Using caesium‐134 and cobalt‐60 as tracers to assess the remobilization of recently‐deposited overbank‐derived sediment on river floodplains during subsequent inundation events

River floodplains act as sinks for fine‐sediment and sediment‐associated contaminants. Increasing recognition of their environmental importance has necessitated a need for an improved understanding of the fate and residence times of overbank sediment deposits over a broad range of timescales. Most existing investigations have focused on medium‐term accretion rates, which represents net deposition from multiple flood events over several decades. In contrast, the fate of recently‐deposited sediment during subsequent overbank events has received only limited attention. This paper presents a novel tracing‐technique for documenting the remobilization of recent overbank sediment on river floodplains during subsequent inundation events, using the artificial radionuclides, caesium‐134 (¹³⁴Cs) and cobalt‐60 (⁶⁰Co). The investigation was conducted within floodplains of the Rivers Taw and Culm in Devon, UK. Small quantities of fine‐sediment (< 63 µm dia.), pre‐labelled with known activities of either ¹³⁴Cs or ⁶⁰Co, were deposited at 15 locations across each floodplain. Surface inventories, measured before and after three consecutive flood events, were used to estimate sediment loss (in g m^–2). Significant reductions provided evidence of the remobilization of the labelled sediment by inundating floodwaters. Spatial variations in remobilization were related to localized topography. Sediment remobilized during the first two events for the River Taw floodplain were equivalent to 63 · 8% and 11 · 9%, respectively, of the original mass. Equivalent values for the River Culm floodplain were 49 · 6% and 12 · 5%, respectively, of the original mass. Sediment loss during the third event proved too small to be attributed to remobilization by overbank floodwaters. After the third event, a mean of 22 · 5% and 35 · 2% of the original mass remained on the Taw and Culm floodplains, respectively. These results provide evidence of the storage of the remaining sediment. The findings highlight the importance of remobilization of recently‐deposited sediment on river floodplains during subsequent overbank events and demonstrate the potential of the tracing‐technique. Copyright © 2013 John Wiley & Sons, Ltd.     

Floodplain sedimentation rates in an alpine watershed determined by radionuclide techniques

Vertical profiles of the activities of ¹³⁷Cs and ²¹⁰Pb were measured on floodplain sediment cores and upland soil cores along the Soda Butte Creek and the Yellowstone River to determine floodplain sedimentation rates. The position of mine tailings from a 1950 impoundment failure was used as a stratigraphic marker to estimate the sedimentation rates and to make comparisons with rates provided by radionuclide‐based methods. Mass accumulation (sedimentation) rates calculated from the position of the mine tailings ranged from 0·00 to 0·17 g cm^?2 yr^?1 and were in good agreement with sedimentation rates calculated from the inventories of ¹³⁷Cs and ²¹⁰Pb. Sedimentation rates calculated from the position of the ¹³⁷Cs peak generally overestimated the sedimentation rates, probably because of increased downward migration of ¹³⁷Cs caused by the low pH of water moving through the mine tailings or the high permeability of floodplain sediments relative to upland reference soils. This study demonstrates that the ¹³⁷Cs and ²¹⁰Pb inventory methods for determining sedimentation rates can be applied to an alpine floodplain where sedimentation events are episodic and where orographic effects on precipitation generate strong downstream gradients in the delivery of atmospheric radionuclides. Copyright © 2007 John Wiley & Sons, Ltd.     

Estimating sedimentation rates and sources in a partially urbanized catchment using caesium-137

While there has been increased interest in determining sedimentation rates and sources in agricultural and forested catchments in recent years, there have been few studies dealing with urbanized catchments. A study of sedimentation rates and sources within channel and floodplain deposits of a partially urbanized catchment has been undertaken using the ¹³⁷Cs technique. Results for sedimentation rates showed no particular downstream pattern. This may be partially explained by underestimation of sedimentation rates at some sites by failure to sample the full ¹³⁷Cs profile, floodplain erosion and deliberate removal of sediment. Evidence of lateral increases in net sedimentation rates with distance from the channel may be explained by increased floodplain erosion at sites closer to the channel and floodplain formation by lateral deposition. Potential sediment sources for the catchment were considered to be forest topsoil, subsurface material and sediments derived from urban areas, which were found to be predominantly subsurface material. Tracing techniques showed an increase in subsurface material for downstream sites, confirming expectations that subsurface material would increase in the downstream direction in response to the direct and indirect effects of urbanization. © 1998 John Wiley & Sons, Ltd.     

