Unlabeled islands! They're everywhere! Look! *drops a large rock in a stream*
For instance, consider the Casiquiare canal (which is a natural river, not a canal). It flows from the Orinoco River into the Rio Negro (which flows into the Amazon). That means that this northeastern bit of South America is, in fact, cut off by water from the rest of South America.
You know what that is? That's an island.
But does it have a name? As far as I can tell, no. I mean -- does anyone even recognize it as an island?
And this case is hardly unique! For instance, there's the region of the US lying inbetween the Mississipi and Atchafalaya rivers! And... well, a whole lot of other examples! Just go look!
I mean there's plenty of examples where the rivers rejoin quickly so it's obvious that you've got an island and so these get names. But my question is about the big ones, where you might not even realize there's an island at all. The Wikipedia article on "distributary" does list one example where people seem to recognize "hey, this forms an island" -- involving the Danube, the Little Danube, and the Vah -- and it even has a name, it's called Žitný ostrov! ("Great Rye Island".) But I don't see any others listed there. And that one might well just be an obvious island.
I mean OK, I guess if you start applying this everywhere you get problems, because you have things like Divide Creek and North Two Ocean Creek and who's to say what's an island then?
But still! Look at that map of South America! Just look at it!