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CONTEMPORARY PROBLEMS OF PHYSICAL GEOGRAPHY: QUATERNARY ICE-DAMMED LAKES IN THE MOUNTAINS OF SOUTH SIBERIA AND THEIR INFLUENCE ON THE DEVELOPMENT OF INTRACONTINENTAL WATER RUN-OFF SYSTEMS OF NORTH ASIA IN LATE PLEISTOCENE
Alexei Rudoy
Laboratory of Pleistocene Geology and Palaeogeography, Geography Department, Tomsk State Pedagogical University, Komsomolski Pr., 75, Tomsk 634041, Siberia,
Russia
Резюме
Возникновение и развитие ледниково-подпрудных озер определялось особенностями оледенения и рельефа. Режим ледниково-подпрудных озер зависел главным образом от режима питающих озера ледников, как индикаторов климатических изменений. Фронтальные части ледниковых покровов Северной Евразии подпру-живали сток текущих на север рек. В результате на равнинах и плоскогорьях перигляциальной зоны скапливались огромные пресноводные ледниково-подпрудные моря. В горах талые воды аккумулировались в гигантских межгорных впадинах, блокированных ледниками. Горные и равнинные ледниково-подпрудные озера сообщались через динамичную дилювиальную сеть каналов стока. Потоки воды объемом в тысячи кубических километров систематически и катастрофически поступали с гор в акваторию приледниковых морей Евразии. Это вызывало периодические сбросы этих морей в бассейн Атлантики.
В статье рассмотрены также возможные механизмы подпруживания и сбросов ледниково-подпрудных озер в горах.
Abstract: Development of ice-dammed lakes and their existence are conditioned by glaciers and relief. The regime of these lakes is defined mostly by the regime of glaciers which are indicators of any climatic change. Front parts of ice covers in Eurasia used to hold and aliment huge amounts of freshwater that resulted in the development of real freshwater seas on pre-glacial territories. In the mountains melt water accumulated within vast intermountain basins blocked with ice. Mountain and plain ice-dammed lakes communicated via a net of diluvial run-off channels. Water flows volumed in thousands of km3 ran catastrophically to the water surface of dammed freshwater intermountain basins in West and East Siberia. An about-a-century periodicity of out-throws of the upper water stratum from pre-glacial plain basins into the Atlantic Ocean.
The paper studies possible mechanisms of the development of ice-dammed lakes of various types in the mountains. The role of surges in the formation of giant Pleistocene ice-dammed lakes is being estimated by the paper.
INTRODUCTION
For several decades catastrophic failures of Pleistocene pre-glacial North-American lakes Spocan and Missoula (Bretz, 1923; Pardee, 1942; and others) have been considered unique and characteristic only of the territory of the Channelled Scabland on the basalt plateau of Columbia and adjoining regions. In monographies and text-books the very territory of the plateau with its complex
of geology-geomorphologic traces of those failures has been suggested as a «special case» of catastrophic transformations of the Earth image against the background of «normal» evolution of the Earth surface.
At the beginning of the 80-s first evidence of systematic outburst floods from giant Pleistocene ice-dammed water bodies of intermountain basins were found in Central Asia, in the first place - in the Altai Mountains (Fig. 1), in the valleys of the Chuya, the Katun, the Chulishman and the Bashkaus rivers (Rudoy, 1981, 1984, and others; Butvilovsky, 1985, and others). There also appears studies of Quaternary Glaciohydrology of North-Mongolian basins (Grosswald, Rudoy, 1986; Grosswald, 1987; Grosswald, Rudoy, 1996, in press) and of the depression of Issyk-Kul Lake (Grosswald et al., 1994). Calculated and analytic data also showed the possibility of immense Late-Pleistocene glacial floods in Zabaikalie and Pribaikalie (Rudoy, 1987 and others; Grosswald, Kuhle, 1995).
The Altai discovery of systematic superfloods the discharges of which strike our imagination and by two-three exceed the discharges of the greatest known modern mud streams has destroyed the idea of unique cataclysmic outbursts from the widely-known Missoula Lake that has been kept for more than 60 years. In its turn it induced the author to look at the recent history of the nature and mountain and plain territories of ancient glaciers from new, palaeoglaciohydrologic, positions. It also made the studies of glacial cataclysmic superfloods actual as a powerful geologic factor the role of which in creation of modern landscapes has not been practically considered before at all.
The Altai Mountains are situated practically in the centre of Asia, in the south of West-Siberian Plain (Fig. 1). The morphostructurai plan of the Altai Mountains is shaped as a system of fan-like diverging mountain ranges separated by river valleys and intermountain basins. The highest mountain ranges are those of the Central and Southern Altai, with the highest peak of Siberia -Belukha Mountain (4506 m). The mountains get lower toward the north and the north-west to 2000-2600 m. The depth of the vertical dissection of the mountains decreases in this direction, too: from more than 2500 m in the south to hardly 800 m in the north of the mountain land.
Mountains of the Central and South-Eastern Altai have all the features of the alpine-type relief: comb-shaped narrow steep watersheds with forms of glacial exaration; talus and rockfall-talus slopes and so on. That is why some Altai ranges are called «Alps», especially in the old literature in Geography: the Katun Alps, the North-Chuya Alps and so on (Fig. 2).
Relatively wet summers and comparatively mild and snowy winters are characteristic of northern regions of the Altai. The mean perennial air temperature does not fall lower than +15°C, and the annual one goes down to -4°C. The maxima of the air temperature in the mountains and at foothills of the Northern Altai are registered in the basin of Teletskoye Lake: +40°C, with the minima at about -50°C, with the mean quantity of precipitations 800-1000 mm per year.
Specific morphostructure of the Central and the South-Eastern Altai, i.e. the presence of high-mountain ranges separated by deep intermountain basins determines peculiarities of the climatic conditions in accordance to changes of altitude and slope expositions. Climate information on the high watersheds in the Altai is scarce for the present. Only at the beginning of the 80-s certain work was started for the estimation of some hydrometeorologic indices in high mountains. They are, in the first place, quantities of snow accumulation in firn zones of Belukha massif and glaciers of Aktru. Some small temporary meteorology stations were organized in the nival-glacial zone at the altitude more than 3000 m above sea-level about 15 years ago. Now it is known that the mean perennial air temperatures in July are +4°C-+6°C here with the extreme marks +17.4°C --7.6°C
kuray intermountain íHtsui Chuya intermountain basin Lymon interfuountain basin íhe Chuya River gorge between ■■~ Chuya and Kuray depressions
Location ia Fig. 5 . - •• A-B - line of the profile represented ¿./on the section (see Fig.4-4)
(53 ft O
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X
s *
p
-Si 06 O"
O ??
and the average daily amplitude of about 7°C. Annual sums of precipitations are about 1500-2500 mm near axial parts of mountain ranges; the greatest amount of them falls down in the solid form in autumn and spring-autumn time. Climate of the Central and South-Eastern Altai is more continental. Special continentality characterizes the climate in the largest Altaian intermountain basins - the Chuya and the Kuray basins.
