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Significance The importance of high-magnitude, short-lived events in controlling the evolution of landscapes is not well understood. This matters because during such events, erosion processes can surpass thresholds and cause abrupt landscape changes that have a long-lasting legacy for landscape morphology. We show that extreme flood events, during which the flow depth exceeds the threshold for erosion through plucking rather than abrasion, are the dominant control on the evolution of a large bedrock canyon in Iceland. The erosive signature of these events is maintained within a dynamic landscape over millennial timescales, emphasizing the importance of episodic extreme events in shaping landscapes. Extreme flood events have the potential to cause catastrophic landscape change in short periods of time (10 ⁰ to 10 ³ h). However, their impacts are rarely considered in studies of long-term landscape evolution (>10 ³ y), because the mechanisms of erosion during such floods are poorly constrained. Here we use topographic analysis and cosmogenic ³He surface exposure dating of fluvially sculpted surfaces to determine the impact of extreme flood events within the Jëökulsëöárgljëöáúfur canyon (northeast Iceland) and to constrain the mechanisms of bedrock erosion during these events. Surface exposure ages allow identification of three periods of intense canyon cutting about 9 ka ago, 5 ka ago, and 2 ka ago during which multiple large knickpoints retreated large distances (>2 km). During these events, a threshold flow depth was exceeded, leading to the toppling and transportation of basalt lava columns. Despite continuing and comparatively large-scale (500 m ³/s) discharge of sediment-rich glacial meltwater, there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive event because the vertical knickpoints have not diffused over time. We provide a model for the evolution of the Jëöáúökulsëöáúöárgljëöáúöáúfur canyon through the reconstruction of the river profile and canyon morphology at different stages over the last 9 ka and highlight the dominant role played by extreme flood events in the shaping of this landscape during the Holocene.

The rate at which transient knickpoints propagate through a landscape fundamentally controls the rate of geomorphic response to tectonic and climatic perturbation. Here we present knickpoint retreat rates upstream of active faults for 19 bedrock catchments in Turkey and 11 bedrock catchments in Italy where we have very good constraints on both the magnitude and timing of the tectonic perturbation and where climate histories are well documented. We show that the knickpoints have average retreat rates of between 0.2 and 2 mm/yr for catchments with drainage areas between 6 and 65 km2 and we test whether differences in rock mass strength and catchment size are sufficient to explain this range in retreat rates. Our analysis suggests that even accounting for these two variables, knickpoint propagation velocities differ markedly, and we show that channels crossing faults with higher throw rates have knickpoints that are retreating faster. The dependence of knickpoint retreat velocity on throw rate is at least as important as catchment drainage area. These results indicate, counterintuitively, that landscapes forced by large amplitude tectonic perturbations will have shorter response times than those perturbed by smaller amplitude changes. The link between the knickpoint propagation velocity and throw rate is largely (but not completely) explained by channel narrowing in areas of high uplift rate. Channel steepening upstream of the active faults may explain all of the residual dependency of knickpoint retreat rate on fault throw rate, but only if the slope exponent, n, in the standard stream power model is greater than 1.3. However, we cannot rule out a role for sediment supply in driving enhanced knickpoint retreat rate in addition to the well‐documented channel narrowing effect. Finally, we find that mean knickpoints retreat rates in Turkey are only half of those in Italy, for catchments of equivalent size, crossing faults with similar throw rates. This difference in fluvial response time is accounted for by long‐term differences in the ratio of precipitation to infiltration in the two areas over the last 1 My. Key Points Knickpoint retreat rates are positively correlated with fault throw rates Landscape response times are smaller for large tectonic perturbations Climate modulates knickpoint retreat rates in Turkey and Italy

Geomorphometric analysis using geomorphic indices is essential to comprehend the evolution of a river basin including denudation, surface runoff, subsurface infiltration, differential erosion, lithological variations, possible surface tilting, landslides, and the influence of geological formations and structure. Research in morphometric measurements continues to face many challenges and difficulties despite all the effort carried out. These include the inaccuracy of morphometric measurements and the time it takes to obtain the expected results in large basins. Under such condition, the purpose of the study is to conduct an analysis for the group of indices which includes SL index, transverse topographic symmetry factor, and hypsometry curve along with its integral value in the Mandakini Catchment. Examining the spatial distribution of knickzones has not been well documented, particularly in the Mandakini Catchment; hence, we further analyzed the spatial distribution of knickpoints, channel steepness index, and chi-index along with the longitudinal river profile. Through this analysis, we aim to determine how these indices collectively contribute to the comprehensive characterization of the landscape evolution within the study area and to find the landscape signatures of the uplift by comparing different river profiles. Various knickpoints were found mainly in the upper reaches at higher elevation, validated through aerial imagery and then through detailed field observation. During the field investigation, various geomorphic indicators such as fluvial terraces, entrenched river meandering, active landslides, extensive toe erosion, and waterfalls associated were observed. The study also found out that the places near the Kedarnath, Sonprayag, and Kalimath-Kotma, show high SL index and high steepness index that may correlate with the presence of active thrust and faults.

