Channel Widening Research Articles

River widening in mountain and foothill areas during floods: Insights from a meta-analysis of 51 European Rivers

River widening, defined as a lateral expansion of the channel, is a critical process that maintains fluvial ecosystems and is part of the regular functioning of rivers. However, in areas with high population density, channel widening can cause damage during floods. Therefore, for effective flood risk management it is essential to identify river reaches where abrupt channel widening may occur. Despite numerous efforts to predict channel widening, most studies have been limited to single rivers and single flood events, which may not be representative of other conditions. Moreover, a multi-catchment scale approach that covers various settings and flood magnitudes has been lacking. In this study, we fill this gap by compiling a large database comprising 1564 river reaches in several mountain regions in Europe affected by floods of varying magnitudes in the last six decades. By applying a meta-analysis, we aimed to identify the types of floods responsible for more extensive widening, the river reach types where intense widening is more likely to occur, and the hydraulic and morphological variables that explain widening and can aid in predicting widening. Our analysis revealed seven groups of reaches with significantly different responses to floods regarding width ratios (i.e., the ratio between channel width after and before a flood). Among these groups, the river reaches located in the Mediterranean region and affected by extreme floods triggered by short and intense precipitation events showed significantly larger widening than other river reaches in other regions. Additionally, the meta-analysis confirmed valley confinement as a critical morphological variable that controls channel widening but showed that it is not the only controlling factor. We proposed new statistical models to identify river reaches prone to widening, estimate potential channel width after a flood, and compute upper bound width ratios. These findings can inform flood hazard evaluations and the design of mitigation measures.

SPATIO-TEMPORAL ASSESSMENT OF LAND USE/LAND COVER CHANGE AND ITS IMPLICATIONS ON DONGA RIVER BASIN, TARABA STATE, NIGERIA

Disturbances due to anthropogenic activities, especially Land use/ Land cover change; modifies runoff and sediment transport leading to adjustment in channel dimension. To this effect, the land use/land cover change of Donga River Basin was conducted with the aid of temporal satellite images of 1985, 2000 and 2020 downloaded from the United States Geological Survey (USGS) achieve. An unsupervised classification was carried out in ARCGIS 10.3 where seven classes were generated namely: built up area, cropland, forest, grassland, plantations, water body, woodland and bare surfaces. Descriptive statistics was further used to analyse the data. The analysis revealed that Built-Up Area, Cropland, Grassland, Woodland and Bare surfaces increased by +3.41, +12.37, +2.88, +3.05 and +3.75 respectively; while Forest, Plantation and Water Body decreased by -20.93, -2.47 and -2.1 respectively. The implications of such changes at basin scale are increase sediment generated during agricultural activities which are capable of entrainment by fluvial processes, siltation, flooding, cutoffs, narrowing of geomorphic threshold, loss of anchorage for bank materials making it prone to under cutting, bank collapse, channel widening and loss of life and properties. It was recommended that healthy agricultural practices should be encouraged so as to reduce the amount of sediments generated from farming activities especially on hill slopes. Overgrazing and annual bush burning should be check mated in order to reduce the effect of nutrient leaching and erosion of the top soil. There’s also need for land use plan that is in line with ecological principles of the study area.

Effects of process‐based floodplain restoration on aquatic macroinvertebrate production and community structure

AbstractRecent stream restoration approaches, such as Stage 0 restoration, aim to restore natural processes to regain lost ecosystem functions, but project implementation can also represent a reach‐scale disturbance. Assumed outcomes of these restoration actions, like greater biological productivity, are rarely evaluated. In this study, we examined the short‐term effects of Stage 0 floodplain restoration on the secondary production of aquatic macroinvertebrate communities in the South Fork McKenzie River, Oregon, 1–2 years following project implementation. We seasonally sampled macroinvertebrates from benthic and submerged wood surfaces in the restored reach, and two unrestored reference reaches located upstream, to estimate annual secondary production. Macroinvertebrate production estimates were 3× lower on a per‐meter‐squared basis in the restored reach than in the upstream unrestored reference reaches (9764 vs. 29,636 mg DM/m2/yr). However, because there was 4.5‐times greater wetted area available in the restored reach, overall macroinvertebrate production per unit of valley length was 3.4× higher in the restored reach than in unrestored reference reaches (2744 vs. 802 kg DM/km). Additionally, the mosaic of aquatic habitats created via restoration (main‐channel, side‐channel, and wetted forest habitats) supported a diversity of macroinvertebrate assemblages both within and among reaches. Our findings suggest Stage 0 project implementation, which can include dewatering and filling incised channels, may reduce aquatic macroinvertebrate production on a per‐unit‐area basis for at least 1 or 2 years following restoration. However, this short‐term disturbance effect may be offset by channel aggradation and widening, which can provide a more wetted area for macroinvertebrate production and may support greater macroinvertebrate community diversity. Future studies are needed to examine the longer term (2–10 years) aquatic macroinvertebrate response to Stage 0 restoration, and the impacts of shifting resource availability on stream fishes.

