Floodplain Cores Research Articles

Variability of rare earth and heavy metal elements in a representative alluvial depositional system.

Heavy metals (Ni, Cr, W, Cd, and Pb) and rare earth elements (REE) were investigated in the flood plain sediments of an island of the lower Yangtze River near Nanjing to determine how the vertical distribution of heavy metals could be affected by natural sedimentation processes and anthropogenic contamination. Stratigraphic analyses of magnetic susceptibility and the mean grain size distribution of the deposits enabled us to identify layers associated with a relatively high influx of suspended sediments that resulted in sudden changes in the concentrations of heavy metals. The results show that layers associated with high sediment influx (0.8m depth) displayed low concentrations of Cr, Ni, W, and Cd that were mainly lithogenic in origin. The Post Archean Australian Shale (PAAS) normalized REE patterns in the flood plain cores were enrichment in Ce and Eu relative to PAAS, indicating that the sediments were most likely derived from a mixture of sediments and not from an anthropogenic source. Sharp increases in Y/Ho ratios, as well as heavy metal (Cd, Cr, Ni, and W) and Y concentrations were observed in the uppermost layer that could have been deposited from the rapid transport of sediment-laden, contaminated waters. The temporal (vertical) trends in Pb concentrations may be strongly influenced by coal burning. Elevated Pb concentrations (350ppm and 1000ppm) correlate with high magnetic susceptibility (> 200 m3 × kg-1) and the history of thermal power plant (1910-2002) activity. The anthropogenic inputs of Pb were, however, not diluted by high suspended sediment loads, which supports the argument that Pb was derived from fly ash.

Geomorphically mediated carbon dynamics of floodplain soils and implications for net effect of carbon erosion

AbstractThe fate of organic carbon deposited in floodplain sediments is an important control on the magnitude and direction of the carbon flux from anthropogenically accelerated erosion and channelization of the riverine network. Globally, carbon deposition rates and mean residence time (MRT) within different geomorphic settings remains poorly constrained. We sampled soil profiles to 0.8 m depth from two geomorphic zones: active channel belt (ACB) and lowland floodplain, under long‐term pasture adjacent to the river Culm in SW England, UK. We evaluated sedimentation rates and carbon storage using fallout radionuclide 137Cs, particle size and total carbon analyses. Variation in decomposition was assessed via empirical (soil aggregate size, density fractionation combined with natural abundance 13C analysis) and modelling simulation (using the RothC model and catchment implications explored using a floodplain evolution model). Sedimentation and carbon accumulation rates were 5–6 times greater in the ACB than the floodplain. Carbon decomposition rates also varied with geomorphic setting. In floodplain cores, faster decomposition rates were indicated by greater 13C‐enrichment and subsoils dominated by mineral‐associated soil organic carbon. Whereas, in the ACB, carbon was less processed and 13C‐depleted, with light fraction and macroaggregate‐carbon throughout the cores, and RothC modelled decomposition rates were 4‐fold less than lowland floodplain cores. Including the ACB in floodplain carbon MRT calculations increased overall MRT by 10%. The major differences in the balance of sedimentation and decomposition rates between active and inactive floodplains suggests the relative extent of these contrasting zones is critical to the overall carbon balance. Restoration projects could enhance soil carbon storage by maximizing active floodplain areas by increasing river channel complexity.

The fate of fluvially-deposited organic carbon during transient floodplain storage

CO2 release from particulate organic carbon (POC) oxidation during fluvial transit can influence climate over a range of timescales. Identifying the mechanistic controls on such carbon fluxes requires determining where POC oxidation occurs in river systems. While field data show POC oxidation and replacement moving downstream in lowland rivers, flume studies show that oxidation during active fluvial transport is limited. This suggests that most fluvial POC oxidation occurs during transient floodplain storage, but this idea has yet to be tested. Here, we isolate the influence of floodplain storage time on POC oxidation by exploiting a chronosequence of floodplain deposits above the modern groundwater table in the Rio Bermejo, Argentina. Measurements from 15 floodplain cores with depositional ages from 1 y to 20 ky show a progressive POC concentration decrease and 13C-enrichment with increasing time spent in floodplain storage. These results from the Rio Bermejo indicate that over 80% of fluvially-deposited POC can be oxidized over millennial timescales in aerated floodplains. Furthermore, POC in the oldest floodplain cores is more 14C-enriched than expected based on the independently-dated floodplain ages, indicating that a portion of this oxidized POC is replaced by autochthonous POC produced primarily by floodplain vegetation. We suggest floodplain storage timescales control the extent of oxidation of fluvially-deposited POC, and may play a prominent role in determining if rivers are significant atmospheric CO2 sources.

