Distributary! Distributary! ;P I'm just razzing, but I've been there too, on both sides in fact. If you have deltas, just browse some maps or photos (depending on scale) and see if what you want to do on your piece is a scenario that is likely to appear in the real world. As for color suggestions... if you haven't found a map you want to emulate, I'd suggest doing an old fashioned photo search on the web until you see any photo with a color scheme that you like and then experiment with those colors. If you're working in photoshop or some other editor with hue adjustment layers that you can clip to the layer below (it's called a clipping mask in ps) then color experimentation will be a whole lot easier; use an adjustment for each element (mountains, rivers/water, forests, land, etc) and tweak them until you get something you like.