River Incision Research Articles

Pseudo-Karst Silicification Related to Late Ni Reworking in New Caledonia

Silicification in New Caledonian pseudo-karsts developed on peridotite was assessed using δ18O and δ30Si pairs on quartz cements. The objective was to document the chronology of pseudo-karst development and cementation relative to geomorphic evolution. The latter began at the end of the Eocene with the supergene alteration of peridotites and the subsequent formation of extended lateritic weathering profiles. Neogene uplift favoured the dismantling of these early lateritic profiles and valley deepening. The river incision resulted in (i) the stepping of a series of lateritic paleo-landforms and (ii) the development of a pseudo-karst system with subvertical dissolution pipes preferentially along pre-existing serpentine faults. The local collapse of the pipes formed breccias, which were then cemented by white quartz and Ni-rich talc-like (pimelite). The δ30Si of quartz, ranging between −5‰ and −7‰, are typical of silcretes and close to the minimum values recorded worldwide. The estimated δ18O of −6 to −12‰ for the fluids are lower than those of tropical rainfall typical of present-day and Eocene–Oligocene climates. Evaporation during drier climatic episodes is the main driving force for quartz and pimelite precipitation. The silicification presents similarities with silcretes from Australia, which are considered predominantly middle Miocene in age.

River incision, 10Be production and transport in a source-to-sink sediment system (Var catchment, SW Alps)

Abstract. Detrital 10Be from continental river sands or submarine sediments has been extensively used to determine the average long-term denudation rates of terrestrial catchments, based on the assumption that the rate of cosmogenic nuclide production by the interaction of source rocks with cosmic radiation balances out the loss of these nuclides by surface denudation. However, the 10Be signal recorded in sediments may be affected at the source by the response time of mountainous catchments to high-frequency forcings. In addition, transient sediment storage in piedmonts, alluvial plains and lakes or near the coast may also induce a difference between the erosive signal and its record in the sedimentary sink. Consequently, a significant part of the signal recorded in shallow-water sediments can be lost, as deep marine sediments may simultaneously record a signal coming from newly eroded source rocks along with one coming from the destabilization of previously deposited sediments. In this paper, we use the landscape evolution model Badlands to simulate erosion, deposition and detrital 10Be transfer from a source-to-sink sedimentary system (the Var River catchment, southern French Alps) over the last 100 kyr. We first compare model-based denudation rates with the ones that would be extracted from the 10Be record of local continental sediments (equivalent to river sands) and from sediments deposited offshore over time in order to examine if this record provides an accurate estimate of continental denudation rates. Then, we examine which conditions (precipitation rate, flexure, ice cover) satisfy published measured river incision rates and 10Be concentration in submarine sediments. Our results, based on the Var catchment cosmic ray exposure dating and modelling indicate that, while river sands do accurately estimate the average denudation rate of continental catchments, this is much less the case for deep submarine sediments. We find that deep-sea sediments have a different and often much smoother 10Be signature than continental ones and record a significant time lag with respect to imposed precipitation rate changes, representing the geomorphological response of the margin. A model which allows us to fit both measured 10Be concentration in marine sediments and river incision rates on land involves an increase in precipitation rates from 0.3 to 0.7 m yr−1 after 20 ka, suggesting more intense precipitation starting at the end of the Last Glacial Maximum.

A 10Be-based paleo-erosion record for the Qilian Shan (NE Tibet) over the past 4.2 Ma from a drillcore in the Hexi Corridor

