Local Base Level Research Articles

Mio‐Pliocene paleo‐course of Indus River in Upper Sutlej‐Zhada basin: Implication of tectonic uplift on river piracy and drainage reorganization in SW Tibet and NW Himalaya

AbstractWe analysed the elevated low‐relief relict landscapes in the transient Upper Satluj‐Zhada basin and the adjoining region in the tectonically active north‐western (NW) Himalaya–south‐western (SW) Tibetan orogen to understand the evolution of the regional landscape and drainage system under the influence of the Karakoram Fault‐Leo‐Pargil Horst system. This elevated low relief landscape represents the Mio‐Pliocene establishment of a new river network, which testimonies the present Sutlej River, which has been experiencing a transient surface uplift‐incision regime since (~4–1 Ma) with a local base level at the confluence of the Sutlej and Spiti River. The Miocene exhumation of the Ayilari Range and Leo‐Pargil Horst across the Karakoram fault (KF) system led to headward erosion, which abandoned the Paleo‐Sutlej‐Indus drainage system, which in turn caused drainage reversal along Qusum detachment (QD) and produced southward migration of the Paleo‐Sutlej River towards the mountain front. Our results indicate that the Upper Indus River has significantly lower χ‐ranges at higher elevations as compared with the adjacent Upper Sutlej River at lower elevations, which corresponds with a river piracy model that incorporates area gain‐loss feedback. The Upper Sutlej River in the Zhada basin is characterized by a comparable series of coplanar slope‐break knickpoints at ~4000–4500 m elevation, and their adjoining divides are in a state of disequilibrium as a consequence of the very high rapid incision across the Leo Pargil Horst, which drives the regional gradation process. The headward‐eroding Upper Indus River captured the proto‐Sutlej due to a base‐level change of >~1500 m, which significantly impacted the regional growth pattern and tectonics. The Mio‐Pliocene sedimentation pattern of the Upper Sutlej‐Zhada basin in the SW Tibet–NW Himalaya reflects this regional drainage capture, tectonic uplift and paleo‐drainage reorganization. The present finding has wider implications for the Mio‐Pliocene reorganization of drainage systems and the possible linkage of the Upper Indus River with the Paleo‐Sutlej over the Zhada basin.

Does Rural Labor Transfer Impact Chinese Agricultural Carbon Emission Efficiency? A Substitution Perspective of Agricultural Machinery

In this paper, referring to Kaya’s method, the ratio of gross agricultural production (GAP) and agricultural carbon emission is defined as agricultural carbon emission efficiency (ACEE). Considering rural labor transfer (RLT) increases the agricultural machinery intensity (AMI), the two are substitutes for each other and may interact with agricultural carbon emission efficiency (ACEE). We constructed a Simultaneous Equations Model (SEM) of ACEE, RLT and AMI and analyzed the interaction mechanism of these three variables using the Three-Stage Least Squares (3SLS). The following conclusions are drawn. First, RLT and AMI significantly promote the improvement of ACEE, while the improvement of ACEE and AMI further promotes RLT. Secondly, the causal relationship and influence mechanism of ACEE, RLT and AMI are interactive and multi-directional. For example, an increase in AMI promotes ACEE, but an increase in ACEE inhibits an increase in AMI. Finally, China has significant regional heterogeneity, and different regions have different interaction mechanisms. Local governments should consider the local economic base and technological level when implementing policies. This paper extends the analytical framework of ACEE, RLT, and AMI and provides a reference for governments to make policies.

From landform inventories to landscape evolution? - Karst development in the Central Styrian Karst (Austria)