Dynamics of floodplain lakes in the Upper Amazon Basin during the late Holocene

To better understand the impact of channel migration processes and climate change on the depositional dynamics of floodplain lakes of the upper Amazon Basin during the late Holocene, we collected three sediment cores from floodplain lakes of the Ucayali River and one from the Marañón River. The cores were dated with 14C, radiographed and described. Bulk density, grain size analysis and total organic carbon (TOC) were determined. The results show that sedimentation in Ucayali floodplain lakes was marked by variations during the late Holocene, with periods of intense hydrodynamic energy and abrupt accumulations, a gap in the record between about 2870 and 690cal yr BP, and periods of more lacustrine conditions. These changes in sedimentation were associated with variations in the river's influence related to changes in its meandering course (2870cal yr BP) and a period of severe flooding between 3550 and 3000cal yr BP. Lake Lagarto on the Marañón River floodplain exhibits a different sedimentary environment of low hydrodynamics with palm trees and macrophytes. Apparently, the lake has not experienced intense migration processes during the last 600cal yr BP (base of the core). Nevertheless, the river sediment flux to the lake was important from 600 to 500cal yr BP, although it decreased thereafter until the present. This decrease in the mineral accumulation rate indicates a decrease in river discharge since 500cal yr BP, which coincides with precipitation records from the central Andes. In the upper part of the three Ucayali floodplain cores, a 30- to 250-cm-thick layer of reworked sediments has been deposited since 1950 AD (post-bomb). In Lake Carmen, this layer is associated with invasion of the lake by the levee of a migrating meander of the Ucayali. In Lakes Hubos and La Moringa, however, the river is still far away and the deposition must be interpreted as the result of extreme flooding. The beginning of the Ucayali meander migration is dated back to 2000 AD, suggesting that these extreme floods could be very recent and linked to hydrologic extremes registered instrumentally in the Amazon Basin.

Legacy sediment, lead, and zinc storage in channel and floodplain deposits of the Big River, Old Lead Belt Mining District, Missouri, USA

The Old Lead Belt of southeastern Missouri was one of the leading producers of Pb ore for more than a century (1869–1972). Large quantities of contaminated mine waste have been, and continue to be, supplied to local streams. This study assessed the magnitude and spatial distribution of mining-contaminated legacy sediment stored in channel and floodplain deposits of the Big River in the Ozark Highlands of southeastern Missouri. Although metal concentrations decline downstream from the mine sources, the channel and floodplain sediments are contaminated above background levels with Pb and Zn along its entire 171-km length below the mine sources. Mean concentrations in floodplain cores >2000mgkg−1 for Pb and >1000mgkg−1 for Zn extend 40–50km downstream from the mining area in association with the supply of fine tailings particles that were easily dispersed downstream in the suspended load. Mean concentrations in channel bed and bar sediments ranging from 1400 to 1700mgkg−1 for Pb extend 30km below the mines, while Zn concentrations of 1000–3000mgkg−1 extend 20km downstream. Coarse dolomite fragments in the 2–16mm channel sediment fraction provide significant storage of Pb and Zn, representing 13–20% of the bulk sediment storage mass in the channel and can contain concentrations of >4000mgkg−1 for Pb and >1000mgkg−1 for Zn. These coarse tailings have been transported a maximum distance of only about 30km from the source over a period of 120years for an average of about 250m/y.About 37% of the Pb and 9% of the Zn that was originally released to the watershed in tailings wastes is still stored in the Big River. A total of ~157millionMg of contaminated sediment is stored along the Big River, with 92% of it located in floodplain deposits that are typically contaminated to depths of 1.5–3.5m. These contaminated sediments store a total of 188,549Mg of Pb and 34,299Mg of Zn, of which 98% of the Pb and 95% of the Zn are stored in floodplain deposits. Most of the metal mass in channel deposits is stored near the mines, with 72% of the Pb and 78% of the Zn occurring in the 25km of channel proximal to the mine source. Although environmental assessments of streams contaminated by mines often focus on evaluating metal concentrations in the geochemically active fine sediment fractions, about 60% of the Pb stored in channels is associated with coarse dolomite tailings fragments deposited in channels within 25km of the mines. The magnitude and basinwide distribution of Pb and Zn storage in legacy floodplain sediments ensures that remobilization by bank erosion will be a continuing problem for water quality far into the future.

