Actually I don't think that's true.

The mississippi river in the US has MANY islands in the river. Here's one off that little bump in illinois;

The largest, Quinsippi island, is about half the size of the river.

It comes right after the tributary from the river out by goose lake meets up with the mississippi.

Right before (almost at) the meeting of those two rivers are 3 smaller islands. None of those is more than 1/5 the width of the mississippi.

People don't see this a *LOT* because most rivers aren't as wide. A narrow river hits a piece of high ground and only has enough pressure to go around it one way. A big ole river like the mississippi will split, because the pressure of all that water hits the high ground and splits...

I dunno how to explain it.

Anyway, what makes an "island" in the middle of a river is simply that the ground there is higher than the water level.

The size of the island compared to the size of the river depends on how big the hill is and how wide your river is.

Quinsippi island:

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You can't see this from further up, but it's there.

The thing about the jetties makes sense though.