The Chuya Basin is a vast (about 80x50 km2) isometric flat depression gently sloping from the east to the west from 2100 m to about 1600 m above sea-level with the absolute bench-mark in the central part of the valley floor (village of Kosh-Agatsh) 1757 m above sea-level. The basin is surrounded by high alpine-type ranges carrying a huge modern glacier. In Kosh-Agatsh the mean January temperature is -32°C, and that of July is +13.8°C. The mean annual temperature in the Chuya Basin is -6.7°C with the absolute minimum at the Kosh-Agatsh station -62°C. The summer maximum reaches +31 °C. The annual temperature amplitude of soil surface according to the mean perennial data is 121°C. Sharp temperature contrasts both annually or during a-day-and-a-night time are explained to greater extent not by their high mountain situation but by fierce cooling under a stable anticyclonic regime over the basins. Winter air temperatures and mean annual temperatures 1-2°C higher within the mountain frame. It can be explained by the wind regime, too. There is only a weak interaction between cold and warm air strata inside the basins because of the absence of winds that contributes to mixing of the air; it leads to a sharp inversion of temperatures. In winter quiet weather reaches 60% and even more, and in summer it's about 44%. Besides the duration of the sunshine is about 110 days a year. There are strong winds on no more than 28 days a year, the mean month velocity of the wind does not exceed 3 m/s. Main wind directions are east and northeast, nearly to the equal amount west and east as well. Local mountain and valley winds change their directions: in the night - from the mountains, in the day-time - up the valleys.
The quantity of precipitations increases also from basins toward watersheds. In the Chuya Basin there are less than 110 mm of precipitations a year, and 70% of them fall down in summer. Their quantity grows up to 160-200 mm per year with the mountain frame of basins, in Aktru valley at North-Chuya Range their quantity exceeds already 700 mm a year, and, as it was already mentioned, it is more than 1000 mm annually within the sites close to the watersheds of North-Chuya Range. The climate is not less rigorous in other parts of the South-Eastern Altai either: in Ulagan Basin, on Chulishman and Ukok tablelands, at the headwaters of the Argut, the Chuya, the Bash-kaus and the Chulishman rivers.
Modern Altai glaciation is represented by all morphologic types of glaciers which can be found in other mountain-glacial regions on the Earth. The total glacier area here is over 910 km2. These glaciers belong mainly to critique, slope, cliff and couloir glacier-types. On the whole the glaciers' number decreases with the increase of their size, and the critical glacier size corresponding to the climate under the given orographic conditions is 3.6 km that co-ordinates very well with mean cirque dimensions and points to the fact that the latter represent the orographic base of the modern glaciation (Reviakin et al., 1979).
The variety of the glacier existence conditions in the Altai is confirmed by heightening of the modem snowline from the north-west towards the south-east from 2100-2300 m up to 3300 m. From the palaeoglacial point of view it is very important that the role of the heat factor grows towards the south of the mountain land, but it manifests itself more sharply in association with the general humidity decrease. It reflects in its turn in the heightening of the firn line east-wards where one can observe the decrease of all indices of glaciation energy.
At the Quaternary Age the direction of climate processes was on the whole analogous to the modem one. Glaciers also possessed high energy and produced a huge amount of geological work the traces of which can be observed everywhere in the mountains. However, these traces are differ-
ently presented in different regions. Marginal glacial and water-glacial formations, as well as forms of glacial exaration and water-glacial erosion have been distinctly preserved in the Central
Fig. 2. Palaeoglaciohydrologic scheme of the Altai.
Cluonologic section - about 14,000 yr. BP. Legends: 1 - boundaries of ice-complex; 2 - probable limit of ice spread at the stage of the last glacial maximum; 3 - ice-dammed lakes; 4 - spillways, 5 - giant current ripples; 6 - directions of diluvial floods; 7 - maximum lake boundaries; 8 - sites of diluvial terraces and ramparts; 9 - modern glaciers; 10 - «dry waterfalls».
Figures on the map are manes of reconstructed lakes (named after the corresponding basins): 1 - Chuya Lake; 2 - Kuray Lake: 3 - Uymon Lakes, 4 - Yaloman Lake; 5 - Ulagan Lake; 6 - Teletskoye Lake; 7 - Julukul Lake; 8 - Jassater Lake; 9 - Tarkhat Lake; 10 - Bertek Lake; 11 -Abay Lake; 12 - Kan Lakes. Lake dimensions are reconstructed according to lake terraces and deposits and also according to trace marks of the glaciers which dammed lake depressions.
Maximum dimensions of ice-dammed lakes are reconstructed according to the absolute marks of spillways and outburst valleys and of diluvial ramparts on watersheds.
Boundaries of glacial complexes are drawn proceeding from the position of the snow-line which was 1200 and 800 metres lower than modern ones 18,000-20,000 and 14,000 years ago.
and especially in the South-Eastern Altai, in the South-West of Tuva and in Northern Mongolia, and they are open for immediate studies. In more humidified northern parts of Central Asia mountains they are partly destroyed or completely demolished by erosion, comouflaged with abundant vegetation and, all in all, look "more ancient" than formations of the same age in more continental intra-mountain regions. Taking into account an insufficient number of datings for loose sediments, this circumstance, in particular, led to different interpretations of the same exposures.
The mean perennial water run-off from the territory of the Altai is 42 x 109 m3. The drainage network is rather thick, the total number of all the watercourses is 20188, and their common length reaches 62555 km. About 95 per cent of streams and 60 per cent of their length are represented by the rivers with the length less than 10 km. Stream gradients reach highest values (up to 130 m/km) in high mountains, fall down from 20 to 3 m/km in the mid course and have practically plain character in the lower reaches (Fashtshevsky, 1971).
The main rivers of the Altai are the Katun, the Bia, the Chuya and the Chulyshman. These rivers, except the Bia, being an orographic continuation of the Chulyshman, have nearly fully-mountain character and belong to the Altaian type according to their high-water regime and water run-off course during the year. The Altain river type is characterized by not high, prolonged floods having a comb discharge hydrograph and by a heightened summer-autumn run-off and minimum winter one. Basic characteristics of the Altai rivers and some confluents are represented in Table 1.