Sedimentation in the floodplains of the Mekong Delta,Vietnam Part II: deposition and erosion

Deposition and erosion play a key role in the determination of the sediment budget of a river basin, as well as for floodplain sedimentation. Floodplain sedimentation, in turn, is a relevant factor for the design of flood protection measures, productivity of agro‐ecosystems, and for ecological rehabilitation plans. In the Mekong Delta, erosion and deposition are important factors for geomorphological processes like the compensation of deltaic subsidence as well as for agricultural productivity. Floodplain deposition is also counteracting the increasing climate change induced hazard by sea level rise in the delta. Despite this importance, a sediment database of the Mekong Delta is lacking, and the knowledge about erosion and deposition processes is limited. In the Vietnamese part of the Delta, the annually flooded natural floodplains have been replaced by a dense system of channels, dikes, paddy fields, and aquaculture ponds, resulting in floodplain compartments protected by ring dikes. The agricultural productivity depends on the sediment and associated nutrient input to the floodplains by the annual floods. However, no quantitative information regarding their sediment trapping efficiency has been reported yet. The present study investigates deposition and erosion based on intensive field measurements in three consecutive years (2008, 2009, and 2010). Optical backscatter sensors are used in combination with sediment traps for interpreting deposition and erosion processes in different locations. In our study area, the mean calculated deposition rate is 6.86 kg/m² (≈ 6 mm/year). The key parameters for calculating erosion and deposition are estimated, i.e. the critical bed shear stress for deposition and erosion and the surface constant erosion rate. The bulk of the floodplain sediment deposition is found to occur during the initial stage of floodplain inundation. This finding has direct implications on the operation of sluice gates in order to optimize sediment input and distribution in the floodplains. Copyright © 2013 John Wiley & Sons, Ltd.     

The application of caesium‐137 measurements to investigate floodplain deposition in a large semi‐arid catchment in Queensland,Australia: a low‐fallout environment

Floodplains comprise geomorphologically important sources and sinks for sediments and associated pollutants, yet the sedimentology of large dryland floodplains is not well understood. Processes occurring on such floodplains are often difficult to observe, and techniques used to investigate smaller perennial floodplains are often not practical in these environments. This study assesses the utility of ¹³⁷Cs inventory and depth‐profile techniques for determining relative amounts of floodplain sedimentation in the Fitzroy River, northeastern Australia; a 143 000 km² semi‐arid river system. Caesium‐137 inventories were calculated for floodplain and reference location bulk soil cores collected from four sites. Depth profiles of ¹³⁷Cs concentration from each floodplain site and a reference location were recorded. The areal density of ¹³⁷Cs at reference locations ranged from 13 to 978 Bq m^–2 (0–1367 Bq m^–2 at the 95% confidence interval), and the mean value ± 2 (standard error of the mean) was 436 ± 264 Bq m^–2, similar to published data from other Southern Hemisphere locations. Floodplain inventories ranged from 68 to 1142 Bq m^–2 (0–1692 Bq m^–2 at the 95% confidence interval), essentially falling within the range of reference inventory values, thus preventing calculation of erosion or deposition. Depth‐profiles of ¹³⁷Cs concentration indicate erosion at one site and over 66 cm of deposition at another since 1954. Analysis of ^239+240Pu concentrations in a depositional core substantiated the interpretation made from ¹³⁷Cs data, and depict a more tightly constrained peak in concentration. Average annual deposition rates range from 0 to 15 mm. The similarity between floodplain and reference bulk inventories does not necessarily indicate a lack of erosion or deposition, due to low ¹³⁷Cs fallout in the region and associated high measurement uncertainties, and a likely influence of gully and bank eroded sediments with no or limited adsorbed ¹³⁷Cs. In this low‐fallout environment, detailed depth‐profile data are necessary for investigating sedimentation using ¹³⁷Cs. Copyright © 2009 John Wiley & Sons, Ltd.     

