Particulate matter fluxes in the southern and central Kara Sea compared to sediments: Bulk fluxes, amino acids, stable carbon and nitrogen isotopes, sterols and fatty acids

The Kara Sea is one of the arctic marginal seas strongly influenced by fresh water and river suspension. The highly seasonal discharge by the two major rivers Yenisei and Ob induces seasonal changes in hydrography, sea surface temperature, ice cover, primary production and sedimentation. In order to obtain a seasonal pattern of sedimentation in the Kara Sea, sediment traps were deployed near the river mouth of the Yenisei (Yen) as well as in the central Kara Sea (Kara) within the framework of the German–Russian project “Siberian River run-off; SIRRO”. Two and a half years of time-series flux data were obtained between September 2000 and April 2003 and were analyzed for bulk components, amino acids, stable carbon and nitrogen isotopes as well as sterols and fatty acids.Sediment trap data show that much of the annual deposition occurred under ice cover, possibly enhanced by zooplanktonic activity and sediment resuspension. An early bloom of ice-associated algae in April/May occurred in the polynya area and may have been very important to sustain the life cycles of higher organisms after the light limitation of the winter months due to no/low insolation and ice cover. The strong river input dominated the months June–August in the southern part of the Kara Sea. The central Kara Sea had a much shorter productive period starting in August and was less affected by the river plumes. Despite different time-scales of sampling and trapping biases, total annual fluxes from traps were in the same order of magnitude as accumulation rates in surface sediments. Terrestrial organic carbon accumulation decreased from 10.7 to 0.3 g C m⁻² a⁻¹ from the riverine source to the central Kara Sea. Parallel to this, preservation of marine organic matter decreased from 10% to 2% of primary productivity which was probably related to decreasing rates of sedimentation.     

Fluvial suspended sediment yields over hours to millennia in the High Arctic at proglacial Lake Linnévatnet,Svalbard

Sediment yield can be a sensitive indicator of catchment dynamics and environmental change. For a glacierized catchment in the High Arctic, we compiled and analyzed diverse sediment transfer data, spanning a wide range of temporal scales, to quantify catchment yields and explore landscape response to past and ongoing hydroclimatic variability. The dataset integrates rates of lake sedimentation from correlated varve records and repeated annual and seasonal sediment traps, augmented by multi‐year lake and fluvial monitoring. Consistent spatial patterns of deposition enabled reconstruction of catchment yields from varve‐ and trap‐based fluxes. We used hydroclimatic data and multivariate modeling to examine annual controls of sediment delivery over almost a century, and to examine shorter‐term controls of sediment transfer during peak glacier melt. Particle‐size analyses, especially for annual sediment traps, were used to further infer sediment transfer mechanisms and timing. Through the Medieval Warm Period and Little Ice Age, there were no apparent multi‐century trends in lake sedimentation rates, which were over three times greater than those during the mid‐Holocene when glaciers were diminished. Twentieth‐century sedimentation rates were greater than those of previous millennia, with a mid‐century step increase in mean yield from 240 to 425 Mg km^?2 yr^?1. Annual yields through the twentieth century showed significant positive relations with spring/summer temperature, rainfall, and peak discharge conditions. This finding is significant for the future of sediment transfer at Linnévatnet, and perhaps more broadly in the Arctic, where continued increases in temperature and rainfall are projected. For 2004–2010, annual yields ranged from 294 to 1330 Mg km^?2 yr^?1. Sediment trap volumes and particle‐size variations indicate that recent annual yields were largely dominated by spring to early summer transfer of relatively coarse‐grained sediment. Fluvial monitoring showed daily to hourly sediment transfer to be related to current and prior discharge, diurnal hysteresis, air temperature, and precipitation. Copyright © 2017 John Wiley & Sons, Ltd.     

