Central Himalayan rivers record the topographic signature of erosion by glacial lake outburst floods

In steep landscapes, river incision sets the pace of landscape evolution. Transport of coarse sediment controls incision by evacuating material delivered to river channels by landslides. However, large landslide-derived boulders that impede bedrock erosion are immobile even in major runoff-driven floods. Glacial lake outburst floods (GLOFs) mobilize these boulders and drive incision, yet their role in regional-scale erosion is poorly understood, largely because of their rarity. Here, we find a topographic signature consistent with widespread GLOF erosion in the Nepal Himalaya. In rivers with glaciated headwaters that generate GLOFs, valleys stay narrow and relatively free of sediment, with bedrock often exposed to erosion. In turn, tributaries to these valleys are steep, allowing less efficient erosional regimes to keep pace with GLOF-driven incision. Where GLOFs are less frequent, valleys are more alluviated and incision stalls. Our results suggest the extent of headwater glaciation may play an important role in erosion of Himalayan river valleys and deserves more attention in future work.

Rapid Holocene bedrock canyon incision of Beida River, North Qilian Shan, China

Located at the transition between monsoon and westerly dominated climate systems, major rivers draining the western North Qilian Shan incise deep, narrow canyons into latest Quaternary foreland basin sediments of the Hexi Corridor. Field surveys show that the Beida River incised 125 m at the mountain front over the Late Pleistocene and Holocene at an average rate of 6 m/kyr. We hypothesize that a steep knickzone, with 3 % slope, initiated at the mountain front and has since retreated to its present position, 10 km upstream. Terrace dating results suggest this knickzone formed around the mid-Holocene, over a duration of less than 1.5 kyr, during which incision accelerated to at least 25 m/kyr. These incision rates are much larger than the uplift rate across the North Qilian fault, which suggests a climate-related increase in discharge drove rapid incision over the Holocene and formation of the knickzone. Using the relationship between incision rates and the amount of base level drop, we show the maximum duration of knickzone formation to be 700 yr and the minimum incision rate to be 50 m/kyr. This period of increased river incision is the result of increasing excess discharge, which likely corresponds to a pluvial lake-filling event at the terminus of the Beida River and correlates with a wet period driven by strengthening of the Southeast Asian Monsoon.

Testing alternative conceptual models of river-aquifer connectivity and their impacts on baseflow and river recharge processes

This study characterizes the dynamics of exchange fluxes between Brazos River Alluvium Aquifer and the Brazos River, TX, USA. Seven alternative conceptual models for the connection between the river and the aquifer were simulated in HYDRUS 2D using small-scale, high-resolution transects across the river. These models assumed varying aquifer lithology and river incision depths and considered processes such as riverbed clogging and seepage face flows. The simulations were forced by observed river stage values and tested against observed hydraulic heads in two nearby monitoring wells. The nearly 1.5 years of sub-hourly measurements spanned both flood and drought periods. The best-fit conceptual model supported a hypothesized hydraulic disconnection between the subsurface near the river and the wider alluvial aquifer. In contrast to the assumptions of previous studies, these data were more consistent with the presence of an abandoned paleochannel rather than riverbed clogging or other low-permeability zones. The implications for groundwater-surface water exchanges, and their modelling, are profound. Across the range of models, the difference in average baseflow predicted was nearly 13 m3/d/m, equivalent to seven times the firm water rights allocated for river users.

