Sediment Storage Research Articles

Quantifying connectivity and its effects on sediment budgeting for an agricultural basin, Chesapeake Bay Watershed, United States

AbstractExcessive sediment runoff as a result of anthropogenic activities is a major concern for watershed ecologic health. This study sought to determine the sources, storage, and delivery of sediment using a sediment budget approach for the predominantly pasture and forested Smith Creek watershed, Virginia United States, a tributary to the Chesapeake Bay. Utilizing a novel combination of the Universal Soil Loss Equation (USLE) model and an index of connectivity along with field surveys of channels, this study indicated that streambanks and pastures were major sources of sediment. Overestimation of fine‐grained sediment flux exported from the watershed according to this study's models (3811 Mg/year) compared to export measured at the outlet (2918 Mg/year) most likely indicates underestimation of storage in the watershed from unaccounted for geomorphic features (ponds, toe slopes, and colluvial slopes). Sediment budget results indicating that streambanks are a major source of sediment in the watershed support previous sediment fingerprinting results and provide a framework for managers to address the sediment problem in Smith Creek and similar tributaries to the Chesapeake Bay.

First Evidence of Microplastic Presence in Bed Load Sediments of a Small Urban Stream in Warsaw

Microplastics (MPs) are considered as a contaminant of widespread global concern. Due to their small size, MPs become bioavailable to many types of organisms and affect them. However, there is still little known about MP release on land, storage in soils and sediments, or their transport by runoff in rivers. Thus, the aim of this work was to present the results of the first, initial investigation on microplastic presence in an urban stream located in Warsaw, Poland. A simple and relatively inexpensive procedure that leads step-by-step to the detection of microplastics in bed load sediment is presented. It consists of sampling, sieving, density separation, organic matter reduction, and Nile Red staining. The presence of MP in the channel of Służew Creek was confirmed. The estimated amount of particles ranged from 191 to 279 pieces per 30 g of bed load sediment for the selected sampling sites. The number of particles seemed to increase with the catchment area. There is a need for further broad research focusing, among others, on the standardization of methods and laboratory procedures leading to microplastic detection.

Testing a novel sonar‐based approach for measuring water depth and monitoring sediment storage in beaver ponds

AbstractWidely available ‘fish‐finder’ echo‐sounding devices are beginning to be used in bathymetric studies to estimate geomorphic change. To date, however, there have been no applications in shallow and complex wetlands, where changes in sediment storage are notoriously dynamic in time and difficult to describe accurately in space. Therefore, in this study, we tested the performance of an ‘off‐the‐shelf’ fish‐finder for mapping bathymetry in a shallow beaver pond. We tested fish‐finder sonar depth readings against a traditional‐sounding lead‐line method across 21 paired Sampling Points with a minimum depth of 0.31 m and a mean of 0.65 m. Spatial accuracy of the unit was also tested against a differentially corrected Global Navigation Satellite System (GNSS) receiver. Measured depths to pond bottom from the fish‐finder were on average within 5%, although significantly 0.015 m (SD = 0.034) less than those obtained by the lead‐line method. Spatial accuracy, however, varied greatly compared to the corrected GNSS receiver readings, with a mean discrepancy of 2.7 m (SD = 1.5) but up to 6.2 m. Given the close match of depth readings between the two methods, we conclude that sonar is a suitable, cost‐effective, and less‐intrusive method than existing techniques, even in moderately vegetated shallow waterbodies. Methods do need to be adopted to account for poor spatial precision with ‘off‐the‐shelf’ fish‐finder models, but this can be rectified with survey design or using a secondary GNSS. Application of this technology will allow rapid one‐off surveys or repeated monitoring of depth, bedform and sediment accumulation in otherwise hard‐to‐access or disturbance‐sensitive wetlands, such as beaver ponds and water treatment or flow attenuation wetlands.

