Floodplain Surface Research Articles

Dust activity and surface sediment characteristics of the dustiest river in southern Africa: the Kuiseb River, Central Namib

Previous remote sensing studies (2005–2008) have identified Namibia’s Kuiseb as the dustiest river in southern Africa. The purpose of this study was to extend the dust event record through to 2014, and to examine the nature of the surfaces from which this satellite imagery indicates these dust plumes originate. The new 10-year record confirms the delta as the dustiest geomorphological unit (54% of plumes), followed by the gravel plain (28%) and then the river (8%). No dust originated from the Namib Sand Sea dunes or interdunes. Field observations provided detail about the geomorphological and sedimentological setting of the landscape components. The laboratory analysis focused on the size characteristics of 153 surface sediment samples collected from the Kuiseb main channel, its terraces, delta, gravel plain surfaces and tributaries, dunes and interdune. This study has identified that surface sediments suitable for dust production increase towards the coast with particular ‘dusty’ floodplain surfaces between Swartbank and Rooibank and the Kuiseb delta. We suggest that silt crusts formed as the flood water dissipate, provide a main source of potentially entrainable material for emission. The crusts consist entirely of silt- and clay-sized materials, with a maximum of 97% <63 μm, 39% <10 μm and 6% <2 μm. Anthropogenic disturbances of the surfaces are potentially playing a role in the production of dust, with the area undergoing significant development.

Dating the Holocene Incision of the Danube in Southern Hungary

AbstractThe alluvial development of the Great Hungarian Plain has greatly been determined by the subsidence of different areas in the Pannonian Basin. The temporal variation of subsidence rates significantly contributed to the avulsion and shifting of main rivers. This was the case in terms of the Hungarian Lower Danube when occupying its present day N-S directional course. The considerable role of tectonic forcing is also supported by the presence of different floodplain levels. Although, several channel forms are identifiable on these the timing of floodplain development has been reconstructed up till now mostly by the means of geomorphological analysis, and hardly any numerical dates were available. The main aim of this study is to provide the first OSL dates for palaeo-channels located on the high floodplain surface of the Hungarian Lower Danube, and to determine the maximum age of low and high floodplain separation on the Kalocsa Plain. For the analysis two meanders were sampled close to the edge of the step slope between the two levels. According to the results, the development of the investigated palaeo-meanders could be rapid. The formation of the older meander was dated to the Late Atlantic, while the possible separation of the high and low floodplain surfaces could start in the beginning of the Subboreal Phase.

Long‐term cottonwood establishment along the Green River, Utah, USA

AbstractCottonwood (Populus spp.) riparian forest and woodlands provide valuable ecological goods and services, especially in arid and semiarid regions of North America. Successful establishment and survival for cottonwoods requires bare, moist alluvium that is relatively safe from fluvial disturbance. These are restrictive requirements in dry regions. Cottonwoods can survive for three to four centuries and thus may provide a long‐term record of specific fluvial and geomorphic events. We used C14 analysis and ring counts of cottonwood stems to document a long‐term record of cottonwood establishment. We related this record to the century‐long gage record at Green River, Utah along with historical photos and paleoflood records. Radiometric carbon dating and ring counts suggest some cottonwood stems established on high alluvial deposits following large floods as early as the late 1600s through the late 1800s. Cottonwoods also established on inset floodplain surfaces following sustained decreases in peak flows resulting from climatic drought and the closing of Flaming Gorge Dam. Finally, establishment occurred on both higher post‐dam flood deposits as well as low, active channel surfaces following floods and during a series of low flow years. With continued peak‐flow reduction, cottonwood recruitment will likely be restricted to spatially limited, lower elevation surfaces and subjected to competition from an array of herbaceous and woody riparian species.

