Sediment Mobilization Research Articles

Investigating the importance of recreational roads as a sediment source in a mountainous catchment using a fingerprinting procedure with different multivariate statistical techniques and a Bayesian un-mixing model

Road construction associated with land development generally increases erosion and sediment yields. Construction of unpaved roads has the potential to alter hydro-sedimentological behavior and catchment sediment source dynamics and, to date, this has largely been investigated in forested environments. The objective of this study, therefore, was to assess the relative importance of unpaved recreational roads as a sediment source alongside hillslope surface soils and stream channel banks in a non-forested mountainous catchment in northern Tehran, Iran, using a fingerprinting procedure. Eleven geochemical tracers were measured on 27 samples collected to characterise the sediment sources and five suspended sediment samples collected at the study catchment outlet. The statistical analysis employed to select three different composite fingerprints for discriminating the sediment sources comprised: (1) the Kruskal–Wallis H test (KW-H), (2) a combination of KW-H and discriminant function analysis (DFA), and (3) a combination of KW-H and principal components & classification analysis (PCCA). A Bayesian un-mixing model was used to ascribe sediment source contributions using the three composite fingerprints. Using the KW-H composite signature, the respective relative contributions (with uncertainty ranges) from recreational roads, hillslope surface soils and channel banks were estimated as 64.5% (57.7–73.1), 1.1% (0.1–4.9), and 33.9% (24.9–41.0), compared to 55.3% (45.5–68.5), 1.9% (0.1–7.9) and 42.1% (27.8–52.4) using a composite signature selected using a combination of KW-H and DFA, or 82.0% (69.7–93.8), 8.2% (0.7–22.7) and 7.3% (0.7–21.0) using a fingerprint selected using KW-H and PCCA. The root mean square difference between the apportionment results using the fingerprints identified on the basis of the three different statistical approaches ranged from 5.5% to 25.7%, highlighting the sensitivity of source estimates to the tracers used. Regardless, the different composite signatures all suggested that unpaved recreational roads were the dominant source of the suspended sediment samples, underscoring the need for mitigation measures targeting these anthropogenic features of the catchment system, including closure to permit re-vegetation, surface ripping and/or mulching to improve infiltration or gravel re-surfacing to reduce exposure of bare surfaces to sediment mobilisation.

Modeling global anthropogenic erosion in the Holocene

A large proportion of natural vegetation has been converted to agricultural use, and this typically accelerates erosion by one to two orders of magnitude. Quantification of this accelerated erosion is important to understand the impact of human activities on soil ecosystem service given that soil erosion induces soil degradation and changes in soil organic carbon (SOC) stocks. Until now, few studies have evaluated the accumulated impact of agricultural erosion, since the start of agriculture (ca. 6000 BC), on the soils system and the carbon cycle. In this study, we mainly focused on the enhanced water erosion by conversion of natural vegetation to crops, while wind erosion on the cropland is not assessed. We first evaluated and constrained existing anthropogenic land cover change (ALCC) scenarios by comparing observed cumulative erosion for the agricultural period under a wide range of global agro-ecological conditions with model simulations. An optimized land-use scenario that makes the best fit between the simulation and the observation was derived in the model calibration. We further applied a spatially distributed erosion model, which was modified based on Revised Universal Soil Loss Equation (RUSLE), under the optimized land-use scenario across globe to estimate the total anthropogenic cumulative erosion and characterize their spatial variability. Simulations suggest that conversion from natural vegetation to cropland has caused a global cumulative agricultural erosion of 27,187 ± 9030 Pg for the period of agriculture. This results in an average cumulative sediment mobilization of 1829 ± 613 kg m−2 on croplands, corresponding to a soil truncation of ca. 1.34 ± 0.45 m. Regions of early civilization, particularly with high cropland fractions such as South Asia, Southeast Asia, and Central America have higher area-averaged anthropogenic erosion than other regions. This results in spatial variability in soil truncation rates because of erosion, which would further affect the soil production rate. Our study shows that observations of long-term anthropogenic erosion at the catchment scale can be used to constrain the reconstructed land-use scenarios.

