From the Dictionary of Science & Creationism, Ronald Ecker, 
1990.

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 Elohim 
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. 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 flood 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," thus to become fossilized 
with "simpler" organisms in a lower rock layer.

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).