Table 1
Hydrographie River Characteristics, the Altai Mountains
(Fashtshevsky, 1971)
River Site Drainage area, km2 Distance to the site, km Mean annual discharge, m3/s Stream gradient, m/km Highest discharge registered, m3/s
Chulyshman Balykcha 16,600 211 155 8.2 2400
Bia Biysk 36,900 230 477 1.1 5040
Katun river mouth 60,900 680 640 2.5 5520
Argut Argut 7070 163 92 9.6 918
Chuya Bely Bom 10,900 281 42 10.0 293
Koksa Ust-Koksa 5600 177 85 6.6 716
The specific discharge of the Altai Mountain streams is high. The Aktru and the Akkem rivers possess the highest discharges in the high-mountain zone: 20-30 1/s. They are alimented mostly by snow melting, ice melting and rainfalls. Confluents of the Upper Chuya basin (the Yustyd, the Bar-Burgazy, the Kokoria, the Shibety, the Ulandrik) have low specific discharge - 3-6 1/s that is explained by Fashtshevsky with the presence of some orographic obstacle to moisture-containing winds in the mountain framing of the Chuya basin. The share of the glacial alimentation is defined by the glacier dimensions and its altitudinal position. The river regime is alternating with the change
of geographical zones. Increase of the total quantity of precipitations and their solid phase with the altitude increase caused by the air temperature lowering calls forth a change in the correlation of the water balance components. The vaster the glacier area - the longer the high-water stage of the high-mountain rivers, and the closer its connection with the temperature course. The flood duration and its characteristics mostly depend on the altitude of the catchment area and its distribution according to altitudinal levels. Presence of the long-term permafrost lying close to the day surface prevents the atmosphere moisture considerably from infiltrating into the soil, that is why high discharges coefficients and sharp seasonal transitions and alternations in the discharge in the high-mountain, nival-glacial and in some cases in the periglacial zones are observed. High discharge coefficients are also caused by deep seasonal freezing of the surface, high local gradients and often by bedrock which is not deposited very deep. Exceptions are the rivers flowing through high-mountain basins where the discharge coefficient is considerably lowered at the expense of much waste for infiltration. The character of river annual alimentation and regime is presented in Table 2.
Table 2
Alimentation Types of the Altai Mountain Rivers
(Fashtshevsky, 1971, specified)
River Site Catchment area, km2 Glaciation area, Catchment area mean altitude, km Annual alimentation share, %
km2 %
Katun Belukha 65 16.3 21.5 3.1 10 27 14 49
Aktru Hydro-meteo station 33.4 14.7 44.1 3.0 10 15 15 60
Chagan river mouth 385 73.0 18.9 2.8 11 32 18 39
Argut settlement of Argut 7000 154 2.2 2.4 9 35 13 43
Chuya Bely Bom 10,600 250 2.7 2.35 33 31 18 18
Akkem Hydro-meteo station 72 13.0 18.1 3.0 9 35 13 43
Bia Biysk 36,900 9.4 0.0 0.9 15 55 30 0.0
Main determinants of the formation of the maximum discharges on the rivers are the snow-store volume by the beginning of melting and warmth coming, the sun radiation being the chief factor of snow and ice melting. The minimum discharges are usually observed in all the rivers at the end of the winter.
The freeze-up duration on the rivers varies from 110 to 200 days, some rivers of the Chuya headwaters are frozen down to bottom during the winter period. A number of streams don't freeze up at individual spots during the whole year.
A shallow-deposited upper level of the long-term permafrost over the vast tundra and steppe areas of the South-Eastern Altai caused a wide spread of icing features; here both vast aufeis fields and flood valley icing remain safe during the summer time of favourable years.
The number of lakes in the Altai Mountains runs to about 7000 with their total area over 600 km2. Their origin varies greatly. In the high-mountain zone mostly cirque, moraine-dammed and rock-bar-dammed lakes can be found. There are moraine-dammed lakes in each mountain-glacial basin and they have a broad age diapason: beginning from modern near-glacial lakes through all the degradation stages of the last glaciation ending with relict lakes in the Chuya and the Julukul basins. The largest lakes among the Holocene moraine-dammed ones are the basins of the Ak-Kol river-valley (South-Chuya Range), Moulta, Akkem and Kochurla Lakes in the periglacial zone of Katun Range, Shavla Lakes and Maashey Lake at North-Chuya Range. The efflux of these lakes is carried out as a rule by means of filtration through moraine swells. Sometimes cirque lakes get empty over the rock bar in waterfalls. About one third of the moraine-dammed lakes have an open efflux.
The largest lakes of the group are cirque lakes, their depth exceeds sometimes 30 m. The lakes occupying depressions in a knob-and-kettle moraine relief though have a vast area, but are relatively shallow. For example, Julukul Lake at the altitude of 2199 m has the maximum depth only about 10 m with its area about 34 km2. Thermokarst lakes are associated with the regions of well-developed long-term permafrost, they can be found on flat surfaces of downworn flood-plains, accumulative interstream plains and high-mountain basins of the South-Eastern Altai. Their depths can reach 5-7 m. Though they are rather great in number, their total area is small.
Teletskoye Lake occupies about 223 km2; it is one of the largest and most beautiful lakes in Siberia. The Teletskoye basin is of the tectonic origin. The sides of the basin once underwent ice exaration. As for its structure, the basin presents a young, obviously Late-quaternary, gapping fault along an extensive submeridional structure. The water mirror of Teletskoye Lake lies at the mark of 436 m. The length of the lake at its meridional part is 76 km, the mean width is 3.2 km, the maximum width is about 5 km. The maximum depth of the lake is 325 m. The lake water volume is 40 km3. The main tributary of the lake is the Chulyshman which contributes about 70 per cent of the summary water receipt. The Bia, the only lake's outflow, carries away about 98 per cent of the coming water. Thus only about 2 per cent are wasted for evaporation.
The mean amplitude of the lake levels is 359 cm per year, the maximum of it is 608 cm. In summer only a 20-40-metre water layers gets warm with the maximum temperature 22°C on the surface.
Summarising this brief physical-geographic review one should put out the fact that the Altai Mountains are a typical representative of the mountains belonging to Central-Asian mountain zone by many components. From the orographic point the Altai takes an intermediate position among the highest mountain systems of Central Asia and among middle- and low-mountain constructions of Middle and South Siberia. Geographic position of the mountain land and relatively deep relief dissection resulted in the variety of climatic conditions over the area and in the contrasting vertical landscape zone division. Presence of a great number of modern glaciers of different morphologic types, high-mountain near-glacial lakes and rivers let us make a vast use of the actualising approach to glacioclimatic reconstructions of ancient-glacial ages according to the pattern of really existing mountain-glacial basins, with regard to the fact that the orographic scheme of the Altai Mountains did not undergo any principle reconstructions during the middle of the Late-Quaternary time, and climatic processes had the same direction at the late age as they have now. An additional favourable point that makes the Altai Mountains especially convenient for the studies of the past ice-dammed lake regime is a rather thorough study of facts of Quaternary glaciers, the traces of which are developed all over the area and, from any point of view, they testify a dominant influence of glaciers or phenomena caused by glaciers on the Earth surface in Pleistocene.