On the sensitivity of the sedimentary system in the East Frisian Wadden Sea to sea-level rise and wave-induced bed shear stress

The paper addresses the individual and collective contribution of different forcing factors (tides, wind waves, and sea-level rise) to the dynamics of sediment in coastal areas. The results are obtained from simulations with the General Estuarine Transport Model coupled with a sediment transport model. The wave-induced bed shear stress is formulated using a simple model based on the concept that the turbulent kinetic energy (TKE) associated with wind waves is a function of orbital velocity, the latter depending on the wave height and water depth. A theory is presented explaining the controls of sediment dynamics by the TKE produced by tides and wind waves. Several scenarios were developed aiming at revealing possible trends resulting from realistic (observed or expected) changes in sea level and wave magnitude. The simulations demonstrate that these changes not only influence the concentration of sediment, which is very sensitive to the magnitude of the external forcing, but also the temporal variability patterns. The joint effect of tides and wave-induced bed shear stress revealed by the comparison between theoretical results and simulations is well pronounced. The intercomparison between different scenarios demonstrates that the spatial patterns of erosion and deposition are very sensitive to the magnitude of wind waves and sea-level rise. Under a changing climate, forcing the horizontal distribution of sediments adjusts mainly through a change in the balance of export and import of sediment from the intertidal basins. The strongest signal associated with this adjustment is simulated North of the barrier islands where the evolution of sedimentation gives an integrated picture of the processes in tidal basins.     

Fluvial suspended sediment transfer and lacustrine sedimentation of recent flood turbidites in proglacial Eklutna Lake,western Chugach Mountains,Alaska

Suspended sediment delivery and deposition in proglacial lakes is generally sensitive to a wide range of hydrometeorologic and geomorphic controls. High discharge conditions are of particular importance in many glaciolacustrine records, with individual floods potentially recorded as distinctive turbidites. We used an extensive network of surface sediment cores and hydroclimatic monitoring data to analyse recent flood turbidites and associated sediment transfer controls over instrumental periods at Eklutna Lake, western Chugach Mountains, Alaska. Close to a decade of fluvial data from primary catchment tributaries show a dominating influence of discharge on sediment delivery, with various interconnections with other related hydroclimatic controls. Multivariate fluvial models highlight and help quantify some complexities in sediment transfer, including intra-annual variations, meteorological controls, and the influence of subcatchment glacierization. Sediments deposited in Eklutna Lake during the last half century are discontinuously varved and contain multiple distinctive turbidites. Over a 30-year period of stratigraphic calibration, we correlate the four thickest flood turbidites (1989, 1995, 2006, and 2012) to specific regional storms. The studied turbidites correlate with late-summer and early-autumn rainstorms with a magnitude of relatively instantaneous sedimentation 3–15 times greater than annual background accumulation. Our network of sediment core data captured the broad extent and sediment variability among the study turbidites and background sediment yield. Within-lake spatial modelling of deposition quantifies variable rates of downlake thinning and sediment focusing effects, and highlights especially large differences between the thickest flood turbidites and background sedimentation. This we primarily relate to strongly contrasting dispersion processes controlled by inflow current strength and turbidity. Sediment delivery is of interest for this catchment because of reservoir and water supply operations. Furthermore, although smaller floods may not be consistently represented, the lake likely contains a valuable proxy record of regional flooding proximal to major population centers of south-central Alaska including Anchorage.     

Influence of varying tidal prism on hydrodynamics and sedimentary processes in a hypertidal salt marsh creek

