Answer:
These areas had the most resources.
Explanation:
When you are settled by rivers, you have the sources you need to grow crops, raise life such as cows, pig, chicken. Not only this but you also have a place to bath, collect drinking water, and wash clothing. Life thrives more on river sides rather than inland during these times as they did not have all the tech we do today.
More buildings and more space for humans to move around
Less dense then water its self
