old cities are typically built on the flood planes of large rivers since you get easy access to clean(ish) water and flat ground. Large rivers tend to engage in

old cities are typically built on the flood planes of large rivers since you get easy access to clean(ish) water and flat ground. Large rivers tend to engage in sedimentation so everything around them either needs to slowly and constantly build upwards to avoid being underwater or will be razed and the ground raised by the next flood with its associated payload of mud.