CAN INVESTOR ATTENTION DEFUSE THE RISK OF CORPORATE ZOMBIFICATION? – EMPIRICAL EVIDENCE FROM LISTED COMPANIES IN CHINA

Solving the risk of zombification of enterprises and relieving their business difficulties, as a key element of supply-side structural reform, is the pain point of the conversion of old and new dynamic energy and the difficulty of economic transformation and upgrading. In the Internet era, the impact on business operations is also expanding with the widening of investor attention channels. This paper selects Chinese listed companies from 2011–2020 as a research sample, and the empirical results show that, first, investor attention can effectively reduce the risk of transforming enterprises into zombie enterprises, i.e., the risk of corporate zombification decreases as the level of investor attention increases; second, there is heterogeneity in the role of investor attention in resolving the risk of corporate zombification; third, further mechanism tests find that along with Third, further mechanistic tests reveal that as the level of investor attention increases, the level of environmental uncertainty decreases and the annual market value of individual stocks increases, thereby reducing the risk of corporate zombification. The findings of this paper provide theoretical support and empirical evidence for further improving the risk mitigation of corporate zombification, promoting the “de-emphasis” of enterprises, and leading the high-quality and healthy development of enterprises.

Gravity currents of viscous fluids in a vertically widening and converging fracture

We are investigating flows in the viscous-buoyancy balance regime in a converging channel with an upward increase in the width, with the gap of the channel varying according to a xkzr power function, being x and z the horizontal and vertical coordinate, respectively, and with 0<k<1 and 0<r<1 in order to be consistent with the model. The fluid rheology is described according to the Ostwald–de Waele model, with a power-law relationship between shear stress and shear rate and with application for shear-thinning, shear-thickening and, as a special case, Newtonian fluids. While for the case of flow in the direction of widening of the horizontal channel, a self-similar solution of the first kind can be expected, for flow toward the origin, with channel narrowing horizontally, the solution is self-similar of the second kind, with the space and a reduced time coupled in a self-similar independent variable but with an unknown parameter of the transformation group that makes the differential problem invariant. The solution is found in the phase plane by numerical integration of the paths connecting pairs of singular points, separately for the pre-closure phase, in which the current front advances invading the channel, and for the post-closure or leveling phase, in which the fluid has reached the origin and the front no longer propagates, while the level progressively increases canceling the pressure gradient. Integration is performed with a trial and error procedure by modifying the unknown parameter, generally named eigenvalue and specifically critical eigenvalue when the path has been successfully integrated. The overall effect of an increasing permeability upward is that of an increase in the front speed, with the current profile also becoming locally steeper. The effect of an increase in the fluid-behavior index is mixed, as it reduces the speed of the front but still increases the steepness of the local current profile. In any case, the model implies that the eigenvalue tends to infinity for k→1 even in the presence of an increase in the vertical permeability (r > 0).