FUNDAMENTALS OF THE THEORY OF DILUVIAL MORPHOLITHOGENESIS
The main peculiarity of the regime of ice-dammed lakes is their cataclysmic emptying (jokul-hlaups). All jokulhlaups are very rapid and rarely take longer than 10-15 days. Periods of maximum discharges of outburst flooding take usually less than 10 per cent of this time. In their turn the magnitudes of water discharges grow up sharply and become immense during these short intervals of their culminations at the hydrograph peaks. Nowadays the number of modern ice-dammed lakes amounts to thousands (the Alps, the Pamirs, the Caucasus, Tien Shan, Kora Korum, Alaska, Pa-tagonian Andes, Scandinavia, Iceland and others). Their outbursts lead to grandiose floods, the damage of which influences considerably not only the budgets of whole states but also can cause tragic, and possibly, irreversible national demographic consequences because of the great number of human victims.
Table 3
Hydrologic Characteristics of the Greatest Quaternary Ice-Dammed Lakes
Name of a lake or a lake system Area, xlO3 km2 Hw, m Vmax> km3 Hj,m Discharge, xlO5 m3/s Source
Stolper, North Germany 5 0,037 Piotrowski, 1994
Kan Lakes, Altai 0,26 (?) 19 (?) 0,10 Rudoy et al., 1989
Porcupine, Alaska 1,34 Thorson, 1989
Ulagan Lakes, Altai 0,12 300 14 10-20 Butvilovsky, 1985
Abay, Altai 0,32 (?) 300 23 (?) 230 0,14 Rudoy et al., 1989
Uymon Lakes, Altai 1,2 (?) 200 200 217 1,9 The same paper
Yaloman Lakes, Altai 0,017-0,04 760 120 830 2,0 The same paper, also: Rudoy, 1995
Jassater Lakes, Altai 0,6 270 100 300 (?) 2,0
Darkhat Lakes, Mongolia 2,6 200 (?) 250 430 4,0 Grosswald, 1987
Missoula, North America 7,5 635 2514 170 Pardee, 1942; O'Connor, Baker, 1992
Chuya-Kuray Lakes, Altai 12 900 3500 >1000 180 Baker, Benito, Rudoy, 1993; Rudoy, 1981,1984, 1995a; Rudoy, Baker, 1993
For comparison - the mean annual run-ofi ? of the Amazon is 7000 km (a )out 15 per cent of the
world annual runoff).
Hw - depth of the lake near the dam; VmflX - lake volume; Hj - ice dam thickness calculated ac-cording to the formula by J.Nye (Nye, 1976) and geomorphologically reconstructed_
Pleistocene glacier of dry land and continental shelves was accompanied by the rise and growth of giant near-glacier water bodies whose dimensions were many times greater than modern ones. Discharges of outburst superfloods out of them exceeded often 1 mln. m7s, water velocities were dozens metres per second, and depths of such floods were more than hundreds of meters (Table 3). The initial surface underwent immense transformations during very short time intervals as a result of geologic work of glacial outburst superfloods. ]
A wide spread of ice-dammed lakes of different types during glacier periods, their systematic failures caused by unstable ice dams because of the low ice density, great, sometimes - cardinal, consequences of these failures, all these facts led the author to distinguish reasonably a specific complex of exogenous processes - diluvial ones. Diluvial processes of relief formation are processes of Earth surface transformation by cataclysmic water streams out of outbursting ice-dammed lakes. Therefore, these very floods are called diluvial (Rudoy, 1987).
Glaciers dam river valleys and aliment lakes appearing within them. The lakes break ice dams, wash away or destroy glacier traces in the main run-off valleys out of lake basins. Glacier processes are more active at high hypsometric levels while diluvial ones - at lower levels, where in most cases their influence exceeds that of the glaciers which gave rise to these processes. This is the essence of the theory.
The author's concept about the cause-consequence connections of glacial and diluvial processes in the Altai Mountains has been tested in all great river valleys of Southern Siberia during the last decade. Presently it can be extrapolated theoreti cally to all the regions of the Earth globe having a similar palaeoglaciohydrologic situation for any time section.
The theory of diluvial morpholithogenesis is an objective possibility to consider the probability of diluvial floods in the river valleys of such mountain countries where the morphostructura! appearance is similar to the Altai, and where the once-present Quaternary glaciation capable of blocking the river outflow from intermountain basins has been confirmed with certainty, hi the first row mountains and uplands of the Baikal region belong to such districts. Here the whole mountainous diluvial morpholithocomplex can be found, moreover, judging by the size of the dammed reservoirs it is even much more grandiose than that in the Altai-Sayany Mountain District. On the other hand, the diluvial morpholithogenesis concept lets us solve a reversed problem: to assume the presence of large pre-glacial lakes at the upper reaches of the rivers going by material traces of the diluvial floods within the river valleys, and consequently to reconstruct the dimensions of the glacier. Geological age of diluvial-accumulative deposits will be adeqate both to the age of corresponding ice-dammed lakes and to the age of glaciers blocking them. Such direct and reversed extrapolations are true for plain territories as well.
Climatic conditions of individual regions at definite time intervals (glacio-eras, -periods, -epochs, ice phases and oscillations) conditioned glacier formation and existence. Glaciers dammed river flows that would lead to the rise of lakes. Ice dams would fail when lake depressions got overfilled with melt water and the release of potential water energy occurred. The energy volume was determined by the water mass, valley declivity and roughness of the out-throw channel, and by the geometry of the latter, too.
Going by the regime of oblation and how quickly the intermountain depressions would be filled with melt water an about-a-century periodicity of outbursts from giant ice-dammed lakes during the age of last glaciation has been ascertained. The repetition of cataclysmic outbursts from the large pre-glacial basins of the Central and South-Eastern Altai is confirmed geologically, too (Rudoy, 1993a).
Cataclysmic outbursts of basinal ice-dammed lakes of Central Asia used to produce water streams with the discharges over 1 mln. m3/s. Maximum water discharges used to be about 18 mln. m3/s when Chuya and Kuray ice-dammed lakes which were the greatest in the Altai Mountains
would fail (Baker, Benito, Rudoy, 1993). Momentary flood velocities would exceed 40 m/s, while the depths of a superflood would reach 400 m. They were the greatest freshwater floods known on the Earth.
