While there have been mega-floods in prehistory, there's a very strange tendency to try to pin particular events for the genesis (so to speak) of flood mythology, especially when cultures all over the world have flood myths.
Somehow it's inconceivable that ancient people could experience normal disastrous flooding, with inundation as far as the eye can see, and then imagine what it would be like if a flood engulfed everything, even trees and hills, and remained for a long time.
To be fair to the ancients, floods were as common an occurrence as they are today. Every living person would have probably experienced at least one.
There exists however, a story about a particularity bad flood in many cultures.
Instead of assuming that the ancients were stupid and thought of every flood as "teh worst evar!!!", it's far more likely that there was in fact an instance of a flood that stood out from the rest enough to write down.
Somehow it's inconceivable that ancient people could experience normal disastrous flooding, with inundation as far as the eye can see, and then imagine what it would be like if a flood engulfed everything, even trees and hills, and remained for a long time.