Phosphorus in the catchment of high sediment load river: A case of the Yellow River, China

Surface sediment samples concurrently collected in the catchment of the Ningxia-Inner Mongolian reach of Yellow River (NIMYR) were analyzed to determine the phosphorus and correlated physiochemical properties of sediments. Samples were obtained from three main areas: the riverbed surface sediment (RSS), the floodplain surface sediment (FSS) and the desert surface sediment (DSS). The sequence of phosphorus-contamination level in this catchment, determined by a cumulative distribution function and multivariate statistical analysis, were FSS>RSS>DSS. Moreover, because of the impacts of anthropogenic activities, the sampling site with the highest and lowest phosphorus concentration (mgkg−1) of this catchment appeared in RSS (749.40) and DSS (200.10) respectively. In addition, this is the first study to present a qualification of the effect of the sediment's physicochemical properties on phosphorus by the multivariate regression tree analysis. Co-precipitation of phosphate with calcite [i], phosphate absorbed onto ferric hydroxides [ii] and grain-size effect [iii] were the three main mechanisms for phosphorus distribution in the sediment of NIMYR. The contributions of these three mechanisms to RSS and FSS were, respectively, [i]>[iii]>[ii] and [i]>[ii]>[iii]. The heaviest phosphorus-contaminated group of RSS was primarily controlled by [i], whereas the heaviest phosphorus-contaminated group of FSS was controlled by [i] and [ii], indicating that the FSS had a higher potential risk of releasing phosphorus from the sediment to overlying water.

Vertical surface water–groundwater exchange processes within a headwater floodplain induced by experimental floods

AbstractRestoring hydrologic connectivity between channels and floodplains is common practice in stream and river restoration. Floodplain hydrology and hydrogeology impact stream hydraulics, ecology, biogeochemical processing, and pollutant removal, yet rigorous field evaluations of surface water–groundwater exchange within floodplains during overbank floods are rare. We conducted five sets of experimental floods to mimic floodplain reconnection by pumping stream water onto an existing floodplain swale. Floods were conducted throughout the year to capture seasonal variation and each involved two replicate floods on successive days to test the effect of varying antecedent moisture. Water levels and specific conductance were measured in surface water, soil, and groundwater within the floodplain, along with surface flow into and out of the floodplain. Vegetation density varied seasonally and controlled the volume of surface water storage on the floodplain. By contrast, antecedent moisture conditions controlled storage of water in floodplain soils, with drier antecedent moisture conditions leading to increased subsurface storage and slower flood wave propagation across the floodplain surface. The site experienced spatial heterogeneity in vertical connectivity between surface water and groundwater across the floodplain surface, where propagation of hydrostatic pressure, preferential flow, and bulk Darcy flow were all mechanisms that may have occurred during the five floods. Vertical connectivity also increased with time, suggesting higher frequency of floodplain inundation may increase surface water–groundwater exchange across the floodplain surface. Understanding the variability of floodplain impacts on water quality noted in the literature likely requires better accounting for seasonal variations in floodplain vegetation and antecedent moisture as well as heterogeneous exchange flow mechanisms. Copyright © 2016 John Wiley &amp; Sons, Ltd.

An index of floodplain surface complexity

Abstract. Floodplain surface topography is an important component of floodplain ecosystems. It is the primary physical template upon which ecosystem processes are acted out, and complexity in this template can contribute to the high biodiversity and productivity of floodplain ecosystems. There has been a limited appreciation of floodplain surface complexity because of the traditional focus on temporal variability in floodplains as well as limitations to quantifying spatial complexity. An index of floodplain surface complexity (FSC) is developed in this paper and applied to eight floodplains from different geographic settings. The index is based on two key indicators of complexity, variability in surface geometry (VSG) and the spatial organisation of surface conditions (SPO), and was determined at three sampling scales. FSC, VSG, and SPO varied between the eight floodplains and these differences depended upon sampling scale. Relationships between these measures of spatial complexity and seven geomorphological and hydrological drivers were investigated. There was a significant decline in all complexity measures with increasing floodplain width, which was explained by either a power, logarithmic, or exponential function. There was an initial rapid decline in surface complexity as floodplain width increased from 1.5 to 5 km, followed by little change in floodplains wider than 10 km. VSG also increased significantly with increasing sediment yield. No significant relationships were determined between any of the four hydrological variables and floodplain surface complexity.