In the Central Styrian Karst (CSK, Styria, Austria) 1158 karst landforms were used to obtain a better understanding of the landscape evolution in this region. An automatically delineated doline inventory was combined with an existing cave inventory to investigate surface and subsurface karst landforms in relation to their elevation above the local base level at the transition zone between the Eastern Alps and the Pannonian Basin. The landform entities analysed consist of 51 % dolines and 49 % caves and are related to various stages of landform evolution and thus ages.The CSK was ice-free during glacial times and therefore allow for a much longer preservation of surface landforms compared to landforms that were reshaped during glaciations. Nevertheless, weathering and downwasting do re-shape and even may lead to the disappearance of surface landforms (i.e., dolines); however, subsurface landforms (i.e., caves), if accessible, might preserve information about older geomorphological stages.An old, a mature, and a young stage of landform evolution can be distinguished in the CSK and be related to planation surfaces preserved in the area, presumably older than five Ma. The chosen approach is likely applicable for other areas in the world too by implying karstification over million years' time scale.Besides those regional implications, there are local (neo-) tectonic inferences derived from the analyses that might explain some discrepancies in correlations of levels and planation surfaces, but requiring further more detailed investigations. The automatic doline delineation in the forested karst of the CSK made an area not yet mentioned in the literature become apparent.With an increasing availability of high-resolution digital elevation models, remote-sensed inventories of dolines (surface landforms) renew the question of dolines as diagnostic landforms for karstification. However, in the CSK it is revealed that an absence of dolines due to e.g., surface erosion does not indicate the absence of karstification. Therefore, landscape evolution should consider both surface (i.e., doline) and subsurface (i.e., cave) landforms.

Groundwater dilution and flow conditions in sulfuric acid caves: the case study of Frasassi (Italy)

This study analyzes the dilution process of the sulfidic groundwater in the Frasassi caves due to the recharge of O2-rich freshwater and its influence on the sulfuric acid speleogenesis and morphogenesis. The drainage pattern and the seasonal changes of the chemo-physical characteristics of the groundwater through a year-long monitoring and the measurements of the groundwater levels are presented. The inflow of water infiltrating from the karst surface influenced the sulfidic groundwater parameters, reflecting the seasonal meteoric cycle. On the contrary, the Sentino River, which represents the local base level, directly influenced the groundwater only in the most external part of the cave. The water level measurements evidenced a low hydraulic gradient (~3‰), due to the high karstification, and also some differences in the permeability depending on the drainage direction. Most cave pools are isolated on the surface and connected to each other through a network of submerged passages. In the present conditions, a surface layer of bicarbonate water forms above the sulfidic water in a large part of the cave, where it impedes subaerial corrosion by released acidic gases. Conversely, the distribution of residual gypsum deposits and corrosional wall features in the upper old cave levels demonstrate that large interfaces between sulfidic water and cave atmosphere existed during some periods of the cave history. Here, the release of acidic gases (CO2 and H2S) and the production of H2SO4 from H2S oxidation caused the widespread subaerial corrosion which significantly contributed to the morphogenesis. The comparison between these residual morphologies and the active processes shows that morphogenesis in the cave has evolved through time, influenced by hydrodynamic conditions, in turn depending on the general morphological and hydrogeological setting of the whole karst area.

Pleistocene Colorado River terraces in the canyonlands region (Utah, USA) record unsteady, transient incision and growth of the Cataract Canyon knickzone by salt tectonics

Abstract Colorado River terraces in the canyonlands region of southeastern Utah can be dated and analyzed to address the controls of incision and nature of the Colorado Plateau’s largest channel-steepness anomaly, Cataract Canyon. Field correlations supported by luminescence and cosmogenic-nuclide ages on strath terraces along Meander Canyon, upstream of Cataract Canyon, reveal a complex record of unsteady incision over the past ~340 k.y. at an average rate of ~0.4 mm/yr. Both an upstream progression of rapid incision and a unique sigmoidal long-profile pattern of terraces indicate incision in response to episodic baselevel fall. Also, terraces converge downstream with the anomalously low-gradient modern channel above Cataract Canyon. We interpret these results as indicating that growth of the Cataract Canyon knickzone is due to an erosion-salt tectonics feedback since at least the Mid Pleistocene, which has imparted unsteady, and currently elevated, local baselevel. More broadly, the canyonlands region is marked by rapid and unsteady incision that is complicated by local geologic controls even while being absent of any regional or mantle-driven uplift.

Assessing the Hazard of Deep-Seated Rock Slope Instability through the Description of Potential Failure Scenarios, Cross-Validated Using Several Remote Sensing and Monitoring Techniques