MODELLING FLOOD HYDRAULICS AND OVERBANK DEPOSITION ON RIVER FLOODPLAINS

This paper outlines a numerical model for the prediction of floodplain inundation sequences, overbank deposition rates and deposit grain size distributions. The model has two main components: first, a simplified hydraulic scheme which predicts floodwater flow depths and velocities, and second, a sediment transport element which employs a mass balance relation describing suspended sediment dispersion by convective and diffusive processes and sediment deposition as a function of particle settling rates. These relationships are solved numerically on a finite difference grid that accurately replicates the complex topographic features typical of natural river floodplains. The model is applied to a 600 m reach of the River Culm, Devon, U.K. using data derived from a range of field and laboratory techniques. Continuous records of river stage and suspended sediment concentration provide the model's upstream boundary input requirements. These are supplemented by measurements of the in situ settling characteristics of the suspended sediment load. The model's sediment transport component is calibrated with the aid of a dataset of measured overbank deposition amounts derived from flood events over a 16 month period. The model is shown to predict complicated floodwater inundation sequences and patterns of suspended sediment dispersion and deposition, which are largely a product of the complex topography of the floodplain. These results compare favourably with observations of overbank processes and are an improvement over those of previous models which have employed relatively simple representations of floodplain geometry. © 1997 by John Wiley & Sons, Ltd.     

A preliminary investigation of the integration of modelled floodplain hydraulics with estimates of overbank floodplain sedimentation derived from Pb-210 and Cs-137 measurements

This paper investigates the possibility of linking finite element modelled two-dimensional hydraulics with floodplain sedimentation rates derived from radionuclide dating techniques. The TELEMAC-2D system is used to simulate the full dynamic velocity vector and water depth distributions for a 1 in 12 year flood event for the River Culm, Devon. These are compared to patterns of medium-term (30– 100 years) sedimentation rates and a degree of correlation is found to exist. This information is used to comment on the complexity of flow and sediment deposition interactions in floodplain environments and to provide a research design for the development of a more fully integrated floodplain morphodynamic model. Copyright © 1999 John Wiley & Sons, Ltd.     

Reconstruction of floodplain sedimentation rates: a combination of methods to optimize estimates

Reconstruction of overbank sedimentation rates over the past decades gives insight into floodplain dynamics, and thereby provides a basis for efficient and sustainable floodplain management. We compared the results of four independent reconstruction methods – optically stimulated luminescence (OSL) dating, caesium‐137 (¹³⁷Cs) dating, heavy metal analysis, and flood bed interpretation – applied at three embanked floodplain sites along lower Rhine River distributaries in the Netherlands. All methods indicate significant sedimentation rates on the floodplains, varying between 2–7 mm/a in the distal zones and 3–9 mm/a in the proximal zones. On a rapidly developing sand bar along a natural levee sedimentation rates of 9 to 25 mm/a were found. Except for some minor inconsistencies in ¹³⁷Cs dating results, all methods show decreasing sedimentation rates with increasing distance from the river channel. Intercomparison of the results of the different dating methods revealed the potential errors associated with each method, particularly where disagreement among the results were found. Uncertainties may arise due to (1) grain‐size dependent downward migration of ¹³⁷Cs, (2) smoothing of the vertical heavy metal and ¹³⁷Cs profiles, (3) delayed sediment‐associated input of ¹³⁷Cs in addition to direct atmospheric fall‐out, (4) overestimation of the burial age in OSL dating due to incomplete resetting of the OSL signal, or (5) non‐linear relationships between sediment deposition and flood magnitude in the count‐from‐the‐top correlation between sediment lamination and past observed flood records. Still, taking the uncertainties associated with each method into account, the results are generally in good agreement. Using the results we indicate the optimal spatial range of application of each method, depending on sediment texture and sedimentation rate. The optimal spatial and temporal ranges differ for each method, but show significant overlap. A combination of the methods will thus provide maximum information for accurate estimation of sedimentation rates on a decadal time scale. Copyright © 2010 John Wiley & Sons, Ltd.     