Knowledge of erosion rates through time is of great importance to understand the effects of tectonics and climate on the topographic evolution of mountain belts. In this study, we present a 10Be-based paleo-erosion record for the northern Qilian Shan (NE Tibet) during the last 4.2 Ma. This new record is based on samples from a magnetostratigraphically dated drillcore from the foreland of the northern Qilian Shan (i.e., the Hexi Corridor). The 10Be-based paleo-erosion rates obtained from the drillcore lie between ~40 and ~150 m/Ma, with the exception of two transient peaks at ~3.4 Ma (~330 m/Ma) and ~2.0 Ma (~220 m/Ma). These high values may result from transitions to a generally cooler climate with more pronounced changes between glacial and interglacial periods. Apart from these peak values, erosion rates during the period from 4.2 to 2.8 Ma are rather low (~40 to ~70 m/Ma; mean: 50 m/Ma), whereas erosion rates over the last ~1.5 Ma are higher, ranging from ~80 to ~150 m/Ma (mean: ~112 m/Ma). We attribute the slower erosion between 4.2 and 2.8 Ma to a warmer climate, with more rainfall than today, and a lower relief in the sediment-source region. These factors led to a stabilization of hillslopes and caused erosion rates to be low. Subsequently, progressive cooling, aridification, and enhanced glacial and periglacial processes reduced the vegetation cover and increased hillslope erosion. In addition, ongoing rock uplift and concomitant river incision in the Qilian Shan may have increased the local relief. These factors explain the higher erosion rates in the past ~1.5 Ma. Available evidence on tectonic boundary conditions, drainage evolution, sedimentary environment, and paleo-climate conditions suggest that the geomorphology of the northern Qilian Shan has not significantly changed over that period.

Late Miocene or older canyon incision in the northern U.S. Cordillera shown by erosion rates, incision models, and basalt flow ages

Deep canyons along the Salmon, Snake, and Clearwater rivers in central Idaho, USA suggest long-lasting transient incision, but the timing and drivers of this incision are not well understood. The perturbation of the Yellowstone hotspot, eruption of flood basalts, and drainage of Lake Idaho all occurred within or near to this region, but the relationship among these events and incision is unclear. Here, we utilized in situ 10Be cosmogenic radionuclide concentrations for 46 samples (17 new) of fluvial sediment across the region to quantify erosion rates, calibrate stream power models, and estimate incision timing. We estimate that transient incision along the Salmon River began prior to ca. 10 Ma. However, canyon age decreases to ca. 5 Ma or earlier farther to the north. For a group of tributaries underlain by basalt, we use the age of the basalt to estimate that local transient incision began between ca. 11.5 and 5 Ma. Based on these timing constraints, the canyons along the Salmon and Clearwater rivers predate the drainage of Lake Idaho. We argue that canyon incision was triggered by events related to the Yellowstone hotspot (e.g., basalt lava damming, subsidence of the Columbia Basin, reactivation of faults, and/or lower crustal flow). Furthermore, our models suggest basalt may be more erodible than the other rock types we study. We show that lithology has a significant influence on fluvial erosion and assumptions regarding river incision model parameters significantly influence results. Finally, this study highlights how geodynamic processes can exert a significant influence on landscape evolution.

Tectonic geomorphology of the Ottawa-Bonnechere Graben, Eastern Canada: implications for regional uplift and intraplate seismicity

In intraplate areas where regional tectonic strain is accommodated by the reactivation of pre-existing structures, the level of seismic hazard associated with faults may be underestimated due to the poor surface expression of faults, scattered earthquake distribution, and long earthquake recurrence intervals. The cause(s) of seismicity in eastern Canada remains unresolved. This is partially because surface expressions of faults have been eroded during glacial and deglacial periods and in the Ottawa-Bonnechere Graben (OBG) are undetectable until a seismic event. Morphotectonic analysis has been widely applied to assess relative tectonic activity in various geological settings. To establish whether active uplift is occurring and to investigate the spatial distribution of relative uplift rates, 131 bedrock drainage basins in OBG were analysed. The aim of this was to (1) test the applicability of geomorphic indices for quantifying active deformation, (2) quantify the spatiotemporal distribution of relative uplift rates, and (3) explore the implications for faulting mechanisms, deformation styles, and ultimately regional seismic hazard. We measured valley floor width-to-height ratio ( Vf), basin elongation ratio ( Re), basin hypsometric integral, and normalized channel steepness index ( ksn). The results show that high relative uplift rates exist in all the six bedrock escarpment sections investigated, suggesting that they are presumably related with regional broad wavelength uplift (epeirogeny) caused by a complex interaction between far-field tectonic stress and glacial isostatic adjustment (GIA). Our analysis showed that Vf, Re, and ksn reveals no considerable spatial differences in high relative uplift rates, consistent with the Canadian Base Network GPS uplift rates. Highlights Geomorphic indices are applied in a low relief and low-medium seismicity area. Geomorphic indices reveal a broad wavelength regional uplift and show relative tectonic activity in the OBG. Relatively high geomorphic indices values provide longer term deformation history compared to global positioning system (GPS) records. Landscape formation and river incision is driven by a complex interaction between GIA and far-field tectonic stresses. Relative uplift rate results show similar patterns to published GPS uplift rates. Possible link between relative tectonic activity and current seismicity.

