Please understand your map is excellent work.
The river issue is just a common misconception, and we want to make better maps just by avoiding geologic anomalies.
A river delta is the exception, this is caused when sediment (sand, mud, silt) being carried by the river's current slows down because the sea pushes back and the sediment falls. As the sand bank fills up it changes the rivers course causing many channels to form over years of deposits. The current will primarily stay in one main channel regardless of the branches formed in the past.
Islands occur on rivers where the river splits on either side, but generally reconnect a soon as possible. This is just a harder point the river could not erode away easier than channels to either side.
It is extremely rare for a river to fork downstream. One that forks hundreds (I don't know the scale of your map) of miles away and both reach the sea, is not possible geologically speaking. Physics will cause the water to flow the best way, and one would dry up, as the other becomes the course of the river.
I'm only trying to explain this so that you understand my point of view.
River issue aside, Great Map!