High discharges and velocities of cataclysmic glacial superfloods determined their ability to produce enormous amounts of work. Systematic failures of giant ice-dammed lakes led to immense transformations of the initial surface. The latter included, on the one hand, development of deep gorges, channels of the water out-throw and evorsional forms, and, on the other hand, accumulation of thick units of loose sediments, namely, ramparts and terraces, giant current ripples and diluvial berms.
Zones of energetic local vortices and still broader areas of backwaters would develop at the expansions of the main run-off valleys (or in the intermountain basins) as well as at curvatures or bends and other alternations in the plan configuration of stream channels (Fig.3).
Destructive and accumulative forms developed in such a way perform paragenetic morpholi-thologic associations of the territories of pre-glacial and glacial zones which undergo or underwent before a repeated influence of catastrophic floods out of failing ice-dammed lakes (scablands). The process of the scabland development was named "diluvial", and the constructive and distinctive work of glacial superfloods performs the essence of diluvial morpholithogenesis (Rudoy, 1996).
Ice-dammed lakes are one of the most important components of Quaternary continental nival-glacial systems at the local, regional and, probably, global levels. Rise of such lakes and their existence are conditioned by glaciers, lake regime is determined mostly by glacier regime. Front parts of glacial covers of Eurasia and North America held and alimented immense amounts of freshwater. As a result, there appeared vast ice-dammed freshwater seas on pre-glacial territories (Bretz, 1923, et al.; Baker & Banker, 1985, et al.; Arkhipov, et al., 1995; Grosswald & Hughes, 1995). In the mountains melt water concentrated within huge intermountain basins blocked by glaciers. Ice-dammed lakes in the mountains and on the plains communicated via a net of diluvial run-off chan-
The area of ice-dammed Mansi Lake in Eastern Siberia was over 600,000 km2, while the total area of all ice-dammed seas of plains and tablelands of North Asia was not less than 3 mln. km (Rudoy, 1995b). Areas of basinal ice-dammed lakes were much lesser. However, owing to the deeply dissected relief their depths were hundreds of metres. Summary volume of basinal ice-
o J
dammed reservoirs in the Altai exceeded 7.300 km with their total area more than 27,000 km (Rudoy, 1995a). Water streams with the volume of thousands of cubic kilometres flew catastrophi-cally to the water surface of dammed freshwater basins of Eastern and Western Siberia. Simultaneous out-throws of ice-dammed lakes in the Altai only gave nearly a 12-metre increase of the level of Mansi Lake, it was 4 metres more than the lake needed to flow over Turgay run-off channel. Cataclysmic emptyings of the ice-dammed lakes of Darkhat and Khubsugul could not but cause the same effect in the dammed Yenisei Basin. As a result, Kas-Ket spillway came to action and it set to work the whole grandiose run-off system of ice-dammed freshwater seas of North Asia.
An about-a-century periodicity of outbursts of the greatest basinal ice-dammed lakes in Southern Siberia led consequently to an about-a-century periodicity of out-throws of the surface water stratum of several metres thick out of pre-glacial plain basins into the Atlantic basin. Moreover, the run-off system of Siberian ice-dammed seas itself is meant to have been extremely dynamic (Arkhipov et al., 1995; Grosswald & Hughes, 1995). Then we may assume that the modern tracery of the hydrographic net of plain territories which bore Quaternary glaciation is conditioned mainly
by climatic, not tectonic reasons. Morphologic associations of plain scablands must have been formed on such territories. Plain scablands, probably, are not only very different from those of the mountains but also possess a set of special features. The latter allow to differenciate this relief both from typical erosional-denudational landscapes and typical accumulative fluvial plains from the positions of the morpholithogenesis concept. In this respect vast territories of Putorana Plateau, Tunguska trapps-plateau and many other regions of Middle and Eastern Siberia are of much prospect. Here the palaeoglaciohydrologic situation of the last glacial time must have been very agressive, and broadly developed basaltic covers of different age could have contributed to the formation and conservation of the scabland similar to the Channelled Scabland of Columbia Pla-
Influence of large pre-glacial lakes upon the Earth crust is an independent problem of Quaternary Glaciohydrology. On the one hand, the load of the lake-water stratum of several hundreds metres deep itself could not be very influential for any noticeable sagging of local parts of the continental crust. On the other hand, The following fact is of great importance: this load appeared and disappeared periodically and very rapidly from the geologic position due to the filling and catastrophic emptying of the lakes. The latter made some limited continental locations rather unstable ("diluvial tremble") and caused inevitable local earth-quakes, fissures and faults within the framing of lake depressions, and rockfalls and landslides as well. From the scientific point of view it is noteworthy that that periodically appearing "limnoisostasy" may serve as another reaction example of the endogenous component of the morpholithogenesis to any alternations in the outer conditions which have exclusively climatic causes. Seismic unstability of large ice-dammed basins is conditioned by periodical and sharp fluctuations of the load on lake-depression bottoms. It could also have served as impulses to ice movements which resulted in new damming of the ice outflow, lake depressions being filled with water again.
DAM AND OUT-THROW MECHANISMS OF QUATERNARY ICE-DAMMED LAKES IN
THE MOUNTAINS
Development Of Ice-Dammed Lakes In The Mountains
Proving the recurrence of cataclysmic outbursts of Quaternary giant ice-dammed lakes (Baker & Banker, 1985; Rudoy, 1984, 1988a, et al.) the modern research focuses mainly on the study of last stages of pre-glacial lake evolution. For the time being the first catastrophe act stays practically out of sight of Quaternary Glaciohydrology, i.e. mechanisms of repeated dammings of lake depressions, their causes, and the dynamic type of the glaciers which would dam intermountain basins.
Most of modern ice-dammed lakes which have had jokulhlaups are dammed by pulsating glaciers. Every repeated filling of the basins with melt water was preceded by a new shift of the dam glacier. If surges did not occur then no lake appeared, i.e. there were no outbursts and, by all means, no diluvial floods occurred either. In such a case neither diluvial sediments nor diluvial relief would have appeared.
