Historical Glacier-dammed Lakes and Outburst Floods in the Karambar Valley (Hindukush-Karakoram)

摘要

At least six devastating glacial floods occurred in the Karambar valley in the 19th and 20th century. Previously mainly the Karambar glacier was considered as the origin of these outburst floods. However, in this project more detailed investigations revealed that up to eight more tributary glaciers could have dammed the Karambar valley in historical and prehistorical times. The ice-dammed lakes reached an approximate length of up to about 5 km and more. 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 probably involved almost synchronously the drainage of at least two lakes resulting in a lake outburst cascade. The Karambar case study highlights the characteristic geomorphological landforms of the glacier dams, their lake basins and the geomorphological impact of the outburst floods. The abundant occurrence of unconsolidated sediments mantling the valley flanks caused 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. Secondary temporary lake formations (back water ponding) during the flood events in consequence of blockages of the ice- and sediment-loaden flood masses occurred at many locations in the narrower valley sections and lasted for several days. Additionally, debris flows in-between the glacier dams have dammed temporarily the Karambar valley. On the basis of losses of settlement area and eye-witness reports, the extent, erosion rates and characteristics of the 1905 flood event could be reconstructed. In order to warn the villagers living downstream, the Karambar people established an early warning fire system (Puberanch) from Sokther Rabot to Gilgit which was operated until 1905. The reconstructed Karambar flood chronology represents one of the longest records for this region and provides also information on historical and recent glacier oscillations, especially on exceptional glacier advances. At present, the Chateboi glacier seals the Karambar valley over a distance of 4 km. An outburst flood would have disastrous impacts to the human infrastructure as the settlement areas expanded to the flood plains in the last decades.