Macroinvertebrate habitat requirements in rivers: overestimation of environmental flow calculations in incised rivers

Flow variability determines the conditions of river ecosystem and river ecological functioning. The variability of ecological processes in river ecosystems gradually decreases. Prediction of the environmental flow allowing to keep biological diversity and river health develops as a response to the degradation of aquatic ecosystems overexploited by humen. The goal of the study was to test the influence of river incision on environmental flow estimation based on the macroinvertebrate BMWP_PL index. The 240 macroinvertebrate assemblages of 12 waterbodies varying in the bed substrate, amplitude of discharge were surveyed in southern Poland. The variations in the distribution of 151 466 macroinvertebrates belonging to 92 families were analysed. The similarity of benthic macroinvertebrates reflects the typological division of the rivers into three classes: mountain Tatra streams, mountain flysch rivers, and upland carbonate and silicate rivers (NMDS, ANOSIM, p < 0.001). As a response variable reflecting the macroinvertebrate distribution in the river, environmental parameters, BMWP_PL index was chosen. Our results show that the BMWP_PL index reached its highest values in shallower zones (by the shores) and at high water velocity in the Tatra Rivers or low velocity in most lowland rivers. The river incision significantly increased the values of e-flow calculations in relation to redeposited channels. The area of habitat suitability decreased with the bed incision intensity. In highly incised rivers, the environmental flow values are close to the mean annual flow, suggesting that a high volume of water is needed to obtain good macroinvertebrate conditions. As a consequence, the river downcutting processes and impoverishment of suitable habitats will proceed.

Landslides, river incision and environmental change: the Ruzizi gorge in the Kivu Rift

&lt;p&gt;The understanding of the interplay between natural and human induced factors in the occurrence of landslides remains poorly constrained in many regions, especially in tropical Africa where data-scarcity is high. In these regions where population growth is significant and causes changes in land use/cover, the need for a sustainable management of the land is on the rise. Here, we aim to unravel the occurrence of landslides in the 40 km-long Ruzizi gorge, a rapidly incising bedrock river in the Kivu Rift in Africa that has seen its landscape disturbed over the last decades by the development of the city of Bukavu (DR Congo). Careful field observations, historical aerial photographs, satellite imagery and archive analysis are combined to produce a multi-temporal inventory of 264 landslides. We show that the lithological context of the gorge and its extremely high incision rate (&gt; 20 mm year&lt;sup&gt;-1&lt;/sup&gt;) during the Holocene explains the presence of a concentration of large landslides (up to 2 km&amp;#178;) of undetermined age (well before the first observations of 1959) whose occurrence is purely natural. They are mostly of the slide type and do not show morphologic patterns of recent activity. The landslides that occurred during the last 60 years are flow-like shallower slope failures of smaller size (up to 0.12 km&amp;#178;) and tend to disappear rather quickly (sometimes within a few years) from the landscape as a result of rapid vegetation growth, land reclamation and (human-induced) soil erosion. They are primarily related to threshold slopes and precipitation plays a frequent role in their onset. However, land use/cover changes also affect their occurrence. This study provides useful information for a more accurate evaluation of the landslide hazard in the area, particularly with respect to the growth of the city of Bukavu that has developed without the consideration of naturally instable slopes. It also stresses the need and added value of building accurate landslide inventories in data-scarce regions.&lt;/p&gt;

