Early civilizations were generally built on great floodplains (the Nile, Euphrates, Indus), and floods were a regular part of their cultures. The annual Nile Flood has heights of 20-50 feet on average, every single year. Ditto for the Indus (which most recently had a disastrous flood in 2010), Tigris-Euphrates, Ganges (which has disastrous floods so regularly that it barely registers as news), and other great river systems.

100-year floods are epic natural disasters, but each of these three great river systems would experience, on average, 30 of these disasters between 4000 BCE and 1000 BCE. 1000-year floods are far worse, yet we should expect about 3 of them in each of these rivers over the same time span, with an even probability of a 10,000-year flood (holy crap!) in at least one of them.

We can say with certainty that many great floods occurred in these cradles of civilization, because even their regular annual floods are major events, and statistically some of these will rise to the level of epic natural disasters, just as they continue to do today.

While many scientists agree that flood myths were based on actual historical events, there's some disagreement on exactly when and where the flood(s) occurred, and what the cause was.

One theory that puts the earliest date on the flood is the Black Sea deluge hypothesis, at approx. 7,000-8,000 years ago. Although it seems likely that a flood of the Black Sea region happened during this time, there is significant doubt as to the severity of the event. This is the only theory that puts the flood significantly earlier than the invention of writing (5,000-6,000 years ago) but is not generally considered consistent with the biblical/Sumerian myth.

The next theory is interesting, because it explains a source of global flood myths -- that is, flood myths outside the Judeo-Sumerian literature -- as a comet impact 5,000 years ago in the Indian Ocean, resulting in a planet wide mega-tsunami -- although evidence for this hypothesis is sparse.

The most widely supported theory is that both the biblical and Sumerian flood myths date back to a single event: Localized flooding of the Euphrates river region in 3,000 BCE as a result of a combination of two different shifts (1, 2) in global climate patterns. Evidence of a regional flood during this time is present both in the fossil record, and in at least three separate cuneiform records, dating as far back as 1,700 BCE, although there's speculation that these are copies of even earlier records.

The other thing I'd like to ask is if "flood myths" are actually all that similar or if we're lumping a bunch of myths that happen to involve floods and survivors but are otherwise dissimilar into an ad hoc category. Lots of cultures are in areas that are prone to flooding(it is after all one of the most common natural disasters out there) so it's not like we would be surprised to see a lot of myths involving floods in the same way we'd be surprised to see a lot of geographically disparate myths about heroes whose names are all variants on "heracles'.

I recently listened to one of the Great Courses called "Lost Worlds of South America," and in it the lecturer off-handedly mentioned a theory that the seeming universality of flood myths across peoples and continents (even the isolated South American peoples) may be based on cultural memory of the end of the last ice age, and the substantial glacial melts. Can anyone speak to this?

Is it possible that many cultures had some stories of flooding because as the glaciers finished retreating about 8,000 years ago, there were a number of melt-fed reservoirs that were held back by dirt or ice dams, and many of these broke, creating multiple large but localized floods?

There's lots of discussion about the biblical area floods, but I am curious about the floods referred to in Native American flood myths. Is there evidence of those occuring? If so, any dated evidence and is that similar time frames to biblical floods?

If I remember correctly, the path taken by humans out of Africa involved a longish pit stop at the land that is currently covered by the waters of the Persian gulf. Apparently there were supposed to be major settlements in the area. At some point that area was flooded forcing the peoples to migrate, possibly in differently directions - eastwards to India (if I'm not wrong - the Hindus have the oldest accounts of the cataclysm, unless the Sumerians did too), northwards to Europe and the steppes of Russia, North-east to Central Asia etc. All those migrants would have woven that major event into their tales.