Applicability of Geomorphic Index for the Potential Slope Instability in the Three River Region, Eastern Tibetan Plateau

Geomorphic indices (e.g., the normalized channel steepness index (Ksn) and the stream length-gradient index (SL)) highlight changes in fluvial shapes and gradients. However, the application of these indices was seldom used to identify potential landslide zones. In this study, we used the Ksn and SL indices to detect the significant variations in the stream power along river reaches, which are anomalies associated with landslides, in the Zengqu River watershed, the upper reaches of the Jinsha River. Most of the landslide anomalies originate along the trunk and surrounding tributaries below the knickpoint of the mainstream. This suggests an erosional wave is migrating upstream throughout the drainage area. The fluvial incision may generate over-steepened hillslopes, which could fail in the future. In addition, the divide asymmetry index (DAI) predicts the direction of the divide as the headwaters migrate toward lower relief, higher elevation surfaces. Landslides are expected to occur as the unstable divide migrates. The proposed methodology can benefit the detection and characterization of potential landslide zones. It should improve hazard and risk analysis and the identification of drainage network areas associated with landslides.

Relationship Between Dams, Knickpoints and the Longitudinal Profile of the Upper Indus River

Mass movements in mountainous areas are capable of damming rivers and can have a lasting effect on the river longitudinal profile. The long profile is commonly used to retrieve regional tectonic information, but how much dams may compromise geomorphometry-based tectonic analysis has not been systematically researched. In this study, we investigate the relationship between river dams and the longitudinal profile of the upper Indus River basin, based on interpretation and analysis of remote sensing imagery and digital elevation models (DEMs) and local field work. We identified 178 landslide, glacier and debris flow dams. Using TopoToolbox, we automatically extracted the river longitudinal profile from the 30 m SRTM DEM, determined the location of convex knickpoints and calculated the channel steepness index. One hundred and two knickpoints were detected with heights above 148 m, of which 55 were related to dams. There is good spatial correspondence between dams, convexities in the river longitudinal profile and relatively high steepness index. Different dam types have different impacts on the river profile; on the upper Indus, debris flow dams have a greater impact than landslide and glacier dams and can form knickpoints of up to 900 m. Therefore, dams may have a significant influence on the river longitudinal profile, knickpoints and steepness index, and should be considered when extracting information on regional tectonics using these indices.

Does lithology control post-orogenic topography and rock erodibility? Insights from Anti-Atlas of Morocco

<p>Rock erodibility plays a central role in setting topographic limits on relief development and is a key parameter in landscape evolution models. However, channel bed erodibility (K) is usually either fixed arbitrary or let varying over a wide range of values (10<sup>-12</sup> – 10<sup>-3</sup>) because it is difficult to estimate. The topography of ancient orogens offers favourable conditions to quantify bedrock erodibility through the stream profile analysis, because the channel steepness is directly related to rock erodibility rather than rock uplift or climate variability.</p><p>The Anti-Atlas is a Variscan (Paleozoic) orogen of NW Africa that has not been drifted for long distances over the late Cenozoic and hence has not experienced an extended shift across climatic zones. Furthermore, it is characterized by a well preserved uplifted relict landscape with rather uniform erosion rates since at least the last 120 - 100 Ma. This specific configuration allows studying in detail landscape erosional dynamics and erodibility.</p><p>Here, we combine geomorphic analysis of stream profiles with in situ-produced cosmogenic concentrations (<sup>10</sup>Be) in river sediments, to decipher the surface evolution of the AntiAtlas and the adjacent Siroua Massif. In the Anti-Atlas, the basin-wide denudation rates determined for the relictal part of the landscape range between 5 and 20 m Ma<sup>-1</sup>, consistent with rates estimated from the volume of volcanics eroded from the Siroua Massif during the last 12 - 10 Ma (10 to 20 m Ma<sup>-1</sup>). The close agreement of short- and long-term erosion rates suggests a steady state landscape.</p><p>Our results demonstrate the main role of rock-type on sustaining post-orogenic landscape. Specifically, we find a striking correlation between erosion rates and normalized channel steepness per different rock-types. This allows estimating the erodibility within a narrower range of values (10<sup>-7</sup> - 10<sup>-4</sup>) as a function of the reference concavity values of the river network.</p>