Can channel banks be the dominant source of fine sediment in a UK river?: an example using 137Cs to interpret sediment yield and sediment source

AbstractCultivated fields have been shown to be the dominant sources of sediment in almost all investigated UK catchments, typically contributing 85 to 95% of sediment inputs. As a result, most catchment management strategies are directed towards mitigating these sediment inputs. However, in many regions of the UK such as the Nene basin there is a paucity of sediment provenance data. This study used the caesium‐137 (137Cs) inventories of lake and floodplain cores as well as the 137Cs activities of present day sediment to determine sediment provenance. Sediment yields were also reconstructed in a small lake catchment.Low 137Cs inventories were present in the lake and floodplain cores in comparison to the reference inventory and inventories in cores from other UK catchments. Caesium‐137 activities in the present day sediments were low; falling close to those found in the channel bank catchment samples. It was estimated that 60 to 100% of the sediment in the Nene originated from channel banks.Pre‐1963 sediment yields were approximately 11.2 t km−2 yr−1 and post‐1963 was approximately 11.9 t km−2 yr−1. The lack of increased sediment yield post‐1963 and low sediment yield is unusual for a UK catchment (where a yield of 28 to 51 t km−2 yr−1 is typical for a lowland agricultural catchment), but is explained by the low predicted contribution of sediment from agricultural topsoils. The high channel bank contribution is likely caused by the river being starved of sediment from topsoils, increasing its capacity to entrain bank material.The good agreement between the results derived using cores and recently transported sediments, highlight the reliability of 137Cs when tracing sediment sources. However, care should be taken to assess the potential impacts of sediment particle size, sediment focusing in lakes and the possible remobilization of 137Cs from sedimentary deposits. Copyright © 2016 John Wiley & Sons, Ltd.

Use of sediment source fingerprinting to assess the role of subsurface erosion in the supply of fine sediment in a degraded catchment in the Eastern Cape, South Africa

Sediment source fingerprinting has been successfully deployed to provide information on the surface and subsurface sources of sediment in many catchments around the world. However, there is still scope to re-examine some of the major assumptions of the technique with reference to the number of fingerprint properties used in the model, the number of model iterations and the potential uncertainties of using more than one sediment core collected from the same floodplain sink. We investigated the role of subsurface erosion in the supply of fine sediment to two sediment cores collected from a floodplain in a small degraded catchment in the Eastern Cape, South Africa. The results showed that increasing the number of individual fingerprint properties in the composite signature did not improve the model goodness-of-fit. This is still a much debated issue in sediment source fingerprinting. To test the goodness-of-fit further, the number of model repeat iterations was increased from 5000 to 30,000. However, this did not reduce uncertainty ranges in modelled source proportions nor improve the model goodness-of-fit. The estimated sediment source contributions were not consistent with the available published data on erosion processes in the study catchment. The temporal pattern of sediment source contributions predicted for the two sediment cores was very different despite the cores being collected in close proximity from the same floodplain. This highlights some of the potential limitations associated with using floodplain cores to reconstruct catchment erosion processes and associated sediment source contributions. For the source tracing approach in general, the findings here suggest the need for further investigations into uncertainties related to the number of fingerprint properties included in un-mixing models. The findings support the current widespread use of ≤5000 model repeat iterations for estimating the key sources of sediment samples.

Trajectories of river chemical quality issues over the Longue Durée: the Seine River (1900S-2010).

