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Seasonal precipitation gradients and their impact on fluvial sediment flux in the Northwest Himalaya
(2010)
Precipitation in the form of rain and snowfall throughout the Himalaya controls river discharge and erosional processes and, thus, has a first-order control on the fluvial sediment flux. Here, we analyze daily precipitation data (1998-2007) of 80 weather stations from the northwestern Himalaya in order to decipher temporal and spatial moisture gradients. In addition, suspended sediment data allow assessment of the impact of precipitation on the fluvial sediment flux for a 10(3)-km(2) catchment (Baspa). We find that weather stations located at the mountain front receive similar to 80% of annual precipitation during summer (May-Oct), whereas stations in the orogenic interior, i.e., leeward of the orographic barrier, receive similar to 60% of annual precipitation during winter (Nov-Apr). In both regions 4-6 rainstorm days account for similar to 40% of the summer budgets, while rainstorm magnitude-frequency relations, derived from 40-year precipitation time-series, indicate a higher storm variability in the interior than in the frontal region. This high variability in maximum annual rainstorm days in the orogenic interior is reflected by a high variability in extreme suspended sediment events in the Baspa Valley, which strongly affect annual erosion yields. The two most prominent 5-day-long erosional events account for 50% of the total 5-year suspended sediment flux and coincide with synoptic-scale monsoonal rainstorms. This emphasizes the erosional impact of the Indian Summer Monsoon as the main driving force for erosion processes in the orogenic interior, despite more precipitation falling during the winter season.
In the high mountains of Asia, glaciers cover an area of approximately 115,000 km² and constitute one of the largest continental ice accumulations outside Greenland and Antarctica. Their sensitivity to climate change makes them valuable palaeoclimate archives, but also vulnerable to current and predicted Global Warming. This is a pressing problem as snow and glacial melt waters are important sources for agriculture and power supply of densely populated regions in south, east, and central Asia. Successful prediction of the glacial response to climate change in Asia and mitigation of the socioeconomic impacts requires profound knowledge of the climatic controls and the dynamics of Asian glaciers. However, due to their remoteness and difficult accessibility, ground-based studies are rare, as well as temporally and spatially limited. We therefore lack basic information on the vast majority of these glaciers. In this thesis, I employ different methods to assess the dynamics of Asian glaciers on multiple time scales. First, I tested a method for precise satellite-based measurement of glacier-surface velocities and conducted a comprehensive and regional survey of glacial flow and terminus dynamics of Asian glaciers between 2000 and 2008. This novel and unprecedented dataset provides unique insights into the contrasting topographic and climatic controls of glacial flow velocities across the Asian highlands. The data document disparate recent glacial behavior between the Karakoram and the Himalaya, which I attribute to the competing influence of the mid-latitude westerlies during winter and the Indian monsoon during summer. Second, I tested whether such climate-related longitudinal differences in glacial behavior also prevail on longer time scales, and potentially account for observed regionally asynchronous glacial advances. I used cosmogenic nuclide surface exposure dating of erratic boulders on moraines to obtain a glacial chronology for the upper Tons Valley, situated in the headwaters of the Ganges River. This area is located in the transition zone from monsoonal to westerly moisture supply and therefore ideal to examine the influence of these two atmospheric circulation regimes on glacial advances. The new glacial chronology documents multiple glacial oscillations during the last glacial termination and during the Holocene, suggesting largely synchronous glacial changes in the western Himalayan region that are related to gradual glacial-interglacial temperature oscillations with superimposed monsoonal precipitation changes of higher frequency. In a third step, I combine results from short-term satellite-based climate records and surface velocity-derived ice-flux estimates, with topographic analyses to deduce the erosional impact of glaciations on long-term landscape evolution in the Himalayan-Tibetan realm. The results provide evidence for the long-term effects of pronounced east-west differences in glaciation and glacial erosion, depending on climatic and topographic factors. Contrary to common belief the data suggest that monsoonal climate in the central Himalaya weakens glacial erosion at high elevations, helping to maintain a steep southern orographic barrier that protects the Tibetan Plateau from lateral destruction. The results of this thesis highlight how climatic and topographic gradients across the high mountains of Asia affect glacier dynamics on time scales ranging from 10^0 to 10^6 years. Glacial response times to climate changes are tightly linked to properties such as debris cover and surface slope, which are controlled by the topographic setting, and which need to be taken into account when reconstructing mountainous palaeoclimate from glacial histories or assessing the future evolution of Asian glaciers. Conversely, the regional topographic differences of glacial landscapes in Asia are partly controlled by climatic gradients and the long-term influence of glaciers on the topographic evolution of the orogenic system.