Late-Holocene Sediment Storage in Upland Valley Systems in the Gamo Highlands of Southern Ethiopia

Part of the eroded soil material from the hillslopes is temporarily stored on hillslopes and in river valleys as colluvial and alluvial storage, respectively. This storage component of a catchment’s sediment budget is an important archive reflecting past erosion and sediment delivery processes in relation to both natural and anthropogenic environmental changes. Information on long-term sediment dynamics (i.e., centennial to millennial timescales) is generally lacking for tropical mountain environments. Here, we quantify long-term floodplain sediment storage and sedimentation dynamics in the Gamo highlands of the southern Ethiopia Rift Valley. In two upstream catchments (Chencha and Dembelle), a detailed survey of the floodplain sediment archive was conducted through hand augering of 37 cross-valley transects. Sediment thicknesses vary between 4 and 8 m and total storage equals 0.03 Mt ha−1 floodplain area for the Chencha area and 0.05 Mt ha−1 floodplain area for the Dembelle area. Radiocarbon dating of organic material retrieved from the sediment archives provided a temporal framework for interpretation of sedimentation processes dynamic. The mean sedimentation rate in the Chencha floodplain is ~3.22 ± 0.33 kt ha−1 catchment area, whereas it is ~3.76 ± 0.22 kt ha−1 catchment area for the Dembelle floodplain. Up to 70% of the total sediment mass is stored in the floodplains within the most recent 2000 years. Cumulative probability function plots of radiocarbon dates show that sedimentation started to increase from ca 2000 to ca 1600 cal BP, roughly coincident with an increase in human presence, as is indicated through archaeological data.

Impact of stream power gradients on storage of sediment and carbon on channel margins and floodplains

Abstract Spatial complexity impacts the resilience of river ecosystems by mediating processes that control the sources and sinks of sediment and organic material. Using four independent geochemical tracers and three morphometric indices, we show that downstream spatial gradients in stream power (Ω) predict storage of material in the channels and margins and/or floodplains. A field test in a 48 km2 watershed demonstrates that reaches with downstream decreases in Ω coincide with wider floodplains and elevated inventories of 137Cs, 210Pbex (ex—excess), and organic matter in locations of the ~3 to 20 yr floodplain. In contrast, reaches with downstream increases in Ω coincide with narrower floodplains and decreased inventories of 137Cs, 210Pbex, and organic matter. The occurrence of in-channel bedrock exposures and the activity of short-lived 7Be in within-channel sediments also correlate with downstream Ω gradients, demonstrating a link, over both short and long time scales, between withinchannel processes and floodplain-forming processes. The combined geochemical and physical characteristics demonstrate the importance of downstream gradients in sediment transport, characterized by downstream changes in stream power rather than at-a-point stream power, in determining spatial complexity in carbon and sediment storage at intermediate scales (102 to 103 m) in river systems.

Controls on sediment storage in wide mountain valleys — a case study from Srinagar (Garhwal) valley, NW Himalaya

Controls on sediment storage in wide mountain valleys — a case study from Srinagar (Garhwal) valley, NW Himalaya

Development of A Two-storage Reservoir Operation Rules for Flood Control Case Study of Wonogiri Multipurpose Reservoir, Central Java, Indonesia

Wonogiri Reservoir, one of the large reservoirs in Indonesia, regulates water for flood control, irrigation, domestic use, and hydropower. It is located at the upper part of Bengawan Solo River Basin, Central Java. A closure dike has recently been constructed as a countermeasure against severe sedimentation that separates the reservoir into two parts, i.e., Sediment Storage Reservoir (SSR) and Main Reservoir (MR). A two-gate spillway was constructed in the SSR to facilitate sediment flushing and sluicing. During flood events, it controls reservoir water level jointly with the MR spillway. This brings the need for an update on the reservoir operation rules, especially on the gates opening of the new spillways to maintain the flood control performance. An assessment of reservoir routing simulation results was carried out by considering the overflow from SSR to MR, or vice versa with two options of Control Water Level (CWL), i.e. +135.8 m and +136.3 m. The flood control performance of the new reservoir operation rules was evaluated by considering the reservoir water level recovery, the maximum total outflow discharge, and the dam safety due to the risk of overtopping. These assessments give a foundation for the selection of CWL and the spillway operation to ascertain an optimal flood control of the reservoir.