River quality trajectories are presented for (i) organic pollution, (ii) eutrophication, (iii) nitrate pollution, and (iv) metal contamination over the Longue Durée (130 to 70years). They are defined by a quantified state indicator (S) specific to each issue, compared to drivers (D) or pressures (P) and to social responses (R) that reflect the complex interactions between society and river quality. The Lower Seine River, naturally sensitive to anthropogenic pressures, greatly impacted by Paris urban growth, industrialization, and intensive agriculture, and well documented by the PIREN-Seine 25-year research program, was chosen to illustrate these trajectories. State indicators, dissolved oxygen, algal pigments, nitrate, and heavy metals (Cd, Cr, Hg, Pb, Zn) in sediments have only been monitored by river basin authorities since 1971. Therefore, their past changes have been reconstructed using three approaches: (i) reassessment of historical sources, (ii) pressure-state models that reconstruct past water quality, and (iii) sedimentary archives of past persistent contamination from dated floodplain cores. The indicators were then transformed into river quality status using contemporary water quality criteria throughout these records. Each environmental issue shows specific trajectories because each has its own relationship between the issue evidence and the social response, but all are characterized by very poor quality in the past, largely ignored: the long-term summer hypoxia (<1880-1995), the summer eutrophication peak (1965-2005), the growing nitrate level since the 1950s, recently stabilized but still high, and the extreme metal contamination (>1935-2000) that peaked in the 1960s. The efficiency of social responses has been highly variable but more efficient in the last 15-25years.

Active overbank deposition during the last century, South River, Virginia

We quantify rates of overbank deposition over decadal to centennial timescales along the South River in Virginia using four independent methods. Detailed mercury profiles sampled adjacent to the stream channel preserve the peak historic mercury concentration on suspended sediment dating from 1955 to 1961 and suggest sedimentation rates of 8 to 50cm/100years. Sediment accumulation over the roots of trees suggest rates of 0 to 100cm/100years, with significantly higher values on levees and lower values on floodplains farther from the channel. Profiles of 137Cs and 210Pb from two eroding streambanks are fit with an advection–diffusion model calibrated at an upland reference site; these methods suggest sedimentation rates of 44 to 73cm/100years. Mercury inventories from 107 floodplain cores, combined with a previously published reconstruction of the history of mercury concentration on suspended sediment, provide spatially comprehensive estimates of floodplain sedimentation: median sedimentation rates are 3.8cm/100years for the <0.3-year floodplain, 1.37cm/100years for the 0.3- to 2-year floodplain, 0.4cm/100years for the 2- to 5-year floodplain, and 0.1cm/100years for the 5- to 62-year floodplain. While these sedimentation rates are relatively low, the total mass of sediment stored from 1930 to 2007 is 4.9±1.7 (95% confidence interval)×107kg, corresponding to an average thickness of 2.5cm (3.2cm/100years). These results demonstrate that floodplains of our 4.5-km-long study reach have stored 8 to 12% of the total suspended sediment supplied to the study reach of the South River. Hydrologic Engineering Center-River Analysis System (HEC-RAS) modeling demonstrates that the floodplain of the South River remains hydraulically connected to the channel: 56% of the 100-year floodplain is inundated every two years, and 83% of the floodplain is inundated every five years. These results, combined with previously published data, provide the basis for a regional synthesis of floodplain deposition rates since European settlement. Floodplain sedimentation rates were high following European settlement, with published estimates ranging from 50 to 200cm/100years. Sedimentation rates decreased by 1 to 2 orders of magnitude during the twentieth and twenty-first centuries; but despite these lower sedimentation rates, floodplains continue to store a significant fraction of total suspended sediment load. Many floodplains of the mid-Atlantic region are active landforms fully connected to the rivers that flow within them and should not be considered terraces isolated from contemporary fluvial processes by post-settlement aggradation.

Pollution assessment using local enrichment factors: the Berounka River (Czech Republic)

