http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== Quaternary superfloods Low-latitude superfloods, Superfloods related to the melting of the last North American ice sheet During the Quaternary, changes in the distribution of global weather <#> systems and the growth and decay of ice sheets in various parts of the world led to complex changes in river activity. In low latitudes, for example, spatial and temporal variations in monsoonal precipitation led on occasions to periods of time during which severe river flooding took place. In areas close to the former ice sheets, numerous large lakes were impounded against the former ice margins. Overflow from these ice-marginal lakes resulted in greatly increased fluvial discharge through particular drainage basins <#>. The greatest floods, however, took place during periods of ice-sheet melting. On occasions, impounded lake waters breached ice barriers and resulted in some of the greatest river floods ever to have taken place across the surface of the Earth. Low-latitude superfloods During the close of the last Ice Age, one of the most significant changes in fluvial activity took place within the Nile <#> drainage basin. It is generally agreed that during the period of the last Quaternary glaciation the Nile drainage basin was extremely arid through much of the time period from 25 000 years bp. Soon after 12 500 years bp, major changes in river activity took place. At this time the waters of Lake Victoria began to overflow into the Nile, while within the Nile Valley in Egypt greatly increased flooding took place, particularly during the time interval 12 000–11 500 years bp. In lower Nubia these floods may have reached as much as +9 m above the flood plain of the time. The floods are thought by several authors to have reflected unusual climatic conditions in sub-Saharan Africa at this time. This period of greatly increased Nile discharge may have been due to the south-westerly monsoon over eastern Africa that had previously failed to traverse the continental landmass during the last glaciation. This major change in low-latitude atmospheric circulation may, in turn, may have been influenced by changes in the position of the Intertropical Convergence Zone. A similar period of monsoonal flooding may also have taken place in west Africa at this time owing to changes in low-latitude atmospheric circulation. Superfloods related to the melting of the last North American ice sheet During the beginning of the retreat of the last (Cordilleran) ice sheet in the western part of North America <#>, numerous ice-dammed lakes were produced at various locations along the southern margin of the ice sheet. The greatest catastrophic river floods associated with the drainage of these lakes took place in association with the emptying of glacial Lake Missoula in Washington and Idaho. Several scientists have proposed that glacial Lake Missoula may have repeatedly been breached on as many as forty occasions during the melting of the last ice sheet in western North America, and that on each occasion the floods drained westwards across the Columbia River Basin in Washington and ultimately drained into the Pacific Ocean <#>. Victor Baker has estimated that the peak discharge associated with the floods that drained out of glacial Lake Missoula may have been of the order of 20 million cubic metres per second (m^3 s^−1 ). This is an astonishing value for former river discharge: it is about twenty times greater than the average present worldwide run-off of 1.1 million m^3 s^−1 . Numerous large ice-dammed lakes were also produced along the southern margin of the (Laurentide) ice sheet east of the Rocky Mountains (Fig. 1). The largest of these lakes was glacial Lake Agassiz, which formed during the melting of the last Laurentide ice sheet and between about 12 800 years bp and 8000 years bp fluctuated in size. Throughout much of the time that glacial Lake Agassiz existed, greatly increased fluvial discharge took place through the Minnesota valley into the Mississippi drainage basin and ultimately into the Gulf of Mexico <#>. It has been estimated that the average overflow discharge from Lake Agassiz at this time was between 40 000 and 100 000 million m^3 s^−1 . During periods when Lake Agassiz increased in size owing to ice retreat or when additional flood waters derived from melting ice <#> masses and other lakes emptied into Lake Agassiz, the rate of discharge from Lake Agassiz may have on occasions been as high as a million cubic metres a second. *Fig. 1.* Reconstruction of the Laurentide ice sheet for (a) 8400 bp and (b) 8000 bp, showing positions of major ice divides (D) and saddles (S). The distribution of ice for 8000 bp may also include a floating ice island (not shown) over Hudson Bay that was produced during the final disintegration of the Laurentide ice mass and the drainage of glacial Lake Agassiz/Ojibway through the Hudson Strait. (Based on Dyke and Prest (1987) /Géographie Physique et Quaternaire/, *41*, 237–64, and Dawson (1992).)reconstruction of the Laurentide ice sheet On several occasions during the melting of the waters of the Laurentide ice-sheet, Lake Agassiz breached ice barriers and drained as a series of catastrophic floods in directions other than through the Mississippi drainage basin. For example, a major flood took place at about 11 000 years bp owing to the eastward escape of lake waters through Lake Superior and the St Lawrence Lowlands into the North Atlantic. Jim Teller has estimated that during each of a series of such flood events, approximately 4 km^3 of water drained from the lakes. As the Laurentide ice continued to thin and retreat, a series of newly created lake overflow routes served to transport water out of Lake Agassiz. On each occasion Lake Agassiz alternately filled and drained, and each emptying event caused a catastrophic flood. These floods ended at 8500 years bp when continued ice retreat established a link between glacial Lake Agassiz and neighbouring glacial Lake Ojibway to the east (Fig. 1). Lakes Agassiz and Ojibway became confluent and bordered the southern margin of the ice sheet along approximately 3100 km of its length. Most probably the remainder of the ice sheet was in an advanced state of decay and undergoing widespread stagnation. At this time the waters of glacial Lake Ojibway suddenly breached catastrophically beneath ice in the Hudson Bay area and through the Hudson Strait into the North Atlantic. This resulted in the lowering of the levels of both Lakes Ojibway and Agassiz, in some areas by at least 250 m. It is difficult to estimate accurately the volume of water that must have drained from these lakes and beneath the ice. The figure is most probably between 75 000 and 150 000 km^3 —representing one of the greatest floods that took place during the Quaternary. In Eurasia large lakes were dammed along the southern margin of the Russian ice sheet. The development of the very large ice sheet in Eurasia resulted in a complete reorganization of the drainage systems of major rivers such that most of the melt water that drained from this ice sheet was diverted southwards into the Black Sea and ultimately into the eastern Mediterranean. According to M. G. Grosswald, in the west Siberian Lowlands two extremely large lakes, the Pur and Mensi Lakes, occupied a vast area of about 1.5 km^2 . These lakes were comparable in size with those that formed in North America at the close of the last ice age, and the scale of the river discharge associated with water that drained southwards from these lakes was similar to the volumes estimated for melt-water drainage rates in the Mississippi region. The ultimate fate of these large lakes in western Siberia is not known in any detail. It is, however, believed that most of the impounded melt-water in these lakes drained northwards catastrophically as soon as widespread ice-sheet stagnation had taken place during the Early Holocene. Bibliography and More Information about /Quaternary superfloods/ * Dawson, A. G. (1992) /Ice Age Earth: Late Quaternary geology and climate <#>/. Routledge, London. Alastair G. Dawson