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.
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(1,2). Variable retreat rates(3-6) and a paucity of glacial mass-balance data(7,8) 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(9,10) or global sea level(11).
The sediment flux through Himalayan rivers directly impacts water quality and is important for sustaining agriculture as well as maintaining drinking-water and hydropower generation. Despite the recent increase in demand for these resources, little is known about the triggers and sources of extreme sediment flux events, which lower water quality and account for extensive hydropower reservoir filling and turbine abrasion. Here, we present a comprehensive analysis of the spatiotemporal trends in suspended sediment flux based on daily data during the past decade (2001-2009) from four sites along the Sutlej River and from four of its main tributaries. In conjunction with satellite data depicting rainfall and snow cover, air temperature and earthquake records, and field observations, we infer climatic and geologic controls of peak suspended sediment concentration (SSC) events. Our study identifies three key findings: First, peak SSC events (a parts per thousand yen 99th SSC percentile) coincide frequently (57-80%) with heavy rainstorms and account for about 30% of the suspended sediment flux in the semi-arid to arid interior of the orogen. Second, we observe an increase of suspended sediment flux from the Tibetan Plateau to the Himalayan Front at mean annual timescales. This sediment-flux gradient suggests that averaged, modern erosion in the western Himalaya is most pronounced at frontal regions, which are characterized by high monsoonal rainfall and thick soil cover. Third, in seven of eight catchments, we find an anticlockwise hysteresis loop of annual sediment flux variations with respect to river discharge, which appears to be related to enhanced glacial sediment evacuation during late summer. Our analysis emphasizes the importance of unconsolidated sediments in the high-elevation sector that can easily be mobilized by hydrometeorological events and higher glacial-meltwater contributions. In future climate change scenarios, including continuous glacial retreat and more frequent monsoonal rainstorms across the Himalaya, we expect an increase in peak SSC events, which will decrease the water quality and impact hydropower generation.
Despite global warming and unlike their Himalayan neighbours, glaciers in the Karakoram mountains do not show signs of significant retreat. Here we report high velocity variations of Biafo Glacier, central Karakoram, which occurred between 2001 and 2009 and which indicate considerable dynamics in its flow behaviour. We have generated a dense time series of glacier surface velocities, based on cross-correlation of optical satellite images, which clearly shows seasonal and interannual velocity variations, reaching 50% in some places. The interannual velocity variations resemble the passing of a broad wave of high velocities, with peak velocities during 2005 and some diffusion down-glacier over a period of at least 4 years. High interannual velocity variations are also observed at other glaciers in the vicinity, suggesting a common cause, although these appear to partly comprise longer acceleration phases. Analysis of weather station data provides some indications of meteorological conditions that could have promoted sustained sliding events during this period, but this does not explain the wave-like nature of the acceleration at Biafo Glacier, and the regular, protracted velocity changes.
TopoToolbox is a MATLAB program for the analysis of digital elevation models (DEMs). With the release of version 2, the software adopts an object-oriented programming (OOP) approach to work with gridded DEMs and derived data such as flow directions and stream networks. The introduction of a novel technique to store flow directions as topologically ordered vectors of indices enables calculation of flow-related attributes such as flow accumulation similar to 20 times faster than conventional algorithms while at the same time reducing memory overhead to 33% of that required by the previous version. Graphical user interfaces (GUIs) enable visual exploration and interaction with DEMs and derivatives and provide access to tools targeted at fluvial and tectonic geomorphologists. With its new release, TopoToolbox has become a more memory-efficient and faster tool for basic and advanced digital terrain analysis that can be used as a framework for building hydrological and geomorphological models in MATLAB.