Modern analogues show that all ice-dammed Pleistocene lakes of South Siberia (and their total area was not less than 100,000 km2 within the volume total over 60,000 km3 of water) were dammed with pulsating glaciers. The number of cataclysmic lake outbursts which have been proved geologically and analytically equals the number of shifts of the glaciers which would dam every individual glacier basin (if those emptyings were realized by means of outbursts, sweeping down of
ice dams). Within the Altai Mountains only there existed several dozens of large ice-dammed lakes during the last ice age (25,000-14,000 yr. BP), Their areas over a hundred of square kilometres each, and at least five of them had the areas over 1000 km with the water volume over 300 km each (Chuya-Kuray, Uymon, Jassater lake systems and Teletskoy, Bertek and Tarkhat Lakes) (Fig. 2). Those lakes occupied intermountain basins of the most various morphologic types. They are nearly equally spread over the whole territory of the Altai Mountains. It means that pulsation glaciers which might dam those basins were equally characteristic of all altitudinal climatic zones of the Altai (at least - during the late ice time), even in case each of dammed lakes emerged and outburst only once. The dimensions of those pulsating glaciers were by one-two orders greater than modern ones as the dimensions of the lakes dammed by them exceeded the modern ones considerably, too. The thickness of ice dams was inversely proportional to the number of their outbursts. Consequently, the diluvial flood discharges were directly dependent on their dam thickness.
Repeated outbursts of big pre-glacial plain lakes in North America and North Eurasia may also testify to great surges of outlet glaciers along the periphery of the Quaternary ice covers which would block river valleys. The fact partly confirms the previously-made conclusions (M. Gross-wald, G. Denton and T. Hughes) about surges which used to be characteristic of both mountain glaciers and ice covers.
Ice-Dammed Lakes and Ledoyoms
Many intermountain depressions used to be fully occupied by glaciers from the surrounding mountains during glaciation maxima and would turn into giant ledoyoms (the Russian term "ledoyom" in word-by-word translation means a "body of ice" by analogy with a "body of water"), which would begin to function as independent glacier centres. Those centres aliment huge valley glaciers within river valleys that came out of the ledoyoms (Chuya, Kuray, Ulagan, Julukul and many other intermountain depressions of South Siberia). Calculations of the melt glacier run-off volume at the maximum and the post-maximum of the last glacial have shown that it used to be much greater than the contemporary one. Within the basin of Chuya intermountain depression (in the Chagan-Usun river valley range on the way out of depression) this volume used to be 8.8 km3 per year, i.e. by over 30 time more than the contemporary one. Thus the transgressions of the pre-glacial lakes and those of the glacier were synchronized (Rudoy, et al., 1989). And it means that under a glacier culmination the intermountain basins must have been already occupied by dammed lakes. The TL- and 14C datings on the lacustrine-glacier varved "clays" confirmed the calculations given above (32,000-25,000 yr. BP) respectively according to the varved clays of the Chagan-Usun River; here there existed a long and narrow bay of Chagan ice-dammed lake during the interglacial of the Late Wurm Ice Age (Rudoy, Kirianova, 1994).
The interrelations of glaciers with ice-dammed lakes had a several-scenario development in South Siberia (depending on the basin topography, the snow-line depression in the surrounding mountains and the glaciation energy): a) a ledoyom only (without a lake) - the so-called "classic ledoyom"; b) a body of water and a ledoyom combined (Fig. 3. 4). This model deals with the case when mountain-valley glaciers (Fig. 4-1, 4-3) which went down from the mountains into lake depressions join floating ("shelf glaciers) on the surface of a large ice-dammed lake and they over-cover the latter. Then "catch lakes" appear (Fig. 4-2). When a "catch lake" throws the water, the glacier ice goes down onto the bottom of the lake and leaves certain geologic forms of "dead" ice. It
also projects the eskers and kames which have been previously developed on the surface of the "shelf' glacier onto the lake bottom (Fig.5) (Rudoy, 1990). The eskers and kames are not laid on the ground moraine, as it normally occurs, but on the lacustrine-glacier sediments; c) ledoyoms of "aufeis" type. This model considers the cases when the limit-line of the glacier alimentation goes lower the lake mirrors (Fig.6). At the same time there appear complicated forms which consist of a thick lens of melt water which is armoured by lake-, "aufeis"- and glacier-ice, and by snow-firn sequence, too. Surfaces of such ice-dammed lakes, consequently, are involved into the zone of glacier alimentation and they turn into independent ice centres with subradial ice outlet. Possible analogues of such an evolution mechanism of pre-glacial lakes and glaciation are thick water lenses under a 3-4-kilometres' units of the ice cover at the location of Dome B and Dome C and Vostok Station in Eastern Antarctica (Rudoy, 1995b); d) an ice-dammed lake only.
Under different extensions of the glacier at different time periods one and the same basin underwent sequence of the lake-glacier events.
Outbursts of Ice-Dammed Lakes
The outbursts of ice-dammed lakes would occur within a broad range of mechanisms: from a slow leakage and thermoerosion to cataclysmic breaches, ice failures. In the main run-off valleys out of some basins there remain moraine fragments of the glaciers that used to dam the lakes. These moraines are associated with the ranges of the outburst sites at the upper levels of the run-off channels along the way out of the depressions. We have studied such forms at the site between Chuya and Kuray basins (Fig. 4, Site "G"), below Kuray basin on the slopes of the site of Barotal, in the Katun river valley below the site of Sok-Yaryk and at some other locations. It is very likely that at the stages of late glaciation the outflows of Chuya, Kuray and Uymon ice-dammed lakes in the direction of the main valleys would mostly run via intra- and near-glacial channels and via under-glacial spillways, too, the lakes not reaching the maximum volumes because of the main outflow decrease and little thickness of the dams. The dams were never completely destroyed at those stages. So was the case, for example, of the failure of Strandline Lake in Alaska in September 1982 (Sturm, Benson, 1985). The volume of the lake was 7x10 m , and the water outflow velocities were estimated by the authors of the paper as equalling 14 m/s. After a cataclysmic failure of the lake which had taken only five hours the run-off channels remained still open for about a year longer, afterwards they got closed again. W. Mathews informs of a cataclysmic outburst of Summit Lake in December 1965 (Mathews, 1973). The lake flowed down via an intra-glacial tunnel of a regular shape with its maximum diameter of 13 m and its length of nearly 13 km, the water discharges being 3200 m3/s. After a cataclysmic outburst of Abdukagor Lake in the Pamirs in 1973 just immediately below it there remained some fragments of both Medvezhy (Bear) Glacier which had dammed the lake and front moraines of Glacier of the Russian Geography Society which was situated 5 km down the valley. But still further down the stream of the Vanch River the valley turned out to be washed by the glacial floods along its full stretch (Rudoy, 1995a).
