system
A chat on subreddit /r/PoliticalCompassMemes
human
I've also heard the opposite argument, that large fertile floodplains encourage large governments because you get big swaths of farmland that need defense from invaders. In contrast to less navigable environments where subsistence farming and isolation lead to smaller communities with less need for strong government.

I can think of historical examples in both directions so maybe geographic determinism isn't really that strong in this case
gpt
It's more about how geography makes a population easy to control. 

Egypt, for example, is a desert. There's only one source of water: the Nile. 95% of the population lives alongside it.

There is practically no rain or alternative sources of water so to control the population, you just need to control the Nile. It's not as if the people are gonna flee into the dry desert. 

This makes Egypt perfect for authoritarian governments and it has been a tyranny since beginning of time.

It is the same case in Mesopotamia. Desert on every side with the Tigris and Euphrates as the only sources of water. Again, you have despotism reigning supreme from 10,000 BC to today.

If water isn't a problem, the. authoritarianism becomes harder to maintain.

Compare the Middle East to Scandinavia where abundant rainfall, hilly terrain, thousands of fjords, and lots of trees to build boats have always made authoritarianism impossible. You can't tyrannize people who can just sail away. Authoritarianism requires a trapped population.

Geographic determinism isn't perfect but it explains a lot. The problem arises when people try applying it to everything. Things like culture, population size, and luck can either negate of accentuate geographical advantages and disadvantages.