TERRACE EVOLUTION AND INCISION RATES ALONG THE UPPERMOST LANGTANG RIVER, NEPAL HIMALAYA

Terrace development in the Nepal Himalaya can reveal long-term river incision as well as depositional events and subsequent river response. We report estimated ages of a sequence of three terraces at Langshisa Kharka on the Langtang River near its source at 4115 m elevation ~9km from the Tibetan border in the Langtang Himal. At Langshisa Kharka the Langtang River is a braided channel and two glaciers, the Shalbachum to the north and the Langshisa to the south, feed into it. The moraines of both glaciers have been dismembered, possibly by glacial lake outburst floods that produced debris flows. Relative elevations of four terrace levels, including the modern river level, were measured using tape and compass. The terraces consist of sand and poorly to moderately rounded cobbles and boulders. Terrace ages were estimated using lichenometry; an apparent growth curve for R. geographicum was previously developed based on lichen sizes on surfaces of known age near Kyanjin Gompa, ~15 km downstream. The sizes of R. geographicum present on boulders on each terrace were measured and the largest on each terrace was identified. Lichen more than 20% larger than the next largest lichen and were excluded as outliers. Results of terrace heights and ages are as follows. The lowest terrace contained no lichen as is considered to be modern. On the first terrace (1.28m above river level) the largest lichen was 32mm in diameter, corresponding to an age of 18.3 ybp (1993). On the second terrace (5.46m above river level) the largest R. geographicum was 40mm, corresponding to an age of 26.3 ybp (1985). The highest terrace (9.68 m above river level), was partially covered by colluvium and an age could not be determined. Based on the above data, the incision rate of the Langtang River was 0.56 m/yr from 1985 – 1993 and 0.07 m/yr from 1993 – 2011, or 0.22 m/yr from 1985 – 2011. These are net incision rates, which take into account both incision and possible episodes of aggradation, and are large compared to the average rate of 0.033 m/yr determined by Barnard (2006) for the past ~800 years for the Langtang River 15-30 km downstream. It is likely that the rapid incision recorded by the Langshisa Kharka terraces reflects rapid downcutting of the river in response to pulses of deposition from debris flows, which in turn suggests at least two major flood events in the past 30 years.