Geomorphic control on stage-area hysteresis in three of the largest floodplain lakes

Hysteresis in floodplain lakes occurs between stage and lake area. Stage-area hysteresis controls the storage and exchange of water and sediments, and is a critical hydrological behavior for lake management. While hysteresis has been repeatedly observed in the floodplain lakes of large rivers, the hydrological mechanism and factors in control have been poorly understood thus far. In this paper, we investigate the role of geomorphology in controlling lake hysteresis, specifically the geologic setting and the lake basin, the lake position relative to the main stem of the river, as well as the influence of lake shape and its internal depositional landforms on inundation dynamics. We study the floodplain lakes along three of the largest rivers around the world: the Curuai Lake of the Amazon River, the Tonle Sap Lake of the Mekong River, and the Poyang Lake of the Yangtze River. The three lakes exhibit a similar counter-clockwise stage-area hysteresis: for a given stage, the lake area is larger in the falling season than in the rising season. Our results indicate that hysteresis is mainly controlled by geomorphology, where the lake shape and basin size lead to delays in the drainage and drop in lake area during the falling season, resulting in counter-clockwise hysteresis. Nevertheless, the lakes are of distinct climatic and geologic-geomorphic settings, representing the variety in the lake types of large rivers. Hence, while geomorphology is the overall driver, unique lake characteristics delay the fall in water extent and shape hysteresis on a case-by-case nature. At Curuai, the complex floodplain morphology (impeded floodplain) complicates and slows the routing of outflow. At Tonle Sap, the lake flows into the river solely through a narrow channel, where a backwater effect restricts drainage. At Poyang, the wide lake shape upstream leads to counter-clockwise hysteresis, while the narrow channel downstream exhibits clockwise hysteresis. Out of the three investigated floodplains, Tonle Sap has the largest degree of hysteresis (0.41), followed by Poyang (0.17) and Curuai (0.13). This trend in hysteresis extent is a result of the different composition of inflow and the lake–river hydrological connectivity, attributed to lake geomorphology. This study is the first to address geomorphology as the primary control over lake hysteresis, which improves understanding of the stage-area curve in empirical and numerical hydrological models, and potentially floodplain management.

Nature‐based solutions enhance sediment and nutrient storage in an agricultural lowland catchment

AbstractIn this paper, nature‐based solutions (NBS) include: (1) natural flood management (NFM) interventions with a primary function of flood risk reduction but with additional multiple benefits for water quality improvements through the mitigation of diffuse pollution; and (2) ponds with a primary function of water quality improvement. This study assesses the ability of these NBS to trap pollutants in run‐off within two small (3.4 km2) agricultural catchments (Upper Thames, UK). The masses of sediment, phosphorus, and organic carbon trapped by 14 features (since construction 2–3 years previously) were quantified through sediment surveying and sampling. Streamflow and suspended sediment monitoring downstream of features enabled catchment yields to be calculated. The features trapped a total of 83 t sediment, 122 kg phosphorus, and 4.3 t organic carbon. Although the footprint of the features was <1% of the catchment area, they drained 44% of the total land area and captured the equivalent of 15% of the total suspended sediment yield, 10% of the total phosphorus yield, and 8% of the particulate organic carbon yield as monitored at the catchment outlet over the monitoring period. Results reveal that accumulation rates were influenced by hydrological connectivity, with greater accumulation in features constructed directly on streams (online ponds), and those offline features that filled from overbank flows. The low to moderate accumulation rates observed in offline features suggests that their floodwater storage potential is only likely to significantly reduce in the medium term, necessitating maintenance after ~10 years. Compared with topsoil in each contributing area, trapped sediment was enriched in phosphorus and carbon in the majority of features, having on average 50% higher phosphorus and 17% higher organic carbon concentrations than surrounding arable soils, highlighting its potential value for redistribution on farmland. Monitoring results demonstrate the potential of NBS, including NFM, to mitigate diffuse pollution in lowland catchments.

Secondary Mineral Formation and Carbon Dynamics during FeS Oxidation in the Presence of Dissolved Organic Matter.

Iron (Fe) minerals constitute a major control on organic carbon (OC) storage in soils and sediments. While previous research has mainly targeted Fe (oxyhydr)oxides, the impact of Fe sulfides and their subsequent oxidation on OC dynamics remains unresolved in redox-fluctuating environments. Here, we investigated the impact of dissolved organic matter (DOM) on FeS oxidation and how FeS and its oxidation may alter the retention and nature of DOM. After the anoxic reaction of DOM with FeS, FeS preferentially removed high-molecular-weight and nitrogen-rich compounds and promoted the formation of aqueous sulfurized organic molecules, according to Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) analysis. When exposed to O2, FeS oxidized to nanocrystalline lepidocrocite and additional aqueous sulfurized organic compounds were generated. The presence of DOM decreased the particle size of the resulting nano-lepidocrocite based on Mössbauer spectroscopy. Following FeS oxidation, most solid-phase OC remained associated with the newly formed lepidocrocite via a monodentate chelating mechanism (based on FTIR analysis), and FeS oxidation caused only a slight increase in the solubilization of solid-phase OC. Collectively, this work highlights the under-appreciated role of Fe sulfides and their oxidation in driving OC transformation and preservation.