Measurable impact of river incision on rift tectonics

&lt;p&gt;Models that couple tectonics and surface processes commonly predict that efficient erosion and sedimentation help focus crustal deformation onto fewer, longer-lived faults. However, because their geomorphic parameters are difficult to calibrate against real landscapes, the sensitivity of tectonic deformation to a realistic range of surface process efficiencies remains poorly known. Here we model the growth of structurally simple half-graben structures subjected to fluvial incision of specified efficiency and sedimentation. Numerical simulations predict that infinitely-efficient erosion and deposition (i.e., complete surface leveling) can more than double the maximum offset reached on a master normal fault before crustal strain localizes elsewhere. Further, leveling footwall relief tends to promote the migration of strain towards the hanging wall to form new grabens instead of horsts.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160; &amp;#160; &amp;#160; &amp;#160; &amp;#160;To test whether the efficiency of river incision can vary sufficiently across real rifts to exert a control on tectonic styles, we analyze the profiles of rivers draining half-graben footwalls and horst blocks in the Basin &amp; Range, Taupo, Rio Grande, and East African Rift. We adapt the standard methodology of equilibrium river profile analysis to account for spatial variations in uplift expected from crustal flexure in a fault-bounded block. Erosional efficiency (EE) is defined as the inverse of the (dimensionless) slope of uplift- and drainage area-corrected river elevation plots. &amp;#160;Measured EEs range between ~0.1 and ~4, reflecting natural variability in lithology, climate, and uplift rates across sites. Incorporating EEs within this documented range in numerical simulations, we find that increasing EE can increase the maximum throw on half-graben master faults by ~50%. Changing EE also affects the geometry of subsequent faults, with lower EEs favoring the transition from half-graben to horsts. These models predict that rifting in a colder, stronger continental crust is less sensitive to surface processes and requires even lower EE to develop horst structures. Our simulations are consistent with a compilation of EE, crustal strength proxies, and fault characteristics across real rift zones. These results suggest that natural variability in climatic conditions and surface erodibility has a measurable impact on the tectonic makeup of Earth's plate boundaries.&lt;/p&gt;

Contrasting exhumation histories and relief development within the Three Rivers Region (south-east Tibet)

The Three Rivers Region in south-east Tibet represents a transition between the strongly deformed zone around the Eastern Himalayan Syntaxis (EHS) and the less deformed south-east Tibetan Plateau margin in Yunnan and Sichuan. In this study, we compile and model published thermochronometric ages for two massifs facing each other across the Mekong River in the core of the Three Rivers Region (TRR), using the thermo-kinematic code Pecube to constrain their exhumation and relief history. Modelling results for the low-relief (< 600 m), moderate-elevation (∼ 4500 m) Baima Xueshan massif, east of the Mekong River, suggest regional rock uplift at a rate of 0.25 km/Myr since ∼ 10 Ma, following slow exhumation at a rate of 0.01 km/Myr since at least 22 Ma. Estimated Mekong River incision accounts for 30 % of the total exhumation since 10 Ma. We interpret exhumation of the massif as a response to regional uplift around the EHS and conclude that the low relief of the massif was acquired at high elevation (> 4500 m), probably in part due to glacial “buzzsaw-like” processes active at such high elevation and particularly efficient during Quaternary glaciations. Exhumation of the Baima Xueshan is significantly higher (2.5 km since ∼ 10 Ma) than that estimated for the most emblematic low-relief “relict” surfaces of eastern Tibet, where apatite (U–Th) / He (AHe) ages > 50 Ma imply only a few hundreds of metres of exhumation since the onset of the India–Asia collision. The low-relief Baima Xueshan massif, with its younger AHe ages (< 50 Ma) that record significant rock uplift and exhumation, thus cannot be classified as a relict surface. Modelling results for the high-relief, high-elevation Kawagebo massif, to the west of the Mekong, imply a similar contribution of Mekong River incision (25 %) to exhumation but much stronger local rock uplift at a rate of 0.45 km/Myr since at least 10 Ma, accelerating to 1.86 km/Myr since 1.6 Ma. We show that the thermochronometric ages are best reproduced by a model of rock uplift on a kinked westward-dipping thrust striking roughly parallel to the Mekong River, with a steep shallow segment flattening out at depth. Thus, the strong differences in elevation and relief of two massifs are linked to variable exhumation histories due to strongly differing tectonic imprint.