Making room for rivers: quantification of benefits from a flood risk perspective

Since 1996, the Netherlands has adopted a flood risk management policy based on making more room for the rivers. Currently, the focus in flood risk management is being adapted again, in view of increasing societal vulnerability and foreseeable effects of climate change. In this context, the choice between making more room for the river and/or strengthening embankments is again of major concern. This calls for further quantification of the effectiveness, costs and benefits of all possible measures. Making room for rivers was originally advocated by referring to a reduction of the probability of failure of the embankments and a reduction of the consequences in case of breaching. These arguments still apply, but the effect of making more room for rivers on flood risk has never been properly quantified yet. In this paper we identify three potential risk reducing effects, and quantify their effect for along the Rhine and Meuse Rivers in the Netherlands. We show that lowering the flood levels means smaller flood probabilities, that larger floodplain surface area significantly influences the relationship between discharge and flood level, and that lower flood levels in the river translate into smaller flooding depths and/or flood extent, and thus reduce the consequences of flooding.

Banking carbon: a review of organic carbon storage and physical factors influencing retention in floodplains and riparian ecosystems

AbstractRivers are dynamic components of the terrestrial carbon cycle and provide important functions in ecosystem processes. Although rivers act as conveyers of carbon to the oceans, rivers also retain carbon within riparian ecosystems along floodplains, with potential for long‐term (&gt; 102 years) storage. Research in ecosystem processing emphasizes the importance of organic carbon (OC) in river systems, and estimates of OC fluxes in terrestrial freshwater systems indicate that a significant portion of terrestrial carbon is stored within river networks. Studies have examined soil OC on floodplains, but research that examines the potential mechanistic controls on OC storage in riparian ecosystems and floodplains is more limited. We emphasize three primary OC reservoirs within fluvial systems: (1) standing riparian biomass; (2) dead biomass as large wood (LW) in the stream and on the floodplain; (3) OC on and beneath the floodplain surface, including litter, humus, and soil organic carbon (SOC). This review focuses on studies that have framed research questions and results in the context of OC retention, accumulation and storage within the three primary pools along riparian ecosystems. In this paper, we (i) discuss the various reservoirs for OC storage in riparian ecosystems, (ii) discuss physical conditions that facilitate carbon retention and storage in riparian ecosystems, (iii) provide a synthesis of published OC storage in riparian ecosystems, (iv) present a conceptual model of the conditions that favor OC storage in riparian ecosystems, (v) briefly discuss human impacts on OC storage in riparian ecosystems, and (vi) highlight current knowledge gaps. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Sedimentology and vertebrate taphonomy of the Moradi Formation of northern Niger: A Permian wet desert in the tropics of Pangaea

Pangaean paleogeographic models place the Tim Mersoi basin of northern Niger in a 5000-km-wide corridor between Gondwana and Laurasia approximately 15 degrees south of the paleoequator. Late Permian paleoclimate models position this basin between tropical summer-wet to the north and desert to the south. Recent investigations of the fossil vertebrates and paleosols in the late Permian (Lopingian) Moradi Formation confirm that the climate was warm and hyperarid with highly seasonal monsoonal rainfall. Possibly as a result of these unusual “wet desert” conditions, the tetrapod fauna shows a high degree of endemism. This study tests existing paleoclimate models by providing additional data on sedimentary environments and vertebrate taphonomic processes. The Moradi red bed sequences accumulated in a gently subsiding sag basin to the west of the tectonically active Massif de l'Aïr. Low angle gravelly alluvial fans prograded westward from the massif and at times impinged on a large stable northward flowing meandering channel system. The interchannel mudrock sequences are over-thickened by the accumulation of loessic silts and preserve isolated skull and post crania of amphibians (Nigerpeton and Saharastega) as well as semi-articulated captorhinids (Moradisaurus). Detailed surface mapping of a fossil-rich exposure revealed an anastomosed network of loess-filled distributary channels incised into the floodplain mudrocks. This provided a locus for the accumulation and rapid burial of at least 15 associated skeletons of the pareiasaurian Bunostegos. Semi-permanent ponds are evidenced by patches of fissile red mudstone containing rare bivalves and spiral coprolites. In the distal floodplains away from the main river channels, the combination of a generally high groundwater table, warm mean annual temperatures, and deflation of fines from the floodplain surface promoted the formation of gypsiferous paleosols and end-point playa lakes. Carbonate-rich mud accumulated around the lake margins and provided ideal conditions for the imprinting and preservation of tetrapod trackways.

