Lateroglacial landforms in subtropical high mountains: A case study from the Karakoram and Hindukush Mountains part of Vignettes:Vignette Collection
Lateroglacial landforms are located at the lateral margins of the glaciers and are part of the glacial landscape systems (Iturrizaga 2001). The term "lateroglacial" defines the ice-marginal areas at the lateral sides of the glacier. The lateroglacial environments pass over glacier downwards into the latero-frontal and finally the proglacial environments. In principal, they can be found along any valley glacier, but they occur preferentially at valley glaciers in high mountain regions with sufficient debris supply areas and their associated forelands. Lateroglacial landforms were investigated systematically as a specific geomorphological unit along 53 glaciers in the Karakoram and Hindukush (72°-79°E; 35°-36°N) in regard to their distribution, evolution and morphodynamics (Iturrizaga 2001, 2003, 2007). They are best-developed in these mountain regions, which show the highest concentration of large valley glaciers outside of the pole regions with glacier lengths of up to 72 km in combination with a surrounding high mountain relief of up to over 8000 m a.s.l.. Lateroglacial sediment complexes may attain a length of up to several to tens of kilometres and are referred to as "lateroglacial valleys". Lateroglacial valleys are ice-marginal depressions between the glacier and the valley flank and sometimes even show a drainage system over short distances. They are not true valleys in sensu stricto, but rather glacier-marginal discontinuous depressions. Where mountain spurs stand out against the glaciated main valley, the lateroglacial valleys may be interrupted. The topographical depression can be filled up with different types of sediments or the deposition of the sediments itself may even be the trigger for the formation of the lateroglacial valleys. These landform assemblages are classified as a specific geomorphologic unit of the glacial environment. One of their main characteristics of the lateroglacial sediment regime is the damming effect by the glacier, which results in a large-scale sediment storage along the glacier margins. The lateroglacial sediments are polygenetic landforms consisting of a variety of different debris source areas. Besides their large horizontal distribution, they are spread over a considerable vertical range in the Karakoram and occur between an altitude of about 25005000 m a.s.l.. The upper limit of the occurrence of lateroglacial sediments deriving from processes of glacial deposition is theoretically given by the altitude of the equilibrium line altitude (ELA). Due to the fact that a major part of the glaciers are avalanche-fed glaciers and therefore show steep inclined head walls of up to 3000 m in height, the distribution of lateroglacial sediments starts usually 10001500 m lower than the ELA. The transition from the valley flank to the glacier has in different morphological variations. One of the most well-known and prominent landform is the lateral moraine valley. It is a mostly linear depression or sediment assemblage between the lateral moraine and the valley flank. It had been often confused with an ablation depression (see below) or commonly referred to as "ablation valley". Visser (1938) carried out the first systematic investigation on these landforms and postulated an insolation-controlled distribution of all lateroglacial landforms. This terminology has been widely criticised by many authors (v. Klebelsberg 1938, Hewitt 1993). Consequently, the non-genetical expression "lateroglacial valleys" has been introduced (Iturrizaga 2001). The formation of lateral moraine valleys is mainly a result of a) dumping processes of the lateral moraine against the valley flank, especially during times of a comparatively smaller extent of the glacier, and b) different type of debris inputs from the adjacent tributary valleys and valley slopes (fig. 1 & 2). Their width attains a size of up to 1 km. The great dimension already indicates that ablation processes can only play a subordinated role in their formation. The lateral moraine is one of the most distinct depositional landform in lateroglacial environments. They may attain a height of about 250 m. Their large size has been attributed to repetitive glacier advances and accordingly to various deposition processes. The lateral moraine is often composed of an older moraine core, which has been superimposed by younger moraine layers and/or processes of moraine accretion. The time period of their formation goes in general back to the Neoglacial and Little Ice Age. Once the lateral moraine has been built up, it prevents the direct debris transfer from the glacier to the interior of the lateroglacial valleys. Consequently, the sedimentary system in the ice-marginal depression is well protected from glacial activities, unless some over spilling or break through of the lateral moraine takes place. In addition to that, the lateral moraine impedes the drainage of the tributary rivers into the glacier system. The lateral moraine valleys are distinguished into two principal types: a) The V-shaped lateral moraine valley, in which the lateral moraine is directly connected with the adjacent valley flank (fig. 3). These landforms develop by dumping processes occurring at the glacier margin. b) The lateral moraine valley with a valley bottom floor (fig. 4). The incorporated sediments are composed of heterogeneous debris sources: 1. Primary processes of rock disintegration such as ice avalanches and freeze-thaw processes as well as glaciofluvial sediments from the main and tributary glaciers provide debris for the formation of lateroglacial sediments, especially for the formation of the lateral moraine. 