The sources and dynamics of fine-grained sediment degrading the Freshwater Pearl Mussel (Margaritifera margaritifera) beds of the River Torridge, Devon, UK

The Freshwater Pearl Mussel (Margaritifera margaritifera) is an endangered organism across its entire range. It has a complex life cycle and stringent habitat requirements and is therefore an indicator species for the general ecosystem health of host rivers. Whereas historical intensive pearl fishing contributed to population declines, excess nutrient and sediment loss associated with current land use pressures in host river catchments, including modern intensive farming practices, are now highlighted as primary contributory factors. Accordingly, this study investigated the sources and dynamics of fine-grained sediment sampled in the mussel beds of the River Torridge, SW England. Sediment source fingerprinting using a combination of colorimetric and radiometric tracers to construct different composite signatures revealed the importance of roads both as a sediment source and delivery pathway for fine-grained sediment mobilised from fields predominantly supporting lowland livestock farming. Grassland fields with evidence of soil poaching were highlighted as important sediment sources, but equally, riparian woodland was also identified as important, especially during the latter stages of consecutive runoff events when its rainfall buffering capacity was exceeded. Bed sediment storage levels (median up to 393 g m−2) were found to be low (41st percentile) compared to typical values reported by a recent strategic scale survey across England and Wales, whereas elevated turbidity peaks were shown to be long duration (days) in conjunction with consecutive days of rainfall and corresponding runoff events. Hysteresis patterns varied but were generally clockwise during the largest runoff events associated with consecutive rain days; again, suggesting mobilisation of sediment from proximal woodland sources following exceedance of rainfall buffering capacity. In combination, the data assembled by this study provides a basis for planning sediment control measures for protecting the Freshwater Pearl Mussel (FPM) beds from excessive fine-grained sediment inputs associated with the intensive use of primarily grazing land.

On the pre-history of the turbidite concept: an Alpine perspective on occasion of the 70th anniversary of Kuenen’s 1948 landmark talk

The recognition of turbidity currents as an important geological agent of sediment transport, sedimentation, and erosion 70 years ago, initiated a bonanza in clastic sedimentology and led to new interpretations of both recent marine sediments and ancient sedimentary rocks. Only scarce attention had been paid to graded bedding, a hallmark of turbidites, before ca. 1930 and almost nobody had linked it to episodic mass flow sedimentation. This article is an addition to the pre-history of the turbidite revolution from an Alpine, and more specifically, a Swiss perspective. It focuses on two time periods, the early and middle part of the eighteenth century and the turn from the nineteenth to the twentieth century. In the former period, Swiss naturalists such as Scheuchzer and Gruner had not only recognized the ubiquity of graded bedding in some Alpine flysch successions, but also proposed hypotheses as to its likely origin. In doing so, Gruner invoked in 1773 periodic sediment mobilization due to bottom currents on the floor of an ante-diluvian sea, resulting in distinct episodes of sediment settling giving rise to normally graded layers. Gruner’s model was inspired by several contemporaneous pioneers of experimental clastic sedimentology. This interest in physical sedimentology declined during the nineteenth century and it was only with the start of the second period discussed herein that geologists began to re-appreciate the importance of sediment movement at the bottom of the oceans. This time, inspiration came from the practical experience of Swiss geologists with shore collapse events and subsequent sedimentary mass flows in lakes. Building on that, they were able to better interpret parts of the Alpine sedimentary record, especially flysch successions. However, it was only with Bailey, Migliorini, Kuenen and others during the 1930s and 1940s that the link between mass flows and graded bedding was finally established.