Determining soil erosion rates on semi‐arid watersheds using radioisotope‐derived sedimentation chronology

This study investigates erosion dynamics of the past 90 years in three small semi‐arid watersheds with histories of grazing and vegetation change. Activity of ¹³⁷Cs and excess ²¹⁰Pb from 18 cores collected from sedimentation ponds were measured using a gamma spectrometer. The sediment was dated using a constant rate of supply (CRS) model. This study represents the first time that reservoir sediment accumulation rates determined from fallout isotopes have been verified by direct volumetric measurements of aggradation based on topographic surveys. Measured sedimentation in the ponds ranged between 1.9 and 2.3 cm y^?1, representing average sediment delivery rates from the watersheds of between 0.6 and 2.0 t ha^?1 y^?1. These sediment delivery rates were in agreement with those established by other methods for similar catchments in the region. Past variations in sedimentation rates were identified and correlated with recorded history of anthropogenic disturbance. ¹³⁷Cs and ²¹⁰Pb methods are suitable for use in arid environments and can complement each other to increase reliability of erosion rate estimates. The abundance of stock ponds in southwestern USA presents an opportunity to quantify historic erosion and sediment transfer dynamics in areas that have not been well studied or instrumented. Published 2016. This article is a U.S. Government work and is in the public domain in the USA     

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.     

Sedimentation in arctic proglacial lakes: Mittivakkat Glacier,south‐east Greenland

Several sediment cores were collected from two proglacial lakes in the vicinity of Mittivakkat Glacier, south‐east Greenland, in order to determine sedimentation rates, estimate sediment yields and identify the dominant sources of the lacustrine sediment. The presence of varves in the ice‐dammed Icefall Lake enabled sedimentation rates to be estimated using a combination of X‐ray photography and down‐core variations in ¹³⁷Cs activity. Sedimentation rates for individual cores ranged between 0·52 and 1·06 g cm⁻² year⁻¹, and the average sedimentation rate was estimated to be 0·79 g cm⁻² year⁻¹. Despite considerable down‐core variability in annual sedimentation rates, there is no significant trend over the period 1970 to 1994. After correcting for autochthonous organic matter content and trap efficiency, the mean fine‐grained minerogenic sediment yield from the 3·8 km² basin contributing to the lake was estimated to be 327 t km⁻² year⁻¹. Cores were also collected from the topset beds of two small deltas in Icefall Lake. The deposition of coarse‐grained sediment on the delta surface was estimated to total in excess of 15 cm over the last c. 40 years. In the larger Lake Kuutuaq, which is located about 5 km from the glacier front and for which the glacier represents a smaller proportion of the contributing catchment, sedimentation rates determined for six cores collected from the centre of the lake, based on their ¹³⁷Cs depth profiles, were estimated to range between 0·05 and 0·11 g cm⁻² year⁻¹, and the average was 0·08 g cm⁻² year⁻¹. The longer‐term (c. 100–150 years) average sedimentation rate for one of the cores, estimated from its unsupported ²¹⁰Pb profile, was 0·10–0·13 g cm⁻² year⁻¹, suggesting that sedimentation rates in this lake have been essentially constant over the last c. 100–150 years. The average fine‐grained sediment yield from the 32·4 km² catchment contributing to the lake was estimated to be 13 t km⁻² year⁻¹. The ¹³⁷Cs depth profiles for cores collected from the topset beds of the delta of Lake Kuutuaq indicate that in excess of 27 cm of coarse‐grained sediment had accumulated on the delta surface over the last approximately 40 years. Caesium‐137 concentrations associated with the most recently deposited (uppermost) fine‐grained sediment in both Icefall Lake and Lake Kuutuaq were similar to those measured in fine‐grained sediment collected from steep slopes in the immediate proglacial zone, suggesting that this material, rather than contemporary glacial debris, is the most likely source of the sediment deposited in the lakes. This finding is confirmed by the ¹³⁷Cs concentrations associated with suspended sediment collected from the Mittivakkat stream, which are very similar to those for proglacial material. Copyright © 2000 John Wiley & Sons, Ltd.     