Recent initiatives directing tidal power development in the Bay of Fundy have raised questions about far‐field environmental impacts related to energy extraction. It is understood that commercial scale tidal power installations in the Minas Passage will result in an overall decrease in tidal amplitude in the Minas Basin. Corresponding changes in sedimentation patterns may or may not be within the natural range of variability, and it is hypothesized that intertidal sedimentation rates will demonstrate a non‐linear response to modification of the tidal energy regime. This research considers acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) data from a sheltered tidal creek in the Minas Basin, for analysis of tidal characteristics in a hypertidal creek environment over spring and neap tidal cycles. Sediment deposition in the creek was also measured. Results show a first‐order control of topography on flow magnitude in the tidal creek, which impacts net sediment deposition through resuspension and removal of newly introduced material. This study demonstrates that tides which peak around the bankfull level show reduced early ebb stage turbulence and flow velocity and encourage an extended depositional period. The dynamics of marshfull tides may be responsible for the maximum sediment deposition in tidal creeks, providing large amounts of material that is eventually distributed to and deposited on marsh surfaces. Copyright © 2012 John Wiley & Sons, Ltd.     

Modelling floodplain sedimentation using particle tracking

Both climate change and river rehabilitation projects induce changes in floodplain sedimentation. Notably along the lower River Rhine, the sediment deposition patterns and rates are subject to change. To assess the magnitude of these changes, we developed the MoCSED model, a floodplain sedimentation model within a geographical information system for the lower Rhine River. We based MoCSED on the ‘method of characteristics’ (MoC), a particle tracking method that minimizes numerical dispersion. We implemented the MoCSED model in the PCRaster dynamic modelling language. The model input comprises initial suspended sediment concentrations, water levels, flow velocities, and longitudinal and transverse dispersivities. We used a combination of the Krone and Chen concepts to calculate the subsequent sedimentation (SED routine). We compared the model results with sediment trap data for the Bemmel floodplain along the Dutch Waal River during the 2003 inundation. This comparison showed that MoCSED was able to simulate the pattern of sediment deposition. In addition, the model proved to be an improvement in comparison with a conventional raster‐based floodplain sedimentation model for the lower River Rhine. In future, MoCSED may serve well to study the impact of a changing discharge regime due to climate change and floodplain rehabilitation plans on deposition of sediments. Copyright © 2006 John Wiley & Sons, Ltd.     

Spatiotemporal variation of streambed quality and fine sediment deposition in five freshwater pearl mussel streams,in relation to extreme drought,strong rain and snow melt

Oxygenated streambeds are considered a key requirement for the successful recruitment of stream fauna, including highly endangered freshwater pearl mussel Margaritifera margaritifera. Excessive amounts of fines impede exchange between open water and interstitial, leading to colmation and low oxygen levels in the juvenile habitat. Understanding the dynamic relationship between sediment delivery, transport, deposition and remobilization in relation to anthropogenic drivers is still poorly understood, yet is essential for conservation and restoration.This study analysed spatiotemporal sediment dynamics and interstitial habitat quality in five pearl mussel streams at the border region between Bavaria, Saxony and the Czech Republic during 2018 and 2019, comparing extremely dry periods with higher discharge events caused by snow melt and rainfall. Physicochemical habitat conditions within the streambed and sediment deposition were recorded in high spatial resolution along the stream courses, with a particular focus on the effects of tributaries and outflows of man-made fishponds.Habitat conditions were unsuitable for juvenile pearl mussels at the majority of sites, indicated by pronounced differences in physicochemical parameters between open water and the substrate, independent of discharge conditions. Sediment deposition varied markedly between discharge events, in terms of both the quality and quantity of deposits. Snow melt resulted in the highest sedimentation rates, but the smallest proportion of fine particles. During low flow conditions, fine sediment deposition was highly variable, ranging from 0.048 to 4.170 kg/week/m², mostly independent of flow velocity. High spatiotemporal variation was observed within and amongst stream systems, revealing different longitudinal patterns of fine sediment deposition, with catchment land use as the main driver. Temporal variability in sediment deposition was mainly associated with the discharge condition while abiotic parameters varied mainly with season.The high site-specificity of sedimentation rates and substrate conditions in response to different discharge events highlights the importance of an adapted conservation management which considers anthropogenic effects at the local scale.     

