You have, it appears, a river violation of a most basic sort. However, it could be done artificially as a canal, but rivers don't split, except quite briefly and not without rejoining.

I've cropped down a bit and circled exactly what I'm talking about.

For coastal deltas, remember that they require a rather flat terrain and are built by the river dumping earth into the sea, so the coastline will generally bulge out into the sea where the delta is.

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