Dating of sediments from Lake Zurich (Switzerland) with210Pb and137Cs

²¹⁰Pb- and¹³⁷Cs-measurements, and varve counting have been used to date sediment cores from Lake Zurich (Switzerland). Two cores from different water depths were dated with²¹⁰Pb/²¹⁰Po and revealed sediment accumulation rates of (0.055±0.015) g·cm^?2·y^?1 and (0.09±0.03) g·cm^?2·y^?1, respectively. A comparable rate of (0.07±0.01) g·cm^?2·y^?1 has been obtained from¹³⁷Cs measurements. These rates were confirmed by annual layer (varve)-counts which lead to rates of 0.07 g·cm^?2·y^?1. Constant²¹⁰Pb activities were observed in the top 6 cm of the sediment cores. This constancy is generally explained in the literature by mixing processes caused by bioturbation and by distortion during coring operations of the uppermost water-rich fluffy sediments. However the distinct¹³⁷Cs-maxima and the regular and undisturbed varve lamination of the top sediment observed in the cores of Lake Zurich contradict this assumption. In addition, measurements of⁷Be at the water/sediment interaces proved complete sediment core recovery and mechanically undisturbed sediments. Remobilization processes are assumed to cause the observed constant²¹⁰Pb activities. Remobilization may also be the reason for an incomplete²¹⁰Pb inventory in the sediments which contain only about 50% of the fallout from atmosphere. The results of the²¹⁰Pb dating should therefore be considered with some care. If existent, varve counting represents the easiest and most reliable means for dating lake sediments.     

Problems of core correlation,sediment source ascription and yield estimation in Ponsonby Tarn,West Cumbria,UK

A suite of 27 short cores, 10 of which have been used for magnetic measurements and four for radiometric dating, provides a framework for reconstructing the processes, patterns and rates of sedimentation in Ponsonby Tarn, a small artificial impoundment created towards the end of the 19th century, close to the Sellafield nuclear reprocessing plant in NW England. Spatial and temporal changes in sedimentation are reconstructed and evidence presented for non-synchroneity in magnetic property changes from core to core in the upper part of the sequence, as a result of sorting and selective deposition at different distances from the inflow to the Tarn. Magnetic measurements alone are therefore not a secure basis upon which to quantify sediment yield for defined time intervals at this site. The chronology, established mainly from ²¹⁰Pb and ¹³⁴Cs analyses, allows estimates of mean sediment yield per annum for four periods: prior to AD 1940, 1940–1964, 1964–1986 and 1986–1991. The rates of sediment accumulation have increased in recent times, especially since 1964, with evidence for input from both magnetically enhanced soils and gleyed alluvial and/or podsolized subsoil sources. Pre-1940 mean annual deposition within the present area of the lake is calculated as 19·5 t a⁻¹ and for the period since 1986 (the period of maximum sedimentation rates), as 111·3 t a⁻¹. These represent yields of 7·0 t km⁻² a⁻¹ and 39·8 t km⁻² a⁻¹, respectively, for the catchment as a whole. Rock magnetic evidence, based on measurements of both bulk samples and the finest particle size separates, suggests that bacterial magnetite, formed within the lake, contributes to the magnetic properties of the sediments, thus modifying the signatures relating to allochthonous sediment input. Copyright © 1999 John Wiley & Sons, Ltd.     

Revisiting lake sediment budgets: How the calculation of lake lifetime is strongly data and method dependent