Tidal and meteorological forcing of sediment transport in tributary mudflat channels

Field observations of flow and sediment transport in a tributary channel through intertidal mudflats indicate that suspended sediment was closely linked to advection and dispersion of a tidal salinity front. During calm weather when tidal forcing was dominant, high concentrations of suspended sediment advected up the mudflat channel in the narrow region between salty water from San Francisco Bay and much fresher runoff from the small local watershed. Salinity and suspended sediment dispersed at similar rates through each tidal inundation, such that during receding ebbs the sediment pulse had spread spatially and maximum concentrations had decreased. Net sediment transport was moderately onshore during the calm weather, as asymmetries in stratification due to tidal straining of the salinity front enhanced deposition, particularly during weaker neap tidal forcing. Sediment transport by tidal forcing was periodically altered by winter storms. During storms, strong winds from the south generated wind waves and temporarily increased suspended sediment concentrations. Increased discharge down the tributary channels due to precipitation had more lasting impact on sediment transport, supplying both buoyancy and fine sediment to the system. Net sediment transport depended on the balance between calm weather tidal forcing and perturbations by episodic storms. Net transport in the tributary channel was generally off-shore during storms and during calm weather spring tides, and on-shore during calm weather neap tides.     

Floodplain evolution in a small,tectonically active basin of northern California

Sediment cores were collected along ?oodplains in the Navarro River basin of coastal northern California to examine the controls on ?oodplain evolution in a tectonically active setting. Sedimentary strata were subsampled for organic content, bulk density, and grain size measurements. Organic samples were analysed for ¹⁴C age, which yielded net‐averaged sedimentation rates for all cores. Overbank deposition rates decreased at all study sites through time and declined in the downstream direction. The ability of intermediary‐order streams to store sediment in ?oodplains decreased the ability of highest‐order streams to record sediment‐pulse events. The effects of anthropogenic disturbance, primarily logging, on long‐term overbank deposition rates were minimal. Climatic variability, by affecting sediment loading in the channel network, is the principal control on ?oodplain evolution through the Holocene. A hypothetical model is proposed to explain overbank deposition rates in the Navarro basin, which may be extrapolated to the northern‐coastal California region during the late Pleistocene and Holocene. The complexities observed in sediment storage and routing in this study imply that caution should be made when extrapolating sediment‐yield measurements obtained at river mouths or coastal shelves to geomorphic events within small, tectonically active basins. Copyright © 2003 John Wiley & Sons, Ltd.     

Floodplain development in an engineered setting

Engineered flood bypasses, or simplified conveyance floodplains, are natural laboratories in which to observe floodplain development and therefore present an opportunity to assess delivery to and sedimentation within a specific class of floodplain. The effects of floods in the Sacramento River basin were investigated by analyzing hydrograph characteristics, estimating event‐based sediment discharges and reach erosion/deposition through its bypass system and observing sedimentation patterns with field data. Sediment routing for a large, iconic flood suggests high rates of sedimentation in major bypasses, which is corroborated by data for one bypass area from sedimentation pads, floodplain cores and sediment removal reporting from a government agency. These indicate a consistent spatial pattern of high sediment accumulation both upstream and downstream of lateral flow diversions and negligible sedimentation in a ‘hydraulic shadow’ directly downstream of a diversion weir. The pads located downstream of the shadow recorded several centimeters of deposition during a moderate flood in 2006, increasing downstream to a peak of ～10 cm thick and thinning rapidly thereafter. Flood deposits in the sediment cores agree with this spatial pattern, containing discrete sedimentation layers (from preceding floods) that increase in thickness with distance downstream of the bypass entrance to several decimeters thick at the peak and then thin downstream. These patterns suggest that a quasi‐natural physical process of levee construction by advective overbank transport and deposition of sediment is operating. The results improve understanding of the evolution of bypass flood control structures, the transport and deposition of sediment within these environments and the evolution of one class of natural levee systems. Copyright © 2008 John Wiley & Sons, Ltd.     