Foreseeing the failure of important unstable volumes is a major concern in the Alps, especially due to the presence of people and infrastructures in the valleys. The use of monitoring and remote sensing techniques is aimed at detecting potential instabilities and the combination of several techniques permits the cross-validation of the detected movements. Supplemented with field mapping and structural analysis, it is possible to define possible scenarios of rupture in terms of volume, mechanisms of failure and susceptibility. A combined observation strategy was applied to the study of major instability located in the Ticinese Alps (Switzerland), Cima del Simano, where the monitoring started in 2006 with the measurement of opened cracks with extensometers. Since 2021, the monitoring has been completed by LiDAR, satellite and GB-InSAR observations and structural analysis. Here, slow but constant movements of about 7 mm/yr were detected along with rockfall activities near the Simano summit. Eight failure scenarios of various sizes ranging from 2.3 × 105 m3 to 51 × 106 m3, various mechanisms (toppling, planar, wedge and circular sliding) and various occurrence probabilities were defined based on the topography and the monitoring results and by applying a Slope Local Base Level (SLBL) algorithm. Weather acquisition campaigns by means of thermologgers were also conducted to suggest possible causes that lead to the observed movements and to suggest the evolution of the instabilities with actual and future climate changes.

Assessing the rock failure return period on an unstable Alpine rock wall based on volume-frequency relationships: The Brenva Spur (3916 m a.s.l., Aosta Valley, Italy)

Defining the relationship between volume and return period is critical when estimating the risk of rockfalls and/or rock avalanche, especially during continued global warming at high altitudes that threatens rock wall stability. Characterizing the volume-frequency relationship based on historical datasets is, however, limited by observation and quantification biases, which have not received enough attention. Here, to monitor recent activities for the Brenva Spur (Mont-Blanc massif, Italy) that is also a rock avalanche scar and estimate the return period of future rock failures based on the volume-frequency relationship (and the corresponding uncertainty), a structure-from-motion photogrammetric survey was conducted from 2017 to 2021. 39 rockfall sources with volumes ranging from 11 to 13,250 m3 were identified within the scar. The total failure volume is 22,438 m3, with an associated erosion rate of 15.5 mm/year, indicating very active morphodynamics possibly linked to the permafrost evolution in the spur. The volumes were characterized by a negative power-law that fits significant two events in 2016 (3.4 × 104 m3) and one in 1997 (2.0 × 106 m3) remarkably well, and the randomness of the fit was evaluated by a Monte Carlo approach. 7 potential failure scenarios ranging from 3.1 × 104 m3 (S1) to 4.8 × 106 m3 (S7) were defined according to a structural analysis and the sloping local base level concept. Their extrapolated return periods derived by the power-law fit indicate a longer return period for the maximum failure scenario than for the smaller scenarios. S1 has a 50% chance of occurring every 3 years, while S7 has a 50% chance of occurring every 31 years. Though the median return period of S7 is 31 years, the 95% and 68.2% confidence intervals range from 8 to 399 years and 14 to 93 years, respectively, which reflects a high level of uncertainty but is realistic when considering global warming, progressive rock failure, etc. In addition to characterizing recent rock failure activities in high mountains, this study offers a preliminary examination of the return periods of some extreme scenarios and provides primary data for risk management in mountainous areas that are very sensitive to global warming.

Archaeological sites in Grand Canyon National Park along the Colorado River are eroding owing to six decades of Glen Canyon Dam operations

The archaeological record documenting human history in deserts is commonly concentrated along rivers in terraces or other landforms built by river sediment deposits. Today that record is at risk in many river valleys owing to human resource and infrastructure development activities, including the construction and operation of dams. We assessed the effects of the operations of Glen Canyon Dam – which, since its closure in 1963, has imposed drastic changes to flow, sediment supply and distribution, and riparian vegetation – on a population of 362 archaeological sites in the Colorado River corridor through Grand Canyon National Park, Arizona, USA. We leverage 50 years of evidence from aerial photographs and more than 30 years of field observations and measurements of archaeological-site topography and wind patterns to evaluate changes in the physical integrity of archaeological sites using two geomorphology-based site classification systems. We find that most archaeological sites are eroding; moreover, most are at increased risk of continuing to erode, due to six decades of operations of Glen Canyon Dam. Results show that the wind-driven (aeolian) supply of river-sourced sand, essential for covering archaeological sites and protecting them from erosion, has decreased for most sites since 1973 owing to effects of long-term dam operations on river sediment supply and riparian vegetation expansion on sandbars. Results show that the proportion of sites affected by erosion from gullies controlled by the local base-level of the Colorado River has increased since 2000. These changes to landscape processes affecting archaeological site integrity limit the ability of the National Park Service and Grand Canyon-affiliated Native American Tribes to achieve environmental management goals to maintain or improve site integrity in situ. We identify three environmental management opportunities that could be used to a greater extent to decrease the risk of erosion and increase the potential for in-situ preservation of archaeological sites. Environmental management opportunities are: 1) sediment-rich controlled river floods to increase the aeolian supply of river-sourced sand, 2) extended periods of low river flow to increase the aeolian supply of river-sourced sand, 3) the removal of riparian vegetation barriers to the aeolian transport of river-sourced sand.

