Super interesting map here, but the river in the southwest corner does bother me. First, as Lukc mentioned, it does seem to contradict the water flow. Second, that seems like an awful lot of water to be draining from an area that appears to be mostly grassland. And third, some of those tributaries feeding the river just don't make sense. Unless there's a line of hills right along the coastline then the river should actually empty into the sea rather than the channel in the middle of the island. In fact I would probably start the river in the mountains and have it run west or south rather than having it curve northeast.

Anyway, still a super cool idea for an island map. Looks like something I'd want to run in a seafaring campaign someday.