Selective deposition response to aeolian–fluvial sediment supply in the desert braided channel of the upper Yellow River, China

Abstract. Rivers flow across aeolian dunes and develop braided stream channels. Both aeolian and fluvial sediment supplies regulate sediment transport and deposition in such cross-dune braided rivers. Here we show a significant selective deposition in response to both aeolian and fluvial sediment supplies in the Ulan Buh desert braided channel. The Ulan Buh desert is the main coarse sediment source for this desert braided channel, and the mean percentage of the coarser (&gt; 0.08 mm) grains on the aeolian dunes surface is 95.34 %. The lateral selective deposition process is developed by the interaction between the flows and the aeolian–fluvial sediment supplies, causing the coarser sediments (&gt; 0.08 mm) from aeolian sand supply and bank erosion to accumulate in the channel centre and the finer fluvial sediments (&lt; 0.08 mm) to be deposited on the bar and floodplain surfaces, forming a coarser-grained thalweg bed bounded by finer-grained floodplain surfaces. This lateral selective deposition reduces the downstream sediment transport and is a primary reason for the formation of an "above-ground" river in the braided reach of the upper Yellow River in response to aeolian and fluvial sediment supplies.

HOLOCENE FAULTS ACTIVITY IN THE SOUTH-EASTERN PART OF WESTERN SIBERIA AND ITS INFLUENCE ON RELIEF AND STRUCTURE OF ALLUVIAL DEPOSITS

The structure of flood plain alluvial deposits in one of the river valleys in the south-eastern part of Western Siberia represents numerous forms of wave-type and flexural deformations and liquefaction (intralayer folding). The river valley corresponds to fault identified on remote images and extended 150 km in north-eastern direction. These deformations indicate the middle or late Holocene earthquake, which causes local displacements of flood-plain surface.

Floodplain complexity and surface metrics: Influences of scale and geomorphology

Many studies of fluvial geomorphology and landscape ecology examine a single river or landscape, thus lack generality, making it difficult to develop a general understanding of the linkages between landscape patterns and larger-scale driving variables. We examined the spatial complexity of eight floodplain surfaces in widely different geographic settings and determined how patterns measured at different scales relate to different environmental drivers. Floodplain surface complexity is defined as having highly variable surface conditions that are also highly organised in space. These two components of floodplain surface complexity were measured across multiple sampling scales from LiDAR-derived DEMs. The surface character and variability of each floodplain were measured using four surface metrics; namely, standard deviation, skewness, coefficient of variation, and standard deviation of curvature from a series of moving window analyses ranging from 50 to 1000m in radius. The spatial organisation of each floodplain surface was measured using spatial correlograms of the four surface metrics. Surface character, variability, and spatial organisation differed among the eight floodplains; and random, fragmented, highly patchy, and simple gradient spatial patterns were exhibited, depending upon the metric and window size. Differences in surface character and variability among the floodplains became statistically stronger with increasing sampling scale (window size), as did their associations with environmental variables. Sediment yield was consistently associated with differences in surface character and variability, as were flow discharge and variability at smaller sampling scales. Floodplain width was associated with differences in the spatial organisation of surface conditions at smaller sampling scales, while valley slope was weakly associated with differences in spatial organisation at larger scales. A comparison of floodplain landscape patterns measured at different scales would improve our understanding of the role that different environmental variables play at different scales and in different geomorphic settings.

