Description: The hydrological cycle in the high Pamir Mountains: how temperature and seasonal precipitation distribution influence stream flow in the Gunt catchment, Tajikistan Earth Surface Dynamics Discussions, 2, 1155-1215, 2014 Author(s): E. Pohl, M. Knoche, R. Gloaguen, C. Andermann, and P. Krause Complex climatic interactions control hydrological processes in high mountains that in their turn regulate the erosive forces shaping the relief. To unravel the hydrological cycle of a glaciated watershed (Gunt River) considered representative of the Pamirs' hydrologic regime we developed a remote sensing-based approach. At the boundary between two distinct climatic zones dominated by Westerlies and Indian summer monsoon, the Pamir is poorly instrumented and only a few in situ meteorological and hydrological data are available. We adapted a suitable conceptual distributed hydrological model (J2000g). Interpolations of the few available in situ data are inadequate due to strong, relief induced, spatial heterogeneities. Instead we use raster data, preferably from remote sensing sources depending on availability and validation. We evaluate remote sensing-based precipitation and temperature products. MODIS MOD11 surface temperatures show good agreement with in situ data, perform better than other products and represent a good proxy for air temperatures. For precipitation we tested remote sensing products as well as the HAR10 climate model data and the interpolation-based APHRODITE dataset. All products show substantial differences both in intensity and seasonal distribution with in-situ data. Despite low resolutions, the datasets are able to sustain high model efficiencies (NSE ≥0.85). In contrast to neighbouring regions in the Himalayas or the Hindukush, discharge is dominantly the product of snow and glacier melt and thus temperature is the essential controlling factor. 80% of annual precipitation is provided as snow in winter and spring contrasting peak discharges during summer. Hence, precipitation and discharge are negatively correlated and display complex hysteresis effects that allow to infer the effect of inter-annual climatic variability on river flow. We infer the existence of two subsurface reservoirs. The groundwater reservoir (providing 40% of annual discharge) recharges in spring and summer and releases slowly during fall and winter. A not fully constrained shallow reservoir with very rapid retention times buffers melt waters during spring and summer. This study highlights the importance of a better understanding of the hydrologic cycle to constrain natural hazards such as floods and landslides as well as water availability in the downstream areas. The negative glacier mass balance (−0.6 m w.e. yr −1 ) indicates glacier retreat, that will effect the currently 30% contribution of glacier melt to stream flow.

Description: Tectonic and climatic controls on the Chuquibamba landslide (western Andes, southern Peru) Earth Surface Dynamics Discussions, 2, 1129-1153, 2014 Author(s): A. Margirier, L. Audin, J. Carcaillet, and S. Schwartz The contribution of landslides to the Quaternary evolution of reliefs is poorly documented in arid contexts. In southern Peru and northern Chile several massive landslides disrupt the arid western Andean front. The Chuquibamba landslide, located in southern Peru, belongs to this set of large landslides. In this area, the Incapuquio fault system captures the intermittent drainage network and localizes rotational landslides. Seismic activity is significant in this region with recurrent M w = 9 subduction earthquakes, however none of the latest seismic events have triggered a major landslide. New terrestrial cosmogenic dating of the Chuquibamba landslide provides evidence that the last major gravitational mobilization of these rotational landslide deposits occurred at ~102 ka, during the Ouki wet climatic event identified on the Altiplano between 120 and 98 ka. Our results suggest that wet events in the arid and fractured context of the Andean forearc induced these giant debris-flows. Finally, our study highlights the role of tectonics and climate on (i) the localization of large Andean landslides and on (ii) the long-term mass transfer to the trench along the arid Andean front.

Description: Seismic monitoring of geomorphic processes Earth Surface Dynamics Discussions, 2, 1217-1267, 2014 Author(s): A. Burtin, N. Hovius, and J. M. Turowski In seismology, the signal is usually analysed for earthquake data, but these represent less than 1% of continuous recording. The remaining data are considered as seismic noise and were for a long time ignored. Over the past decades, the analysis of seismic noise has constantly increased in popularity, and this has led to develop new approaches and applications in geophysics. The study of continuous seismic records is now open to other disciplines, like geomorphology. The motion of mass at the Earth's surface generates seismic waves that are recorded by nearby seismometers and can be used to monitor its transfer through the landscape. Surface processes vary in nature, mechanism, magnitude and space and time, and this variability can be observed in the seismic signals. This contribution aims to give an overview of the development and current opportunities for the seismic monitoring of geomorphic processes. We first describe the common principles of seismic signal monitoring and introduce time-frequency analysis for the purpose of identification and differentiation of surface processes. Second, we present techniques to detect, locate and quantify geomorphic events. Third, we review the diverse layout of seismic arrays and highlight their advantages and limitations for specific processes, like slope or channel activity. Finally, we illustrate all these characteristics with the analysis of seismic data acquired in a small debris-flow catchment where geomorphic events show interactions and feedbacks. Further developments must aim to fully understand the richness of the continuous seismic signals, to better quantify the geomorphic activity and improve the performance of warning systems. Seismic monitoring may ultimately allow the continuous survey of erosion and transfer of sediments in the landscape on the scales of external forcing.