It is very likely that the mechanism of underglacial out-throws of the lakes would become prevailing at the glacier culminations. Though the out-throws themselves would occur much more rarely. They might be similar to those described by J. Piotrowskij (1994) for the Late-Pleistocene Stolper Lake in Northern Germany in the North Sea, and by Brennard and Shaw (1994) for the Late-Quaternary glaciers of southern Ontario Lake.
o\
Fig. 3- Palaeohydrological sketch of the Kuray intermontane basin, the Altay. Tim slice of about 1*1.5 yr. BP (Rudoy, 1995)
Legend: 1 - directions of diluvial floods (established); 2 - the same, inferred; 3 fields of giant current ripples; 4 - spillways, gorges of outbursts and oversplashes; 5 - end moraines; 6 - boundaries of intermontane basins; 7 - modern glaciers.
Fields of giant current ripples in the zone of backwater of the basin center are presented in Figs. 10 and 11 also.
a
a ?
05> o-
»N
Ci ?!
Fig A. STAGES OF THE "CATCH LAKES" DEVELOPMENT WITHIN INTERMOUNTAIN BASINS OF SOUTH SIBERIA 25-22 THOUSAND YR. BP. K and C - KURAY and CHUYA ICE-DAMMED LAKES. G - THE CHUYA RIVER GORGE BETWEEN CHUYA and KURAY
DEPRESSIONS „ , „>.
B-A - Profile through Chuya
and Kuray basins (see4-3)
¿jjr fZT When the "shelf glaciers joined (lowing an ,t— ¡he surface they usetf to armour completely r/je surface at the ice-dammed lakes, whic h had turned Into catch lakes' ¡22-20 f*T*!-? thousand //-. BP). As the snow-linc went
¿¿^¿zEZ-_— <*own by 300 m more (about 20-13
mKy^-——' ____' thousand yr. BP), there appeared
V^y Independent glacier centres )n the
Intemountain depressions. They consisted of a lense of lake water covercd by take-.
О
l is- А
Marginal icc formations in Uymon intermountain basin it) the Central Altai (see Fig. 1 and space photograph). 3- (l) - an cskcrs exposure in the central part of the basin: I) brown loess; 2) layered sands, coarse- and fine-grained, grey and washed types. Aside from the grain orientation in concordant bedding the layered structure is emphasized by alternating coarse- and line-grained horizons. "Pockets" of hold rock are often to be found; .V) brown layered sands involved in all the horizons as long bands, more l ately as lenses; 4) unclcarly bedded gravel and pebble gravel. The bedding has a dome-shaped, "anticlinal" appearancc irrespective of the section orientation; 5) lake sands of coarse-grained type (even gravels) horizontally bedded. Section i - (t) is orientated in concord with the cskcrs trelch. S-Q) - an cskcrs exposure at the location of the settlement of Ntzhniy l.'ymon; I) sod; 2) brown focss; 3) dark-grey coarse-grained sands. Exposed as lenses, wedges, "pockets";
a 4) loess-type loamy sand with
moraine intcrlaycrs and lenses; 5) clearly bedded lake gravel and pebble gravel. The bedding foot goes under the faces, Eskers do not lie on ground moraines but on lake pebble gravels. They were developing on the ice covering which used to armour the whole or the greater part of the lake surfacc (sec l-ig. 4- (1) and ; 2 ') With the emptying of the lake the glacier would go down onto the bottom. 5- (3) - marginal cskcrs of the rim of the glacicr tongue which had developed at the contact of the glacicr and (lie ice-dammed lake, a - plan: b - section: I) brown loess; 2) gravel and pebble grave! with rare small boulders. Clear bedding copies the surface topography; 3) dark-brown water-saturated loamy soil; 4) moram.
I i .У, ;* л•:* *. .t .'Г. *.. * *- -"_-- Г_ 7,;%
Fig. 5. Marginal ice formation in Uymon intermountain basin in Central Altai
Thousand yr. BP
15 я
25
35
и 1200
|_L
Volume of 200 water, km 3 1„ .....
400 1—
600
800 km
Fig. 6. Principle scheme of the mechanism of an aufeis-type iedoyom development within ice-dammed lakes of the intermountain basins of South Siberia (Chuya intermountain basin taken as an example) 1 - water body stage; 2 - aufeis-type Iedoyom stage; 3 - snowline depression A Hs.
PROBABLE CHRONOLOGY AND DISCUSSION
Observations in modern mountain-glacial basins show that alternations in ice run-off volumes are closely connected with the glacier area (Dikikh, 1993; Panov, 1993; and others). Generally, the total area of the glaciers increases simultaneously or somewhat belated with the firn-line getting lower and ice-accumulation areas expanding. Consequently, the quantity of the ice-melt outflow increases in its absolute expression, too. At ice ages glacial discharge shares grew considerably as related to initial or modern ones. It would happen under great depressions of the snow-line (in the Altai and Sayany they were over 1200 m). The ice-melt outflow would respectively increase relating to initial or modern ones. It is proved by our calculations, too (Rudoy, et al., 1989). Therefore intermountain basins having the only narrow and deep run-off channel would react immediately with the melt water concentration within the basins as a reply to ice-damming. That is why, there always were pre-glacial lakes in the mountains and on the plains when the glacier grew large enough to block river valleys. This important theoretical premise let us reconstruct the chronology of lake-glacial events while there was a limited set of reliable absolute dates. It was done mainly proceeding from the speed of filling of intermountain basins with melt water and from the hydraulics of diluvial floods (Rudoy, Baker, 1996).
The difficulty with the determining of the absolute age of the lake outbursts is also that the duration of the diluvial form development and the period of formation of the sediments of a different genesis under dating which could serve as certain registration marks for the age can be confronted by no means. We speak of minutes, hours and days for the former, while for the latter there are at least many tens and hundreds of years. That is why the period of accumulation of the sediments of several floods may go into one error of the defining of the absolute age. The dating of the sediments that belong to the intermediate events between the floodings can be generally of no more success for two reasons: firstly, for the one mentioned above, and, secondly, - it's geologic or bio-geographic methods that the investigator should use to decide what particular events took place and the way they may be deciphered now in concrete exposures and landscapes.
Determining synchronisity of basin filling with melt-water with ice ages at least up to the levels which are singularly interpreted as lake terraces, determining the speed of this filling proceeding from the melt outflow (it is on the average about 100 years for Chuya basin and about 30 years for Kuray basin) up to 2200 m above sea level we come to the following conclusion. Further filling of the basins with the water ceases because the lakes got empty and (or) because the snow-line went down to the mirror of water bodies and even lower.
Maximum lake transgressions, i.e. the maximum storage of melt water, synchronized with the glacier maxima. But it is just at that time that the ice dams were so thick that it is very unlikely for the lakes to have those about-a-century periodical outbursts which have been mentioned above. Regular water outflows out of ice-dammed lakes must have occurred at the starting and the final points of the glacials when the ice dams were unstable. Besides, under culminations of the Quaternary glaciers the snow-line depression would cause the conservation of the lakes, the latter turned into "aufeis" ledoyoms. At that time the lakes which were buried under lake-, aufeis- and glacier-ice would not break up for thousands of years all completely.