Pleistocene Accelerated Exhumation Within the Sumatran Fault: Implications for Late Cenozoic Evolution of Sumatra (Indonesia)

AbstractThe 1,900‐km‐long Sumatran fault plays an important role in accommodating oblique plate motion between the Indo‐Australian and Sunda plates. Although fault geometry, kinematics, and seismic activity are well‐studied, the onset of dextral motion on the Sumatran fault and uplift of adjacent Barisan Mountains is unclear; this hinders an understanding of the late Cenozoic evolution of Sumatra and the forearc region. In this study, we use low‐temperature thermochronology to measure cooling histories of rocks within and outside of the Sumatran fault. An accelerated exhumation within the fault zone began at ∼2 Ma. The Barisan Mountains may have experienced uplift and associated river incision in the late Miocene to Pliocene. The fault systems in the forearc region were inferred to be kinematically linked to the Andaman Sea and Sunda Strait and accommodated early relative plate convergence, then relocated the strike‐slip component of deformation to the Sumatran fault at ∼2 Ma.

Reconstruction of a Holocene landslide-dammed lake in the Yalong basin, eastern Tibetan Plateau

The failures of natural dams formed by landslides and glaciers in mountain areas have triggered the most destructive flood events on Earth. Outburst floods are effective agents in modifying landscapes, such as carving bedrock gorges and transporting surface masses. Reconstructing the ages and magnitudes of prehistoric landslide damming events allows us to capture the full range of frequencies and magnitudes of similar events and deepen our understanding of the interaction between large landslides and trunk rivers in tectonically active high-relief mountains. Previous studies have revealed many paleo-landslide-dammed lakes in the Yarlung Tsangpo River and its tributaries, the Jinsha River and the Min River. However, most studies associated with paleo-event reconstruction lack an estimation of the extent and volume of paleo-landslide-dammed lakes. The paleo-level of a landslide-dammed lake can be recorded in the variation in the sedimentary sequence and the geometry of the breach channel of a landslide dam. This information has not been fully exploited. In this study, we identified sedimentary evidence formed by an ancient landslide-dammed lake in the Liqiu River, a left-bank tributary of the Yalong River. The 12 exposures that we investigated mainly consist of typical clay or silt varves deposited in deep water and the Bouma sequence deposited by subaqueous turbidity flows, as well as climbing ripples that formed in a near-shore environment. Based on sedimentary analysis, we inferred that the minimal lake volume of this ancient lake was approximately 2.4 × 108 m3, corresponding to a lake level at 3,187 m a.s.l. The maximal lake extent derived from relict dam morphology reveals a lake volume of 3.2 × 108 m3 at the level of 3,200 m a.s.l. An initial drainage that occurred as the lake water overtopped the landslide dam was suggested to explain the small discrepancy in the lake level. The relict lake has remained stable over a long period of time and has accommodated continuous clay varve deposits. Furthermore, the optically stimulated luminescence dating results illustrate that the dammed lake initiated before 7.9 ka BP and persisted until at least 3.9 ka BP. The long-term cumulative coupled effect of tectonic movement and the rapid river incision rate provided favorable conditions for the failure of the Yuting landslide.