Conceptual Framework for Sediment Management in Torrents

Torrential catchments are complex systems showing a huge diversity of sediment and flow regime reactions. Therefore, it seems reasonable to decrease the complexity by introducing a conceptual framework to obtain a general idea of the torrential system’s behavior. The aim of the manuscript is to provide a conceptual framework and a stepwise procedure to develop a reproducible sediment management concept. This involves starting with a definition of torrential process types. These processes are then routed through the drainage system, which is divided into homogeneous reaches. Each reach transfers a flow process from the upstream node to the adjacent downstream reach, based on driving geomorphic factors of sediment mobilization and deposition. Additionally, a sediment budget, derived from cumulative erosion and deposition volumes, is calculated. The estimated flow process together with the sediment budget enables the selection of a sediment management strategy. The implementation of the strategy is based upon the selection of functions, which define the desired modification with regard to the torrential system. Ultimately, the sum of each function’s performance must lead to the fulfilment of the overall protection objective.

Shapefiles with the position of the studied edifices for the study "Subsurface Sediment Mobilization in the Southern Chryse Planitia on Mars"

Shapefiles with the position of the studied edifices for the study "Subsurface Sediment Mobilization in the Southern Chryse Planitia on Mars"

Field observations of turbulence in the intertidal and shallow subtidal zones

Turbulence is critical for sediment mobilisation in shallow water and hence beach evolution. Present large-scale morphological models often exclude the effects of bed- and surface-generated turbulence on the short-wave sediment transport, which might be part of the reason why they tend to result in incorrect estimates of the onshore-directed sediment transport under low-energetic conditions. A first step for model improvements is to be able to couple turbulence intensity and orbital motion at intra-wave time scale. Field measurements of flow velocities at three vertical elevations at two cross-shore locations in the intertidal and shallow subtidal zones were used to examine turbulence beneath shoaling waves, spilling breakers and surf bores. Beneath shoaling waves, near-bed turbulent kinetic energy (TKE(t)) was generated by bed friction and phase-coupled to the cross-shore orbital velocity (u(t)). Beneath spilling breakers, turbulence intensity increased, especially in the upper part of the water column due to injection of breaker-generated turbulence. The surface-induced TKE(t) peaked just after the wave crest. Near the bed, TKE(t) had a broad maximum under the wave trough possibly due to increased shear from stronger offshore-directed mean currents. Beneath surf bores, TKE(t) peaked at the wave front throughout the water column which could be due to both acceleration skewness and a turbulent bore front.

Sediment transport by tropical cyclones recorded in a submarine canyon off Bangladesh

Frequent cyclones originating in the Bay of Bengal landfall on the delta coast of the Ganges and Brahmaputra rivers. The cyclones are well recorded in the sediments of a canyon that is deeply incised into the shelf offshore Bangladesh. The large mud supply by the two rivers forms temporary deposits on the innermost shelf, where they are mobilized by waves and currents during the passage of cyclones. The resulting, highly concentrated fine sand-silt-clay suspension is moved by wind-induced currents and eventually plunges into the shelf canyon. These gravity flows are deposited as graded beds on the broad canyon floor. In a 362-cm-long section of a dated sediment core covering the period from 2006 to 1985, nearly all 59 graded beds can be correlated with 42 cyclones observed in that period. The threefold decrease in the sedimentation rate of the last decade compared to the period from 1994 to 1954 is due to the decreased number and power of cyclones. Compared to the sediment transfer by cyclones, the input by local sediment slumps, tidal currents, and monsoonal floods is small. Thus, cyclones dominate the mobilization and distribution of sediment on the Bangladesh shelf. This sediment dispersal mechanism is probably also typical for other shelf areas crossed by tropical cyclones.

