"5. The Genesis myth is composed, it seems, of at least three distinct elements. First, historical memory of a cloudburst in the Armenian mountains which, according to Woolley's Ur of the Chaldees, flooded the Tigris and Euphrates about 3200 B.C.

covering villages over an area of 40,000 square miles with eight feet of clay and rubble. Only a few cities perched high on their mounds, and protected by brick walls, escaped destruction."

Hebrew Myths, p 116.

This flood is described in the Gilgamesh Epic.

See also some material from the net which indicates that the flood might have been some time between 5,000 B.C. and 3,000 B.C.