The only way to do it without doing it all by hand is to do it in 3D and then convert to 2D. Maybe use ViewingDale's Instant Islands
ViewingDale
or Wilbur?
Current Wilbur Version

Doing it in 3D first might seem like overkill, and in a way it is, but going off the map you posted doing it by hand would not be that challenging/ time consuming-
it's not got that many rivers in it. There are a couple of tuts on here on creating rivers in Photoshop- but again that's 2D and by hand.

monks