Active back-arc thrust in North West Java, Indonesia

<p>The Baribis-Kendeng Fault System crosscuts the northern part of Java Island (Indonesia). It seems that the fault systems is the continuation westward from the active Flores thrust in the northern offshore of the Lesser Sunda Islands. While the Flores thrust in the east is well documented as an active fault in the back-arc platform (e.g., source of the 2018 Lombok 6.9 Mw earthquake), the nature, timing, and activity of the Baribis-Kendeng Fault Systems, particularly the Baribis Fault Zone (BFZ) in the westernmost part of the system remain elusive. Yet, understanding the geological risk associated with the BFZ is crucial, as it crosscuts densely-populated regions, possibly up to 30 million inhabitants in the megalopolis of Jakarta. Previous studies mostly identified the BFZ by first-order morphotectonic observations, as well as large-scale geodetic and seismotectonic investigations, and assigned historical earthquakes (estimated up to 8.5 Mw in 1780) in northern Java to the BFZ. Ground-truthing the structure and activity of the BFZ from geological arguments is a cornerstone to evaluate associated geohazards.</p><p>We first focus on the Cikamurang Ridge, nearly at the eastern part of the BFZ, where uplifted Pliocene-Recent sediment sequences outcrop. Morphotectonic data include an 8-m resolution digital elevation model that we used to map fault lineaments and calculate the channel steepness index of the rivers crossing the mapped fault segments. Field data, including paleoseismological trenching at the central part of Cikamurang Ridge and sediment dating (OSL and radiocarbon) provide temporal constraints on the BFZ activity. Subsurface geophysical data include seismic reflection and resistivity imaging provide better image of the fault geometry in the sub-surface.</p><p>Our results suggest that the BFZ has been active in the Cikamurang Ridge during the late-Pleistocene to Holocene times, with deformed sediment sequences dated between 55 and 7 ka. Eastward, the BFZ crosses the Cisanggarung River where the fault deformed ~13-ka old sediments. Westward of the Cikamurang Ridge, both fault lineament interpretation and channel steepness index indicates that the fault continues from Subang regency to Jatiluhur and reaches the area between Jakarta and Bogor. Even though in the area between Jakarta and Bogor the surficial trace of the BFZ is not as clear as the Cikamurang and Subang, the seismic reflection data reveal the blind fault propagation fold. We conclude that the BFZ has a high seismic hazard that requires a careful risk evaluation along its trace, as it threats the numerous infrastructures of the extremely densely-populated West Java.  Comparing to the Flores back-arc thrust, the existence of the BFZ indicate the whole island of Java affected with the back-arc compressive regime as well as the existence of the Kendeng Fault Zone, in the easternmost of the Baribis-Kendeng Fault Systems.</p>

Orographic precipitation patterns, stream power, and river longitudinal profiles: Predictions and implications for detecting climate’s influence in mountain landscapes

<p>Dynamic climates featuring spatially and temporally variable precipitation patterns are ubiquitous in mountain settings. To understand the role of climate on landscape evolution in such settings, and how climate change-related signals might be translated into the sedimentary realm, this variability must be addressed. Here, we present an analysis of how spatial gradients and temporal changes in rainfall combine to affect both the steady state form and transient evolution of river profiles of large transverse river basins as predicted by the stream power model. Where rainfall is uniform, the stream power model predicts that topographic metrics, like fluvial relief and normalized channel steepness index (k<sub>sn</sub>), vary inversely and monotonically with rainfall at steady state. In contrast, we find that these relationships are more complex and can be inverted in many circumstances, even at steady state, in the presence of orographic rainfall gradients. An important consequence of this is that correlations between average rainfall (climate) and topography are always weaker in catchments that experience rainfall gradients relative to expectations based on uniformly distributed rainfall. Moreover, dispersion caused by rainfall gradients is systematic, varying both with the polarity (i.e., generally increasing vs. decreasing downstream) and intensity of the gradient. Therefore, even in quasi-steady-state, rainfall gradients have the potential to obscure or distort the influence of climate on landscapes if they are not accounted for. In addition, we find that temporal changes in spatially variable rainfall patterns can produce complex erosional and morphological responses that can be contrary to expectations based on the change in mean rainfall. Specifically, enhanced incision and surface uplift may occur simultaneously in different parts of a landscape in a pattern that evolves during the transient response to climate change, complicating prediction of the net erosional and topographic response to climate change. Thus, transient responses to the orographic distribution of rainfall may misleadingly appear inconsistent with erosional or morphological responses expected for a relative change in average climate. Additionally, topographic indications of transient adjustment, even to a dramatic change in orographic precipitation, can be subtle enough that a landscape can appear to be in quasi-steady-state. In such cases, spatial gradients in erosion rate driven by a change in orographic precipitation pattern may be mistakenly interpreted as recording spatial gradients in rock uplift rate, potentially at once obscuring an important influence of climate and misinterpreting tectonic drivers of landscape evolution. Finally, we explore the use of a variant of normalized channel steepness index (k<sub>sn-q</sub>) that is able to incorporate the influence of spatially variable in rainfall based on the stream power model. Importantly, we find that k<sub>sn-q</sub> preforms well to help diagnose and quantify the role of climate acting in a landscape, in particular during transient adjustment to changes in rainfall patterns where the standard channel steepness metric (k<sub>sn</sub>) may be misleading.</p>

