Abstract:Among the large rivers rising on the Tibetan Plateau and adjacent high mountains, the discharge and suspended sediment load of the Ayeyarwady (Irrawaddy) River are the least well known. Data collected between 1969 and 1996 at Pyay (Prome) are analysed to provide the best available modern estimate of discharge (379 š 47 ð 10 9 m 3 /year) and suspended sediment load (325 š 57 ð 10 6 t/year) for the river upstream of the delta head. A statistical comparison with data collected in the nineteenth century (1871 to 1879) shows discharge has significantly decreased in the last ¾100 years. Regression and correlation analyses between discharge in the modern period and indices of El Niño-Southern Oscillation (ENSO) show a relationship.

Field measurements conducted 4 years after the construction of a new portion of the Weixi-Shangri-La road in Yunnan, China, reveal that unprecedented rates of mass wasting occurred along the road with much of this sediment directly impacting the headwaters of the Mekong River. Landslide erosion (including dry ravel) exceeded 33,000 t ha -1 year -1 along the most severely eroded sections of the road and averaged more than 9,600 t ha -1 year -1 along the surveyed 23.5 km of road; these values are the highest ever reported for road-related landslides. While surface erosion was only about 7% of the total erosion from the road, it is still more than an order of magnitude higher than typical surface erosion rates from disturbed lands in Southeast Asia. Combined landslide and surface erosion from this road delivered an estimated 19 times more sediment to the river than the remaining 99.6% of the contributing catchment. These sediment inputs are aggrading local channels, promoting downstream sediment transport, degrading aquatic habitat, and creating the possibility for a future debris flood or hyperconcentrated flow.

Many shallow landslides were triggered by this M w 6.6 (M j 6.7) earthquake. The basement complex in the affected area (sedimentary rocks) is covered with thick pyroclastic fall deposits derived from the Tarumae Volcano, etc., and the strong seismic shocks triggered shallow landsliding of them. Shallow landslides moving along valley type topography traveled greater distances than those moving along planar slope topography. Some shallow landslides occurred on relatively gentle slopes (< 30°). The earthquake also induced several large-scale deep-seated landslides, including one that has formed a landslide dam in the Hidaka-horonai River. Landslides were densely distributed over hilly regions (elevation: 200-400 m) within an area of approximately 400 km 2 in Atsuma (landslides caused 36 deaths), Abira, and Mukawa, and the number of landslides and the total area of the landslides were the largest in Japan ever since the Meiji Era . The catchments where shallow landslides were concentrated were severely devastated.