Theoretical investigation on niobium doped LiFePO4 cathode material for high performance lithium-ion batteries

In this work, the first principle study has been conducted to investigate the properties of LiFePO4 (LFP) cathode material for the Li-ion batteries, and the effects of doping on their crystalline structures with Nb atoms was explored. The obtained results showed that doping of the LFP structure with Nb at the Fe site significantly improved the conductivity of LFP and decreased the band gap from 3.502 to 0.658 eV, which is consistent with the acquired density of state data. Moreover, energy formation results proved that the stability of the system after doping was ideal. The geometrical analysis of the structure showed that the Nb doping leads to widening of the Li passage channel, increased bond length, and reduction in covalent strength among atoms which in turn mitigated energy barrier. Furthermore, our calculation proved that Nb doping has no adverse effect on the theoretical voltage of LFP and, its value remained constant at about 3.1 V. Then, the nudged elastic band method was examined for the different pathways of Li ion diffusion in the LFP crystal. The finding represented that energy barriers of Li migration along all paths were decreased through Nb doping. In addition, the diffusion coefficient of Li-ion in the doped systems was improved by four orders of magnitude compared to undoped LFP. Moreover, the effective mass of electron theory and theoretical voltage of pristine LFP and Nb-doped LFP were evaluated. Finally, the electron localization function theory was implemented, for in-depth study and illustration of the difficulty of Li-ion diffusion and bond characteristics.

Modelling fire-induced perturbations in sediment flux based on stream widening and accelerated bank migration

Watersheds impacted by wildfires undergo alterations in water, sediment, and biogeochemical fluxes owing to, e.g., soil destabilization and loss in evapotranspiration potential. Enhanced sediment availability and surface runoff are known to make burnt slopes susceptible to localised slope failure, whereas larger watershed-scale (>1,000 km2) changes in sediment flux are largely unconstrained. This shortcoming results from the scarcity of direct measurements from most large, remote watersheds, which makes modelling efforts particularly relevant. Here, we develop a two-fold model to estimate sediment flux from hillslopes to a river based on characteristic rates of catchment erosion and relationships with riverbank migration. We apply this model to the watershed of the Bonaparte River (British Columbia, Canada), which was heavily impacted by the Elephant Hill megafire in the summer of 2017. Photogrammetric data collected from three meandering reaches reveal sharp post-fire channel widening and accelerated migration (up to 130% and 230%, respectively) in the fire’s aftermath. Field observations link these trends to enhanced post-fire suspended- and wash-load fluxes to floodplain reaches, whereas input of bedload sediments was likely restricted to bedrock reaches. Our model predicts a power-law relationship between fire-induced sediment-supply enhancement from hillslopes and riverbank mobility, consistent with the hypothesis that sediment flux affects meander migration. Future integrations of these results with sub-catchment-scale mapping of burn severity and direct data on sediment capacity may help better inform alterations in sediment and biogeochemical fluxes in the aftermath of wildfires elsewhere.

Combining exposed tree roots and UAV imagery to quantify land denudation in central Mexico

Approximately 42 % of Mexico is affected by soil denudation resulting from moderate to severe sheet erosion and gullying processes. At Huasca de Ocampo (central Mexico), soil degradation has been linked to intense land use dating back to pre-Hispanic times as well as to unfavorable geological, geomorphic, and climatic conditions. Here, we quantify erosion rates with high precision at annual to multi-decadal timescales by combining, for the first time, dendrogeomorphic reconstructions and UAV-based remote sensing. To assess rates of sheet erosion and gullying processes over the longer-term erosion rates (10–60 yrs), we assessed the age and first exposure of 159 roots to determine sheet erosion rates and gullying processes. At shorter timescales (<3 yrs), we employed an Unmanned Aerial Vehicle (UAV) to develop digital surface models (DSMs) for February 2020 and September 2022. Exposed roots provided evidence of sheet erosion ranging between 2.8 and 43.6 mm yr−1 and channel widening ranging between 11 and 270 mm yr−1, with highest erosion rates found along gully slopes. The UAV-based approach pointed to intense gully headcut retreat with rates between 164.8 and 870.4 mm yr−1; within gullies, channel widening rates ranged between 88.7 and 213.6 mm yr−1 and gully incision rates were between 11.8 and 109.8 mm yr−1. The two approaches yielded very comparable results regarding gully erosion and channel widening; this underlines the potential of using exposed roots to quantifying soil degradation processes retrospectively and considerably beyond the period covered by UAV imagery.

