Maybe try "down" in the bevel settings (if you're not already). I would, in general make them much more subtle. As Ascension said, you might not see them from this altitude...I don't see a scale but judging by the lat/long markers you've got showing this is a pretty big piece of land. The shapes of the rivers are quite pleasing but maybe try making the headwaters just a single pixel thick and taper that up as you get toward the coast. I like to "bury" my rivers under the vegetation layer a bit...either by actually putting that layer under the other, or by using a grungy type brush to erase out bits when the river's small.
M