Hydrologic and geomorphic effects on the reduction of channel discharge capacity in the Middle Yangtze River

A systematic increase in the occurrence of flood events has been noted in rivers worldwide. However, our understanding remains unclear of how the flood level is affected by the change in channel discharge capacity. Therefore, the reduction of channel discharge capacity was quantified in the Middle Yangtze River (MYR) based on extensive measured data. It is confirmed that the variation in channel discharge capacity is a superposition result of channel-morphology and channel-resistance changes, as well as the change in local base-level at the outlet. Then a series of numerical simulations were carried out to quantitatively identify the contribution of each factor to the reduction of channel discharge capacity in the MYR for the years of 2004 and 2020. Simulated results reveal that channel degradation increased the flow passage area, helping the channel to convey floods, with the effect being weakened along the reach; however, the decline in flood level under the same discharge was not observed, which was primarily attributed to the increases in channel resistance and local base-level at the outlet. These two factors accounted for 16–91 % and 9–84 % of the total impact at four hydrometric stations in the MYR. It should be noted that the dominant factor to influence the discharge capacity varied greatly at different stations, which depended on the degree of bed-material coarsening, the distance from the confluence of lakes, etc. At the stations immediately downstream of the dam, the contribution of the increased movable bed roughness usually played a more essential role; while at the stations close to the outlet with the confluence of large lakes, the increase in local base-level became the main factor that accounted for the rise of high-flood levels under the specified large discharge.

Tectonic, climatic and autogenic controls on the Late Quaternary evolution of the Someș fluvial fan, North-East Pannonian Basin, Central Europe

The Someș fluvial fan is located in the NW extremity of the Great Hungarian Plain (Pannonian Basin). It was formed by the left-side tributaries of the Tisa River (a tributary of the Danube) as they developed westward, following the avulsion of the main Tisa channel. Drainage reorganisation after the avulsion has occurred via a complex interplay between tectonic, climatic and autogenic controls over the past ~50–30 ka. In this study, we discuss the role of these factors in the spatial and temporal dynamics of the fluvial system that constructed the Someș fluvial fan during the second half of the last glacial cycle, using a combination of cartographic, sedimentary, and chronological tools. Our data suggests that, within a general setting of subsidence, spatial and temporal variation in the rate of this subsidence between different tectonic blocks created four local base levels, while autogenic factors play only a secondary role. The modern drainage configuration results from spatial channel adjustments during the last ca. 3000 cal BP, related to the current subsidence centre located in the NW extremity of the Someș fluvial fan. Over the entire period analysed, the rivers draining across the fluvial fan predominantly meandered, except when the river switched to a braided pattern. This braided phase likely occurred before ~30–32 ka ago, apparently coeval with a similar short, braided phase that has been documented along the middle Tisa River. The braiding phase is coincident with intense deglaciation in the Someș catchment area and development of open forest vegetation at lower elevations. Climatic changes during and after the Last Glacial Maximum had a reduced impact on the style of fluvial flow, which returned to a meandering pattern. Ca. 5000 cal BP these changes impacted suspended sediment delivery to drainage networks resulting in the present-day channel dimensions, at least along their lower reaches (our study area). Our results highlight the lower sensitivity of rivers draining the Great Hungarian Plain to the Late Quaternary climate changes compared to the far more responsive rivers of Western European. This behaviour is, in our opinion, most likely due to the presence of glacial forest refugia in the catchment areas, which modulated the response of discharge and fluvial dynamics to climatic changes.