Exhumed fluvial landforms reveal evolution of late Eocene–Pliocene rivers on the Central and Northern Great Plains, USA

Abstract Cenozoic strata on the Great Plains are the products of a longlived, continental sediment routing system, and yet strikingly little is known about these ancient rivers. This article details the discovery of ~3100 fluvial ridges—erosionally inverted alluvial-fan, channel-fill, channel-belt, and valley-fill deposits—extending from the Rocky Mountain front to the eastern margin of the Great Plains. The direct detection of these channel bodies reveals new insights into late Eocene–Pliocene drainage evolution. Late Eocene–Oligocene streams were morphologically diverse. Alluvial fans adjacent to the Rocky Mountain front changed eastward to parallel or downstream-divergent, fixed, single-thread, straight to slightly sinuous (S = 1.0–1.5) streams &amp;lt;50 m in width. At ~100 km from the Rocky Mountain front, streams became sinuous and laterally mobile, forming amalgamated channel bodies as much as 3 km in width. Streamflow in all these systems was highly dispersed (southeast to northeast) and temporally variable. These characteristics reveal a nascent Great Plains alluvial apron hosting small, poorly integrated drainages undergoing abrupt changes. By the Miocene, more uniform streamflow generally trended east-northeast. Channel deposits are identifiable 500 km from the Rocky Mountain front. Middle Miocene valley fills gave way to fixed, multithread channels a few kilometers in width by the late Miocene. These patterns evince a mature alluvial apron hosting bigger rivers in well-integrated drainages. We interpret the systematic changes between fixed and mobile channel styles to record spatially and temporally variable aggradation rates. The widening of channels in the late Miocene likely reflects increased discharge relating to wetter climates upstream or the integration of once-isolated Rocky Mountain drainage basins into a continental-scale drainage system.

What drives major channel widening in mountain rivers during floods? The role of debris floods during a high-magnitude event

Our capability to accurately predict at the local scale the morphological changes in a mountain river affected by a severe flood is currently limited. Given the importance of including geomorphological hazards in flood risk evaluation, some improvements in this direction are required. In fact, on the basis of the controlling factors typically considered in the literature, that are the unit stream power (ωpk) and the confinement index (CI), a relatively broad spectrum of width ratios (WR = ratio between channel width after a flood and channel width before a flood) occurring in response to major floods is commonly observed in streams. We extended previous research focused on parameters that influence the morphological response of a channel to a severe flood by investigating the role played by the type of sediment transport process that occurs in a mountain river. In particular, we explored whether a relationship exists between debris flood (i.e. a condition characterised by extremely high bedload) and intense channel widening. The case study was the Cordevole River (the Dolomites, Italy) and four of its tributaries, which were affected by the severe flooding induced by the Vaia storm in October 2018. WR was determined at the sub-reach scale, and ωpk was calculated with consideration for the flood peak discharge ascertained via hydrological modeling. Despite the significant relationships found among WR, ωpk and CI, 21 sub-reaches (out of 144) of the studied stream network widened intensely reaching a WR higher than approximately 4, often to a substantially larger extent than predicted by statistical models. Such sub-reaches were certainly or probably affected by debris floods, leading us to infer that debris floods can induce widening processes that are more intense than those occurring in response to water flows. Having said that, debris floods do not automatically imply very intense widening. In addition to hydraulic and morphological constraints, the sediment transport processes possibly taking place at a sub-reach of a mountain river should be regarded as another driver of channel modifications during high-magnitude hydrological events.

Spatiotemporal variations of large wood and river channel morphology in a rapidly degraded reach of an intermittent river

AbstractThe links between rapid channel degradation and related large wood (LW) recruitment and storage are still not precisely understood, especially in aquatic systems with high human and natural pressures such as intermittent rivers in the peri‐Mediterranean region. In this study, by using multidisciplinary research (analysis of satellite images time series, detailed field inventory, and simulated hydrological data), we focused on rapid upstream propagation of channel degradation (i.e., incision and consequent channel widening) in an intermittent Mediterranean river (Evrotas River, southern Greece) as a response to short‐term intensive gravel extraction from the active channel and removal of riparian vegetation in the 2013–2016 period. Subsequently, our objective was to link this channel transformation with the spatiotemporal changes of LW storage in the active channel. We found that short‐term and spatially limited human impact, coupled with high flows, allowed rapid upstream progression (~2 km) of erosion processes (approximately twofold widening of the active channel) during the relatively short 2‐year period (2017–2019). This morphological response accelerated LW recruitment through bank erosion and its subsequent storage in the active channel. Incision and channel widening processes were interrupted in the 2019–2021 period due to the prevalence of relatively low, geomorphologically non‐effective flows. However, the storage of LW in the channel continued to increase as a result of the higher trapping efficiency of the widened channel, a delay between tree death and its final uprooting, or by LW recruitment processes not directly related to flood events. We suggest that freshly recruited LW has the potential to accelerate the establishment of a new channel equilibrium and increase channel heterogeneity through its interaction with flows and sediments.