Distribution and Genesis of Organic Carbon Storage on the Northern Shelf of the South China Sea.

The sediments distributed in the marginal seas of the continental shelf are important burial materials for global organic carbon (OC). There have been many estimates of the global continental shelf OC reserves, but due to the limited acquisition of measured data, the estimated results have great uncertainty. The vast continental shelf in the northern part of the South China Sea (SCS) provides a good place for the storage of OC. Based on a large amount of sediment OC data obtained from the northern coast of the SCS, the OC storage in the surface sediment (0~10 cm) in the study area (approximately 8.63 × 104 km2) was accurately calculated as 51 Tg. The study area covers different regions, such as estuaries, open seas, strait areas and upwelling development areas, and the OC content of each area is quite different. According to provenance analysis, the source of OC in sediments is mainly from the input of Pearl River runoff. The OC content is significantly higher and less affected by sediment particle size in the Pearl River Estuary and the surrounding areas; meanwhile, the OC content gradually decreases with the distance from the Pearl River Estuary. Far from the western Pearl River Estuary, the sediment OC content is mainly controlled by the particle size of the sediments and is significantly correlated with silt and clay content. The deposition rate is also an important factor affecting the burial of OC, for the high deposition rates correspond to the high levels of OC in the nearshore estuarine areas, as well as the low deposition rate region having low OC content in the sediments even though it has a high productivity of OC, such in as the upwelling sea area on the eastern side of Hainan.

Remote sensing of broad-scale controls on large river anabranching

River patterns reflect complex geomorphological processes and affect ecosystems and human development along floodplains. Physical controls on anabranching development have been studied primarily at local scales on relatively small rivers (i.e., discharge <1000 m3 s−1). However, there has been little systematic quantification of river anabranching (development of stable multiple channels) for large rivers globally. Here, we use remote sensing, cloud computing, and geospatial analysis to explore the importance of water surface slope, floodplain topography, sediment supply, substrate lithology, and permafrost to anabranching throughout 20 of the world's largest river basins. We use Landsat-derived surface water extent to compute an anabranching index (Ai) for ∼1 M km of river reaches, and compare it with global datasets of water surface slope, available floodplain extent, substrate lithology, and permafrost. We find that ∼49 ± 19% of large-river mainstems are anabranching, ranging from at least ∼17% (Mekong) to as much as ∼84% (Ob’). At the basin scale, anabranching channels comprise at least ∼17% (Yenisey) to as much as ∼55% (Kolyma) of all Landsat-observable river reaches, with a mean global value of 35 ± 11%. Anabranching channel patterns are most commonly associated with low water surface slope (normalized slope < 0.2; absolute slope < 0.2 m km−1) accompanying wide floodplains (normalized floodplain width > 0.6; absolute width > 42 km). Cross-sectionally averaged channel width increases in the most intensely anabranching reaches, suggesting net sediment storage due to reduced stream power. Unconsolidated sedimentary substrates promote both the prevalence and intensity of anabranching, whereas the presence of permafrost enhances intensity only. Overall, our results identify pervasive channel anabranching and some important control mechanisms throughout the world's large river basins, with strong implications for satellite-based discharge retrievals. We conclude that global analyses afforded by remote sensing and cloud computing offer additional, complementary approaches to traditional field-based approaches for understanding large river geomorphic processes and channel forms.