2. A considerable part of the debris supply for the lateroglacial sediment complexes derives from the tributary valleys, in particular at glaciers framed by highly dissected mountain relief (fig. 1). The sediment cones, such as alluvial fans, debris flow cones and avalanche cones drain towards the glacier either into an existent ice-marginal depression or even onto the glacier. Especially large-sized, catastrophical debris flows can even initiate the formation of a lateroglacial valley. Rockslides, debris flows and snow avalanches, deposited into a lateroglacial valley, frequently dam lateroglacial rivers and cause the deposition of lacustrine sediments. As a result a considerable proportion of the lateroglacial sediments is of non-glacial origin. This fact has to be taken into consideration regarding glacier reconstruction in recent non-glaciated mountain valleys. Relict lateroglacial valleys occur as high as 1000 m above the present glacier surfaces and are important landforms for reconstructing the Pleistocene glacier thickness.Dendritic glaciers in which the individual tributary glaciers recede and loose the contact to the main glacier are prone to the formation of lateroglacial sediment complexes. 3. The lateroglacial sediment landscape is built up to a great extent by the secondary debris supply in form of the reworking of older glacigenic deposits mantling the valley flanks (fig. 2). Many glaciers show a close interfingering of Late Glacial slope moraines and younger lateroglacial landforms. After the gradual down-melting of the Pleistocene glacier surface, moraine deposits remain along the lateroglacial margins, partly as terraces, partly as amorphous deposits and are dislocated into the lateroglacial valley by different types of mass movements. 4. In some parts, the sediments are deposited in form of small scaled lateroglacial sandar. These are glaciofluvial deposits which originate directly from the melt water of the main or the tributary glacier. They are located between the glacier and the valley flank or the lateral moraine. After the deglaciation, these sediments are preserved as kame terraces. The formation of the lateroglacial valleys might be triggered by the formation of an ablation depression as it can provide initial sediment traps (Iturrizaga 2003). The ablation depression is a lateroglacial landform which is a void located between the valley flank and the adjacent glacier (Oestreich 1906, Visser 1938) or between the sediment covering the valley flank and the glacier. Its formation is a direct consequence of insolation and reflection effects by the bedrock or the sediment (fig. 5). Due to the heating of the rock surface and the subsequent emission of long-wave radiation, the glacier ice is melting back at its margins forming a striking void. The width of ablation depressions varies from some meters to decameters. Their optimal distribution area is located along glaciers in subtropical latitudes with high rates of insolation close to the solar constant and low rates of humidity promoting a high transparency of the air. They may also occur in high latitudes, but they are smaller in size. However, the overall distribution of lateroglacial sediments is dominated by topographical factors rather than by insolation.
Glacier lake outburst cascades and backwater lake formations in the Hindukush Mountains part of Vignettes:Vignette Collection
Glacier lake outburst floods in the Hindukush-Karakoram Mountains are among the most important types of current geomorphological processes below an altitude of 4500 m. Since 1826, at least 35 glacier lake outbursts have been recorded in the upper Indus catchment area, in which 12 glaciers were involved (Fig. 1, Hewitt 1998). The glacier lakes are mainly the result of the blockage of the main river by advancing tributary glaciers, flowing down from catchment areas of over 7000 m into the semiarid valley floors at about 3000 m. Due to the high discharge rates of the rivers, large-sized lakes, several kilometres in length, can be impounded over a very short time period. The lakes exist only for some months or at most some years. The failures of the glacier lakes occur often periodically and in general between July and August during the time of the highest discharge. Three localities have been notorious for glacier lake outbursts in the last two centuries in the Hindukush-Karakoram: the tributary glaciers of the Shyok valley (East Karakoram, Mason 1935), the Shimshal valley (North-West-Karakoram, Iturrizaga 1997, 2005a) and the Karambar valley (E-Hindukush, Iturrizaga 2005b,c, 2006). The Karambar valley shows one of the highest concentrations of potential glacier dams and had been selected for a case study for demonstrating paradigmatic characteristics for reconstructing former glacier dams and the impacts of multiple glacier lake outbursts. The 90 km long Karambar valley is situated in the Eastern Hindukush (36°30'36°45' N/73°45' 74°10' E) with the Kampir Dior (7168 m) as highest peak of the catchment area. The glaciers are avalanche-fed glacier types and their lengths range between 4 and 23 km. The corresponding glacier dams are located at altitudes between 2830 and 3850 m (fig. 2). Most of the Karambar dams are framed by lateral moraines up to 250 m in height. At least six major flood events occurred in the years 1844, 1860/1861, 1865, 1893, 1895 and 1905 in the Karambar valley, from which the 1905-flood supposed to be one of the most disastrous outburst events. Previously the lowermost and best accessible glacier dam, the Karambar glacier, was primarily made responsible for the outbursts (Photo 1), but the field investigations revealed a different scenario: Nine tributary glaciers in the Karambar valley have blocked the main valley in former times (Photos 2-4). Among them, six glaciers impounded lakes since the mid of the 19th century, which drained catastrophically