Fluvial evolution in a growing thrust-fold range of the Yumu Shan, NE Tibetan Plateau

In an active thrust-fold belt, the drainage evolution, such as the alternation between transverse and longitudinal rivers, will influence the surface erosion within the range and the sedimentary sequence in the foreland basin. However, knowledge of the controls of changes in transverse or longitudinal drainage patterns in an uplifting range is limited. The Yumu Shan, on the northeastern margin of the Tibetan Plateau, is an example for a young and active thrust-fold range that shows evidence for drainage reversals since ∼3 Ma. The materials we used are the late Cenozoic depositional sequences around the Yumu Shan. Analyses of the lithology, grain size, roundness, and clast orientation of gravel deposits on the northern and southern flanks, combined with the results of cosmogenic nuclide burial dating, reveal that a transverse river, the Liyuan River, crossed the Yumu Shan range during 3.0–1.8 Ma by eroding weak Cenozoic sedimentary rocks. After 1.8 Ma, however, this transverse river was deflected and flowed along the edge of the range, and a longitudinal river developed along the back flank of the range. This process was synchronous with the exposure of resistant Paleozoic rocks by the continuous uplift of the range. After comparison with other potential controlling factors for drainage reversals, such as variations in rock uplift, climate or upstream aggradation, we find that the change in bedrock resistance was the most likely control on the change from a transverse to a longitudinal drainage pattern in this relatively young thrust-fold belt. Furthermore, through comparison with a series of active fold-thrust belts worldwide, we propose a simple conceptual model for the drainage evolution of active fold-thrust belts. In the earlier stage of the uplift, the exposed soft sedimentary rocks will cause transverse (antecedent) rivers to cross the range. After the continuous uplift of the range, resistant rocks cause the deflection of a transverse river and its change to a longitudinal river along the back flank of the range. In the final stage, after the several million years to around ten million years of uplift, with the development of a mature range, the local base level of the foreland will greatly fall, promoting a return to a transverse drainage pattern by headward erosion and capture of longitudinal rivers.

Sulfuric acid speleogenesis and surface landform evolution along the Vienna Basin Transfer Fault: Plavecký Karst, Slovakia

Hypogene caves in the Plavecký hradný vrch Hill (Western Slovakia, Central Europe) were formed by waters ascending along faults in fractured Triassic carbonates related to the horst-graben structure at the contact of the Malé Karpaty Mountains and the NE part of the Vienna Basin. The Plavecká jaskyňa and Pec caves mostly contain horizontal passages and chambers with flat corrosion bedrock floors, fissure discharge feeders, wall water-table notches, replacement pockets, as well as a few other speleogens associated with sulfuric acid speleogenesis. The low-temperature sulfuric acid development phases of the Plavecká Jaskyňa are also indicated by the presence of sulfate minerals (i.e., gypsum and jarosite).Subaerial calcite popcorn rims were precipitated from water condensation at the edges of feeding fissures that were still active as thermal vents when the water table dropped. Hydrogen sulfide involved in the sulfuric acid speleogenesis was likely derived from anhydrites and/or hydrocarbon reservoirs with sulfate-saline connate waters in the fill of the adjacent Vienna Basin. It ascended to the surface along deep-rooted sub-vertical fault zones at the contact of the Vienna Basin with neighboring mountains. Three cave levels at 295 to 283 m asl in the Pec Cave, and five levels at 225 to 214 m asl in the Plavecká jaskyňa corresponded to phases of stable local erosional base levels in the bordering part of the Vienna Basin, most likely during periods of strongly decelerated and/or interrupted subsidence. Cave levels separated by vertical differences of only a few meters may also be related to the Pleistocene climatic cycles. The subhorizontal parts of the Pec Cave are probably of late Early Pleistocene age (˃0.99–1.07 Ma?). The two highest levels of the Plavecká jaskyňa developed during the early Middle Pleistocene (˃600 ka). Fine-grained sediments in the passage at 225 m asl with normal magnetic polarity contain jarosite. The middle level of the Plavecká jaskyňa at 220 m asl was formed in the mid-Middle Pleistocene, while the lower and lowermost levels formed in the late Middle Pleistocene (˃270 ka). The water table in the lowermost cave level probably dropped after the tectonic reactivation of the Podmalokarpatská zníženina Depression just in the front of a marginal horst structure of the Malé Karpaty Mountains.