Implant-dependent behavior of tunnel widening and clavicular button migration after arthroscopic-assisted treatment of acromioclavicular joint instabilities.

Despite ongoing discussions for the previous few decades, there is still no consensus regarding the optimal surgical technique for acromioclavicular joint instabilities. The purpose of this study was to determine the impact of various implant materials following arthroscopically assisted stabilization of acromioclavicular joint instabilities on tunnel widening and implant migration. We hypothesized an implant-dependent behavior for tunnel widening and implant migration with differences when comparing acute and chronic acromioclavicular joint instabilities. This study compared 105 patients with acromioclavicular joint instabilities that were managed operatively with coracoclavicular double button constructs. Two of the groups containing acute cases were treated with either a double button construct with a wire (TR) or a tape (D) as a central pillar. The two groups with chronic cases were either treated with a wire (T+) or tape D+ as a central pillar and additional hamstring tendon augmentation (+). One central transclavicular-transcoracoidal drill channel was made in the acute cases, while additional medial and lateral drill channels to augment the central pillar with the hamstring tendon were made. The central drill channel in all cases and the medial/lateral drill channels in chronic cases were subsequently radiologically analyzed immediately postoperatively and at follow-up. Following this, additional radiological analysis of the implant migration of the clavicular button took place. All groups showed significant tunnel widening of the central drill channel at follow-up (p ≤ 0.001). The TR+ technique demonstrated significant widening in both the medial (p ≤ 0.001) and lateral (p ≤ 0.001) drill channels. The D and D+ group displayed significant higher rates of clavicular button migration to a cortical and intraosseous level (p ≤ 0.002). Tunnel widening and implant migration following arthroscopically assisted management of acromioclavicular joint instabilities are dependent on the chosen implant. The stable tape showed a significantly increased degree of tunnel widening with respect to the central drill channel in comparison with the wire. In contrast, a higher degree of load capacity of the tape ultimately favors a protected intraosseous graft healing for chronic cases, which leads to less tunnel widening of the medial and lateral drill channel. Finally, the D/D+ implant groups showed higher rates of implant migration due to lower contact surface area of the implant at the upper clavicular cortex.

Examining meandering stream by using geomorphological characteristics with GIS-based analysis

An assessment of the meandering stream type using its classification system to geomorphology characteristics combined with GIS-based analysis is presented in this paper. It describes geomorphology characteristics consisting of 8 parameters with GIS-based analysis that differ in the zone of sediment position, stream width, stream sinuosity, amplitude, wavelength, bend sharpness, meander pattern, and slope. The selected case study in this paper is the Barito Stream, South Kalimantan, Indonesia. Based on the results, the variability varied in all the geomorphology characteristics except bend sharpness and slope. The transport zone is the longest zone with classified as a very wide river with moderate sinuosity and high amplitude (&lt; 1500). It is also categorized as a moderate wavelength and sharp bend with a relatively shallow slope. This approach is a simple, appropriate, and easy-to-use practice in examining meandering stream since there is no data or lack of supporting field data. The implementation of this meandering stream classification method is suitable for stream restoration projects, fish habitat enhancement, and water resource management. Further research is the study of possible geomorphic responses of a channel to natural and anthropogenic disturbances including channel-bed degradation, channel-bed aggradation, channel widening, and channel straightening.