The Erosional Signature of Drainage Divide Motion Along the Blue Ridge Escarpment

AbstractThe planform rearrangement of river basins is recognized as an important process for landscape evolution. The boundaries of river basins can shift either through gradual drainage divide migration or discrete river captures, but the methods for identifying these processes often rely on topographic evidence that remains otherwise untested. Moreover, efforts to understand the relative importance of either process are hampered by a lack of age constraints on river captures. We use 10Be‐derived erosion rates to test whether, and how, divide motion is occurring at three locations along the Blue Ridge Escarpment in the Appalachian Mountains. In the Pee Dee River basin, we find that the escarpment is migrating inland up to 45 m/Myr, consistent with topographic evidence for gradual divide migration. In the Dan River basin, erosion rates support the topographic evidence for river capture, and we use a forward model of river incision to estimate that the capture likely occurred in the past 12.5 Myr. In the South Fork Roanoke River basin, where the presence of a knickzone has been interpreted as evidence that a river capture initiated a pulse of faster erosion, we instead measure nearly uniform tributary erosion rates above and within the mainstem knickzone. Simulations show that river incision into a more erodible layer of rock, with or without a river capture, could produce the observed topography and erosion rates in the South Fork Roanoke River. Our results show how multiple lines of evidence can illuminate the rates and mechanisms of river basin reorganization.

Insights Into Episodic Exhumation of the Western Tibetan Plateau Since the Late Cretaceous From Low‐Temperature Thermochronology

AbstractThe western Tibetan Plateau (TP) is characterized by low relief with high elevation separated by deep river valleys. When and how this characteristic topography developed remains ambiguous. Here, we present apatite (U‐Th)/He and fission‐track ages from three groups of samples with different geomorphic and geological settings. Thermal histories and exhumation rates were extracted from the thermochronological data. An early stage of exhumation (ca. 0.1 km/Ma) during 70–40 Ma was recorded by all samples, followed by slow exhumation (0.03–0.06 km/Ma) since the Eocene for the sample farthest away from faults and incised valley. The second stage of faster exhumation (ca. 0.13–0.15 km/Ma) during 30–23 Ma was revealed by samples from the hanging wall of thrust faults, whereas the third stage of exhumation during 15–12 Ma (ca. 0.1 km/Ma) was identified from samples in the Dingzi Zangbo valley. Combined with the regional geological setting, we propose that (a) the Late Cretaceous‐Early Eocene exhumation in the western TP was related to regional thrust‐induced crustal thickening and led to the formation of the proto‐TP; (b) the proto‐TP was modified by local structures and river incision. The Oligocene exhumation might be caused by local thrust activity, whereas the Miocene exhumation might be related to the transition from internal to external drainage. The continuous activity of the Karakoram fault resulted in another reorganization of drainage which led to the slow exhumation since 9 Ma. Our results highlight that tectonic and drainage network reorganization play an important role in shaping the geomorphology of the western TP.

Mud redeposition during river incision as a factor affecting authigenic 10Be/9Be dating: Early Pleistocene large mammal fossil‐bearing site Nová Vieska, eastern Danube Basin

ABSTRACTThis study examines the suitability of the authigenic 10Be/9Be dating method to the dating of the deposits of an incising river, taking as an example the Nová Vieska river terrace, which accumulated during the neotectonic inversion of the Danube Basin (western Slovakia). The succession was formed by a wandering river with minor preservation of proximal floodplain muds. The frequent occurrence of mud intraclasts reflects significant input of eroded material from underlying, older successions. The ages of 13 authigenic 10Be/9Be dating samples formed three groups: (1) samples from below the base of the river terrace yielded dates of ~4.13–3.70 Ma (including uncertainties); (2) muddy intraclasts from the river terrace gave an age range of ~2.79–1.96 Ma; and (3) in situ muddy layers had ages in the range of ~1.91–1.39 Ma. The large mammal fossil assemblage from channel thalweg deposits yielded a biostratigraphic age of ~3.6–2.2 Ma, matching the age of intraclasts, and thus emphasising the redeposited origin of those fossils. The relatively wide range of authigenic 10Be/9Be dating ages is interpreted as a result of the redeposition of mud from older strata on three scales: decimetre‐scale intraclasts, millimetre‐scale rip‐up clasts mixed into the newly formed beds, and formation of two authigenic rims with different age and 10Be/9Be records around individual particles. Considering these observations, an age range of in situ layers of ~1.91–1.39 Ma is proposed as the depositional age of the river terrace, with the most probable age falling within the most recent part of this interval. The effect of redeposition is thus shown to be potentially limiting to the application of authigenic 10Be/9Be dating to incising rivers, and stands in marked contrast to aggrading river settings, where redeposition of older sediments is limited and the degree of 10Be/9Be variability is low.