Late Oligocene-early Miocene Origin of the First Bend of the Yangtze River explained by thrusting-induced river reorganization

The origin of the First Bend of the Yangtze River is key to understanding the birth of the modern Yangtze River. Despite considerable efforts, the timing and mechanism of formation of the First Bend remain highly debated. Inverse river-profile modeling of three tributaries (Chongjiang, Lima, and Gudu) of the Jinsha River, integrated with regional tectonic and geomorphic interpretations, allows the onset of incision at the First Bend to be constrained to 28–20 Ma. The spatio-temporal coincidence of initial river incision and activity of Yulong Thrust Belt in southeastern Tibet highlights thrusting to be fundamental in reshaping the pre-existing stream network at the First Bend. These results enable us to reinterpret a change in sedimentary environment from a braided river to a swamp-like lake in the Jianchuan Basin south of the First Bend, recording the destruction of the hypothesized southwards-flowing paleo-Jinsha and Shuiluo Rivers at ~36–35 Ma by magmatism. During the late Oligocene-early Miocene, the paleo-Shuiluo River was diverted to the north by focused rock uplift due to thrusting along the Yulong Thrust Belt, which also led to exhumation of the Jianchuan Basin. Diversion of the paleo-Shuiluo River can be explained by capture from a downstream river in the footwall of the Yulong Thrust Belt. Subsequent rapid headward erosion, that was caused by thrusting-induced drop of local base level, is recorded by upstream younging ages for the onset of incision and led to the formation of the First Bend. The combination of new ages for the onset of incision at 28–20 Ma at the First Bend and younger ages upstream indicates northwards expansion of the Jinsha River at a rate of 62 ± 18 mm/yr. Our results suggest that the origin of the First Bend was likely triggered by thrusting at 28–20 Ma, after which the Yangtze River formed.

Intensified paraglacial slope failures due to accelerating downwasting of a temperate glacier in Mt. Gongga, southeastern Tibetan Plateau

Abstract. Topographic development via paraglacial slope failure (PSF) represents a complex interplay between geological structure, climate, and glacial denudation. Southeastern Tibet has experienced amongst the highest rates of ice mass loss in High Mountain Asia in recent decades, but few studies have focused on the implications of this mass loss on the stability of paraglacial slopes. We used repeat satellite- and unpiloted aerial vehicle (UAV)-derived imagery between 1990 and 2020 as the basis for mapping PSFs from slopes adjacent to Hailuogou Glacier (HLG), a 5 km long monsoon temperate valley glacier in the Mt. Gongga region. We observed recent lowering of the glacier tongue surface at rates of up to 0.88 m a−1 in the period 2000 to 2016, whilst overall paraglacial bare ground area (PBGA) on glacier-adjacent slopes increased from 0.31 ± 0.27 km2 in 1990 to 1.38 ± 0.06 km2 in 2020. Decadal PBGA expansion rates were ∼ 0.01 km2 a−1, 0.02 km2 a−1, and 0.08 km2 in the periods 1990–2000, 2000–2011, and 2011–2020 respectively, indicating an increasing rate of expansion of PBGA. Three types of PSFs, including rockfalls, sediment-mantled slope slides, and headward gully erosion, were mapped, with a total area of 0.75 ± 0.03 km2 in 2020. South-facing valley slopes (true left of the glacier) exhibited more destabilization (56 % of the total PSF area) than north-facing (true right) valley slopes (44 % of the total PSF area). Deformation of sediment-mantled moraine slopes (mean 1.65–2.63 ± 0.04 cm d−1) and an increase in erosion activity in ice-marginal tributary valleys caused by a drop in local base level (gully headward erosion rates are 0.76–3.39 cm d−1) have occurred in tandem with recent glacier downwasting. We also observe deformation of glacier ice, possibly driven by destabilization of lateral moraine, as has been reported in other deglaciating mountain glacier catchments. The formation, evolution, and future trajectory of PSFs at HLG (as well as other monsoon-dominated deglaciating mountain areas) are related to glacial history, including recent rapid downwasting leading to the exposure of steep, unstable bedrock and moraine slopes, and climatic conditions that promote slope instability, such as very high seasonal precipitation and seasonal temperature fluctuations that are conducive to freeze–thaw and ice segregation processes.