Application of a confluence‐based sediment‐fingerprinting approach to a dynamic sedimentary catchment,New Zealand

Fine sediment is a dynamic component of the fluvial system, contributing to the physical form, chemistry and ecological health of a river. It is important to understand rates and patterns of sediment delivery, transport and deposition. Sediment fingerprinting is a means of directly determining sediment sources via their geochemical properties, but it faces challenges in discriminating sources within larger catchments. In this research, sediment fingerprinting was applied to major river confluences in the Manawatu catchment as a broad‐scale application to characterizing sub‐catchment sediment contributions for a sedimentary catchment dominated by agriculture. Stepwise discriminant function analysis and principal component analysis of bulk geochemical concentrations and geochemical indicators were used to investigate sub‐catchment geochemical signatures. Each confluence displayed a unique array of geochemical variables suited for discrimination. Geochemical variation in upstream sediment samples was likely a result of the varying geological source compositions. The Tiraumea sub‐catchment provided the dominant signature at the major confluence with the Upper Manawatu and Mangatainoka sub‐catchments. Subsequent downstream confluences are dominated by the upstream geochemical signatures from the main stem of Manawatu River. Variability in the downstream geochemical signature is likely due to incomplete mixing caused in part by channel configuration. Results from this exploratory investigation indicate that numerous geochemical elements have the ability to differentiate fine sediment sources using a broad‐scale confluence‐based approach and suggest there is enough geochemical variation throughout a large sedimentary catchment for a full sediment fingerprint model. Combining powerful statistical procedures with other geochemical analyses is critical to understanding the processes or spatial patterns responsible for sediment signature variation within this type of catchment. Copyright © 2015 John Wiley & Sons, Ltd.     

Carbonate sedimentation in seagrass beds on Ishigaki‐jima,Ryukyu Islands,southwestern Japan

In order to investigate biota and sedimentary facies, and to delineate processes of carbonate sedimentation in seagrass beds, we conducted sedimentological investigations along three onshore–offshore transects at two sites (Nagura and Yoshihara) on Ishigaki‐jima, Ryukyu Islands. Along the transects, the seagrass beds extended seaward 20–40 m from shore, and their widths parallel to the shore ranged from 60 to >110 m. The seagrass was dominated by Thalassia hemprichii, Cymodocea rotundata and subordinate C. serrulata. Seasonal changes in seagrass coverage were evident, with mean coverage relatively higher in summer and fall (July and October) than in winter and spring (January and April). The surface sediment throughout the seagrass beds was dominated by medium to very coarse sand‐sized bioclasts displaying grainstone/packstone fabrics. Bioclasts were dominated by corals and coralline algae, with lesser benthic foraminifers, mollusks, echinoids, and Halimeda. The grainstone/packstone was underlain by gravelly sediment with coral clasts, showing a rudstone fabric, at the Nagura Site. The lower part of the core sediment was blackened, indicating a reducing environment. Two dates of corals collected at the Nagura and Yoshihara sites (24.5 cm and 16.5 cm below the sea bottom) were 2781–2306 and 4374–3805 cal BP (2σ age range), respectively, suggesting extremely low sedimentation rates (<0.1 mm/year). Sediment influx was higher during July–January than during January–July. The relatively large influx during summer and fall is caused by massive sediment transport during typhoons and storms. The total sediment influx (i.e., suspension‐load sediment transportation) is 74–96 kg CaCO₃/m²/year at the Nagura Site and 21–57 kg CaCO₃/m²/year at the Yoshihara Site. Sediment influx was significantly greater in the seagrass beds than in surrounding areas, providing supporting evidence for an sediment trapping function of seagrass beds. Our data indicate that seagrass beds in the Ryukyu Islands are characterized by high sediment fluxes and extremely low sedimentation rates.     

