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

Natural features were very important in the ancient times. In the early ancient days, it was important to have a river or a lake in order for the city to unite. Being near a river/lake unites the city because the settlers / groups can travel or immigrate to other places in order to get trade. With trade, the city is richer resulting in greater communities.
However, there is also a bad point to it. Mountains, rivers and lakes can isolate people. If one group lives on the north side of the mountain, and the other lives on the south, they become isolated because they are separate- which is very sad. Lakes can be very long, like the Nile, and isolate people because the settlers wouldn’t be able to cross a large lake. If they would try, it could be dangerous and they would probably die on the way.