Climatic and tectonic controls on source-to-sink processes in the tropical, ultramafic catchment of Lake Towuti, Indonesia

Humid tropical landscapes are subject to intense weathering and erosion, which strongly influence sediment mobilisation and deposition. In this setting, we aimed to understand how geomorphology and hydroclimate altered the style and intensity of erosion and sediment composition in a tropical lake and its tectonically active catchment. Lake Towuti (2.75°S, 121.5°E) is one of the oldest and deepest lakes in Indonesia, with uninterrupted lacustrine sedimentation over several glacial–interglacial cycles. Here we present results from a novel set of Lake Towuti surface sediment, bedrock and soil samples from the catchment, and two existing sediment cores that extend to 30,000 and 60,000 years before present. We studied the catchment morphology, soil properties, geochemistry, and clay and bulk mineralogy. Results from several river long profiles show clear signs of tectonic activity, which enhances river incision, favours mass movement processes, and together with remobilisation of fluvial deposits, strongly influences modern sedimentation in the lake. Material from the Mahalona River, the lake’s largest inflow, dominates modern sediment composition in Towuti’s northern basin. The river transports Al-poor and Mg-rich sediments (mainly serpentines) to the lake, indicating river incision into the Mg-rich serpentinised peridotite bedrock. Relatively small, but important additional contributions of material, come from direct laterite-derived input and the Loeha River, which both provide Al-rich and Mg-poor sediment to the lake. Over time, the Al/Mg and kaolinite-to-serpentine ratios varied strongly, primarily in response to lake-level fluctuations driven by hydroclimatic changes. In the past 60,000 years, both the Al/Mg and kaolinite-to-serpentine ratios showed variations sensitive to changes in climate boundary conditions across glacial-interglacial cycles, while tectonic activity had less influence on changes in sediment composition on these short time-scales.

Influence of Hydrodynamic Processes on the Fate of Sedimentary Organic Matter on Continental Margins

AbstractUnderstanding the effects of hydrodynamic forcing on organic matter (OM) composition is important for assessment of organic carbon (OC) burial in marginal seas on regional and global scales. Here we examine the relationships between regional oceanographic conditions (bottom shear stress), and the physical characteristics (mineral surface area and grain size) and geochemical properties (OC content [OC%] and carbon isotope compositions [13C, 14C]) of a large suite of surface sediments from the Chinese marginal seas to assess the influence of hydrodynamic processes on the fate of OM on shallow continental shelves. Our results suggest that 14C content is primarily controlled by organo‐mineral interactions and hydrodynamically driven resuspension processes, highlighted by (i) positive correlations between 14C content and OC% (and surface area) and (ii) negative correlations between 14C content and grain size (and bottom shear stress). Hydrodynamic processes influence 14C content due to both OC aging during lateral transport and accompanying selective degradation of OM associated with sediment (re) mobilization, these effects being superimposed on the original 14C characteristics of carbon source. Our observations support the hypotheses of Blair and Aller (2012, https://doi.org/10.1146/annurev‐marine‐120709‐142717) and Leithold et al. (2016, https://doi.org/10.1016/j.earscirev.2015.10.011) that hydrodynamically driven sediment translocation results in greater OC 14C depletion in broad, shallow marginal seas common to passive margin settings than on active margins. On a global scale, this may influence the extent to which continental margins act as net carbon sources and sinks. Our findings thus suggest that hydrodynamic processes are important in shaping the nature, dynamics, and magnitude of OC export and burial in passive marginal seas.

The expression of early Aptian to latest Cenomanian oceanic anoxic events in the sedimentary record of the Briançonnais domain