Hillslope Sediment Supply Limits Alluvial Valley Width

AbstractRiver‐valley morphology preserves information on tectonic and climatic conditions that shape landscapes. Observations suggest that river discharge and valley‐wall lithology are the main controls on valley width. Yet, current models based on these observations fail to explain the full range of cross‐sectional valley shapes in nature, suggesting hitherto unquantified controls on valley width. In particular, current models cannot explain the existence of paired terrace sequences that form under cyclic climate forcing. Paired river terraces are staircases of abandoned floodplains on both valley sides, and hence preserve past valley widths. Their formation requires alternating phases of predominantly river incision and predominantly lateral planation, plus progressive valley narrowing. While cyclic Quaternary climate changes can explain shifts between incision and lateral erosion, the driving mechanism of valley narrowing is unknown. Here, we extract valley geometries from climatically formed, alluvial river‐terrace sequences and show that across our dataset, the total cumulative terrace height (here: total valley height) explains 90%–99% of the variance in valley width at the terrace sites. This finding suggests that valley height, or a parameter that scales linearly with valley height, controls valley width in addition to river discharge and lithology. To explain this valley‐width‐height relationship, we reformulate existing valley‐width models and suggest that, when adjusting to new boundary conditions, alluvial valleys evolve to a width at which sediment removal from valley walls matches lateral sediment supply from hillslope erosion. Such a hillslope‐channel coupling is not captured in current valley‐evolution models. Our model can explain the existence of paired terrace sequences under cyclic climate forcing and relates valley width to measurable field parameters. Therefore, it facilitates the reconstruction of past climatic and tectonic conditions from valley topography.

Yukon River incision drove organic carbon burial in the Bering Sea during global climate changes at 2.6 and 1 Ma

Abstract. River erosion affects the carbon cycle and thus climate by exporting terrigenous carbon to seafloor sediment and by nourishing CO2-consuming marine life. The Yukon River–Bering Sea system preserves rare source-to-sink records of these processes across profound changes in global climate during the past 5 million years (Ma). Here, we expand the terrestrial erosion record by dating terraces along the Charley River, Alaska, and explore linkages among previously published Yukon River tributary incision chronologies and Bering Sea sedimentation. Cosmogenic 26Al/10Be isochron burial ages of Charley River terraces match previously documented central Yukon River tributary incision from 2.6 to 1.6 Ma during Pliocene–Pleistocene glacial expansion, and at 1.1 Ma during the 1.2–0.7 Ma Middle Pleistocene climate transition. Bering Sea sediments preserve 2–4-fold rate increases of Yukon River-derived continental detritus, terrestrial and marine organic carbon, and silicate microfossil deposition at 2.6–2.1 and 1.1–0.8 Ma. These tightly coupled records demonstrate elevated terrigenous nutrient and carbon export and concomitant Bering Sea productivity in response to climate-forced Yukon River incision. Carbon burial related to accelerated terrestrial erosion may contribute to CO2 drawdown across the Pliocene–Pleistocene and Middle Pleistocene climate transitions observed in many proxy records worldwide.