The Berounka River is considered a relatively clean river, but recent studies have reported various levels of pollution. The purpose of this work is to identify anthropogenic pollution by metals (i.e. Pb, Zn, Cu) and magnetic particles in the sediments of that river and its tributaries. Samples were obtained from hand-drilled cores taken from representative areas within the fluvial system; in distal floodplains (overbank fines) and closer to the channel (laterally deposited sediments). Samples were subjected to analysis using mass magnetic susceptibility (MS), X-ray fluorescence spectrometry (XRF) and also by inductively coupled plasma mass spectrometer (ICP MS) which allowed for a determination of 206Pb/207Pb isotope ratios. Macroelement ratios (K/Ti and Ti/Al) were used to distinguish variegated sediment provenance in the Berounka system. Normalization of trace elements by Ti (in the case of trace elements) and by Fe (magnetic susceptibility) allowed us to establish lithogenic background functions of trace elements and magnetic susceptibility within these two geologically different areas. A pollution assessment of the study area was performed using magnetic susceptibility and local enrichment factors (LEFs) for the risk elements. By comparing 1/(LEF Pb) and Pb isotopic composition, the origins of Pb within the catchment were determined. This unique method was able to distinguish Pb from various origins. The upper parts of the floodplain cores contained higher levels of trace elements and magnetic particles (anthropogenically polluted), but samples taken from the cores in the active channel belt exhibited considerably higher concentrations of trace elements and magnetic particles than the upper parts of the floodplain cores and to much greater depths. We interpreted the deeper parts of the floodplain cores as a local lithogenic background. The upper parts of floodplain sediments hence showed moderate pollution (LEF of Pb and Zn ∼2, MS ∼2.5); whereas laterally deposited sediments showed significantly higher LEF values (LEF of Pb ∼6, Zn ∼9, MS ∼8). The analysis of the sediments confirmed that the Berounka River system contains higher concentrations of trace elements and magnetic particles than can be accounted for by natural geological processes. Our pollution assessment of the Berounka River and its tributaries demonstrated that their sediments are moderately polluted from sources situated on its tributaries: Ag–Pb mining near the city of Střibro in the Mže catchment; Pb–Zn mining in the Přibram ore district in the Litavka catchment and Fe ore processing and smelting in the Klabava catchment.

The application of sediment fingerprinting to floodplain and lake sediment cores: assumptions and uncertainties evaluated through case studies in the Nene Basin, UK

Fine sediment has been shown to be a major cause of the degradation of lakes and rivers and, as a result, research has been directed towards the understanding of fine sediment dynamics and the minimisation of sediment inputs. The use of tracers within a sediment fingerprinting framework has become a heavily used technique to investigate the sources of fine sediment pressures. When combined with the use of historically deposited sediment, the technique provides the opportunity to reconstruct past changes to the environment. However, alterations to tracer signatures during sediment transport and storage are a major potential source of uncertainty associated with tracer use. At present, few studies have quantified the uncertainties associated with tracer use. This paper investigated uncertainty by determining the differences between sediment provenance predictions obtained using lithogenic radionuclide, geochemical and mineral magnetic signatures when fingerprinting lake and floodplain sedimentary deposits. It also investigated the potential causes of the observed differences. A reservoir core was fingerprinted with the least uncertainty, with tracer group predictions ∼28 % apart and a consistent down-core trend in changing sediment provenance produced. When fingerprinting an on-line lake core and four floodplain cores, differences between tracer group predictions were as large as 100 %; the down-core trends in changing sediment provenance were also different. The differences between tracer group predictions could be attributed to the organic matter content and particle size of the sediment. There was also evidence of the in-growth of bacterially derived magnetite and chemical dissolution affecting the preservation of tracer signatures. Simple data corrections for sediment organic matter content and particle size did not result in significantly greater agreement between the predictions of the different tracer groups. Likewise, the inclusions of weightings for tracer discriminatory efficiency and within-source variability had minimal effects on the fingerprinting results. This paper highlights the importance of tracer selection and the consideration of recognising tracer non-conservatism when using lake and floodplain sediment deposits to reconstruct anthropogenic changes to the environment and changing sediment dynamics. It was recommended that future research focus on the assessment of uncertainty using the artificial mixing of sediment source samples, the limitation of the fingerprinting to narrow particle size fractions and the development of specific particle size and organic matter correction factors for each tracer.

Effects of a small-scale, abandoned gold mine on the geochemistry of fine stream-bed and floodplain sediments in the Horsefly River watershed, British Columbia, Canada

Mining is known to be a major source of metal contamination for fluvial systems worldwide. Monitoring and understanding the effects on downstream water and sediment quality is essential for its management and to mitigate against detrimental environmental impacts. This study aimed to examine the effects of the small-scale, abandoned, hydraulic Black Creek gold mine on the geochemical content of fine (&lt;63 μm) stream bed and floodplain sediment in the Horsefly watershed (British Columbia, Canada), which is a part of the Quesnel River basin. Concentrations of As, Cd, Se and Zn were determined byaqua regiadigestion and the modified BCR (European Community Bureau of Reference) sequential extraction procedure followed by inductively coupled plasma-optical emission spectroscopy or inductively coupled plasma-mass spectrometry. Age-depth models for the floodplain cores were constructed using excess Pb-210 and Cs-137 activity concentration profiles. The results show that the mine caused local As contamination of the Horsefly River floodplain sediment during the first years of operation, but at present the contamination is mainly contained in the Black Creek. Present-day and recent concentrations of Cd, Se and Zn in the Horsefly River are elevated and this is probably derived from other upstream mining activities. The spatial and temporal changes in heavymetal concentrations suggest a slight, but not particularly widespread, mining effect on the finesediment geochemistry in the Horsefly River system.