Abstract The Brianconnais domain is an important paleogeographic unit in the Alps, which represents a structural high within the former Alpine Tethys Ocean. Expressions of the most important Cretaceous oceanic anoxic events (OAE1a-d and OAE2) are exceptionally well preserved in the Subbrianconnais unit at Roter Sattel in the Fribourg Prealps, Switzerland. The main objective of this study was to reconstruct oxygen contents and trophic levels in the water column for each OAE. To accomplish this we used an integrated multi-proxy approach, including total organic carbon (TOC), phosphorus (P), and redox and productivity-sensitive trace-element contents (RSTE, PSTE). RSTE enrichments associated with the highest TOC values (up to 5.4 and 4.8 wt%) and with the lowest Mn contents in sediments equivalent to the Selli and Paquier OAEs (1a and 1b) indicate the presence of intermittent anoxic to euxinic conditions during these events. Sediments equivalent to Bonarelli OAE2 near the Cenomanian-Turonian (CT) boundary are also characterized by significant trace-element enrichments in comparison to average shale values. Mo and U enrichment factors (EFs) in the lower Aptian and lower Albian intervals are characteristic of an unrestricted marine environment, and in the Cenomanian-Turonian boundary interval they indicate a relationship with a particulate Fe-Mn-oxyhydroxide shuttle within the water column. In the OAE1a interval the elevated total P content associated with higher Corg/Ptot ratios and maxima in TOC values suggests that a part of the remobilized P remained trapped in the sediments. By contrast, the lower Ptot values associated with RSTE and Corg/Ptot enrichments in the OAE1b and OAE2 intervals indicate that a significant part of P was remobilized and escaped to the bottom water. Our results highlight the combined roles of regional and global parameters in the development of oxygen-deficient conditions that may lead to anoxia and increase in organic matter preservation during the OAEs. The differentiated topography of the Brianconnais domain associated with global climate and sea-level change and its influence on weathering in general, and more specifically on the mobilization of sediments on the adjacent ridge, modulated nutrient availability in different proportions during the different “mid”-Cretaceous OAEs.

Assessment of the Sustainability of the Territories Affected by Gully Head Advancements through Aerial Photography and Modeling Estimations: A Case Study on Samal Watershed, Iran

Gully erosion is considered one of the major issues of land sustainability because it can remove considerable volumes of sediment and productive soils. Once started, gullies can continue to move by headcut retreat, or slumping of the side walls. Studies of gully development require constant monitoring activities which are not possible in not-well-explored areas, such as the arduous region of Iran, due to costs and a lack of geoinformation. Thus, the present research attempts to assess gully evolution using only two digital aerial photographs of different periods (1968 and 1994) and field assessment (2009) to estimate the gully head advancement based on frames geometry and rigorous procedure in southwestern Iran. Also, the gully head advancement was estimated and compared among them by different empirical equations. The results indicated that the mean of gully head advancement was 1.4 m year−1 and 1.2 m year−1 during 1968–1994 and 1994–2009, respectively, and the annual average of sediment mobilization was 26.8 m3 ha−1 in 2009. The model assessment indexes indicated that SCS (Soil Conservation Service) II was the best model for gully head advancement estimations in this study area. The main reasons for this can be associated with the Rp factor (previous gully head advancement) and the local environmental conditions. We conclude that the sustainability of the territory has been greatly affected due to this advancement. We also hypothesize that gully head changes could be related to the susceptibility of geological formations, climate, soil properties, and the coincidence of other gullies’ formation with common drainage networks in the study area. Based on the obtained results, land managers can use the results to distinguish the gullies in this region with a higher environmental risk, and to decide an effective implementation of soil conservation measures in order to include them in the land management plans.

The effects of dredging and environmental conditions on concentrations of polycyclic aromatic hydrocarbons in the water column

Sediment dredging can cause damage to the marine environment due to mobilization of sediments and contaminants. The effects of dredging and boundary environmental conditions on the concentration of Polycyclic Aromatic Hydrocarbons (PAHs) in water were evaluated during dredging of the Oil Port of Genoa-Multedo (Italy). Results showed that turbidity and PAH concentrations increased in the water during dredging. However, the scenario was complex due to the high number of interacting physical-chemical factors influencing PAH concentrations and transport. Due to these, PAH distribution is different in water, where low-molecular-weight PAHs were predominant (maximum concentration 0.105 μg L−1), and in bottom sediments, where high-molecular-weight PAHs had the highest concentrations (from 299.3 to 1256.5 ng g−1). Moreover, mainly during dredging the PAH concentrations in water were significantly higher inside than outside the port as a consequence of the lower dynamics within the port basin. Turbidity was the main parameter related to PAH concentrations.

