I'm not a physical geographer or anything but is that how you intend to have the islands arranged ? The composition doesn't somehow chime with me, I guess it might be because you used the word archipelago and I associate that with island chains rather than clusters. I see what you mean about the rivers and the banding though! Not sure what you are using to make this but might it be possible to ditch the colour to contour scheme, keep the shading and paint the islands in photoshop on some other principle like land coverage or biomes (not sure if that's the right word)?