Bridging the Gap between Long–Term Orogenic Evolution (>10 Ma Scale) and Geomorphological Processes That Shape the Western Alps: Insights from Combined Dating Approaches

Constraining the relative roles of erosion and tectonics in the evolution of mountain belts is a challenging scientific goal. In this review article on the Western Alps, we show how it becomes possible to “bridge the gap” between the long–term (>Ma) orogenic evolution controlled by tectonics and exhumation processes and the recent geomorphological evolution that is accessible on an annual–decadal basis. Advances in mineral dating that have grown in relation to deformation in the ductile and brittle crustal fields have allowed us to constrain the evolution of deformation through time and depth. A drastic change from early collision, dominated by rapid underthrusting of the European plate, to a more stagnant syn–collisional tectonic context is documented since about 26–20 Ma by syn–kinematic phengites and vein–hosted monazites along the Alpine arc. The overall dextral kinematic context is accompanied by local extensional domains in the Simplon and High Durance Valley. Activation of the Simplon ductile fault is documented from 20 Ma, whereas the High Durance extensional system commenced after 10 Ma. The application of cosmogenic nuclide dating of incised river gorges demonstrates that the erosion pattern of the Western Alps follows a different evolution within the valleys dominated by upstream glacial erosion than in peripheral watersheds devoid of glaciers. The very low peripheral incision is found to be similar to the vertical GPS signal, suggesting equilibrium of tectonic uplift and incision, whereas the glacial–dominated valleys exhibit significantly increased and transient river incision during interglacials and a constant ongoing tectonic regime.

Middle and Late Pleistocene evolution of the Ardèche Valley archaeological landscapes (France)

Most archaeological and palaeo-environmental archives are preserved in specific environments (buried sediments, rock shelters, cave environments). Hence, the information we can obtain is usually incomplete, and lacking spatial and morphological significance. Studying landscape evolution can help us to understand the location and distribution of past societies and their relation to Quaternary environments. In the Ardèche Valley, most Middle and Late Palaeolithic sites are preserved in caves and rock-shelters and in rare cases in fluvial sediments. Here we apply an integrated geomorphological approach to the study of the evolution of the Ardèche Valley combining multi-method dating, geomorphological mapping and surveys of cave and fluvial formations. We review chronological results obtained through multi-method dating (electron spin resonance (ESR), infrared stimulated luminescence (IRSL), uranium-series (U-series), combined electron spin resonance/uranium-series (ESR/U-series)) on cave and riverine objects and combine them with topographic and stratigraphic observations in an integrated Bayesian hierarchical model. The results are in agreement with those obtained from other rivers in mountainous environment around the Mediterranean Sea and show sedimentation/incision phases that match the 100 ka glacial/interglacial cycles. The mean rate of river incision deduced from the established chronology for the Middle and Late Pleistocene period, 76 ± 7 m.Ma−1, shows adjustments to a moderate uplifting dynamic that shaped the main topographical features seen today in the landscape of the Ardèche Gorge. The spatially reconstructed alluvial features provide a palaeogeographic framework that can be used directly as a chronological and topographical constraint for archaeological research in the Ardèche catchment.

Fluvial bedrock gorges as markers for Late-Quaternary tectonic and climatic forcing in the Southwestern Alps

Fluvial incision is one of the major erosive processes acting at Earth's surface and is highly sensitive to tectonic, isostatic and climatic variations. The aim of this study is to distinguish between the short-term climatic fluctuations versus the long-term tectonic forcing contribution to Late-Quaternary fluvial incision, to better understand its timing and driving mechanism(s). To achieve this goal, we measured in situ-produced 36Cl concentrations along several river-polished gorge walls in Jurassic limestones of the Southwestern Alps. We then compared our dating results to previously-dated river gorges from nearby catchments. This allows us to highlight three trends of distinct incision dynamics, and to discuss their relationships with climate and tectonics. Trend 1 shows the direct impact of a paraglacial crisis in the rivers connected to glaciated areas. Trend 2 suggests an incision wave propagating along the non-glaciated tributaries following enhanced incision in the main streams. Trend 3 displays steady and low incision rates in gorges disconnected from any fluvial response to glacier retreat. Trend 3 also seems to highlight the potential of resistant lithologies to isolate portions of the river network from post-glacial incision propagation. Our analysis shows that gorges connected to upstream glaciers exhibit a significant response of fluvial incision to climatic fluctuations, evidenced by high-amplitudes incision rate variations hindering the long-term tectonic signal. In contrast, incision rates inferred from disconnected gorges are in agreement with previously-estimated long-term denudation and rock-uplift rates in the area. Based on the latter, we can conclude that Late-Quaternary river incision in the Southwestern Alps is readjusting to both short-term climatic forcing and long-term tectonic forcing.