Influence of fluvial environments on sediment archiving processes and temporal pollutant dynamics (Upper Loire River, France)

Floodplains are often cored to build long-term pollutant trends at the basin scale. To highlight the influences of depositional environments on archiving processes, aggradation rates, archived trace element signals and vertical redistribution processes, two floodplain cores were sampled near in two different environments of the Upper Loire River (France): (i) a river bank ridge and (ii) a paleochannel connected by its downstream end. The base of the river bank core is composed of sandy sediments from the end of the Little Ice Age (late 18th century). This composition corresponds to a proximal floodplain aggradation (<50m from the river channel) and delimits successive depositional steps related to progressive disconnection degree dynamism. This temporal evolution of depositional environments is associated with mineralogical sorting and variable natural trace element signals, even in the <63-μm fraction. The paleochannel core and upper part of the river bank core are composed of fine-grained sediments that settled in the distal floodplain. In this distal floodplain environment, the aggradation rate depends on the topography and connection degree to the river channel. The temporal dynamics of anthropogenic trace element enrichments recorded in the distal floodplain are initially synchronous and present similar levels. Although the river bank core shows general temporal trends, the paleochannel core has a better resolution for short-time variations of trace element signals. After local water depth regulation began in the early 1930s, differences of connection degree were enhanced between the two cores. Therefore, large trace element signal divergences are recorded across the floodplain. The paleochannel core shows important temporal variations of enrichment levels from the 1930s to the coring date. However, the river bank core has no significant temporal variations of trace element enrichments and lower contamination levels because of a lower deposition of contaminated sediments and a pedogenetic trace elements redistribution.

Metal Contamination from Gold Mining in the Cid District, North Carolina

The purpose of this paper was to assess contamination from 19th century gold (Au) mining in the Cid district, North Carolina. Sediment samples collected from active channel sediments and floodplain cores were analyzed for mercury (Hg), copper (Cu), lead (Pb), and zinc (Zn). Analysis of trace metal concentrations shows that although Hg contamination exists at relatively low levels (i.e., no samples exceeded the probable effect concentration for Hg), the active channel sediments and historical floodplain deposits are contaminated by Hg downstream from all mines in the district. We also found significant contamination by Cu, Pb, and Zn. The use of Hg and other metals as tracers associated with mining activities suggests that long-term rates of floodplain sedimentation in the Cid district (0.3-0.9 cm/yr) were less than half as high as those in the nearby Gold Hill district. This suggests that the intensity of land disturbance in the Cid district was less than in the more intensively mined Gold Hill district.

A 100 ka record of fluvial activity in the Fitzroy River Basin, tropical northeastern Australia

This study reports the nature and timing of Quaternary fluvial activity in the Fitzroy River basin, which drains a diverse 143,000 km 2 area in northeastern Queensland, before discharging into the Great Barrier Reef Marine Park. The catchment consists of an extensive array of channel and floodplain types that we show have undergone large-scale fluvial adjustment in-channel planform, geometry and sinuosity. Optically stimulated luminescence (OSL) dating of quartz sediments from fifteen (3–18 m) floodplain cores throughout the basin indicates several discrete phases of active bedload activity: at ∼105–85 ka in Marine Isotope Stage (MIS) 5, at ∼50–40 ka (MIS 3), and at ∼30–10 ka (MIS 3/2). The overall timing of late Quaternary fluvial activity correlates well with previous accounts from across Australia with rivers being primarily active during interstadials. Fluvial activity, however, does not appear to have been synchronous throughout the basin’s major sub-catchments. Fluvial activity throughout MIS 2 (i.e. across the Last Glacial Maximum) in the meandering channels of the Fitzroy correlates well with regional data in tropical northeastern Queensland, and casts new light on the river response to reduced rainfall and vegetation cover suggested by regional palaeoclimate indicators. Moreover, the absence of a strong Holocene signal is at odds with previous accounts from elsewhere throughout Australia. The latitudinal position of the Fitzroy across the Tropic of Capricorn places this catchment at a key location for elucidating the main hydrological drivers of Quaternary fluvial activity in northeastern Australia, and especially for determining tropical moisture sources feeding into the headwaters of Cooper Creek, a major river system of the continental interior.