The Himalayan mountains are dissected by some of the deepest and most impressive gorges on Earth. Constraining the interplay between river incision and rock uplift is important for understanding tectonic deformation in this region. We report here the discovery of a deeply incised canyon of the Yarlung Tsangpo River, at the eastern end of the Himalaya, which is now buried under more than 500 meters of sediments. By reconstructing the former valley bottom and dating sediments at the base of the valley fill, we show that steepening of the Tsangpo Gorge started at about 2 million to 2.5 million years ago as a consequence of an increase in rock uplift rates. The high erosion rates within the gorge are therefore a direct consequence of rapid rock uplift.
The Indus River, one of Asia's premier rivers, drains the western Tibetan Plateau and the Nanga Parbat syntaxis. These two areas juxtapose some of the lowest and highest topographic relief and commensurate denudation rates in the Himalaya-Tibet orogen, respectively, yet the spatial pattern of denudation rates upstream of the syntaxis remains largely unclear, as does the way in which major rivers drive headward incision into the Tibetan Plateau. We report a new inventory of Be-10-based basinwide denudation rates from 33 tributaries flanking the Indus River along a 320 km reach across the western Tibetan Plateau margin. We find that denudation rates of up to 110 mm k.y.(-1) in the Ladakh and Zanskar Ranges systematically decrease eastward to 10 mm k.y.(-1) toward the Tibetan Plateau. Independent results from bulk petrographic and heavy mineral analyses support this denudation gradient. Assuming that incision along the Indus exerts the base-level control on tributary denudation rates, our data show a systematic eastward decrease of landscape downwearing, reaching its minimum on the Tibetan Plateau. In contrast, denudation rates increase rapidly 150-200 km downstream of a distinct knick-point that marks the Tibetan Plateau margin in the Indus River longitudinal profile. We infer that any vigorous headward incision and any accompanying erosional waves into the interior of the plateau mostly concerned reaches well below this plateau margin. Moreover, reported long-term (>10(6) yr) exhumation rates from low-temperature chronometry of 0.1-0.75 mm yr(-1) consistently exceed our Be-10-derived denudation rates. With averaging time scales of 10(3)-10(4) yr for our denudation data, we report postglacial rates of downwearing in a tectonically idle landscape. To counterbalance this apparent mismatch, denudation rates must have been higher in the Quaternary during glacial-interglacial intervals.
Some of the largest and most erosive floods on Earth result from the failure of glacial dams. While potentially cataclysmic ice dams are recognized to have repeatedly formed along ice-sheet margins, much less is known about the frequency and longevity of ice dams caused by mountain glaciers, and their impact on landscape evolution. Here we present field observations and results from cosmogenic nuclide dating that allow reconstructing a > 100-k.y.-long history of glacial damming in the Shyok Valley, eastern Karakoram (South Asia). Our field observations provide evidence that Asia's second-longest glacier, the Siachen, once extended for over 180 km and blocked the Shyok River during the penultimate glacial period, leading to upstream deposition of a more than 400-m-thick fluvio-lacustrine valley fill. Be-10-depth profile modeling indicates that glacial damming ended with the onset of the Eemian interglacial and that the Shyok River subsequently incised the valley fill at an average rate of similar to 4-7 m k.y.(-1). Comparison with contemporary ice-dammed lakes in the Karakoram and elsewhere suggests recurring outburst floods during the aggradation period, while over 25 cycles of fining-upward lake deposits within the valley fill indicate impounding of floods from farther upstream. Despite prolonged damming, the net effect of this and probably earlier damming episodes by the Siachen Glacier is dominated by glacial erosion in excess of fluvial incision, as evidenced by a pronounced overdeepening that follows the glaciated valley reach. Strikingly similar overdeepened valleys at all major confluences of the Shyok and Indus Rivers with Karakoram tributaries indicate that glacial dams and subsequent outburst floods have been widespread and frequent in this region during the Quaternary. Our study suggests that the interaction of Karakoram glaciers with the Shyok and Indus Rivers promoted valley incision and headward erosion into the western margin of the Tibetan Plateau.