How do natural features such as rivers, lakes, and mountains hep unite people or keep them isolated?

Natural features can keep people united but also isolated. Rivers oftentimes unite people, because they can trade and travel down the river to establish communication with other town or villages along the river. But a river can also isolate people, if they don't have boats or transportation to get over the river. Lakes can also act like rivers you can get to the other towns and villages and communicate and unite. But people can also unite walking around the lake which is also and option. Mountains mostly isolate people especially if there tall. It takes time to climb onto a mountain or over it, mountains can also be very dangerous and treacherous.