Evolution of a landslide-dammed lake on the southeastern Tibetan Plateau and its influence on river longitudinal profiles

Abstract

Landslide damming of valleys may have a significant effect on the evolution of fluvial landscapes. The southeastern Tibetan Plateau is highly prone to landslide damming due to its deeply incised valleys and young, active geology. Here we present a sedimentological investigation of a landslide-dammed lake in the upper reach of Lulang River on the eastern Himalayan syntaxis. Detailed interpretation of lacustrine varve, lakeshore, and subaqueous delta sediment shows that Lulang landslide-dammed lake (LLDL) had a surface elevation of 3360 m a.s.l., an area of 3.3 × 106 m2, and a volume of 6.5 × 107 m3. Radiocarbon dating of six organic clay lacustrine samples demonstrate that LLDL formed before 24.2 ka cal. BP and persisted for at least 8.8 ka. Sediment infill reached full capacity soon after 15.4 ka cal. BP and fluvial conditions then prevailed. Sedimentary evidence and morphological analysis of cross sections along the LLDL outlet channel revealed a larger, earlier lake with a total volume of 4.3 × 109 m3 and water surface elevation at 3570 m a.s.l. that failed catastrophically; the resulting outburst flood drained 98.5% of the impounded lake. The remnant lake maintained a relatively stable water level of 3360 m a.s.l. for its lifetime of at least 8.8 ka. Long profile and steepness index analysis indicate that the location of the catastrophic dam failure and subsequent long-term blockage is associated with a knickpoint. The origin of the knickpoint may have been polygenetic, however, the landslide damming seems to have increased its distinctiveness and persistence. The relatively wide valley and low channel slope characteristic of reaches upstream of LLDL is likely to have resulted from glacial activity, rather than backwater aggradation which had only a local influence.