Long-term geomorphic adjustments following the recoupling of a tributary to its main-stem river

River restoration and rehabilitation projects are widespread, but rarely include the data needed to fully evaluate if they are successful in achieving their goals or how long the process of readjustment takes before a new ‘recovered’ regime state is reached. Here we present a seven-year post-project dataset detailing the morpho-sedimentary responses of a river to the reconnection of a formerly diverted tributary, and relate observed changes to conditions in the river prior to the reconnection. We describe changes in the tributary and main-stem channels, including changes in channel planform, morphology, and the export of coarse and fine sediment from the tributary to the main-stem river. We use the data to develop a conceptual model of the system's response to the reconnection.Marked geomorphic changes occurred within the first two years after the reconnection. Changes during this ‘shock phase’ included dramatic erosion and subsequent deepening and widening of the tributary channel, rapid development of a confluence bar and an increase in fine sediment delivered to the main-stem. After this shock phase, and despite the continued occurrence of high magnitude flow events, the rate of geomorphic change in the tributary began to decrease, and the rate of growth of the confluence bar slowed. Fine sediment volumes in the main-stem also decreased steadily. After an adjustment phase lasting a total of approximately 4.5 yr (including the initial 2-yr shock phase), the tributary to mainstem system appeared to reach a new dynamic equilibrium that we consider the adjusted regime state. This new regime state was characterised by, among other things, an increase in geomorphic heterogeneity in the tributary and main-stem channels.Changes in both fluvial processes and forms indicate that within 4.5 yr the project was successful in achieving its goal of augmenting sediment and increasing geomorphic heterogeneity. Our conceptual model of adjustment mirrors that developed by Petts and Gurnell (2005), with the river passing through a complex and dynamic adjustment phase before reaching a new regime state. However, unlike the responses to impoundment represented by Petts and Gurnell, our model of river response to rehabilitation charts increases in dynamism and heterogeneity.

Runnels mitigate marsh drowning in microtidal salt marshes.

As a symptom of accelerated sea level rise and historic impacts to tidal hydrology from agricultural and mosquito control activities, coastal marshes in the Northeastern U.S. are experiencing conversion to open water through edge loss, widening and headward erosion of tidal channels, and the formation and expansion of interior ponds. These interior ponds often form in high elevation marsh, confounding the notion applied in predictive modeling that salt marshes convert to open water when elevation falls below a critical surface inundation threshold. The installation of tidal channel extension features, or runnels, is a technique that has been implemented to reduce water levels and permit vegetation reestablishment in drowning coastal marshes, although there are limited data available to recommend its advisability. We report on 5 years of vegetation and hydrologic monitoring of two locations where a total of 600-m of shallow (0.15-0.30-m in diameter and depth) runnels were installed in 2015 and 2016 to enhance drainage, in the Pettaquamscutt River Estuary, in southern Rhode Island, United States. Results from this Before-After Control-Impact (BACI) designed study found that runnel installation successfully promoted plant recolonization, although runnels did not consistently promote increases in high marsh species presence or diversity. Runnels reduced the groundwater table (by 0.07-0.12 m), and at one location, the groundwater table experienced a 2-fold increase in the fraction of the in-channel tidal range that was observed in the marsh water table. We suggest that restoration of tidal hydrology through runnel installation holds promise as a tool to encourage revegetation and extend the lifespan of drowning coastal marshes where interior ponds are expanding. In addition, our study highlights the importance of considering the rising groundwater table as an important factor in marsh drowning due to expanding interior ponds found on the marsh platform.

Sediment supply control on morphodynamic processes in gravel‐bed river widenings

AbstractReach‐scale widening of channelized rivers can locally reactivate morphodynamic processes and increase habitat heterogeneity. Following process‐based restoration principles, construction efforts are often limited to bank protection removal and local bank erosion initiation. Despite significant advances in our understanding of fluvial processes, the influence of sediment supply on widening morphodynamics is still inadequately understood. Using mobile‐bed laboratory experiments, we studied the morphological development of river widenings in channelized subalpine gravel‐bed rivers in response to different sediment supply levels. The initial setup included a channel with a longitudinal slope of around 1% and a floodplain on one side, which was 25 channel widths long and three wide. After removing the fixed bank, the floodplain was available for erosion and channel shifting. Seven experimental series with sequences of steady bed‐forming discharge and unsteady larger floods were conducted. Sediment was supplied at rates of 20%, 60%, 80%, and 100% of the initial channel's transport capacity. The laboratory experiments were combined with two‐dimensional hydrodynamic numerical simulations to closely track both the morphological development and the resulting flow conditions. Sediment supply was found to be a critical driver of morphodynamic activity in river widenings, with sediment supply rates close to channel transport capacity promoting active channel widening, sediment relocation, and lateral channel‐floodplain connectivity. In contrast, low sediment supply maintained a stable, single‐thread channel disconnected from its floodplain. Further, our results indicate that an increase in sediment supply or structural measures triggering local flow deflection can reactivate morphodynamic processes in previously sediment‐starved reaches after a transition period of several years to decades. Our results highlight the fundamental importance of sediment transport in reach‐scale river restoration.

