High mountains and access to water (the sea, not rivers) meant that
navies were critical for war and trade.
This also meant that the different city states of Ancient Greece were
physically separated from each other.
This allowed each culture to have similarities (like language and
religion), but also significant differences like the Spartan vs. Athens
systems. This also led to a particular
type of farming and a limited ability to fight in open plains. Hence the Hoplite system with phalanxes
became dominant.
There are many explanations. One is that it caused many wars because countries wanted to take the resources and food of other republics who were good at agriculture. Another is that it brought the rise of things like oppression and inequality. People started getting enslaved as prisoners of war while women were often considered inferior because they couldn't be as efficient at farming because physical strength. Also, it brought separations into classes based on wealth and also gender based division with men being the "strong bread winners" while women were just "housewives", since they couldn't work on the land as strongly as men. And it also made people shorter, weaker and changed nature. People stopped getting a nutritious diet because they kept eating the same food. While hunters and gatherers would have meat, fruit, vegetables and grain.
Because as a river is a corridor, it can help to travel and transport goods. It can also provide food as it might have fish.