Age attribution to a karst system using river long profile analysis (Hyblean Plateau, Sicily, Italy)

In steady-state tectonic-climate systems, the fluvio-karst processes attain a stable base level, whether it be local or global. In contrast, in an uplifting region, relative base-level changes force river incision processes and a concurrent lowering of the karst water-table. Even at local scales (e.g., single fault-controlled valley), combined tectonically- and climatically-driven base-level drops induce geomorphic and hydrogeological disequilibrium. Thus, the karst system develops vertically and the water-table lowers trying to attain the new local base level. Conversely, the paleo-karst network dries, and becomes abandoned as a hanging relic with aligned karst morphologies. They are exhumed by the entrenchment of the fluvial network, as base-level fall related knickpoints migrate upstream. We propose a geomorphic approach to build fluvio-karst age models, based upon the analysis of river long-profiles. This approach is complementary to altimetric correlation between karst horizons and coastal paleo-sea level markers. Our approach is useful for an inland study area that is far from the marine terraced coast or where active faulting cuts off the coastal area from the inland landscape. We tested the approach in the eastern sector of the Hyblean Plateau (Sicily, Italy), where a carbonate sequence experienced Middle-Late Pleistocene tectonic uplift combined with active faulting. Specifically, we investigated the Cassibile River basin, at the footwall of the active Cassibile-Noto Fault. By reconstructing river paleo-long profiles we fixed time-space reference lines, connecting the abandoned and hanging fluvio-karst levels to the correlative marine strandlines carved along the fault-controlled coastal landscape.

PIRATARIA FLUVIAL NA SERRA DO ESPIGÃO: DIVISÓRIA DOS RIOS URUGUAI E IGUAÇU-AMÉRICA DO SUL

Stream capture is a drainage rearrangement where a flux transference occurs and a contribution area from a drainage basin is incorporated to another. It is about expanding a river system over another one, caused by erosive advantage earned by conditioning factors such as lithostructure, pluviometric regime, topographic gradient, and base level. However, in the southern region of Brazil, a rare dynamic of stream piracy was verified between the drainage basins of the Uruguay and Iguazu (Paraná) rivers. Stream captures were observed along the Serra of Espigão, part of the drainage divide between the two basins. Still, it was not possible to identify which basin was advancing over the other. This paper investigated the occurrence of stream captures, identifying which factors are responsible for this atypical stream dynamic. Mapping the stream captures by remote sensing and further validation with fieldwork, it was verified that there is stream piracy for both sides of the drainage divide. Still, it is not clear which basin is behaving more aggressively. A longitudinal profile analysis of the channels involved in the stream captures showed a local control in the drainage network. This control sets a local base level to the Iguazu river tributaries. It indicates the lithostructural limit between two different geological units: the Serra Geral group basalts and the Botucatu formation sandstones. Depending on the geographic location of this base level, channels that drain to the Iguazu (Paraná) river become more aggressive or less aggressive than the ones that drain to the Uruguai basin, and so they capture or lose area for the other basin. Therefore, it is impossible to identify a river basin that exclusively pirates the other, prevailing, in this case, a mutual competition between the Uruguai and Iguazu (Paraná) rivers basins. This fact highlights the significance of local base levels to promote stream capture processes.

Autocyclic (avulsion‐induced) base‐level change, its upstream and downstream controls, and effects on floodbasin sedimentation: The 1870s avulsion of the Saskatchewan River, Cumberland Marshes, Canada

AbstractThis study examines centennial‐scale hydrological and sedimentological effects of floodplain inundation by avulsion and its upstream and downstream controls. The 1870s avulsion in Cumberland Marshes diverted the Saskatchewan River flow towards Cumberland Lake, a local base level. It invaded a poorly drained sub‐basin of Cumberland Marshes floodplain linked to the parent Saskatchewan River by two small outlets in the resistant substrate. The rapid increase in inflow (~5× on average) during the earlier stages of the avulsion resulted in the base‐level rise and floodplain inundation by the avulsion lake. Since the early 20th century, the forced regression of the avulsion lake occurred, caused by ~5× outlet channel enlargement by ‘hungry‐water’ outflows, whereas the mean lake inflows experienced little change. The avulsion lake served as an effective sediment trap and was filled by predominantly progradational sandy and silty avulsion deposits up to 3–4 m thick, covering about 700 km2. Elsewhere, fluviodeltaic settings with ‘negative relief’ and limited hydrologic connectivity with the rest of the floodplain may be prone to avulsion lakes that form if the rates of inflow increased by avulsion exceed the rates of outflow. Avulsion lakes can last for ~100 years or more before they drain and/or become filled by overbank sediments. On well‐drained floodplains, inundations by avulsions are expected to be short‐term and result in little progradational deposition. This study demonstrates that in some local hydrographic basins, base level becomes a variable of an evolving avulsion rather than its fixed external control. Although avulsion‐induced base‐level changes are short‐lived, they affect 102–103 km2 of a floodplain and occur rapidly, accompanied by high aggradation rates.