Morphological changes currently occurring in sand-filled gully channels on Mars: Implications for the role of substrates inside channels

Long-term monitoring of dune/non-dune gullies has substantiated extensive and diversified morphological changes whose timing correlates with the time of removal of CO2 frost, thereby implying seasonal control on the active processes. We present evidence for present-day changes in sand-filled gully channels consistent with ripples. These channels are ubiquitous in the mid-latitudes and found in gullies formed over a variety of substrates on crater walls. We investigated 1483 gullied craters having coverage of at least two High Resolution Imaging Science Experiment (HiRISE) images between 30°S and 75°S and identified 98 craters (6.6% of the total) with sand-filled gully channels. Among these 98 craters, sand-filled gully channels in 25 craters (25.5% of the total) exhibit morphological changes indicative of avulsion, narrow channel incision, formation of channels with potential terminal pits, channel widening and extension, downslope flow of mobilized sand inside channels, dark flows and rapid boulder movement inside channels, and local sand rearranging and erasing of channels. Investigation of the sequence of events revealed CO2 frost inside the sand-filled gully channels in the image acquired during winter and prior to the image in which recent changes have been observed. This suggests a connection with the sublimating frost inside the channels and a seasonal control that subsequently produces extensive changes in the sand-filled gully channels during defrosting seasons in approximately a Mars year. Comparison of changes on both sand-filled gully channels and dune gully channels substantiates that the currently active processes have similar effects on the loose, unconsolidated substrate in both the gully types. For example, CO2 frost processes are able to erode both sand-filled gully channels and dune gully channels to form linear channels with terminal pits. In contrast, we have not found similar evidence of linear channels with terminal pits on bedrock and/or LDM substrate on the crater wall suggesting that the bedrock and LDM substrate may not be able to be eroded by CO2 frost processes in a manner similar to the loose, unconsolidated substrate in sand-filled gully channels investigated in this study. Together, the results signify that the sand-filled gully channels represent a significant component of present-day modification of gully channels on Mars.

Large deltas, small deltas: Toward a more rigorous understanding of coastal marine deltas

Deltas are subaerial landforms that cap underlying deposits with subaqueous extensions that result from a river feeding sediment directly into a standing body of water at a rate that overwhelms any effective dispersal processes derived from the ambient basin. This definition encapsulates both the terrestrial surface expression and the geological focus on the entire sediment mass. Environmental studies also focus on the ecology of deltaic wetlands, their drowning history, and related sustainability issues including societal considerations, history, and culture. A mean 76 ± 16% drop in hydraulic energy occurs in all subaerial deltas regardless of size, given the break in gradients separating fluvial and deltaic surfaces, driving an ever-decreasing bed-material transport, shallowing of distributary channels and concomitant overbank flooding. A delta's sediment mass grows from the addition of new river loads but can also include aeolian and marine sediment derived from outside the delta domain, growth of peat and other biomass, and inputs from human action. Removal of sediment is via river plumes interacting with marine currents, wave-induced transport, sediment failures and gravity flows, high-tide inundation onto the delta plain, tidal channel widening and deepening, and human action (peat, clay, sand and gravel mining). A delta's trapping efficiency ranges from 0 for small-load rivers that discharge directly into an energetic ocean, to 80% for large deltas, and up to 100% for some semi-enclosed bayhead deltas, including fjords. The global (ensemble) subaerial delta aggradation rate is ∼1.6 mm/y if 70% of the global sediment load exits the river mouth(s), a reminder of how much sediment can be expected to be delivered to the surfaces of global deltas at a time when the 2022 CE sea level rise is ∼4 mm/y. At the planetary scale, deltas are environmentally complex given Earth's range in climate, hydrodynamics, tectonic settings, relative sea-level provinces, sediment input, redistribution processes, and human actions. Under natural conditions, the subaerial portion of deltas adapt to change by advancing, retreating, switching, aggrading, and/or drowning, whereas many modern deltas are structurally constrained by societal needs. The 89 large and mud-rich coastal marine deltas (i.e. subaerial area > 1000 km2) account for 84.3% of Earth's total deltaic area that hosts >89% of all humans occupying deltas, many living within megacities. The 885 medium-size deltas (i.e. subaerial areas 10–1000 km2) account for 15.5% of the global delta area and 10.5% of humans living on deltas, with characteristics that fall between the small and large delta categories. The 1460 small and essentially sandy deltas (1–10 km2), including all fjord deltas, are impacted less from human action (with exceptions) and most are better able to withstand climate change. Recognizing the limits of big data in capturing delta complexity, field data remains a necessary gold standard for site investigators.