Decadal sediment dynamics of a perturbed fluvial system: the case of the man-made Marecchia River canyon, Northern Apennines

&lt;p&gt;River canyons are transient geomorphic systems shaped by river incision into bedrock and coupled by instability of the adjacent valley walls. Investigating the evolution of river canyons is typically challenging due to the geologic time scales involved. In this context, the Marecchia River, which hosts in its intermediate portion a 6-km canyon, developed since the early 1950&amp;#8217;s following intense gravel mining, may be instructive. Indeed, this setting offers the opportunity to: (i) document canyon development through highly erodible pelitic rocks; and (ii) evaluate relevant upstream and downstream effects on fluvial morphodynamics. To these ends, we subdivide the 50-km stretch of the Marecchia River main stem into 22 homogeneous reaches and evaluate decadal geomorphic changes through analysis of LiDAR-derived digital elevation models (i.e., 2009 and 2019) in conjunction with planimetric changes of active channel width delineated on orthophoto-mosaics (i.e., 2009, 2012, 2014, 2017, 2019). The estimation of patterns and rates of fluvial erosion into bedrock and its geomorphic effects are essential for understanding landscape evolution and for applying sustainable sediment management plans.&lt;/p&gt;&lt;p&gt;In terms of volumetric changes, the entire river stretch recorded a decadal degradation of 2,516,150 m&lt;sup&gt;3&lt;/sup&gt; (57%) and 1,884,700 m&lt;sup&gt;3&lt;/sup&gt; of aggradation (43%), with a corresponding net volume loss of -631,450 m&lt;sup&gt;3&lt;/sup&gt;. Highest specific volumes of aggradation were observed in a homogeneous reach located in the lower part of the study segment (0.5 m&lt;sup&gt;3&lt;/sup&gt;/m&lt;sup&gt;2&lt;/sup&gt;), while highest values of degradation were observed in the upper reach of the canyon (-2.3 m&lt;sup&gt;3&lt;/sup&gt;/m&lt;sup&gt;2&lt;/sup&gt;). During the 2009-2019 period, knickpoint headward migration within the canyon has progressed for approximately 500 m, producing an average bedrock incision of about 10 m. As documented by area and volume changes, both rates of fluvial incision and canyon widening, as modulated by landslide activity and valley wall collapses, are highest in proximity of the main knickpoint and tend to decrease progressively downstream. By March 2019, when the second LiDAR survey was conducted, the main knickpoint had reached the foundations of a major check dam, which eventually collapsed two months later. Upstream of the canyon, channel reaches displayed narrowing dynamics with an alternation of degradation and aggradation processes. In terms of total volumetric changes, these reaches presented an indirect correlation with confinement, with the most confined reaches acting as sediment transfer zones. In contrast, the segment downstream of the canyon displayed widening dynamics (+ 11 m on average) together with an increase of aggradation processes. Due to the pelitic nature of the hosting bedrock, despite the high geomorphic change observed, most of the material supplied by the canyon walls gets transported in suspension, contributing very little to the estimated budget of the Marecchia River's distalmost reaches. In this way, we argue that most part of the aggradation observed in this segment was originated upstream, bypassing the canyon.&lt;/p&gt;

Late Quaternary Climate Variability Constrains River Incision and Aggradation in the Spiti Valley, Western Himalaya