Bedrock gorge incision via anthropogenic meander cutoff

Abstract Bedrock river-gorge incision represents a fundamental landscape-shaping process, but a dearth of observational data at >10 yr timescales impedes understanding of gorge formation. I quantify 102 yr rates and processes of gorge incision using historical records, field observations, and topographic and image analysis of a human-caused bedrock meander cutoff along the North Fork Fortymile River in Alaska (USA). Miners cut off the meander in 1900 CE, abruptly lowering local base level by 6 m and forcing narrowing and steepening of the channel across a knickpoint that rapidly incised upstream. Tectonic quiescence, consistent rock erosivity, and low millennial erosion rates provide ideal boundary conditions for this 102 yr gorge-formation experiment. Initial fast knickpoint propagation (23 m/yr; 1900–1903 CE) slowed (4 m/yr; 1903–1981 CE) to diffusion (1981–2019 CE) as knickpoint slope decreased, yielding an ~350-m-long, 6-m-deep gorge within the pre–1900 CE channel. Today, diffusion dominates incision of a 500-m-long knickzone upstream of the gorge, where sediment transport likely limits ongoing adjustments to the anthropogenic cutoff. Results elucidate channel width, slope, discharge, and sediment dynamics consistent with a gradual transition from detachment- to transport-limited incision in fluvial adjustment to local base-level lowering.

Late Cenozoic erosion pattern of the eastern margin of the Sichuan Basin: Implications for the drainage evolution of the Yangtze River

Evolution of the drainage network of the Yangtze River plays an important role in landscape evolution across East Asia during the Cenozoic. The mountains on the eastern margin of the Sichuan Basin form a drainage divide between the tributary rivers of the modern Upper and Middle Yangtze, and the erosion history of these mountains has major implications for the evolution of the Yangtze River. Linear inversion of long profiles of two Yangtze tributaries draining the area allows us to estimate their incision processes, and reveals contrasting erosion patterns between the west and east sides of the mountain belt. Along the Wu River, which drains into the Sichuan Basin, higher incision rates are focused on lower channels near the river's outlet on the Upper Yangtze. In contrast, within the catchment of the Yuan River, which drains into the Jianghan Basin of the Middle Yangtze, the inverted fluvial erosion rate is distributed relatively uniform in space. We calibrate the inferred incision history using previously published cosmogenic 10Be-derived basin-averaged erosion rates, and the results show that the contrasting erosion patterns between the two rivers emerged since the early Miocene (~21–16 Ma). At this time, the incision rates of the lower Wu River started to increase from ~0.04 km/Ma towards the Quaternary average at ~0.07 km/Ma, while the rates of the Yuan River remained low (<0.04 km/Ma). By comparing our results with erosion histories of the eastern Sichuan Basin and Three Gorges, we suggest that during the early Miocene, connection between the Sichuan and Jianghan Basins through the Three Gorges led to additional lowering of the local base level in the Sichuan Basin, which triggered an acceleration in incision rates of the Upper Yangtze tributaries draining into the basin.

Impact of lake water level decline on river evolution in Ebinur Lake Basin (an ungauged terminal lake basin)

Most of the terminal lakes in inland basins, which account for half of the world's lake reserves, have been shrinking at an alarming rate in recent years. In a Terminal Lake Basin, changes in the water level of the lake can lead to variations in the local erosion base level. From the perspective of local erosion base level, we revealed the response of river evolution to change in the water level of lake in Ebinur Lake Basin for the past 5000 years. Our results verified the three geomorphic development stages of natural decline, balance maintenance, and imbalanced decline. In modern times, the decline in the number of rivers entering the lake due to human activities has made the water level of the lake drop 15–30 times that observed from 5 kaBP–0.1 kaBP. The results showed that the average undercut erosion rate of the river entering the lake tends to increase with a rapid decrease in the water level of the lake in the past 5000 years. The instantaneous undercutting rate of the Bortala river section in the basin was 1.6–4.2 times that observed from 5 kaBP–0.1 kaBP. This result showed that a rapid decline in the water level of the lake due to human activities will accelerate the erosion of rivers. Therefore, from the perspective of geomorphology, the river erosion triggered by the rapid decline in the local erosion base level is an important reason for the continuous shrinkage of Ebinur Lake.