That's a fun little map - I like the 'narrative notes'. The swirly water is a neat touch. Ditto the above comments on legibility - the red gets lost way too easily. Yeah, the delta is kind of a fail - there's not enough river upstream to gather a lot of sediments to settle out when the river meets the sea. A delta isn't so much what can happen when a river nears the shore, as it is what happens to the dropped mud and silt when the water slows upon entering the ocean. So they tend to stick out, not extend in. It would work fine to break those distributaries free and make several smaller streams separately entering the ocean; maybe erase others altogether.

You might could keep the same effect you have going with the ocean by lightening and desaturating it some - your text blocks would suddenly work better. I like the overall palette - nice work.