I really like the look of your map. The palette is right up my street, as is the style. But to be honest, I was really having trouble concentrating on your rivers because the dark brown blurred stroke you have used that seems to denote borders is overpowering. I'd lower the opacity on that by a large amount personally.
But aside from that yeah, I can see your rivers aren't great. I can see several that stretch from coast to coast and lakes with several outflows.
With those fixed and those dark brown borders this would be a real looker.