As promised, today we’re talking about mountains. Now that we have our coastline defined, we can chop up our big, flat island into more digestible chunks. Mountains work really well for this purpose because they are both a physical boundary but also permeable. Mountain slow you down, but they don't stop you necessarily.
This makes them useful for the players as internal boundaries inside the island. And they work well for the DM because mountains have frequently served as cultural boundaries in the real world. The Alps marked the boundary between Italy and Gaul just as the Pyrenees mark the boundary between France and Spain. Mountains work better for this purpose than rivers because rivers are facilitators of mobility instead of inhibitors. Rivers encourage motion; mountains discourage it.
So the players will use our mountains to define geographic subdivisions of our island. We will encourage this behavior by using mountains to separate cultural and racial groups.
More about that later. Right now, we just need to chop up this big Island into more digestible bits. So I'll go ahead and slap down some chains of mountains radiating from the center of our island out towards some of the points.
(If you wanted to, you could extend these mountains out into the sea as islands. You can see this sort of thing in the Aleutian Islands of Alaska. But keep in mind we want to encourage our players to stay on the island and not give them small, tempting distractions just offshore. So we’ll stop our mountain chains well short of the coast.)
As you can see, I've chopped my island into six sections. Some are a little big, especially the ones on the eastern side, so I'll probably want to vary the terrain a little bit over there. The northern section is the smallest, so I'll probably make that the "safe zone" of civilization. Finally, I put a ring of mountains in the center of the island. This is going to be our mysterious and difficult to reach plateau. It's perfect for a “lost world” teeming with dinosaurs, ancient lost civilizations, or crashed spaceships à la Expedition to the Barrier Peaks. I have no idea yet what I'm going to put there, but that's okay. We're still only just getting started.
Now that we have mountains and the coastline defining our highest and lowest spots, we can put in rivers. Assuming your world works under the same general principles as ours does, water will flow downhill. Find some likely spots, and draw some squiggly river lines heading towards the coast. Rivers tend to do one of two things when flowing from a mountain to the sea; they either separate into smaller rivers, or merge into bigger rivers. If you want some massive artery like the Mississippi or the Amazon, have lots of rivers flow into one. If you have a good fan-shaped piece of coastline your river can fan out into a delta like the Nile.
Don't go too crazy with the rivers. Not every nook and cranny of your coastline requires a river flowing into it. On a map this size, a handful should serve you well. If you've already located what you consider to be choice spots for civilization, go ahead and put a river by that. Communities love rivers. But we'll get to those later. We've still got to finish our geography first.