Concentration–discharge relationships describe solute and sediment mobilization, reaction, and transport at event and longer timescales

AbstractConcentration–discharge (C‐Q) relationships reflect material sources, storage, reaction, proximity, and transport in catchments. Differences in hydrologic pathways and connectivity influence observed C‐Q patterns at the catchment outlet. We examined solute and sediment C‐Q relationships at event and interannual timescales in a small mid‐Atlantic (USA) catchment. We found systematic differences in the C‐Q behaviour of geogenic/exogenous solutes (e.g., calcium and nitrate), biologically associated solutes (e.g., dissolved organic carbon), and particulate materials (e.g., total suspended solids). Negative log(C)–log(Q) regression slopes, indicating dilution, were common for geogenic solutes whereas positive slopes, indicating concentration increase, were common for biologically associated solutes. Biologically associated solutes often exhibited counterclockwise hysteresis during events whereas geogenic solutes exhibited clockwise hysteresis. Across event and interannual timescales, solute C‐Q patterns are linked to the spatial distribution of hydrologic sources and the timing and sequence of hydro‐biogeochemical source contributions to the stream. Groundwater is the primary source of stormflow during the earliest and latest stages of events, whereas precipitation and soil water become increasingly connected to the stream near peakflow. This sequence and timing of flowpath connectivity results in dilution and clockwise hysteresis for geogenic/exogenous solutes and concentration increase and counterclockwise hysteresis for biologically associated solutes. Particulate materials demonstrated positive C‐Q slopes over the long‐term and clockwise hysteresis during individual events. Drivers of particulate and solute C‐Q relationships differ, based on longitudinal and lateral expansion of active channels and changing shear stresses with increasing flows. Although important distinctions exist between the drivers of solute and sediment C‐Q relationships, overall solute and sediment C‐Q patterns at event and interannual timescales reflect consistent catchment hydro‐biogeochemical processes.

Monitoring ephemeral river changes during floods with SfM photogrammetry

Monitoring detailed topographic changes in rivers is an essential tool for understanding the morpho-sedimentary behavior of rivers. However, as the resolution of topographic reconstruction techniques improves, survey costs and the time consumed increase. In this paper, the emerging Structure from Motion Multi-View Stereo (SfM-MVS), a high resolution but low cost technique, is scrutinized to assess whether it constitutes a viable option for ‘change detection’ in ephemeral rivers. To that end, three photogrammetric flights were carried out to reconstruct the subsequent digital elevation models (DEMs) and quantify fluvial change caused by two floods in 2015 along a 6.5 km reach in an ephemeral gravel-bed river (Rambla de la Viuda; eastern Spain). The application of SfM-MVS resulted in a maximum point cloud density of 1295.4 pts/m2 and a DEM resolution of 3.11 cm/pix. Individual flights registered errors of 0.101, 0.171 and 0.083 m. Level of detection (LoD) of DEMs of difference (DoDs), i.e. topographic change detection, resulted in 0.198 and 0.190 between the first and second, and second and third flights respectively. Orthomosaics were also successfully created at a maximum resolution of 2.50 cm/pix. Analysis of the best configuration of SfM-MVS for ephemeral river monitoring indicated that a high overlap of photographs and, therefore, a large number of projections were critical for an efficient workflow and a high-quality model. To ensure model quality and survey efficiency, the following configuration is recommended: (1) overlap index of more than 20 projections, (2) flight distribution at two heights, and (3) the use of ground control points. SfM-MVS topographies and DoDs showed a discontinuous pattern represented by a succession of erosive-depositional sequences. Evolution of one of these sequences has been studied in detail and the legacy of a past mining pit was pointed out to be the principal driver for this morphosedimentary pattern. Change detection quantified a net erosion of (−)3118 m3 for Flood #1 and a net deposition of (+)787 m3 for Flood #2, at a 95% confidence. Knickpoint retreat, riverbed lowering and bank erosion were identified as the principal sources of sediment. Analyzing separately each flood highlighted that the mobilization of sediments was not proportional to peak discharges (98 and 80 m3/s, respectively). Interpretation of this behavior was hypothesized to be produced by the difference in total water volume (32.5 and 7.1 hm3), longer discharge period (24 and 11 days), different entrainment thresholds of each flood, or a clockwise hysteresis effect in the sediment transport, probably due to varying sediment availability. Thus, it was concluded that SfM-MVS worked especially well for change detection in ephemeral rivers and served as a basis for understanding morphological river patterns associated to floods and human impacts.

