical Names for British Official Use requesting that the name the Gambia retain the definite article, in part to reduce confusion with Zambia which had also recently become independent
At least in California, there’s a tendency to refer to freeways with a definite article, such as “the 5” for I-5.
As I said in my initial post, most geographical features that span over an area have a definite article. Rivers, deserts, forests. I don’t think mountains usually do because they are a specific place, but mountain ranges do. (It’s Mount Everest, not The Mount Everest, but it is in The Himalayas.)
Maybe that’s the most common denominator? As alluded to by @What_Exit. If you can point to a particular spot, there is no article, but if it spreads out there is. Think New York City versus the New York Metropolitan Area. Rivers pass through large areas, while lakes do not. Seas and oceans tend to spread out though, so they often have articles. It seems to me, that’s the determining factor in the English language.
The Matterhorn?
El Capitan, too, maybe?
Before the Bay Area hippies weigh in on this, let me note that using “the” before a freeway number is a Southern California thing. It’s how we keep from being lumped in with the riff-raff up north. 
I think I picked that up from living in the Santa Barbara area for a few years early in my relationship with DH (I grew up in Oregon, moved to California at age 24, am now 52). We both still do that, even after over 15 years in the Central Valley, which if I’m not mistaken is considered northern California, and I don’t get funny looks from other people here.
Central California gets a pass. Only right since they have to have Fresno.
That is one of those unusual exceptions, yes.
Also the Eiger, the Monch, and the Jungfrau.
I’m guessing it’s a German thing.
There’s also The Villages in Florida.
I’m not very familiar with the place, but I have the impression it’s composed of neighborhoods and not individual villages.
My own theory is that they added the S so it wouldn’t be confused with The Village in The Prisoner, to which I think it has a more than passing similarity.
Strangely there is a community outside Annapolis called The Village.
What about El Paso, La Brea, Las Vegas, and Los Angeles?
A.k.a. the La Brea Tar Pits, or “the the tar tar pits.”
The Great Salt Lake covers a lot less territory than Lake Erie, Lake Superior, etc. Though all of those lakes together are “The Great Lakes”. And while seas might usually be very large, The Sea of Galilee should really just be called a lake.
Does “The Ozarks” referring to the mountainous are in Southern Missouri/Northern Arkansas count? They are the Ozark mountains, but commonly referred to as “The Ozarks”, at least in Missouri.
And, The Colony, Texas.
It’s the Rover, isn’t it? All of the places with giant homicidal white balloons tend to get mixed up in my head too.
Many cities in California with El, La, Los, or Las and even a few with Del.
El Cajon. El Sobrante. La Mirada. La Crescenta. Los Gatos. Los Banos. Del Norte County.
Texas must have quite a few also. Las Cruces.
And yet, in the U. S. we say “he is at school” (meaning he is there at the moment) or “he is in school” (meaning he is a student there). No the anywhere to be seen.