Description: Hitting rock bottom: morphological responses of bedrock-confined streams to a catastrophic flood Earth Surface Dynamics Discussions, 2, 1093-1128, 2014 Author(s): M. Baggs Sargood, T. J. Cohen, C. J. Thompson, and J. Croke The role of extreme events in shaping the earth's surface is one that has held the interests of Earth scientists for centuries. A catastrophic flood in a tectonically quiescent setting in eastern Australia in 2011 provides valuable insight into how bedrock channels respond to such events. Field survey data (3 reaches) and desktop analyses (10 reaches) with catchment areas ranging from 0.5 to 169 km 2 show that the predicted discharge for the 2011 event ranged from 400 to 900 m 3 s −1 , with unit stream power estimates of up to 1000 W m −2 . Estimated entrainment relationships predict the mobility of the entire grain size population and field data suggests the localised mobility of boulders up to 4.8 m in diameter. Analysis of repeat LiDAR data demonstrates that all reaches (field and desktop) were areas of net degradation via extensive scouring of mantled alluvium with a strong positive relationship between catchment area and normalised erosion ( R 2 = 0.8). The extensive scouring in the 2011 flood decreased thalweg variance significantly with the exposure of planar bedrock surfaces, marginal bedrock straths and bedrock steps, along with the formation of a plane-bed cobble morphology. Post-flood field data suggests a slight increase in thalweg variance as a result of the smaller 2013 flood, however the current nature and distribution of channel morphological units does not conform to previous classifications of upland river systems. This suggests that extreme events are significant for re-setting the morphology of in-channel units in such bedrock systems. As important, is the exposure of the underlying lithology to ongoing erosion.

Description: Impact of change in erosion rate and landscape steepness on hillslope and fluvial sediments grain size in the Feather River Basin (Sierra Nevada, California) Earth Surface Dynamics Discussions, 2, 1047-1092, 2014 Author(s): M. Attal, S. M. Mudd, M. D. Hurst, B. Weinman, K. Yoo, and M. Naylor The characteristics of the sediment transported by rivers (e.g., sediment flux, grain size distribution – GSD –) dictate whether rivers aggrade or erode their substrate. They also condition the architecture and properties of sedimentary successions in basins. In this study, we investigate the relationship between landscape steepness and the grain size of hillslope and fluvial sediments. The study area is located within the Feather River Basin in Northern California, and studied basins are underlain exclusively by tonalite lithology. Erosion rates in the study area vary over an order of magnitude, from 〉 250 mm ka −1 in the Feather River canyon to 〈 15 mm ka −1 on an adjacent low relief plateau. We find that the coarseness of hillslope sediment increases with increasing hillslope steepness and erosion rates. We hypothesize that, in our soil samples, the measured ten-fold increase in D 50 and doubling of the amount of fragments larger than 1 mm when slope increases from 0.38 to 0.83 m m −1 is due to a decrease in the residence time of rock fragments, causing particles to be exposed for shorter periods of time to processes that can reduce grain size. For slopes in excess of 0.7 m m −1 , landslides and scree cones supply much coarser sediment to rivers, with D 50 and D 84 more than one order of magnitude larger than in soils. In the tributary basins of the Feather River, a prominent break in slope developed in response to the rapid incision of the Feather River. Downstream of the break in slope, fluvial sediment grain size increases, due to an increase in flow competence (mostly driven by channel steepening) but also by a change in sediment source and in sediment dynamics: on the plateau upstream of the break in slope, rivers transport easily mobilised fine-grained sediment derived exclusively from soils. Downstream of the break in slope, mass wasting processes supply a wide range of grain sizes that rivers entrain selectively, depending on the competence of their flow. Our results also suggest that in this study site, hillslopes respond rapidly to an increase in the rate of base-level lowering compared to rivers.

Description: Morphology of the Kosi megafan channels Earth Surface Dynamics Discussions, 2, 1023-1046, 2014 Author(s): K. Gaurav, F. Métivier, O. Devauchelle, R. Sinha, H. Chauvet, M. Houssais, and H. Bouquerel We study the morphology of streams flowing on the alluvial megafan of the Kosi River in north Bihar, India. All streams develop on a uniform sandy sediment and under a similar climate, allowing for statistically significant comparisons. Our data set includes both channels from the braid of the Kosi River and channels from isolated single-thread rivers. Using an Acoustic Doppler Current Profiler, we measure the width, depth and water discharge of the channels. Their average slope is also acquired with a kinematic GPS. These morphological characteristics are strongly correlated with the discharge. However, rescaling the data according to the threshold channel theory removes most of this dependency. The rescaled data suggest that the threads of the Kosi River braid are morphologically similar to isolated channels.