Sedimentary carbon on the continental shelf: Emerging capabilities and research priorities for Blue Carbon

Continental shelf sediments store large amounts of organic carbon. Protecting this carbon from release back into the marine system and managing the marine environment to maximize its rate of accumulation could both play a role in mitigating climate change. For these reasons, in the context of an expanding “Blue Carbon” concept, research interest in the quantity and vulnerability of carbon stored in continental shelf, slope, and deep ocean sediments is increasing. In these systems, carbon storage is physically distant from carbon sources, altered between source and sink, and disturbed by anthropogenic activities. The methodological approaches needed to obtain the evidence to assess shelf sea sediment carbon manageability and vulnerability within an evolving blue carbon framework cannot be transferred directly from those applied in coastal vegetated “traditional” blue carbon habitats. We present a toolbox of methods which can be applied in marine sediments to provide the evidence needed to establish where and when marine carbon in offshore sediments can contribute to climate mitigation, focusing on continental shelf sediments. These methods are discussed in the context of the marine carbon cycle and how they provide evidence on: (i) stock: how much carbon is there and how is it distributed? (ii) accumulation: how rapidly is carbon being added or removed? and (iii) anthropogenic pressures: is carbon stock and/or accumulation vulnerable to manageable human activities? Our toolbox provides a starting point to inform choice of techniques for future studies alongside consideration of their specific research questions and available resources. Where possible, a stepwise approach to analyses should be applied in which initial parameters are analysed to inform which samples, if any, will provide information of interest from more resource-intensive analyses. As studies increasingly address the knowledge gaps around continental shelf carbon stocks and accumulation – through both sampling and modelling – the management of this carbon with respect to human pressures will become the key question for understanding where it fits within the blue carbon framework and within the climate mitigation discourse.

Late Pleistocene to Holocene glacio-eustatic history as recorded in the Pescara paleovalley system (Central Italy, Adriatic basin)

A buried paleovalley system, about 50 m deep and 2 km wide, is documented from the Pescara coastal plain. Based on stratigraphic, sedimentological, paleontological, chronological and geotechnical data, the paleovalley profile and 3D facies architecture of the paleovalley fill (PVF) were reconstructed.The lowermost PVF is a laterally extensive fluvial gravel body, up to 13 m-thick, that represents the lowstand systems tract (LST; pre-11.3 ka cal BP). Above lowstand deposits, the transgressive systems tract (TST), 21 m-thick, shows a deepening-upward trend, from freshwater/inner-estuarine to brackish/outer-estuarine facies associations (11.3–8.0 ka cal BP). The upper part of the succession (highstand systems tract – HST) shows a shallowing-upward tendency from paludal to fluvio-deltaic deposits.Seven millennial-scale parasequences (Ps) were identified within the Pescara Holocene (TST + HST) succession. Transgressive Ps1-3 exhibit a distinctive retrogradational stacking pattern. Highstand Ps4-7 are aggradationally-to-progradationally stacked. During the aggradational phase (P4), the estuary was gradually filled and swamp environments spread onto the valley interfluve. Because of subsequent progradation (Ps5-7), delta plain conditions established. In the research core, TST parasequences show higher accumulation rates (up to 9.4 mm/y) than HST ones (1.3–1.8 mm/y). Thus, the study area evolved from a region of sediment storage (11.3–8.0 ka cal BP) into a sector of prevalent sediment bypass (last 8.0 ky).Major early Holocene flooding events were possibly triggered by Melt-Water Pulses (MWPs) 1B, 1C and 1D. The eustatic rise linked to MWP-1B reasonably caused the P1 flooding event (11.3 ka cal BP). Post-MWPs 1C and 1D sea-level rises likely provoked the complete drowning of the paleovalley system and the subsequent maximum landward migration of the shoreline (about 8.0 ka cal BP).This study provides new evidences, in terms of sedimentary response, of the poorly-documented MWPs 1C and 1D, and the first documentation of MWP-1B eustatic effects in an onshore sector of the Central Adriatic.