&lt;p&gt;The intensity of the Asian summer-monsoon circulation varies over decadal to millennial timescales and impacts surface processes, terrestrial environments, and marine sediment records. The duration and magnitude of this climatic forcing on erosion processes varies, depending on duration and intensity of the climatic events, as well as on the tectonic and geomorphologic preconditioning of the landscape. In this study, we focus on a region in the transition zone between continuous and episodic monsoon impacts: the Spiti River, the largest tributary (12x10&lt;sup&gt;3&lt;/sup&gt; km&lt;sup&gt;2&lt;/sup&gt; ) to the Sutlej River in the western Himalaya. The river valley is located in the northern lee of the Himalayan orographic barrier in a presently arid environment. The Spiti Valley has received significant precipitation during intensified monsoon periods during the late Pleistocene and Holocene and thus constitutes an ideal location to evaluate effects of episodic moisture transport into an arid, high-relief mountainous region.&lt;/p&gt;&lt;p&gt;Here we present 21 new surface-exposure ages of fluvial-fill terraces combined with previously published data to quantify temporal patterns in river incision and erosion rates. Our data include catchment-wide erosion rates and in-situ cosmogenic nuclide ages derived from &lt;sup&gt;10&lt;/sup&gt;Be, &lt;sup&gt;26&lt;/sup&gt;Al, and &lt;sup&gt;21&lt;/sup&gt;Ne and document that terrace formation (i.e., terrace abandonment) occurred during intensified monsoon phases at &amp;#8764;100 ka, &amp;#8764;65 ka, &amp;#8764;43 ka, and &amp;#8764;12 ka, although dating uncertainties prevent the calculation of exact correlation between monsoonal strength and terrace formation. We show that incision into Late Pleistocene valley fills that integrate over several cut-and-fill cycles at 10&lt;sup&gt;5&lt;/sup&gt; y are comparable to exhumation rates determined from thermochronology studies averaging over 10^6 y in that area. We argue that the limiting factor for sediment removal and river incision on shorter, millennial timescales is due to large bedrock landslides that impounded the river network and formed transient sedimentary basins lasting for 10&lt;sup&gt;3&lt;/sup&gt; -10&lt;sup&gt;4&lt;/sup&gt; years. We suggest a feedback process between sediment removal and landsliding, where large landsliding predominantly occurs when the transiently-stored valley fills have been carved out, leading to exposed valley bottoms, bedrock erosion, lateral scouring of rivers, and ultimately to the over-steepening of hillslopes. We suggest that Late Quaternary climatic variability is the main forcing factor in filling and evacuating transiently stored sediments in high mountain ranges and thus plays a direct role in controlling bedrock incision.&lt;/p&gt;

Influence of discharge variability on denudation rates and relief : example from the south-eastern margin of the Massif Central, France

&lt;p&gt;The evolution of continental relief results from the combined action of tectonic and climatic forcings. These processes do not act continuously but often through punctual events (earthquakes, major floods, landslides) whose integrated action over time (100 Kyr to Myr) leads to the formation of landscapes. The distribution of these extreme events is often described by statistical functions involving power-law relationships between frequency and magnitude, which, coupled with the non-linearity of the geomorphological response and threshold effects for the activation of erosion agents, leads to a complex and often poorly understood relief dynamics.&lt;/p&gt;&lt;p&gt;Studying the influence of discharge variability helps to better constrain river incision and long-term relief evolution. The south-eastern margin of the Massif Central (France) is a very interesting target for such investigations because it presents episodes of very intense precipitation focused on the relief resulting in marked differences in the statistical discharges distributions across the landscape. Some theoretical river incision models incorporate such variability (Lague et al., 2005) but they have been confronted with real data only in a limited number of cases (DiBiase et al., 2011; Scherler et al., 2017; Campfort et al., 2020). Here we test these models&amp;#160; in the Massif Central area and in particular on C&amp;#233;vennes, Ard&amp;#232;che and Margeride mountains by quantifying denudation rates using cosmogenic nuclides (10Be), characterizing discharges variability and performing morphological analysis on longitudinal rivers profiles.&lt;/p&gt;&lt;p&gt;The analysis of 326 river gauging stations allow us to observe a strong gradient in discharge variability from the external SE border to the interior of the Massif Central. The&amp;#160;&lt;sup&gt;10&lt;/sup&gt;Be concentrations measured from river sediments in 36 catchments imply a large variation of denudation rates between 29 mm/kyr and 126 mm/kyr.&amp;#160;We compare these denudation rates with the spatial distribution of mean annual precipitations, local relief, slope and concavity index, and also integrate all the observations in the frame of a stochastic threshold incision model.&amp;#160;Our results confirm the complex model predictions of non-linear relationships between mean denudation rates and the channel steepness index and their dependence on hydrological variability and run-off.&lt;/p&gt;&lt;p&gt;key-words : extreme events, stochastic threshold incision model, denudation rates, discharge variability, morphometric parameters, Massif Central&lt;/p&gt;