Spatiotemporal variations of suspended sediment transport in the upstream and midstream of the Yarlung Tsangpo River (the upper Brahmaputra), China

The Yarlung Tsangpo River, which flows from west to east across the southern part of the Tibetan Plateau, is the longest river on the plateau and an important center for human habitation in Tibet. Suspended sediment in the river can be used as an important proxy for evaluating regional soil erosion and ecological and environmental conditions. However, sediment transport in the river is rarely reported due to data scarcity. Results from this study based on a daily dataset of 3 years from four main stream gauging stations confirmed the existence of great spatiotemporal variability in suspended sediment transport in the Yarlung Tsangpo River, under interactions of monsoon climate and topographical variability. Temporally, sediment transport or deposition mainly occurred during the summer months from July to September, accounting for 79% to 93% of annual gross sediment load. This coincided with the rainy season from June to August that accounted for 51% to 80% of annual gross precipitation and the flood period from July to September that accounted for approximately 60% of annual gross discharge. The highest specific sediment yield of 177.6 t/km²/yr occurred in the upper midstream with the highest erosion intensity. The lower midstream was dominated by deposition, trapping approximately 40% of total sediment input from its upstream area. Sediment load transported to the midstream terminus was 10.43 Mt/yr with a basin average specific sediment yield of 54 t/km²/yr. Comparison with other plateau‐originated rivers like the upper Yellow River, the upper Yangtze River, the upper Indus River, and the Mekong River indicated that sediment contribution from the studied area was very low. The results provided fundamental information for future studies on soil and water conservation and for the river basin management. Copyright © 2017 John Wiley & Sons, Ltd.     

Gravel transport and morphological change in braided sunwapta river,Alberta, Canada

The relation between morphological change and patterns of variation in bedload transport rate in braided streams was observed by repeated, daily topographic surveys over a 25 day study period in a 60 m reach of the proglacial Sunwapta River, Alberta, Canada. There are two major periods of morphological change, each lasting several days and each involving the complete destruction and reconstruction of bar complexes. Bar complex destruction was caused by redirection of the flow and by downstream extension of the confluence scour zone upstream. Reconstruction involved accretion of unit bars on bar head, flank and tail and in one case was initiated by disection of a large, lobate unit bar. High rates of sediment movement, measured from net scour and fill of the cross-sections, coincided with these morphological changes. Sediment was supplied from both bed and bank erosion, and patterns and distances of transfer were highly variable. Rates of transport estimated by matching upstream erosional volumes with downstream deposition were much greater than those estimated from either a step-length approach or a sediment budget. Measurements of scour and fill and observations of morphological change indicate that step lengths (virtual transport distances) were typically 40–100m during a diurnal discharge cycle. Shorter step lengths occurred when transfer was confined to a single anabranch and longer steps involved channel changes at the scale of the entire reach. Sediment budgeting was used to describe the spatial patterns of sediment transport associated with the morphological changes and to estimate minimum daily reach-averaged transport rates. Mean bedload transport rates correlate with discharge, but with considerable scatter. The largest deviations from the mean relation can be tied to phases of channel incision, bank erosion, scour hole migration, bar deposition and channel filling apparently controlled by changes and fluctuations in sediment supply from upstream, independent of discharge. These are interpreted as field evidence of ‘autopulses’ or ‘macropulses’ in bedload transport, previously observed only in laboratory models of braided streams.     

Rapid formation of hyperpycnal sediment gravity currents offshore of a semi-arid California river

