I love the look and challenge of perspective-type maps
I mean all of the grass; as a plane/flat surface it would perhaps read better next to the lighter ground areas you have... not quite that light, maybe just half or so toward the lighter value of the other flat ground. Under heavy forest it would obviously be shaded and what you have currently would work well.
For the falls, I would say... soften the hard edge at the top to suggest the falls have a bit more force flowing over the edge of the cliff instead of just dropping straight down in a trickle... there's usually a bit of a curve as water flows off the edge of stuff, but you probably won't need much.
Also, you might pull the bottom left edge of the falls more to the right, and shade a bit of the cliff to the left of the falls where it curves away from the sunlight so it looks like it goes back in behind the fall [especially toward the bottom, to give it some breathing room as the fall disappears into the hole]. As it is, your falls appear to hug the shape of the cliff a lot and these adjustments should help it read better, unless you want the water to be a thin sheet racing down a steep slope... which could be interesting as well.
Let me know if that didn't clear things up.
Edit: Also, take a look at some of the waterfalls in Iceland for some similar examples of what I'm talking about on that second point... particularly Skogafoss, Svartifoss, and Haifoss.