Floodplain development in an engineered setting

AbstractEngineered 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 &amp; Sons, Ltd.

Sediment load and floodplain deposition rates: Comparison of the Fly and Strickland rivers, Papua New Guinea

Rates of aggradation and infilling of accommodation space along lowland channels in response to postglacial sea level rise should depend on sediment supply. The Strickland and Fly rivers join at just 6 m above sea level and have experienced the same Holocene sea level rise. Historically, the Strickland has carried about 7 times the sediment load and 1.4 times the water discharge as the Fly. Therefore we hypothesize that the lowland Strickland floodplain should be more developed and consequently should presently be capturing proportionately less sediment than the floodplain of the lowland Fly River. We use mine‐derived elevated Pb and Ag concentrations in 111 shallow (&lt;1 m) floodplain cores collected in 2003 to determine deposition rates across the lower Strickland floodplain. Sediment deposition rates decrease across the floodplain with distance from the channel bank, and the average rate of deposition is 1.4 cm/a over the first 1 km. Overbank deposition along the lowland sandbedded Strickland results in ∼13% loss of the total load, ∼0.05%/km of channel length of the main stem. Deposition rates over the first 1 km from the channel bank on the Strickland are about 10 times those on the Fly (for estimated natural sediment loads); however, the proportional loss per channel length on the Strickland is less than that on the Fly (0.09%/km of main stem channel length) because of an extensive network of tributary and tie channels that convey sediment to the floodplain on the Fly. Furthermore, the lateral migration of the Strickland channel is ∼5 times that on the Fly, such that most overbank deposits on the Strickland are returned to the channel, causing the net loss of sediment to the floodplain to be small. We conclude that the Strickland River, which has a much higher overbank deposition rate than the Fly River as a function of distance from channel bank, nonetheless has significantly less net accumulation than the Fly because (1) a large proportion of the sediment load is conveyed up tributaries and tie channels on the Fly, and (2) lateral migration (which presumably results from the higher load) on the Strickland sweeps sediment back into the channel. Hence our field observations support our initial hypothesis, though the primary reason for this is because of active lateral migration rather than low overbank deposition rates.

Spatial and temporal dynamics of sediment accumulation and exchange along Strickland River floodplains (Papua New Guinea) over decadal‐to‐centennial timescales

We investigated the processes of sediment exchange between the Strickland River and its lowland floodplain, documenting (1) the rates, textures, and distributions of sediment accumulation, (2) temporal variations in these rates, (3) the remobilization of sediment by erosive processes, and (4) whether net storage is significant over century timescales. We used 210Pb geochronology of floodplain cores from 11 transects to measure deposition rates over the past ∼65 years, finding a decline from 5.5 cm/a (0–10 m distance from the channel) to 0.9 cm/a (∼400 m) to ∼0.1 cm/a (&gt;1 km). Rates are elevated along curved sections of channel and are stable over time, and episodic accumulation is prevalent and may be correlated to floods. Integrated temporally and spatially, the average rate is 1.6 cm/a. Integrating along both sides of ∼318 river km we studied for the lowland Strickland, we calculate 12–19 Mt of annual sediment accumulation (∼0.05 Mt/km or 0.07% of the annual load per km of river length), representing 17–27% of the total annual sediment flux. We used georeferenced Landsat images (1972–2001) to quantify channel migration and the resulting return of sediment to the channel. Mean lateral migration was 5.1 ± 0.8 m/a. Given the floodplain width of ∼10 km, this implies a waiting time of ∼1 ka between floodplain formation and subsequent reentrainment of the bank as the channel migrates laterally. This raises the possibility that the net return of material from the floodplain due to channel migration could balance the overbank deposition we observed. The exchange flux between cut bank erosion and point bar deposition is 20–40 Mt, highlighting the significance of sediment recycling (∼50% of the total load). Such sediment trapping and recycling affects the transport, storage, and evolution of biogeochemically reactive particles, the evolution of the floodplain, and the morphodynamics of basin infilling.

