How did natural features such as lakes, rivers, and mountains help unite people or keep them isolated from each other?

Lakes and rivers might unite people by providing them with a food source. For example, if there was plenty of fish in a lake people would come to the lake to fish, and catch food together. However, a lakes and rivers could also isolate people by being in the middle of two civilizations, like a natural barrier, and when people would naturally have been united it would have prevented them from being together. Mountains could unite people if there was a lot of wildlife, food, and natural resources. Then people would gather and live in the mountains, and be brought together. However, like the lake and the river, if there was a mountain in between two civilizations, this would probably prevent them from uniting, and keep them separated. In this way, natural barriers both bring people together and keep them apart.