Reconstructions of Kuray and Chuya ice-dammed lakes were fulfilled according to the preserved lake terraces. It is obvious that the ancient shore-lines cannot fix the greatest water store in the basins, since maxima of water volumes, as it has been written above, synchronized with maximum glacier dimensions, excluding periods of "aufeis" ice bodies. At these stages foothill and "shelf' glaciers from the valleys alimenting the basins were mainly destroyed within the shore-line of the water bodies. Th ey were lakes in ice-"baths" which left practically
no material traces as their walls just melted out during the glacial degradation process.
To the same degree of precision and certainty we may speculate of the number of the "super outbursts" of the Chuya-Kuray lake system the discharges of which used to be over 1 mln. mVs. Such maximum discharges were calculated for the cases of entire water out-throw from the lakes. Examples of modern pre-glacial lakes show that complete emptyings of pre-glacial lakes are far from being so often as partial water out-throws under the most various mechanisms of ice-dam deformations (Post, Mayo, 1971; Nye, 1976; and others). Thus, the most likely scenario for the discussion seems that, suggested above, or rare, individual (possibly, a single) phenomenal jokulhlaups with the discharges over 15-20 mln. m3/s against the background of systematic outbursts of ice-dammed lakes with the period of about a century and discharges less than 1 mln. m3/s. Of course, the boundary is rather conventional for the present.
The speed of the lake-depression filling has been defined as 100-130 years for the Late-Pleistocene ice-dammed lake of Darkhat in North Mongolia (Grosswald, 1987). The calculations are based on the correlation of the volume of atmosphere precipitations and evaporation with the annual run-off in the basin of Darkhat depression at the glacial maximum of Wiirm and the modern large river basin. The time of the depression filling of 100-130 years long gives an about-a-century periodicity of the repeated cataclysmic emptyings of Darkhat Lake into the Yenisei basin, too. For the present we can probably consider such an about-a-century period of cataclysmic dryings (before we've got more precise data) a characteristic feature of large ice-dammed lakes of Central Asia, if we mean similar climatic conditions of their alimentation. The task now is that we should find some trustworthy geological proofs of that fact. They might be members or layers of diluvium in the flood terraces in the mountains. Search for repeated series within geologic exposures of the diluvial piedmont cone in the south of West and Middle Siberia might turn out to be more fruitful.
Bearing in mind synchronism of diluvial processes with initial and final ice stages, one should also comprehend that the scabland structure observed nowadays is nevertheless conditioned mainly by the work of last superfloods out of last ice-dammed lakes broken up during the degradation period of the last glaciation.
Nowadays there are several trustworthy radiocarbon datings from various parts of the Altai Mountains: beginning from the foothills up to the high-mountain basins. They show that the last phenomenal cataclysmic failure of the Chuya-Kuray system of ice-dammed lakes left, in particular, giant current ripples in Kuray and probably Yaloman basins and at the site of Platovo-Podgornoye, too, at the Altai foothills. It happened not later that 13 thousand years ago. We make the conclusion on the basis of two facts. The age of vegetation remnants from lacustrine loamy soil in the frost mounds of the plateau of Yeshtikkol in the western part of Kuray basin equals 10845 ± 80 years (CO AH - 2346) according to V.A. Panishev. That means that there was no water in that part of Kuray basin further than approximately 11 thousand years ago. According to marlstones and vegetation remnants within the sediments of the North Altai the absolute datings of 13890 ± 200 and 12750 ± 65 years (CO AH - 779) have been gained. A.M. Maloletko associates these datings with the time of evorsional depression of Ai and the carrying of erratic boulders over the foothills (Maloletko, 1980). Thus, Maloletko concludes, the cataclysmic flooding took place in the Katun river valley 13 thousand years ago. The statement is difficult to disagree. After that geologic date the lake would degradate simultaneously with the degradation of the glaciers which alimented them. This does not, of course, exclude their cataclysmic outbursts, but the hydraulics of those failures was not, probably, so grandiose.
A.N. Rudoy discovered some vegetation remnants in the exposure of a big frost mound (pingo) in the central part of Chuya intermountain basin. V.A. Panishev and L.A. Orlova estimated
the absolute age of the hold rock according to those remnants: 3810 ± 105 years (CO AH - 2146). There are some reasons to think that the frost mounds themselves developed even later - about 2140 years ago (Rudoy, 1988b). Consequently, there was no lake already in the basin by that time. In other words, it's possible to say for sure, that ice-dammed lakes of the inter mo untain basins is Siberia disappeared completely later than 5,000 yr. BP, that the once-vast water bodies splitted into a number of small lakes. The relicts of the latter have been preserved up to now.
CONCLUSIONS
1. Ice-dammed lakes in the mountains and on plains existed always when the glacier grew large enough to block river valleys.
2. Discovery of systematic cataclysmic glacial superfloods which had formed the Central Asian scabland destroyed a more than 60-year-old idea about unique grandiose failures of North-American Missoula Lake. Numerous outbursts of all discovered ice-dammed lakes in intermountain basins in the mountains of South Siberia and America mean that diluvial processes pass from the sphere of phenomena into the row of normal exogenous processes of the relief-formation which produce a certain amount of work under certain orographic conditions.
3. The diluvial morpholithogenesis theory is based on the proposition of some connection between glacial and diluvial processes. It lets us reveal the latter in palaeoglaciohydrologic situations similar to the ones under study in any region of the Earth and to reconstruct them and to make prognoses for any chronologic sections.
4. Diluvial processes of relief-formation belong to well-known powerful exogenous Pleistocene processes. Diluvial processes generated by glaciers transformed glacial and pre-glacial territories of many regions on the Earth and created landscapes characteristic of plain and mountain scablands.
5. The 90-s of this century were declared by the UNESCO to be a decade of struggle against natural calamities. In the respect of a rapid pace of assimilating mountain territories it is desirable to consider the role of short-term but powerful and systematically reoccurred processes when estimating the degree of natural risk. At individual geologic moments these processes transform those forms that have been created by the Nature during thousands of years. The consequences of these processes are the more tragic, the more unexpected the calamity is.
Acknowledgements
The author gratefully acknowledges the participating of Marina Kirianova in all recent high-mountain Altai expeditions and her active discussion of the paper.
Special thanks to Elena Hubert who both has translated all the author's materials into English and shared the hardships of the expeditions to the mountains of Siberia in 1993-1995.
The research is supported by the Russian Foundation for Fundamental Research, Grant No. 97-05-65878.
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