A Compilation of New and Existing Thermochronology Data Concerning the Lhasa Terrane Combined with a Geological Synthesis of the Area

AbstractThe history and geodynamic mechanisms of Cenozoic exhumation and denudation of the Lhasa terrane (LT), southern Tibetan Plateau, remain poorly constrained. Most previous studies investigating the exhumation of the LT have relied on extrapolating interpretations based on a few well‐studied but small areas, which inevitably introduce bias. Here, our compilation of new and published thermochronologic ages are plotted against latitude, longitude, and elevation, to illustrate the distribution of cooling ages across broad regions of the LT. Primary results reveal that (1) the north LT experienced significant exhumation during 80–40 Ma; (2) compared with north LT, exhumation and erosion in the south LT continued after ∼40 Ma; and (3) N–S striking rifting, E–W striking river incision, and thrusts led to continued local denudation of the south LT between 20–5 Ma.

Dynamic bedrock channel width during knickpoint retreat enhances undercutting of coupled hillslopes

AbstractMountain landscapes respond to transient tectonic and climate forcing through a bottom‐up response of enhanced bedrock river incision that undermines adjoining hillslopes, thus propagating the signal from the valley bottom to the valley ridges. As a result, understanding the mechanisms that set the pace and pattern of bedrock river incision is a critical first step for predicting the wider mechanisms of landscape evolution. Typically, the focus has been on the impact of channel bed lowering by the upstream migration of knickpoints on the angle, length and relief of adjoining hillslopes, with limited attention on the role of dynamic channel width. Here, we present a suite of physical model experiments that show the direct impact of knickpoint retreat on the reach‐scale channel width, across a range of flow discharges (8.3 to 50 cm3 s−1) and two sediment discharges (0 and 0.00666 g cm−3). During knickpoint retreat, the channel width narrows to as little as 10% of the equilibrium channel width, while the bed shear stress is >100% higher immediately upstream of a knickpoint compared to equilibrium conditions. We show that only a fraction of the channel narrowing can be explained by existing hydraulic theory. Following the passage of a knickpoint, the channel width returns to equilibrium through lateral erosion and widening. For the tested knickpoint height, we demonstrate that the lateral adjustment process can be more significant for hillslope stability than the bed elevation change, highlighting the importance of considering both vertical and lateral incision in landscape evolution models. It is therefore important to understand the key processes that drive the migration of knickpoints, as the localized channel geometry response has ongoing implications for the stability of adjoining hillslopes and the supply of sediment to the channel network and export from landscapes onto neighbouring depositional plains.

Coupling a large-scale hydrological model (CWatM v1.1) with a high-resolution groundwater flow model (MODFLOW 6) to assess the impact of irrigation at regional scale

Abstract. In the context of changing climate and increasing water demand, large-scale hydrological models are helpful for understanding and projecting future water resources across scales. Groundwater is a critical freshwater resource and strongly controls river flow throughout the year. It is also essential for ecosystems and contributes to evapotranspiration, resulting in climate feedback. However, groundwater systems worldwide are quite diverse, including thick multilayer aquifers and thin heterogeneous aquifers. Recently, efforts have been made to improve the representation of groundwater systems in large-scale hydrological models. The evaluation of the accuracy of these model outputs is challenging because (1) they are applied at much coarser resolutions than hillslope scale, (2) they simplify geological structures generally known at local scale, and (3) they do not adequately include local water management practices (mainly groundwater pumping). Here, we apply a large-scale hydrological model (CWatM), coupled with the groundwater flow model MODFLOW, in two different climatic, geological, and socioeconomic regions: the Seewinkel area (Austria) and the Bhima basin (India). The coupled model enables simulation of the impact of the water table on groundwater–soil and groundwater–river exchanges, groundwater recharge through leaking canals, and groundwater pumping. This regional-scale analysis enables assessment of the model's ability to simulate water tables at fine spatial resolutions (1 km for CWatM, 100–250 m for MODFLOW) and when groundwater pumping is well estimated. Evaluating large-scale models remains challenging, but the results show that the reproduction of (1) average water table fluctuations and (2) water table depths without bias can be a benchmark objective of such models. We found that grid resolution is the main factor that affects water table depth bias because it smooths river incision, while pumping affects time fluctuations. Finally, we use the model to assess the impact of groundwater-based irrigation pumping on evapotranspiration, groundwater recharge, and water table observations from boreholes.

