Altai Flood

The Altai flood refers to the cataclysmic flood(s) that, according to some geomorphologists, swept along the Katun River in the Altai Republic at the end of the last ice age. These glacial lake outburst floods were the result of periodic sudden ruptures of ice dams like those triggering the Missoula floods.

Background

In the USA, large glacial outburst floods have been researched since the 1920s. In the 1980s, Russian geologist Alexei N. Rudoy proposed the term ''diluvium'' for deposits created as a result of catastrophic outbursts of Pleistocene giant glacier-dammed lakes in intermontane basins of the Altai. The largest of these lakes (the conjoined Chuya and Kuray) had a water volume of hundreds of cubic kilometers.

Evidence

Gravel ripples

Giant current ripples (gravel wave trains, diluvial dunes and antidunes) up to 18 meters high and 225 meters in wavelength were created in several locations along the lake bottom. They are best developed just east of the Tyetyo River in the eastern part of the Kuray Basin, but several other smaller fields of giant current ripples also occur there. They are made up of rounded pebble gravel.

Giant bars

Giant bars are found along the lower Chuya and the Katun rivers, rising as much as 300m above modern river levels, with lengths up to five kilometers. Well-developed on the Katun River below its confluence with the Chuya River, the bars appear to have formed like giant point bars on the inner bends of the river, as opposed to the scoured bare bedrock walls of the ‘cut bank’ on the outer bends. These bars diminish in height and thickness downstream to about 60 m near Gorno-Altaisk.

Some of these bars form lakes when blocking small tributaries of the Katun.

Suspension gravels

Much of the gravel deposited along the Katun valley lacks a stratigraphic structure, showing characteristics of a deposition directly after suspension in a turbulent flow.

Large blocks

Unique block deposits
diluvial berms
of Rudoy) cap erosional terraces that are a few kilometers long, tens of
meters wide, and about 4 m above the lower bars. Block sizes range up to 20 m on the long axis, and show no
evidence of rounding ig. 31 Cuspate erosional hollows
and accumulation ridges are associated with individual
blocks. Rudoy 003, pers. comm.estimates the discharge
required to transport these blocks in suspension was about 1
million m³, with a duration of maximum flow of about 10
minutes.

Ice-rafted blocks

Ice-rafted boulders up to several meters in diameter.

Eddy deposits

Eddy deposits are seen along the Katun River between Inya and Mali Yaloman.

Multiple flood hypothesis

Dating of the gravel bars has yielded at least 3 times of deposition, suggesting that multiple floods occurred.

The current understanding

Towards the end of the last glacial period, 12,000 to 15,000 years ago, glaciers descending from the Altai mountains dammed the Chuya River, a large tributary of the Katun River, creating a large glacial lake including the Chuya and the Kurai basins. As the lake grew larger and deeper, the ice dam eventually failed, causing a catastrophic flood that spilled along the Katun River. This flood may have been the greatest discharge of freshwater on Earth, since its magnitude has been estimated similar to that of the Missoula flood in North America.

Timing

The age of catastrophic flooding is not tightly constrained, and may involve several events. The mechanisms of lake filling and ice dam failure would suggest an early or late glacial time, whereas conditions at glacial maxima would seem to preclude such events. The catastrophic flood(s) occurred between 12000 BC and 9000 BC.

Most of the water discharge is thought to have occurred during one day, with peak discharges of 10⁷ m³/s (Herget, 2005). The maximum lake volume was 6x10¹¹ m³ (600 km³) with an area of 1.5x10⁹ m². The ice dam was about 650 m high.

Flood route

When the ice dam failed, floodwaters coursed down the Chuya River to the confluence with the Katun River, followed the Katun into the Ob River, and then into Lake Mansi, a large proglacial Pleistocene lake, ~600,000 km² in area. The fast inflow raised its level by only ~12 m but some authors argue that, because the Turgay spillway of Lake Mansi was only 8 m above the lake level at the time, much of the floodwater continued into the Aral Sea. From there the flooding waters may have followed through the Uzboy spillway into the Caspian Sea, then through the Manych spillway into the Black Sea, and eventually into the Mediterranean Sea.Grosswald, M.G., 1998, New approach to the ice age paleohydrology of northern Eurasia. Chapter 15. (P. 199-214)— Palaeohydrology and Environmental Change / Eds: G. Benito, V.R. Baker, K.J. Gregory. — Chichester: John Wiley & Sons Ltd, 1998. 353 p.

See also

*
*
*
*

References

External links

Lee, 2004, The Altay Flood

Alexei N. Rudoy, 2005. Giant current ripples (History of the Research, their diagnostics and palaeogeographical significance). - Tomsk. - 224 pp. In Russian, Eng. summary: pp. 134-211 pp.
{{Webarchive, url=https://web.archive.org/web/20171008181456/http://ice.tsu.ru/index.php?option=com_content&view=article&id=385%3A-q-q-2005-&catid=17%3A2011-02-24-10-44-05&Itemid=48 , date=2017-10-08

Geology of Asia
Megafloods