Morphometry is an important method for understanding geomorphic processes, such as drainage development, impact of tectonics on the landscape, differential erosion, and categorizing the erosion stages of a varied topographic relief. The precision of a morphometric analysis depends on the high-resolution digital elevation models and a powerful estimation tool. The purpose of the study is to calculate the geomorphic indices such as Stream length gradient Index & SL-Hot Spot, Topographic swath profile, Transverse topographic symmetry factor, Channel Steepness Index, Hypsometry, and Erosion-Uplift Rate. The comparative analysis was done for the 3 sub-catchments of the Alaknanda Basin namely Upper Alaknanda, Dhauli and Middle Alaknanda. The results obtain from each of the catchment shows the spatial distribution of hot and cold spot that helps to select the zone with high SL anomalous values and principal knickzones. This information is associated with major thrusts such as the South Tibetan Detachment System (STDS), Main Central Thrust (MCT), Vaikrita Thrust (VT) of Munsiari group, Alaknanda Fault (AF) and other minor faults with the proof of landscape signature. Various knickpoints were found along the trunk stream and have been precisely analyzed using the SL-HCA approach, validated through aerial imagery and, finally, through the detailed field observations. The study also found that convexity in the river profile and knickpoints around Karnaprayag, Nandaprayag, and Joshimath, the uplift rate exceeds the rate of incision. Further, the findings show that these outputs are used in geomorphological investigations like tectonic activity, rock differential erosion, and hillslope formation.

A waterfall is a very steep (commonly nearly vertical) fall of some magnitude in a river course. Waterfalls are widespread fluvial landforms that have been described from many parts of the world. Thirty-eight World Heritage Properties include waterfalls in their designation. In addition, some waterfalls are actual or potential geomorphosites. Waterfalls occur in almost all climatic environments, though they are particularly common in formerly glaciated areas. They occur on a huge diversity of rock types, although in general, they do not form persistent or large falls on soft or unconsolidated rocks. Waterfalls also occur in a wide range of geomorphological settings: glaciated areas, areas of active tectonism, areas of sea-cliff retreat and sea-level change, great escarpments on passive margins, basins with river capture, rifted and faulted areas and areas that have been subjected to megaflooding. Multiple processes account for waterfall retreat and varying rates of recession. Although the greatest interest has been in rates of waterfall recession, there are examples of waterfalls that prograde as a result of tufa deposition.

Increasingly frequent glacier-rock avalanches (GRAs) – events triggered by the detachment of both glacier and rock materials – have occurred in recent years in the Sedongpu gully in the eastern Himalayan syntaxis, blocking the course of the Yarlung Zangbo River repeatedly. To identify spatio-temporal patterns and influencing factors of these GRAs, we analyzed remote sensing images and produced high-resolution digital surface models. At least eight GRAs, originating from the same source areas, have occurred in the past decades: one in 1974, one in 2014, and six between 2017 and 2018. The GRAs that occurred since 2014 were responsible for the loss of > 70 Mm3 of glacier and rock and > 150 Mm3 of moraine deposits and increased the elevation of the basin outlet by 60–120 m. Climate change-induced glacier retreat, steep topography, and a recent strong earthquake were found to be related to the occurrence of the recent GRAs, which resulted in the formation of a knickpoint in the Yarlung Zangbo River.

We designed a workflow to investigate areas of potential neotectonic deformation, making use of well-developed techniques, but applied to a site characterized by low relief and low or moderate tectonic activity. In this pilot study, we targeted the Temiskaming Graben, in Eastern Canada, where recent and ongoing geophysical and sedimentological investigations have revealed recent activity along this ancient structure. The dataset compiled for this experimental study covers an area of nearly 147 square km across the provinces of Ontario and Quebec. For efficiency in terms of computational resources, we first performed cluster analysis on knickpoint location, identifying seven areas with a high density of disruptions along river profiles. We then performed more detailed morphometric analysis at 30 m resolution, identifying knickpoints along river profiles, calculating the hypsometric integral across the landscape with a moving window, and mapping and comparing lineaments with known structural features. The results of our workflow showed that these three techniques can be efficiently combined for neotectonic analysis, and the synergistic approach strengthens the reliability and accuracy of our results. Our research extends the application of morphometric analysis, commonly used for exploring areas with intense tectonism and high topography, to areas that are characterized by low relief and low or moderate tectonic activity. The new areas identified with the workflow proposed in this research require ground-truthing through mapping and shallow geophysical investigations.