Analisis Karbon dalam Sedimen pada Ekosistem Lamun di Teluk Gilimanuk, Bali

&#x0D; &#x0D; &#x0D; &#x0D; The increased CO2 emissions in the atmosphere have caused several environmental changes. Therefore, a larger CO2 absorption capacity is required. Seagrass can absorb carbon relatively fast and able to store it. Seagrass beds can trap sediment, so the absorption of organic carbon from the sediment by seagrass can be influenced by the size of the substrate. It has been found that the larger the substrate grain, the lower the ability of plants to absorb the organic matter. This study aimed to determine the seagrass density and cover as well as carbon storage in seagrass bed sediments. The variable measured was Sediment-Organic Carbon Content (S-OCC) and Sediment-Organic Carbon Stock (S-OCS) of seagrass ecosystems in Gilimanuk Bay. Employing purposive sampling, 20 sampling points were selected randomly where seagrass beds were present. The data were obtained using a 1×1m quadrant transect. The carbon content of the sediment was calculated by the Loss of Ignition (LOI) method by removing organic matter at a temperature of 600? in a furnace. This study found four seagrass species: Enhalus acoroides, Cymodocea rotundata, Cymodocea serrulate, and Halophila ovalis. The average density of seagrasses in this habitat ranged from 17 – 179 #/m2, with seagrass cover values ranging from 25 to 100%. The sediment carbon content, measured as carbon stock, on seagrass ecosystems at Gilimanuk Bay was 0.21% - 5.51% for S-OCC and 0.002 – 0.059 g Corg cm-3 for S-OCS. Total S-OCS in Gilimanuk Bay was 11165.84 Mg g Corg.&#x0D; &#x0D; &#x0D; &#x0D;

Geomorphic effects of natural flood management woody dams in upland streams

AbstractOne popular Natural Flood Management (NFM) technique involves the construction of channel‐spanning woody dams in low‐order streams that maintain a clearance height above base flows. While extensive research has examined the geomorphic effects of natural wood accumulations, little has been documented of NFM woody dams, which are structurally distinct from natural accumulations and may produce different patterns of erosion and deposition. This consideration is crucial because changes in physical habitat characteristics have implications for flood management objectives as well as ecosystem structure and functioning. This study adopted a Before‐After Control‐Impact (BACI) design to assess the geomorphic effects of NFM woody dams in the upper River Cover catchment, United Kingdom. One baseline survey prior to and three monitoring surveys up to 2 years following dam construction were conducted. Structure‐from‐Motion (SfM) photogrammetry was employed to capture topographic change, supplemented by bathymetric surveys. Results highlight that where the dams remained secure in place, they promoted in‐stream habitat diversity by creating underflow pools. Sediment storage was observed only where the dams had clearance heights &lt;0.3 m from the stream bed. Additionally, the dams commonly led to bank erosion, likely enhanced by inherent bank instability in the study catchment as observed along the control reaches. However, volumes of sediments eroded and deposited were not statistically different between the control and woody dam reaches. Longer monitoring is required to determine whether these effects on channel morphology and habitat diversity will persist, amplify, or diminish over time, and to better understand the longevity of NFM woody dams.

Hydraulics, sedimentation, and nutrient deposition within the middle Mississippi River Batture, USA

The Mississippi River batture (the land between the channel's low-water elevation and the flood mitigation levee) is an important sink for fluvial sediments and nutrients. However, the batture's role in sediment and nutrient storage remains poorly understood. To gain a better understanding of sediment and nutrient deposition within the batture, a 2D hydrodynamic model for a 30 km segment of the middle Mississippi River (MMR; between the confluences of the Missouri and Ohio Rivers) was developed to quantify flood extent and shear stresses for batture inundating discharges. Comparing modeled water surface elevations and shear stress to sedimentation estimates revealed which discharge conditions were conducive for deposition. Batture areas that experienced sedimentation during the 2019 flood were sampled for analysis of total organic carbon (TOC) and total nitrogen (TN) to gain insights into where these nutrients were deposited.Hydraulic modeling revealed MMR batture inundation includes six phases across both the rising and falling limb of the flood hydrograph: (1) back flooding (i.e., downstream to upstream inundation), (2) mixed back and overbank flooding, (3) overbank flooding conducive for deposition on the rising limb, (4) overbank flooding conducive for erosion on both the rising and falling limbs, (5) overbank flooding conducive for deposition on the falling limb, and (6) a floodplain draining phase. Phases two, three, and five (discharge conditions >20 to 86 % annual exceedance probability [AEP]), are most suitable for fine-grained deposition because of direct transfer of sediment-laden water from the river to batture, and large areas of the batture experiencing shear stress below the critical threshold for deposition. During the transition from phases three to four, flood depths increase, and hydraulic conditions rapidly favor erosion over the entire batture suggesting the batture may transition from sink to source of sediment for discharges that are ≤20 % AEP.Flood sediment TOC and TN concentrations were more than three times greater in backswamp clay and silt deposits, than in levee and splay sand. Within the backswamp, the most favorable discharge conditions for fine-sediment deposition are >20 to ≤86 % AEP on the rising limb, and >20 to ≤90 % AEP on the falling limb of the hydrograph. However, up to 40 % of the backswamp does not experience shear stress conditions below the threshold for fine-grained deposition. These findings highlight the impact hydraulic conditions have on sediment and nutrient retention in this important river connected landscape.

