Diamond is just trying to help you, Brother- your rivers are unrealistic. Which this is a fantasy map so if you just want to wave your hands and scream "Magic!", well, so be it. That's fair. But otherwise rivers do not split, they merge. At least on larger scales- on smaller scales, sure, there are such things as islands, braiding, and river deltas. But that's not what your map seems to show.

If ever water has two possible outlets then eventually one of them erodes more than the other, and all flow will go that way. The other channel dies. Exceptions are incredibly rare, and doomed to be very short-lived (on geologic timescales), yet you have three of them on one map.

This is a very common error for people new to the guild, so don't feel bad about it. The forum even has a "River Police" badge for people who help others with their rivers. But it really jumps out at us as an error.