The Earth's surface is shaped by tectonics and climate. This simple statement implies that we should, in principle, be able to use the landscape as an archive of both tectonic rates and of changes to climate regime. To solve this inverse problem, and decipher the geomorphic record effectively, we need a sound understanding of how landscapes respond and erode in response to changes in tectonic or climatic boundary conditions. Rivers have been a major focus of research in this field because they are patently sensitive to tectonic and climatic forcing via their channel gradient and discharge. Theoretical, field, and numerical modeling techniques in the last few years have produced a wealth of insight into the behavior of fluvial landscapes, while the increasing availability of high-resolution topographic models have provided the data sets necessary to address this research challenge across the globe. New work by Miller et al. (2012) in Papua New Guinea highlights the progress we have made in extracting tectonics from topography due to these developments, but also illustrates the problems that still remain. This paper reviews our current knowledge of how fluvial landscapes record tectonics at topographic steady-state and under \"transient\" conditions, assesses why the climate signal has proven so challenging to interpret, and maps out where we need to go in the future.

Incising rivers may be confined by low-slope, erodible hillslopes or steep, resistant sidewalls. In the latter case, the system forms a canyon. We present a morphodynamic model that includes the essential elements of a canyon incising into a plateau, including 1) abrasion-driven channel incision, 2) migration of a canyon-head knickpoint, 3) sediment feed from an alluvial channel upstream of the knickpoint, and 4) production of sediment by sidewall collapse. We calculate incision in terms of collision of clasts with the bed. We calculate knickpoint migration using a moving-boundary formulation that allows a slope discontinuity where the channel head meets an alluvial plateau feeder channel. Rather than modeling sidewall collapse events, we model long-term behavior using a constant sidewall slope as the channel incises. Our morphodynamic model specifically applies to canyon, rather than river–hillslope evolution. We implement it for Rainbow Canyon, CA. Salient results are as follows: 1) Sediment supply from collapsing canyon sidewalls can be substantially larger than that supplied from the feeder channel on the plateau. 2) For any given quasi-equilibrium canyon bedrock slope, two conjugate slopes are possible for the alluvial channel upstream, with the lower of the two corresponding to a substantially lower knickpoint migration rate and higher preservation potential. 3) Knickpoint migration occurs at a substantially faster time scale than regrading of the bedrock channel itself, underlying the significance of disequilibrium processes. Although implemented for constant climactic conditions, the model warrants extension to long-term climate variation.

River incision results from interactions among tectonics, climate change, and surface processes, and yet the role of each process operating at different time scales remains poorly understood. In this study, we address this issue by reconstructing the late Quaternary spatiotemporal variation of aggradation and incision rates along the Lancang River (Upper Mekong River) in southeast Tibet. Our work combined field observations, topographic data analysis, and optically stimulated luminescence (OSL) and cosmogenic radionuclide (CRN) dating of geologically well-defined fluvial terrace deposits, and it reveals five levels of fluvial terraces with strath heights up to 200-240 m and a 300-km-wide knickzone along the Lancang River. The new data indicate that: (1) the Lancang River has experienced four aggradation events at >120-100 ka, 90-70 ka, 25-15 ka, and <9 ka, with each event followed by rapid incision at ca. 100 ka, ca. 45 ka, ca. 15 ka, and ca. 6 ka; (2) river incision rates since the late Pleistocene decrease upstream across the knickzone from <2.8-2.3 and <2.1-1.7 to <0.5 mm/yr; and (3) they decrease with time at the knickzone from <2.1 mm/yr at ca. 100 ka to <1.1 mm/yr at 15-6 ka. The terrace-derived incision rates since the late Pleistocene from this study are more than an order of magnitude higher than the existing landscape-scale erosion rates derived from both thermochronological dating of bedrock bounding the river valley at million-year scales and cosmogenic nuclide concentrations of river sand at millennial scales. These findings imply decoupling of hydrologically induced river incision rates since the late Pleistocene from regional erosion rates on million-year and millennial time scales. Specifically, the hydrologically driven incision in a large fluvial system like the Lancang River in southeast Tibet, most likely related to local climate conditions, is much more efficient than tectonically driven erosion at a time scale of 100-10 k.y.