from time to time. There are various geomorphological indicators for reconstructing a former glacier dam (fig. 3). A marked change from light to dark rock colours at the opposite valley flank of the glacier tongues indicates the former extent of the glacier surface as well as shallow undercutting lines at the bedrock rock. The valley flanks are highly stressed by repetitive oscillations of the glacier tongues. The exerted pressure of the glacier tongue against the lower part of the mountain valley flank destabilises the foot slope generating rock failures following glacier retreat. Moraine remnants might outline the former extent of the glacier tongue, but in most cases the glacier advances are too short for moraine deposition or the sediments have already been washed away. The lateral moraines are often transformed by spill-over tongues during the glacier advance which might also result in a break-through of the lateral moraine. The advancing glacier ice against the valley flank generates a back pressure effect on the glacier tongue and causes in turn a local rise of the glacier surface and ice bulges. Accreted moraines are common along the glacier dams, as the valley widens when the tributary glacier enters the main valley. Glacier-dammed lakes rarely deposit lake sediments because the sedimentation time is comparatively short. Therefore the reconstruction is based on travel reports of early researchers and interviews with local inhabitants. The glacier lakes reached a length of up to about 5 km and more and an estimated volume of about 100150 x 106 m3. Within the Chateboi lake basin, another remarkable dam was identified which played a crucial role in the lake outburst history of the Karambar valley. A small hanging glacier, the Saklei Shuyinj glacier, has blocked as well the Karambar valley in historical time (Fig. 4, Photo 5). At present the glacier tongue is located about 2 km from the left Karambar valley flank on a confluence step. The Chateboi glacier even blocks the Karambar today over a distance of 4 km and forms small sized lakes from time to time. When a glacier blocks a valley, the river does not necessarily have to be dammed. The river can drain over long periods subglacially. Repetitive outbursts are common for ice-dammed lakes. The runout distances of the floods, reconstructed by local reports about damages of settlement areas, measure about 120150 km. The 1905-Karambar flood caused a 7 m high flood level above the normal level and several bridges were destroyed. The dense concentration of the glacier dams along a horizontal distance of only 40 km results in a complex interfingering of lake basins and flooded valley sections. In the individual flood events were involved almost synchronously the drainage of at least two lakes resulting in a lake outburst cascade. The geomorphological impacts of the floods are well visible downstream of the glacier dams. The broad gravel floor, up to 2 km in width, shows imbricated boulder clusters, with individual boulder sizes of up to 2 m in diameter, indicative for high-energy fluvial transport processes. The boulder size decreases gradually downstream. The abundant occurrence of unconsolidated sediments mantling the valley flanks cause a high sediment load and enhanced the erosion potential of the flood. The erosion cliffs of sediment cones, up to 100 m high, wash limits along the slopes and longitudinal bars in the gravel floors are main characteristics of the flood landscape. During the flood, backwater lakes formed at valley constrictions at several locations, even about 100 km downward of the glacier dam. These temporary lakes can be caused by fluvial undercutting of the slope sediments generating landslide-dammed lakes. Due to the high sediment concentrations, the flood waters themselves might form temporary blockages at narrow sections in the valley course. Downvalley, the topographical setting of the Karambar valley alternates between expansive, basin-like valley sections and narrow gorge-like valley sections, providing favourable conditions for temporary lake formation by producing a bottle-neck effect for the flood masses. Eroded wood lands in the flood plain provide further material for a jam of the flood waters. Moreover, the glacier tongues of the tributary valleys which project into the main valley cause themselves obstructions to the flood waters. The gap between the glacier tongue and the opposite valley flank measures at the narrowest point only about 50 m. Consequently, a high amount of sediment can be taken up by the flood waters, transforming it more and more to a debris flow. The flood waters of the higher glacier lake are then abruptly backed up by one of the lower glacier dams leading to temporary lake formations. Geomorphological evidences for these processes are ripple marks and eddy bars in the lake basins. The temporary lakes can in turn also drain abruptly causing small-sized floods. In order to warn the villagers living downstream, the Karambar people established an early warning fire system (Puberanch) which was operated until 1905 (Fig. 2). On good visible hill sides in an altitude of up to 4000 m a.s.l., local posts were set up, in order to sent fire signals to the lower posts down to Gilgit over a horizontal distance of over 100 km. To sum up, the overall occurrence of the formation of glacier-dammed lakes has decreased in the course of general glacier shrinkage in the Hindukush Mountains during the last century. However, glacier surges and rapid glacier advances present a high and unpredictable hazard in this region, as many glacier tongues are located in close vicinity of confluence areas and might suddenly block the main valley. Fig. 5. Photos of selected glacier dams in the Karambar valley and their lake basins. Glacier Lake Outburst Photos (Acrobat (PDF) 2.8MB Apr8 09)