Factors controlling floodplain sediment storage in two tropical upland river catchments in the Lake Tana basin, Ethiopia

Floodplain sediment storage is an important component of a catchment’s sediment budget. Here, we present the first quantification of floodplain sediment storage for tropical river catchments draining to Lake Tana, NW Ethiopia. The catchments are characterized by relatively gently sloping to flat lowlands towards the lake (1788 m a.s.l.) and steeper river reaches in the uplands towards Mount Guna (4120 m a.s.l.). Sediment storage for 65 homogeneous floodplain segments was estimated by combining information on floodplain spatial extent obtained through the field- and remote sensing-based approaches, with information on sediment thickness obtained through sediment coring and the analysis of river cut-banks. Extrapolation to the entire catchment was done making use of floodplain typology and following the classification scheme of Nanson and Croke (1992) and other floodplain properties and catchment-wide variables. The results showed average sediment storage of 284 Mg m−1 river length or 21,760 Mg ha−1 floodplain area for the Gumara and approx. 227 Mg m−1 river length or 16,909 Mg ha−1 floodplain area for the Rib River catchments. Total floodplain sediment storage at the catchments scale, above the upper gauging stations, amounts to 92.9 Mt and 35.5 Mt for the Gumara and Rib River catchments, respectively. At the scale of homogeneous floodplain segments, sediment storage is related to floodplain width, floodplain slope, upstream catchment area, the average slope of the upstream catchment area, sinuosity, and floodplain type. Overall, floodplain geomorphology largely controls sediment storage. Approx. 70 % of the sediment is stored in low-energy cohesive floodplains which only take up 11 % of the total river length in both catchments. For most of the total river and valley extent, where rivers are incised and have more energy, storage is limited. Comparing our findings with similar studies in temperate catchments of Europe and N-America showed that sediment storage in the Gumara and Rib Rivers is lower by approx. 1 order of magnitude, for similar-sized catchments.

Debris Cover Limits Subglacial Erosion and Promotes Till Accumulation.

Glaciers are commonly conceptualized as bodies composed of snow and ice. Yet, many glaciers contain a substantial amount of rock, especially those abutting steep mountains. Mountain slopes erode, depositing rocks on glaciers below. This loose rock (or debris) is buried in glaciers and melts out lower down creating a debris cover. Debris cover reduces ice melt, which changes the shape and movement of glaciers. Glacier movement, specifically basal sliding, efficiently sculpts landscapes. To date, we know little about the impacts of surface debris on conditions below glaciers. To help remedy this, we run numerical model simulations which show that debris‐covered glaciers erode slower than glaciers unaffected by debris. Reduced melt under surface debris lowers sliding speeds and causes sediment to accumulate at the bed, potentially establishing conditions for surging. The influence of surface debris cover on the subglacial environment may hold substantial implications for alpine sediment storage and landscape evolution.

Linking sediment flux to river migration in arid landscapes through mass balance

ABSTRACT Lateral migration is a key process shaping sinuous rivers and controlling sediment exchange with floodplains. The rate at which channels migrate is affected by bend curvature, bank erodibility, and sediment supply. The relationship between migration rate and sediment supply is poorly understood in dryland regions, where direct measurements are scarce. Here we propose a simple mass-balance model to estimate the sediment flux of ephemeral streams in North America's Great Basin and establish a comparison with timelapse photogrammetric data of lateral migration. The model takes into consideration variables such as long-term hillslope erosion, transient sediment storage in intra-catchment lowlands, and sediment bypass to depocenters. Our results point to first-order similarities in how sediment supply drives channel migration across diverse hydro-climatic regimes. However, we find that, for a given sediment supply and channel width, and despite their ephemeral discharge, dryland streams with minimal bank vegetation migrate about three times faster than humid-climate, vegetated ones. This difference in migration pace likely results from the compound effect of bank erodibility and bend geometry. Our model sheds new light on the driving mechanisms of channel mobility in dryland streams and may find application in assessing the sediment budgets of ungauged streams, reservoir trapping, and morphodynamic adjustments in stressed watersheds.