Downstream grain-size changes using PebbleCounts in the south-central Andes: Relations to channel steepness and SAR observations

<p>Grain-size data are imperative for understanding erosional and fining processes in steep terrain as rivers are the primary conduits for sediment transport. However, collecting hundreds of pebble measurements at multiple channel bed survey sites in steep and dynamic high-mountain river settings remains a challenging roadblock in studying downstream variations in grain-size. Using <em>PebbleCounts </em>(https://github.com/UP-RS-ESP/PebbleCounts), we survey large (~1,000+ m<sup>2</sup>) channel cross sections and measure thousands of grains per survey to build robust grain-size distributions in the Quebrada del Toro, Northwest Argentina. Because of imagery resolution considerations, we only examine the grain sizes in the ≥ 2.5 cm fraction of the distribution. We gather measurements via a careful counting and validation process to constrain uncertainties, which highlights the dominant over- and under-segmentation errors that occur in <em>PebbleCounts </em>in this challenging geographic setting. Despite uncertainties, we are able to study downstream changes in grain-size percentiles at seven survey sites along a 100-km stretch of the trunk stream, which traverses a steep topographic and environmental gradient. We find that only the upper-most percentiles (≥ 95th) are sensitive, whereas the 50th and 84th percentiles show little downstream variability in this rapidly eroding catchment. In particular, we note a strong relation between increases in these upper percentiles and the along-channel junctions with large, oversteepened tributaries, where extreme channel steepness reaches are > 200 m<sup>0.8</sup> (θ=0.4). Furthermore, independent spaceborne synthetic aperture radar (SAR) coherence and amplitude observations show clear relations to mass transfer and channel bed roughness changes, which also relate to the grain-size variability that we find.</p>

Channel Profile Response to Abrupt Increases in Mountain Uplift Rates: Implications for Late Miocene to Pliocene Acceleration of Intracontinental Extension in the Northern Qinling Range-Weihe Graben, Central China

Cenozoic extension of the Qinling range-Weihe Graben system has occurred in response to the uplift and growth of the Tibetan Plateau. Rapid exhumation of the northern Qinling range since the late Miocene is also regarded as resulting from the eastward expansion of the northeast part of Tibet. Tectonic evidence of this in the landscape remains unclear, but the fluvial system can provide a sensitive proxy record of tectonic forcing through space and over time scales of 105–107 a. Here, we present a study of channel profiles in the northern Qinling range, which forms a footwall highland separated from the southern Weihe Graben by active normal faults. We identify a population of knickpoints that separate river profiles with a gentle upstream gradient from steeper downstream reaches. Above the knickpoints, steepness indices increase from the central part towards the west and east, whereas channel steepness shows its highest values in the Huaxian-Huayin section. We observed no systematic changes of channel steepness pattern as a function of rock resistance, drainage area, or channel concavity. Correlation analysis between channel steepness and basin elevation and relief documents the control of tectonic forcing on regional topography. While bearing no relation to geological outcrop boundaries, the knickpoints show a strong correlation between retreat distance, catchment area, and river length. We infer that the knickpoints formed in response to an increase in mountain uplift rates and retreated as a kinematic wave. Under linear slope exponent n, we calibrated channel erodibility K~1.00±0.44×10−6 m0.1/a and derived knickpoint ages of 5.59±1.80 Ma. Combining the ages of onset of active faulting and mountain growth in the NE Tibetan Plateau (8–10 Ma, e.g., Liupan Shan, Jishi Shan, and eastern segments of the Haiyuan and Kunlun faults) and in the southwest Qinling range (9–4 Ma), we conclude that growth of the NE Tibetan Plateau began in the mid-Miocene time and expanded eastwards to the Qinling range-Weihe Graben during the late Miocene and early Pliocene.