Why does a increase in the number of cities cause more rivers to flood?

what problems might this cause?

Uneven land. In my opinion, more building would affect bodies of water making them more shallow to flood, or allowing them to be emptied out. This is a reason why most lakes, are normally set a little far aways from cultural city life. You might want to google it, this is just my opinion. But, I believe this is the reason, why rivers flood, is because they are too close to city land/buildings.