See, I don't necessarily agree w/ this. In an actual civilization, wouldn't everyone get together and DECIDE where to go next? As in, hmm, there's this awesome fish resource, so let's send people to live on the coast and fish those fishes, rather than sending those same people out into the desert. Again.
It would be natural (easier) to settle on a river/flatland instead of hills and mountains, but after a while, wouldn't you want to go mine instead of farm to diversify your exports?