In my experience, rivers would NOT be visible in an aerial photo of that scale (just figure out what one pixel equates to), but must be shown on a MAP.

Remember, maps are not photos, and the intent of a map is to convey information!

I have both cut maps out of the land (like in my GIMP tutorial) and drawn them on top (in my Niagara map) and I think drawing them on top gives the most flexibility in that they are not destructive (though by just erasing them via a layer mask they aren't destructive anyway.....)

I also do not stroke rivers, because it makes them look too fat in most cases. I get the best results with a tablet, and setting it to vary the brush size with pen weight. I can start up in highlands with a light stroke and carry it down to the ocean/lake with a increasingly heavy stroke.

I also like to apply a slight pillow emboss to make them look "set-in"

-Rob A>