The Genesis Flood narrative is believed by most biblical scholars to be a conflation of two main literary sources, designated (in what is called the documentary hypothesis) as J (the Yahwist account) and P (the Priestly account). The two accounts were composed about 500 years apart, and there are contradictions between them. Thus Noah is told by God (Yahweh) in the J document to take into the ark seven pairs of all clean animals and single pairs of all others (7:2), while in the P source all animals enter by twos (7:8-9). In J the Flood lasts 40 days and nights (7:17), in P 150 days (7:24). J and P can generally be distinguished from each other (in the original Hebrew) throughout the Flood story. The prescribed dimensions of the ark (6:14- 16), the opening of the firmament and upsurging of the subterranean waters (7:11), the landing of the ark "upon the mountains of Ararat" (8:4), and God's covenant with Noah (9:1-17) are details from the P source. Noah's sending out of the raven and the dove (8:6-12), and his burnt offering for the pleasure of Yahweh ("The Lord smelled the pleasing odor") (8:20-22), are from the J source.
The story of Noah's Flood so strikingly resembles that of Utnapishtim in the ancient Babylonian epic of Gilgamesh that some degree of direct or indirect Hebrew borrowing seems indisputable [but this does not mean there is direct evidence of plagiarism -- JM]. In the Babylonian tale (see Speiser 1969, 72-99), Utnapishtim is warned by the god Ea of the coming deluge, builds a ship, takes animals aboard, and weathers the storm. The ship grounds atop a mountain, whereupon Utnapishtim sends out a dove, a swallow, and a raven, and then burns an offering, the gods smelling the odor and crowding around "like flies."
Ancient floor traditions are found among many peoples, with two now fragmentary Mesopotamian myths -- one in which the Noah character is the Sumerian king Ziusudra (Kramer 1969, 42-44), the other in which he is named Atrahasis (Speiser 1969, 104-106) -- predating the one in the Gilgamesh epic. What distinguishes the Genesis myth from its predecessors is firstly the elimination by the biblical writers of the original polytheistic context, and secondly the presentation of the Flood as divine judgement against sin. (In the Atrahasis myth, by way of contrast, a flood is sent because people have become so numerous and noisy that the supreme god Enlil can't sleep.)
The Institute for Creation Research (ICR), headed by America's leading "scientific" creationist Henry Morris, considers NOah's Flood to be literal history. In its quasi-official textbook _Scientific Creationism_ (Morris 1985b, 252), the ICR calls the Genesis Flood "the real crux" of the creation/evolution dispute, for the ICR's creationists consider the Flood, not evolution, to be the answer to the order of the fossil record. That order is one of simpler to more complex fossil organisms as we move from older to younger rock strata -- exactly the order that we would expect to find if life on earth has evolved.
But the ICR (Morris 1985b, 117-120) argues that the fossils are of organisms that were "originally created contemporaneously" and were drowned in a "great hydraulic cataclysm" that laid down all the earth's beds of sedimentary rock. Such a flood, the ICR maintains, would leave its drowned victims roughly in the order of their habitat elevations and their ability to flee from the waters. We are apparently supposed to attribute to coincidence the fact that this order of drowning turns out to look the same as an evolutionary order. We must also share the amazement (rhetorical, of course) of philosopher Michael Ruse (1982, 315) that in fleeing the Flood "not *one* human being," or horse, or cow, or tortoise, was so slow or stupid or crippled as to lag behind and get caught "at the bottom of the hill," [or to be so smart as to advance up the mountains until the waters closed over their heads -- JM] thus to become fossilized with simpler organisms in a lower rock layer [or fossilized on mountaintops in little groups of drowned people].
We also have to wonder where all the water came from, not to mention where all of it went. It has been estimated (Newell 1985, 38) that over 4.4 billion cubic kilometers of water, to a height of 9 kilometers above present sea level, would be needed to completely cover the earth's surface ("and all the high hills, that were under the whole heavens, were covered" [Gen. 7:19]). In other words, Noah's Flood would have required over three times the amount of water (oceans, lakes, atmospheric moisture, and so on) that is presently in the earth's hydrologic cycle. Even a flood only three kilometers above present sea level -- enough to cover about one-third of Mount Everest -- would require more water than the earth has available (Newell 1985, 38). Morris and his fellow "scientific" creationists must therefore resort to fancy. They hypothesize that before the Flood there existed "a vast thermal blanket of water vapor" in the earth's atmosphere, and that this "vapor canopy" not only provided the pro-Flood world with a "marvelous greenhouse effect" but provided ample atmospheric water for the Flood (Morris 1985b, 124-125). Not only does the creationist "vapor canopy theory" have no basis in either science or the Bible, but the atmospheric pressure and composition resulting from such a canopy would have prevented earthly life from existing (see Strahler 1987, 195- 197).