Lake sedimentation has a fundamental impact on lake lifetime. In this paper, we show how sensitive calculation of the latter is to the quality of data available and assumptions made during analysis. Based on the collection of a large new dataset, we quantify the sediment masses (1) mobilized on the hillslopes draining towards Lake Tana (Ethiopia), (2) stored in the floodplains, (3) transported into the lake, (4) deposited in the lake and (5) delivered out from the lake so as to establish a sediment budget. In 2012–2013, suspended sediment concentration (SSC) and discharge measurements were made at 13 monitoring stations, including two lake outlets. Altogether, 4635 SSC samples were collected and sediment rating curves that account for land cover conditions and rainfall seasonality were established for the 11 river stations, and mean monthly SSC was calculated for the outlets. Effects of the floodplain on rivers' sediment yield (SY) were investigated using measurements at both sides of the floodplains. SY from ungauged rivers was assessed using a model that includes catchment area and rainfall, whereas bedload and direct sediment input from lake shores were estimated. As a result, the gross annual SY was c. 39.55 (± 0.15) Mt, dominantly from Gilgel Abay and Gumara Rivers. The 2.57 (± 0.17) Mt sediment deposited in floodplains indicate that the floodplains serve as an important sediment sink. Moreover, annually c. 1.09 Mt of sediment leaves the lake through the two outlets. Annual sediment deposition in the lake was c. 36.97 (± 0.22) Mt and organic matter accumulation was 2.15 Mt, with a mean sediment trapping efficiency of 97%. Furthermore, SSC and SY are generally higher at the beginning of the rainy season because soils in cultivated fields are bare and loose due to frequent ploughing and seedbed preparation. Later in the season, increased crop and vegetation cover lead to a decrease in sediment production. Based on the established sediment budget with average rainfall, the lifetime of Lake Tana was estimated as 764 to 1032 years, which is shorter than what was anticipated in earlier studies. The sedimentation rate of Lake Tana (11.7 ± 0.1 kg m^?2 yr^?1) is in line with the sedimentation rates of larger lakes in the world, like Lake Dongting and Lake Kivu. Copyright © 2017 John Wiley & Sons, Ltd.     

Floodplain sedimentation: Quantities,patterns and processes

This paper presents the result of measurements of floodplain sedimentation using sediment traps. The study was carried out on two embanked floodplains along the Rivers Rhine and Meuse in The Netherlands during a 3 day flood in January 1993. Raster maps of sediment accumulation were made by interpolating the measurements from the traps using block kriging. The sediment maps show clear patterns in sediment accumulation, together with the estimated interpolation errors. Average sediment accumulation ranges between 0·57 and 1·0 kg m^?2. High sediment accumulation is found on the levees (4 kg m^?2 or more) and on low lying areas (1·6 kg m^?2); sediment accumulation decreases with distance from the main channel. The sedimentation patterns are related to floodplain topography and sediment transporting mechanisms. Sediment transport by turbulent diffusion as well as by convection can be recognized. Also, flood duration and the process of sediment settling out in ponding water in closed depressions are important. The applied method allows comparison of the results with raster-based sedimentation models.     

Spatial variability of floodplain sedimentation at the event scale in the Rhine–Meuse delta,The Netherlands

This article addresses spatial variability of comtemporary floodplain sedimentation at the event scale. Measurements of overbank deposition were carried out using sediment traps on 11 floodplain sections along the rivers Waal and Meuse in The Netherlands during the high-magnitude flood of December 1993. During the flood, sand sheets were locally deposited behind a natural levee. At distances greater than 50 to 100 m from the river channel the deposits consisted mainly of silt- and clay-sized material. Observed patterns of deposition were related to floodplain topography and sediment transporting mechanisms. Though at several sites patterns were observed that suggest transport by turbulent diffusion, convection seems the dominant transporting mechanism, in particular in sections that are bordered by minor embankments. The average deposition of overbank fines ranged between 1·2 and 4·0 kg m⁻² along the river Waal, and between 1·0 and 2·0 kg m⁻² along the river Meuse. The estimated total accumulation of overbank fines (not including sand sheets) on the entire river Waal floodplain was 0·24 Mton, which is 19 per cent of the total suspended sediment load transported through the river Waal during the flood. © 1998 John Wiley & Sons, Ltd.     