Description: Trail formation by ice-shoved "sailing stones" observed at Racetrack Playa, Death Valley National Park Earth Surface Dynamics Discussions, 2, 1005-1022, 2014 Author(s): R. D. Lorenz, J. M. Norris, B. K. Jackson, R. D. Norris, J. W. Chadbourne, and J. Ray Trails in the usually-hard mud of Racetrack Playa in Death Valley National Park attest to the seemingly-improbable movement of massive rocks on an exceptionally flat surface. The movement of these rocks, previously described as "sliding stones", "playa scrapers", "sailing stones" etc., has been the subject of speculation for almost a century but is an exceptionally rare phenomenon and until now has not been directly observed. Here we report documentation of multiple rock movement and trail formation events in the winter of 2013–2014 by in situ observation, video, timelapse cameras, a dedicated meteorological station and GPS tracking of instrumented rocks. Movement involved dozens of rocks, forming fresh trails typically of 10s of meters length at speeds of ~5 cm s −1 and were caused by wind stress on a transient thin layer of floating ice. Fracture and local thinning of the ice decouples some rocks from the ice movement, such that only a subset of rocks move in a given event.

Description: Erosional response of an actively uplifting mountain belt to cyclic rainfall variations Earth Surface Dynamics Discussions, 2, 971-1004, 2014 Author(s): J. Braun, C. Voisin, A. T. Gourlan, and C. Chauvel We present an approximate analytical solution to the stream power equation describing the erosion of bedrock in an actively uplifting mountain range subject to periodic variations in precipitation rate. It predicts a time lag between the climate forcing and the erosional response of the system that increases with the forcing period. The predicted variations in the sedimentary flux coming out of the mountain are also scaled with respect to the imposed rainfall variations in a direct proportion to the discharge exponent, m , in the stream power law expression. These findings are confirmed by 1-D and 2-D numerical solutions. We also show that the response of a river channel is independent of its length and thus the size of its catchment area, implying that all actively eroding streams in a mountain belt will constructively contribute to the integrated signal in the sedimentary record. We show that rainfall variability at Milankovitch periods should affect the erosional response of fast uplifting mountain belts such as the Himalayas, Taiwan or the South Island, New Zealand, and predict 1–10 thousand years offsets between forcing and response. We suggest that this theoretical prediction could be used to independently constrain the value of the poorly defined stream power law exponents, and provide an example of how this could be done, using geochemical proxy signals from an ODP borehole in the Bengal Fan.

Description: High natural erosion rates are the backdrop for enhanced anthropogenic soil erosion in the Middle Hills of Nepal Earth Surface Dynamics Discussions, 2, 935-969, 2014 Author(s): A. J. West, M. Arnold, G. Aumaître, D. L. Bourlès, K. Keddadouche, M. Bickle, and T. Ojha Although agriculturally accelerated soil erosion is implicated in the unsustainable environmental degradation of mountain environments, such as in the Himalaya, the effects of land use can be difficult to quantify in many mountain settings because of the high and variable natural background rates of erosion. In this study, we present new long-term denudation rates, derived from cosmogenic 10 Be analysis of quartz in river sediment from the Likhu Khola, a small agricultural river basin in the Middle Hills of central Nepal. Calculated long-term denudation rates, which reflect background natural erosion processes over 1000+ years prior to agricultural intensification, are similar to present-day sediment yields and to soil loss rates from terraces that are well-maintained. Similarity in short- and long-term catchment-wide erosion rates for the Likhu is consistent with data from elsewhere in the Nepal Middle Hills, but contrasts with the very large increases in short-term erosion rates seen in agricultural catchments in other steep mountain settings. Our results suggest that the large sediment fluxes exported from the Likhu and other Middle Hills rivers in the Himalaya are derived in large part from natural processes, rather than from soil erosion as a result of agricultural activity. Because of the high natural background rates, simple comparison of short- and long-term rates may not reveal unsustainable soil degradation, particularly if much of the catchment-scale erosion flux derives from mass wasting. Correcting for the mass wasting contribution in the Likhu implies minimum catchment-averaged soil production rates of ~0.25–0.35 mm yr −1 . The deficit between these production rates and soil losses suggests that terraced agriculture in the Likhu may not be associated with a large systematic soil deficit, at least when terraces are well maintained, but that poorly managed terraces, forest and scrubland may lead to rapid depletion of soil resources.

Description: Are longitudinal ice-surface structures on the Antarctic Ice Sheet indicators of long-term ice-flow configuration? Earth Surface Dynamics Discussions, 2, 911-933, 2014 Author(s): N. F. Glasser, S. J. A. Jennings, M. J. Hambrey, and B. Hubbard Continent-wide mapping of longitudinal ice-surface structures on the Antarctic Ice Sheet reveals that they originate in the interior of the ice sheet and are arranged in arborescent networks fed by multiple tributaries. Longitudinal ice-surface structures can be traced continuously down-ice for distances of up to 1200 km. They are co-located with fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of m yr -1 and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. Associating these structures with the AIS' surface velocity field reveals (i) ice residence times of ~ 2500 to 18 500 years, and (ii) undeformed flow-line sets for all major flow units analysed except the Kamb Ice Stream and the Institute and Möller Ice Stream areas. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow and ice-velocity configuration of the ice sheet may have remained largely unchanged for several thousand years, and possibly even since the end of the last glacial cycle. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.