ALBERT

Description: [1]  As rivers transport water and sediment across Earth's surface they radiate elastic and acoustic waves. We use seismic and infrasound observations during a controlled flood experiment (CFE) in the Grand Canyon to show that three types of fluvial processes can be monitored from outside the channel. First, bed-load transport under conditions of evolving bed mobility is identified as the dominant seismic source between 15 – 45 Hz. Two lower-frequency seismic bands also excited by the CFE exhibited greater power increases, and are consistent with source processes related to fluid rather than sediment transport. The second fluvial seismic source is inferred to be fluid tractions on the rough riverbed, which drive the maximum seismic power increase at 0.73 Hz, but do not excite infrasound. Waves at the fluid-air interface are suggested as a third source, which generates a common 6 – 7 Hz peak in seismic and infrasound responses to the CFE.

Description: High Asian glacial landscapes have large variations in topographic relief and the size and steepness of snow accumulation areas. Associated differences in glacial cover and dynamics allow a first-order determination of the dominant processes shaping these landscapes. Here we provide a regional synthesis of the topography and flow characteristics of 287 glaciers across High Asia using digital elevation analysis and remotely sensed glacier surface velocities. Glaciers situated in low-relief areas on the Tibetan Plateau are mainly nourished by direct snowfall, have little or no debris cover, and have a relatively symmetrical distribution of velocities along their length. In contrast, avalanche-fed glaciers with steep accumulation areas, which occur at the deeply incised edges of the Tibetan Plateau, are heavily covered with supraglacial debris, and flow velocities are highest along short segments near their headwalls but greatly reduced along their debris-mantled lower parts. The downstream distribution of flow velocities suggests that the glacial erosion potential is progressively shifted upstream as accumulation areas get steeper and hillslope debris fluxes increase. Our data suggest that the coupling of hillslopes and glacial dynamics increases with topographic steepness and debris cover. The melt-lowering effect of thick debris cover allows the existence of glaciers even when they are located entirely below the snow line. However, slow velocities limit the erosion potential of such glaciers, and their main landscape-shaping contribution may simply be the evacuation of debris from the base of glacial headwalls, which inhibits the formation of scree slopes and thereby allows ongoing headwall retreat by periglacial hillslope processes. We propose a conceptual model in which glacially influenced plateau margins evolve from low-relief to high-relief landscapes with distinctive contributions of hillslope processes and glaciers to relief production and decay.

Description: Spatially variable response of Himalayan glaciers to climate change affected by debris cover Nature Geoscience 4, 156 (2011). doi:10.1038/ngeo1068 Authors: Dirk Scherler, Bodo Bookhagen & Manfred R. Strecker Controversy about the current state and future evolution of Himalayan glaciers has been stirred up by erroneous statements in the fourth report by the Intergovernmental Panel on Climate Change. Variable retreat rates and a paucity of glacial mass-balance data make it difficult to develop a coherent picture of regional climate-change impacts in the region. Here, we report remotely-sensed frontal changes and surface velocities from glaciers in the greater Himalaya between 2000 and 2008 that provide evidence for strong spatial variations in glacier behaviour which are linked to topography and climate. More than 65% of the monsoon-influenced glaciers that we observed are retreating, but heavily debris-covered glaciers with stagnant low-gradient terminus regions typically have stable fronts. Debris-covered glaciers are common in the rugged central Himalaya, but they are almost absent in subdued landscapes on the Tibetan Plateau, where retreat rates are higher. In contrast, more than 50% of observed glaciers in the westerlies-influenced Karakoram region in the northwestern Himalaya are advancing or stable. Our study shows that there is no uniform response of Himalayan glaciers to climate change and highlights the importance of debris cover for understanding glacier retreat, an effect that has so far been neglected in predictions of future water availability or global sea level.

Description: [1]  Erosion in the Himalaya is responsible for one of the greatest mass redistributions on Earth, and has fueled models of feedback loops between climate and tectonics. Although the general trends of erosion across the Himalaya are reasonably well known, the relative importance of factors controlling erosion is less well constrained. Here, we present 25 10 Be-derived catchment-average erosion rates from the Yamuna catchment in the Garhwal Himalaya, northern India. Tributary erosion rates range between ~0.1-0.5 mm yr -1 in the Lesser Himalaya and ~1-2 mm yr -1 in the High Himalaya, despite uniform hillslope angles. The erosion-rate data correlate with catchment-average values of 5-km radius relief, channel steepness indices, and specific stream power, but to varying degrees of non-linearity. Similar non-linear relationships and coefficients of determination suggest that topographic steepness is the major control on the spatial variability of erosion, and that 2-3-fold differences in annual runoff have only minor effects in this area. Instead, the spatial distribution of erosion in the study area is consistent with a tectonic model in which the rock-uplift pattern is largely controlled by the shortening rate and the geometry of the Main Himalayan Thrust fault (MHT). Our data support a shallow dip of the MHT underneath the Lesser Himalaya, followed by a mid-crustal ramp underneath the High Himalaya, as indicated by geophysical data. Finally, analysis of sample results from larger main-stem rivers indicates significant variability of 10 Be-derived erosion rates, possibly related to non-proportional sediment supply from different tributaries and incomplete mixing in main-stem channels.