Observations of sediment dispersal from the Santa Clara River of southern California during two moderately sized river discharge events suggest that river sediment rapidly formed a negatively buoyant (hyperpycnal) bottom plume along the seabed within hours of peak discharge. An array of acoustic and optical sensors were placed at three stations 1 km from the Santa Clara River mouth in 10-m water depth during January–February 2004. These combined observations suggest that fluid mud concentrations of suspended sediment (>10 g/l) and across-shore gravity currents (∼5 cm/s) were observed in the lower 20–40 cm of the water column 4–6 h after discharge events. Gravity currents were wave dominated, rather than auto-suspending, and appeared to consist of silt-to-clay sized sediment from the river. Sediment mass balances suggest that 25–50% of the discharged river sediment was transported by these hyperpycnal currents. Sediment settling purely by flocs (∼1 mm/s) cannot explain the formation of the observed hyperpycnal plumes, therefore we suggest that some enhanced sediment settling from mixing, convective instabilities, or diverging plumes occurred that would explain the formation of the gravity currents. These combined results provide field evidence that high suspended-sediment concentrations from rivers (>1 g/l) may rapidly form hyperpycnal sediment gravity currents immediately offshore of river mouths, and these pathways can explain a significant portion of the river-margin sediment budget. The fate of this sediment will be strongly influenced by bathymetry, whereas the fate of the remaining sediment will be much more influenced by ocean currents.     

Suspended sediment fluxes at an intertidal flat: The shifting influence of wave,wind, tidal,and freshwater forcing

Using in situ, continuous, high frequency (8–16 Hz) measurements of velocity, suspended sediment concentration (SSC), and salinity, we investigate the factors affecting near-bed sediment flux during and after a meteorological event (cold front) on an intertidal flat in central San Francisco Bay. Hydrodynamic forcing occurs over many frequency bands including wind wave, ocean swell, seiching (500–1000 s), tidal, and infra-tidal frequencies, and varies greatly over the time scale of hours and days. Sediment fluxes occur primarily due to variations in flow and SSC at three different scales: residual (tidally averaged), tidal, and seiching. During the meteorological event, sediment fluxes are dominated by increases in tidally averaged SSC and flow. Runoff and wind-induced circulation contribute to an order of magnitude increase in tidally averaged offshore flow, while waves and seiching motions from wind forcing cause an order of magnitude increase in tidally averaged SSC. Sediment fluxes during calm periods are dominated by asymmetries in SSC over a tidal cycle. Freshwater forcing produces sharp salinity fronts which trap sediment and sweep by the sensors over short (∼30 min) time scales, and occur primarily during the flood. The resulting flood dominance in SSC is magnified or reversed by variations in wind forcing between the flood and ebb. Long-term records show that more than half of wind events (sustained speeds of greater than 5 m/s) occur for 3 h or less, suggesting that asymmetric wind forcing over a tidal cycle commonly occurs. Seiching associated with wind and its variation produces onshore sediment transport. Overall, the changing hydrodynamic and meteorological forcing influence sediment flux at both short (minutes) and long (days) time scales.     

Impact of natural reforestation on floodplain sedimentation in the Dragonja basin,SW Slovenia

Changes in floodplain sediment dynamics have profound effects on riverine habitats and riparian biodiversity. Depopulation due to socio‐economic changes in the Dragonja catchment (91 km²) in southwestern Slovenia resulted in the abandonment of agricultural fields, followed by natural reforestation since 1945. This profoundly changed the water and sediment supply to the streams, as well as floodplain sediment deposition. This paper presents a reconstruction of the development of the Dragonja floodplain due to these land use changes during the last 60 years. The reconstruction is based on dating of floodplain sediments using ¹³⁷Cs profiles, measurement of actual sedimentation rates using artificial grass sedimentation mats, and linking this information to the present‐day hydrological behaviour of the river. The sedimentation mats showed that floodplain sedimentation was restricted to peak flows of considerable magnitude. Due to the reforestation, the return period of such high flows increased from 0·31 year in the period 1960–1985 to 0·81 year between 1986 and 2003, with commensurate changes in sedimentation rates. At the 1·5 m river terrace (formed about 60 years ago), ¹³⁷Cs‐based sedimentation rates (1960–1986) were roughly twice the rates inferred from the artificial grass mats (2001–2003). This finding matches the increase in the return period for larger peak events during the 1986–2003 period, which caused fewer major inundations at this level. Conversely, sedimentation rates determined for the lowest terrace at 0·5 m were similar for both techniques (and periods) because the return periods of the peak events responsible for sediment deposition at this lower level did not change much over the period 1986–2003. Copyright © 2006 John Wiley & Sons, Ltd.     