Three-dimensional control of alluvial fans by rock uplift in an extensional regime: Aydın Range, Aegean extensional province

Tectonics imparts a first-order control on the overall morphology of alluvial fan systems in extensional settings by influencing sediment flux and accommodation space, while other factors such as climate, catchment lithology, and fault footwall characteristics are secondary. Previous alluvial fan modeling studies have focused on the link between the three-dimensional development of alluvial fans and rock uplift, however, despite the potential influence of tectonics on the overall three-dimensional morphology of alluvial fans, the controlling mechanisms, as well as their relative importance, remain largely unquantified in a natural setting with a targeted source-to-sink approach. Here, we examine 45 alluvial fans and their catchments along the southern mountain front of the Aydın Range, delimited by segmented normal faults in the western Anatolia Extensional Province, to quantify the role of rock uplift. We quantify river incision rates and catchment-wide erosion rates together with a series of topographic analyses across the southern flank of the Aydın Range as a proxy for rock uplift. Our results indicate that the spatial distribution of thicker and steeper alluvial fans fit well with higher rock uplift rates along the strike of the mountain front. In contrast, a lower uplift rate is responsible for prograding alluvial fans with decreasing thickness and gradients. Also, our data shows that alluvial fan thickness compared to other alluvial fan metrics strongly associated with the pattern of the rock uplift. This study demonstrates a field-based, quantitative linkage between three-dimensional alluvial fan morphology and rock uplift which has significant implications for improving alluvial fan models and understanding how alluvial fans respond to tectonics in extensional regions.

The influence of bedrock river morphology and alluvial cover on gravel entrainment: Part 1. Pivot angles and surface roughness

AbstractSediment entrainment in bedrock rivers is a key process for river incision and landscape evolution. Bedrock riverbeds are typically comprised of both exposed bedrock and alluvial patches, meaning grains can be entrained from positions on both surfaces. The critical shear stress needed to entrain a grain will be affected by the topography that the grain is located on, as it determines grain pivot angle and exposure, and impacts the local flow profile. The aim of this pair of articles is to determine how the properties of bedrock surfaces with and without sediment cover affect the grain‐scale geometry of sediment grains, and consequently their critical shear stress. We report experiments using 3D‐printed scaled replicas of fluvial bedrock surfaces, with 0 to 100% additional sediment cover. For each surface, grain pivot angles were measured using a tilt table. In this first article, we report how surface roughness and grain pivot angle vary between surfaces and with different amounts of sediment cover, and we explore the relationship between pivot angles and different metrics for measuring surface roughness. We find that: (1) surface roughness is not necessarily a linear combination of the individual roughness of bedrock and alluvial areas, and the underlying bedrock topography can still influence surface roughness at 100% sediment cover; (2) pivot angles generally, but not always, decrease with increasing grain size relative to surface roughness; (3) changes in pivot angles with increasing sediment cover are best explained by changes in surface roughness at spatial scales comparable to the grain size; and (4) pivot angles are also best explained by roughness metrics that incorporate the direction of roughness with respect to the tilt direction, and the surface inclination. This work provides new insights into the processes behind grain entrainment in bedrock rivers that are critical for determining how landscapes may evolve.