Hazard potential of widespread but hidden historic offshore heavy metal (Pb, Zn) contamination (Gulf of Cadiz, Spain)

Natural and human-induced seabed sediment disturbances affect wide areas of the global coastal ocean. These recurrent to chronic disturbances mobilize significant amounts of material, including substances that have the potential to significantly harm the environment once re-released. This very challenging issue is difficult to deal with if sub-surface contaminant concentrations are unknown. Based on the analysis of 11 new, up to 5-m long sediment cores taken offshore in the Gulf of Cadiz, the contamination history (using the trace elements lead and zinc) is well documented over major parts of the gulf. Ore mining and metal processing industries on the southwestern Iberian Peninsula started five thousand years ago and experienced a first peak during the Roman Period, which can be detected over the entire gulf. The Industrial Era added a massive, shelf-wide heavy metal excursion of unprecedented dimension. This metal contamination to the coastal ocean decreased in the 1990s and appears to be today limited to larger areas off the Tinto/Odiel and Guadiana River mouths. The unforeseen, significant finding of this study is that the gulf-wide, peak heavy metal concentration, stemming from the Industrial Era, is widely overlain by a modern sediment veneer just thick enough to cover the contaminant horizon, but thin enough to have this layer within the reach of natural or human-induced sediment mobilization events.

Disastrous sediment discharge due to typhoon-induced heavy rainfall over fossil periglacial catchments in western Tokachi, Hokkaido, northern Japan

In August 2016, Typhoon 1610 (Lionrock) caused heavy rainfall in Hokkaido, which led to the discharge of a large volume of sediment and water from catchments on the eastern slope of the northern Hidaka Range. The eight catchments examined in this study are characterized by granitic lithology and late Pleistocene periglacial landforms with weakly cohesive, low-resistance periglacial debris thickly covering the weathered bedrocks. This characteristic of the landscape presumably provided a transport-limited condition where some debris flows were initiated by shallow landslides. As they moved, the debris flows grew larger through mobilization and erosion of sediment in channel beds and sidewalls. This sediment mobilization and erosion continued for an extensive distance along the course of the river. Morphological changes induced by channel aggradation and bank erosion were considerable and distinctive from upstream to downstream. Granitic periglacial sediments are amply present on the mountain slopes, river channels, and river banks in the area, likely due to the rarity of intensive rainfall events. These distinctive features of fossil periglacial catchments are important for disaster prevention and catchment-scale sediment management in sub-boreal areas, particularly in the context of climate change, which may generate more frequent and intensive rainfall events.

Modeling the impacts of changing climatic extremes on streamflow and sediment yield in a northeastern US watershed

Study regionWe investigate the impacts of local temperature and precipitation trends on discharge and sediment loading by applying the model to a watershed in the northeastern US, where trends in increasing precipitation exceed those of other regions in North America. Study focusIn this study we simulate the response of watershed sediment loading to changing frequencies and magnitudes of extreme precipitation events using a coupled model that explicitly simulates streambank erosion and failure within a distributed watershed model. To drive the model, we use meteorological inputs from general circulation models (GCMs) as well as from a statistical weather generator (WG). New hydrological insights for the regionChanges in the timing and magnitude of snow melt and spring flows, as well as associated sediment mobilization, resulted from increases in temperature. Increases in discharge and sediment load resulted from increases in precipitation events exceeding the 95th percentile. In runs driven by WG weather data, positive trends were evident in peak (as well as annual) discharge and suspended sediment yields over the years modeled. No clear trends were seen in GCM-driven runs, which do not capture historically-observed trends in extreme precipitation. This work is consistent with other studies in that it shows important changes in discharge and sediment yields from a watershed resulting from ongoing changes in climate.