Analysis of historical changes in planform geometry of a mountain river to inform design of erodible river corridor

The analysis of changes in planform geometry of the Biała River during the last 130 years is used to diagnose hydromorphological river degradation and thus to verify the need of river restoration through establishing an erodible river corridor. Outcomes from the analysis are then used for delimiting the erodible corridor and predicting the potential for future river widening in the corridor. The study analyses presentations of the active river zone and the structure of river geomorphic units on topographic maps from 1878, 1935 and 1962, aerial photos from 1967, 1977 and 1987, and orthophotos from 1998 and 2009. As a result of river channelization followed by channel incision, during the study period the width of the Biała was reduced to 16%–57% of its width in 1878. The river narrowing was associated with a reduction in the proportion of channel bars in the total river width and nearly complete elimination of islands. All these changes demonstrated the degradation of hydromorphological integrity of the river and the need for its restoration by allowing free channel development in an erodible corridor. The belt of the river migration during the last 130 years identified on the basis of overlays of the river position from all dates was ⁓5 times wider than the contemporary river. This justified delimiting the erodible river corridor on a substantially larger area of the valley floor than the area of the river in 2009. In 2009 the river was narrower than the largest river width recorded between 1878 and 1998 along nearly the whole length of the study river sections. This indicates that the concentration of flood flows in the narrow channel, increasing their unit stream power, is a factor increasing the potential for future bank retreat and channel widening in the erodible corridor of the river.

Drainage reorganization induces deviations in the scaling between valley width and drainage area

Abstract. The width of valleys and channels affects the hydrology, ecology, and geomorphic functionality of drainage networks. In many studies, the width of valleys and/or channels (W) is estimated as a power-law function of the drainage area (A), W=kcAd. However, in fluvial systems that experience drainage reorganization, abrupt changes in drainage area distribution can result in valley or channel widths that are disproportional to their drainage areas. Such disproportionality may be more distinguished in valleys than in channels due to a longer adjustment timescale for valleys. Therefore, the valley width–area scaling in reorganized drainages is expected to deviate from that of drainages that did not experience reorganization. To explore the effect of reorganization on valley width–drainage area scaling, we studied 12 valley sections in the Negev desert, Israel, categorized into undisturbed, beheaded, and reversed valleys. We found that the values of the drainage area exponents, d, are lower in the beheaded valleys relative to undisturbed valleys but remain positive. Reversed valleys, in contrast, are characterized by negative d exponents, indicating valley narrowing with increasing drainage area. In the reversed category, we also explored the independent effect of channel slope (S) through the equation W=kbAbSc, which yielded negative and overall similar values for b and c. A detailed study in one reversed valley section shows that the valley narrows downstream, whereas the channel widens, suggesting that, as hypothesized, the channel width adjusts faster to post-reorganization drainage area distribution. The adjusted narrow channel dictates the width of formative flows in the reversed valley, which contrasts with the meaningfully wider formative flows of the beheaded valley across the divide. This difference results in a step change in the unit stream power between the reversed and beheaded channels, potentially leading to a “width feedback” that promotes ongoing divide migration and reorganization. Our findings demonstrate that valley width–area scaling is a potential tool for identifying landscapes influenced by drainage reorganization. Accounting for reorganization-specific scaling can improve estimations of erosion rate distributions in reorganized landscapes.