Historical perspective of heavy metals contamination (Cd, Cr, Cu, Hg, Pb, Zn) in the Seine River basin (France) following a DPSIR approach (1950–2005)

The Driver-Pressures-State-Impact-Response approach is applied to heavy metals in the Seine River catchment (65,000 km(2); 14 million people of which 10 million are aggregated within Paris megacity; 30% of French industrial and agricultural production). The contamination pattern at river mouth is established on the particulate material at different time scales: 1930-2000 for floodplain cores, 1980-2003 for suspended particulate matter (SPM) and bed-sediments, 1994-2003 for atmospheric fallout and annual flood deposits. The Seine has been among the most contaminated catchments with maximum contents recorded at 130 mg kg(-1) for Cd, 24 for Hg, 558 for Pb, 1620 for Zn, 347 for Cu, 275 for Cr and 150 for Ni. Today, the average levels for Cd (1.8 mg kg(-1)), Hg (1.08), Pb (108), Zn (370), Cu (99), Cr (123) and Ni (31) are much lower but still in the upper 90% of the global scale distribution (Cr and Ni excepted) and well above the natural background values determined on pre-historical deposits. All metal contents have decreased at least since 1955/65, well before metal emission regulations that started in the mid 1970's and the metal monitoring in the catchment that started in the early 1980's. In the last 20 y, major criteria changes for the management of contaminated particulates (treated urban sludge, agricultural soils, dredged sediments) have occurred. In the mid 1990's, there was a complete shift in the contamination assessment scales, from sediment management and water usage criteria to the good ecological state, now required by the 2000 European Directive. When comparing excess metal outputs, associated to river SPM, to the average metal demand within the catchment from 1950 to 2000, the leakage ratios decrease exponentially from 1950 to 2000 for Cd, Cr, Cu, Pb and Zn, meanwhile, a general increase of the demand is observed: the rate of recycling and/or treatment of metals within the anthroposphere has been improved ten-fold. Hg environmental trajectory is very specific: there is a marked decontamination from 1970 to 2000, but the leakage ratio remains very high (10 to 20%) during this period. Drivers and Pressures are poorly known prior to 1985; State evolution since 1935 has been reconstructed from flood plain cores analysis; Impacts were maximum between 1950 and 1970 but remained unknown due to analytical limitation and lack of awareness. Some Responses are lagging 10 y behind monitoring and have much evolved in the past 10 y.

Causes of River Avulsion: Insights from the Late Holocene Avulsion History of the Mississippi River, U.S.A.

Abstract The emphasis on gradient advantages in studies of avulsion is misleading. While gradient advantages are necessary for an avulsion to occur, the late Holocene avulsion history of the Mississippi River in Louisiana suggests that factors such as substrate composition and floodplain channel distributions are more important. Cross-valley to down-valley slope ratios of the modern floodplain range from 16 to 110 and are typically > 30. The slope ratio is 35 at the location of the Mississippi–Atchafalaya diversion (Old River) yet slope ratios are 83 to 110 immediately upvalley of Old River. All values of Mississippi River floodplain slope ratios are significantly larger than values of avulsion threshold calculated by numerical models. Shallow floodplain cores, 14C dating of organic remains, and geologic mapping show that the Mississippi River has avulsed only four times over the past 5 ky in the southern Lower Mississippi Valley (LMV). Gradient advantages are widespread, yet avulsions are rare. These observations indicate that factors other than gradient advantage control Mississippi River avulsion. Several examples of Mississippi and Red River avulsion by channel reoccupation support the idea that channel distributions and substrate compositions are primary influences on avulsion. Incipient Mississippi River avulsion and development of the Atchafalaya River involved reoccupation of abandoned Mississippi River channels and a Red River crevasse-splay complex. The modern Atchafalaya River also incises buried Mississippi River channel-belt sands. Abandoned channel belts and crevasse-splay complexes consist of sandy substrates that facilitate scour and the development of channels capable of capturing the Mississippi River. Abandoned channels provide ready-made conduits for Mississippi River flow that can efficiently develop into avulsive channels. Multi-storied sheet sandstones in ancient fluvial deposits may provide additional support for the idea that erodible substrates and floodplain channel distributions are critical influences on avulsion. These features record episodic reoccupation of channel belts, which at least in some cases, may simply reflect successive avulsions rather than major changes in aggradation rate or extrabasinal factors such as climate.