Temporal variability of contemporary floodplain sedimentation in the Rhine–Meuse delta,The Netherlands

It is often believed that extreme but infrequent events are most important in the development of landforms. When evaluating the overall effect of large floods on floodplain sedimentation, quantitative measurements of both high- and low-magnitude events should be considered. To analyse the role of flood magnitude on floodplain sedimentation, we measured overbank sedimentation during floods of different magnitude and duration. The measurements were carried out on two embanked floodplain sections along the rivers Rhine and Meuse in The Netherlands, using sediment traps made of artificial grass. The results showed an increase in total sediment accumulation with flood magnitude, mainly caused by enhanced accumulation of sand. At low floodplain sections the increase in sediment deposition was smaller than expected from the strong increase in suspended sediment transport in the river. Spatial variability in sediment accumulation was found to depend both on flood magnitude and duration. Deposition of sand on natural levees mainly takes place during high-magnitude floods, whilst low floods and slowly receding floods are important for the deposition of silt and clay in low-lying areas, at greater distance from the main channel. © 1998 John Wiley & Sons, Ltd.     

FINE SEDIMENT OF THE UPPER MISSISSIPPI AND ILLINOIS RIVERS AND THE DISASTROUS CONSEQUENCES

IINTRODUCTIONLanddevelopmentandlandusepatternsinthewatershedcaninduceincreasedsedimentloadsinriversandstreams.AGREATIllstudy(1982)illustratedthatsedimentyieldsfromagriculturallandcouldbeseveralfoldsmorethanothertypeoflandusesanderosionsources.ThesamestudyalsodemonstratedthatfinesedimentsweretheheaviestportionoftotalsoillossesfromeachtypeoflandusesinthetwelvehydrologicareasitinvestigatedintheUMRS.Thesamecouldbetrueforotheruplandareasalso.Howeverfinesediments,formthewashloadofthestream…     

Proglacial sediment trapping in recently formed Silt Lake,upper Lillooet Valley,Coast Mountains,British Columbia

The sedimentology of proglacial Silt Lake was assessed by lake sediment coring and monitoring of lacustrine processes during a late‐summer period of high glacier melt to characterize sediment delivery from the heavily glacierized catchment and investigate the sediment trapping dynamics of this upland lake. A complete varve chronology was established for a distal basin of the lake which was exposed by Lillooet Glacier retreat between 1947 and 1962. The varve record showed decreasing sedimentation rates in the basin while the glacier retreated, and as the lake became free of ice contact in the early 1970s. Although recession has continued over recent decades, and glacier proximity to the lake has, therefore, continued decreasing, lacustrine sedimentation rates are now accelerating due to changing basin morphometry caused by delta progradation. Over shorter time scales, lake sedimentation patterns respond to changing runoff conditions, including late‐summer glacier melt intensity, intra‐annual flooding events, diumal runoff fluctuations, and within‐lake turbidity currents. Turbidity currents included quasi‐regular flows during high diurnal discharges and an episodic flushing of temporarily stored sediment from the sandur or delta at a time of low stage. Suspended sediment yield to Silt Lake is estimated to exceed 10³ Mg km^?2 a^?1, a magnitude that surpasses previous local and regional yield estimates for the glacierized headwaters of the Lillooet River valley. Since Silt Lake currently traps a significant prooportion of that upland sediment supply, and the trapping efficiency of the basin has been variable at decadal time scales, the formation and continued development of Lilt Lake has likely had a significant influence on downstream sediment delivery. Lacustrine sediment‐based proxies of long‐term hydroclimatic variability being developed in glacially distal settings should include provisions for dynamic sediment trapping effects in upstream water bodies that often form in the active proglacial environment. Copyright © 2008 John Wiley & Sons, Ltd.