Floodplain downed wood volumes: a comparison across three biomes

Downed large wood (LW) in floodplains provides habitat and nutrients for diverse organisms, influences hydraulics and sedimentation during overbank flows, and affects channel form and lateral migration. Very few studies, however, have quantified LW volumes in floodplains that are unaltered by human disturbance. We compare LW volumes in relatively unaltered floodplains of semiarid boreal lowland, subtropical lowland, and semiarid temperate mountain rivers in the United States. Average volumes of downed LW are 42.3 m³ ha^?1, 50.4 m³ ha^?1, and 116.3 m³ ha^?1 in the semiarid boreal, subtropical, and semiarid temperate sites, respectively. Observed patterns support the hypothesis that the largest downed LW volumes occur in the semiarid temperate mountain sites, which is likely linked to a combination of moderate‐to‐high net primary productivity, temperature‐limited decomposition rates, and resulting slow wood turnover time. Floodplain LW volumes differ among vegetation types within the semiarid boreal and semiarid temperate mountain regions, reflecting differences in species composition. Lateral channel migration and flooding influence vegetation communities in the semiarid boreal sites, which in turn influences floodplain LW loads. Other forms of disturbance such as fires, insect infestations, and blowdowns can increase LW volumes in the semiarid boreal and semiarid temperate mountain sites, where rates of wood decay are relatively slow compared with the subtropical lowland sites. Although sediment is the largest floodplain carbon reservoir, floodplain LW stores substantial amounts of organic carbon and can influence floodplain sediment storage. In our study sites, floodplain LW volumes are lower than those in adjacent channels, but are higher than those in upland (i.e. non‐floodplain) forests. Given the important ecological and physical effects of floodplain LW, efforts to add LW to river corridors as part of restoration activities, and the need to quantify carbon stocks within river corridors, we urge others to quantify floodplain and instream LW volumes in diverse environments. Copyright © 2016 John Wiley & Sons, Ltd.     

Elevated sedimentation in lake records linked to agricultural activities in the Ishikari River floodplain,northern Japan

The purpose of this study was to examine the historical change in sedimentation rates in lakes that have been impacted by river regulation and agricultural activities in the Ishikari River floodplain. We dated sediment cores using caesium‐137 (¹³⁷Cs) dating and tephrochronology, and we estimated sediment sources from ¹³⁷Cs concentrations in the topsoil of representative land covers. We found that, between 1739 and 1963, the distance between the lake and the main river channel and whether or not the lake was connected to the river affected the sedimentation rates. After 1963, agricultural drainage systems were established in the Ishikari River floodplain. The average sedimentation rate before and after the construction of drainage ditches varied between 1–66 and 87–301 mg cm^–2 a^–1, respectively. The increase in the sedimentation rate after 1963 was caused by the construction of a number of drainage networks, as well as extensive cultivation activity and/or fragmentation of the swamp buffers surrounding the lakes. The ¹³⁷Cs activities at the surfaces of the lake as well as the catchment‐derived ¹³⁷Cs contributions and ¹³⁷Cs inventory in the lake profiles were used to examine the sediment influx from the various drainage areas after the establishment of the drainage system. Our results indicate that the majority of the lake sediments were derived from cultivated areas, and therefore the catchment‐derived ¹³⁷Cs contribution in the lakes was strongly correlated with the sedimentation rate. The ¹³⁷Cs inventory across all of the lake profiles was also significantly greater than the atmospheric fallout. We identified a negative correlation between the ¹³⁷Cs lake profile inventory and the sedimentation rate. This is because the sediment originating from the drainage areas contained low ¹³⁷Cs concentrations, which diluted the overall concentration of ¹³⁷Cs in the lake sediment. Copyright © 2009 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.     

Quantitative model of the growth of floodplains by vertical accretion

A simple one‐dimensional model is developed to quantitatively predict the change in elevation, over a period of decades, for vertically accreting floodplains. This unsteady model approximates the monotonic growth of a floodplain as an incremental but constant increase of net sediment deposition per flood for those floods of a partial duration series that exceed a threshold discharge corresponding to the elevation of the floodplain. Sediment deposition from each flood increases the elevation of the floodplain and consequently the magnitude of the threshold discharge resulting in a decrease in the number of floods and growth rate of the floodplain. Floodplain growth curves predicted by this model are compared to empirical growth curves based on dendrochronology and to direct field measurements at five floodplain sites. The model was used to predict the value of net sediment deposition per flood which best fits (in a least squares sense) the empirical and field measurements; these values fall within the range of independent estimates of the net sediment deposition per flood based on empirical equations. These empirical equations permit the application of the model to estimate of floodplain growth for other floodplains throughout the world which do not have detailed data of sediment deposition during individual floods. Copyright © 2000 John Wiley & Sons, Ltd.