@Thesslian: at first I wanted the lake to the north was a kind of closed lake with no emissaries, but only tributaries, with water drained through underground aquifers. But, thanks to your comment, I realized that it was very unintuitive so, since I didn't like the shape, I replaced it with a river. Regarding the size of rivers, it is made ​​to be consistent with the scale of the map: now I used the burning tool to make the flowing path more brighter and visible.

@bradlavario: thanks!