Reoxidation of estuarine sediments during simulated resuspension events: Effects on nutrient and trace metal mobilisation

Estuarine environments are considered to be nutrient buffer systems as they regulate the delivery of nutrients from rivers to the ocean. In the Humber Estuary (UK) seawater and freshwater mixing during tidal cycles leads to the mobilisation of oxic surface sediments (0–1 cm). However, less frequent seasonal events can also mobilise anoxic subsurface (5–10 cm) sediments, which may have further implications for the estuarine geochemistry. A series of batch experiments were carried out on surface and subsurface sediments taken from along the salinity gradient of the Humber Estuary. The aim was to investigate the geochemical processes driving major element (N, Fe, S, and Mn) redox cycling and trace metal behaviour during simulated resuspension events. The magnitude of major nutrient and metal release was significantly greater during the resuspension of outer estuarine sediments rather than from inner estuarine sediments. When comparing resuspension of surface versus subsurface sediment, only the outer estuary experiments showed significant differences in major nutrient behaviour with sediment depth. In general, any ammonium, manganese and trace metals (Cu and Zn) released during the resuspension experiments were rapidly removed from solution as new sorption sites (i.e. Fe/Mn oxyhydroxides) formed. Therefore Humber estuary sediments showed a scavenging capacity for these dissolved species and hence may act as an ultimate sink for these elements. Due to the larger aerial extent of the outer estuary intertidal mudflats in comparison with the inner estuary area, the mobilisation of the outer estuary sediments (more reducing and richer in sulphides and iron) may have a greater impact on the transport and cycling of nutrients and trace metals. Climate change-associated sea level rise combined with an increasing frequency of major storm events in temperate zones, which are more likely to mobilise deeper sediment regions, will impact the nutrient and metal inputs to the coastal waters, and therefore enhance the likelihood of eutrophication in this environment.

Peat bog and alluvial deposits reveal land degradation during 16th‐ and 17th‐century colonisation of the Western Carpathians (Czech Republic)

AbstractWallachian (shepherd) colonisation of the upper parts of the Carpathians, the second largest mountain range in Europe, provides a unique opportunity to study human‐induced ecological changes and subsequent sediment mobilisation within slope and fluvial systems. The Wallachians came to the nearly pristine landscape in the Czech part of the Western Carpathians during the 16th to 17th century bringing large‐scale deforestation and grazing to the upper parts of its ridges. Despite the importance of this event, there is a lack of high‐resolution multiproxy reconstructions to help decipher the relative influence of anthropogenic and climate factors on this landscape. Here, we provide an approximately 2.1‐kyr record obtained from a peat bog where, using chronological, sedimentological, and pollen analyses, we were able to differentiate between environmental conditions before, during, and after colonisation. Prior to colonisation, climate deterioration following the onset of Little Ice Age caused changes in forest composition and erosion events (causing a ~ad 0–1500 gap in the record). Abrupt human‐induced deforestation detected in the pollen record, together with the abundant fine‐grained minerogenic content of peat deposits between ad ~1640 and ad 1870, corresponds to increased run‐off and sheet erosion on slopes, enhanced by Little Ice Age climate deterioration. The sedimentary record in alluvial deposits downstream indicates that the colonisation of the mountain slopes in this region not only had a local effect on soil degradation, but it also increased the net aggradation of overbank deposits within valley floors. After reforestation, net aggradation was replaced by river incision into alluvia.