Measuring floodplain spatial patterns using continuous surface metrics at multiple scales

Interactions between fluvial processes and floodplain ecosystems occur upon a floodplain surface that is often physically complex. Spatial patterns in floodplain topography have only recently been quantified over multiple scales, and discrepancies exist in how floodplain surfaces are perceived to be spatially organised. We measured spatial patterns in floodplain topography for pool 9 of the Upper Mississippi River, USA, using moving window analyses of eight surface metrics applied to a 1×1m2 DEM over multiple scales. The metrics used were Range, SD, Skewness, Kurtosis, CV, SDCURV, Rugosity, and Vol:Area, and window sizes ranged from 10 to 1000m in radius. Surface metric values were highly variable across the floodplain and revealed a high degree of spatial organisation in floodplain topography. Moran's I correlograms fit to the landscape of each metric at each window size revealed that patchiness existed at nearly all window sizes, but the strength and scale of patchiness changed within window size, suggesting that multiple scales of patchiness and patch structure exist in the topography of this floodplain. Scale thresholds in the spatial patterns were observed, particularly between the 50 and 100m window sizes for all surface metrics and between the 500 and 750m window sizes for most metrics. These threshold scales are~15–20% and 150% of the main channel width (1–2% and 10–15% of the floodplain width), respectively. These thresholds may be related to structuring processes operating across distinct scale ranges. By coupling surface metrics, multi-scale analyses, and correlograms, quantifying floodplain topographic complexity is possible in ways that should assist in clarifying how floodplain ecosystems are structured.

Flooding with constraints: water meadow irrigation impacts on temperature, oxygen, phosphorus and sediment in water returned to a river

AbstractWater meadows are pasture irrigation systems found across Europe. They potentially mitigate local flooding where a thin sheet of water is allowed across an engineered floodplain surface. In southern England, functioning ‘bedwork’ systems require investigation for their impact on water returned to a river in terms of temperature, dissolved oxygen, sediment trapping and the transport of phosphorus. Temperature change in water returned to the river is found to be minimal, and it remains well oxygenated. Water meadow systems reduce %silt in deposited sediment by an average of 14% and reduce mobile phosphorus by around one third. Soils show a decline in plant‐available phosphorus by one half. Evaluation of the impact on rivers is important because water meadows are typically managed under agri‐environmental schemes, and there is realisation of their historically demonstrated potential for floodwater retention.

Trace metal storage in recent floodplain sediments along the Dill River, central Germany

Trace metals are stored in near-channel floodplain sediments along many rivers in industrial and mined watersheds of western European countries such as Germany. In this paper, I document the distribution of Cu, Pb, and Zn in 13 cores collected from near-channel floodplain sediments along a 25-km reach of the Dill River in central Germany. Mean concentrations of the three trace metals exceed background concentrations, an indication of anthropogenic enrichment along the study reach; many individual samples have concentrations several times the background levels and exceed standards for trace metals contained in the German Federal Soil Protection Act. Metal concentrations generally peak at depths of 0.2 to 0.7m below the floodplain surface and are assumed to represent the period of maximum metal releases to the environment through upstream industrial activity, ore mining, or both. In eight of the cores, radiocarbon ages obtained below the depth of peak metal concentrations provide maximum dates for sediment accumulation of between 90 and 1700years ago, with most ages clustering in the period of 90 to 300years ago. The ages and depth to peak metal concentrations indicate 0.6 to 1.0m of floodplain sedimentation over the last 150 to 300years (0.3–0.4cmy−1), which exceed sedimentation amounts and rates found in surrounding watersheds. Surface sediments contain lower metal concentrations, suggesting deposition recently of cleaner sediments. Trace metals stored along the Dill River serve as a reminder that stores of contaminants exist in geomorphically sensitive locations of the fluvial system. These legacy sediments bear the chemical imprint of industrial or mining activities that no longer occur in the watershed.

Late Quaternary chronostratigraphic framework of terraces and alluvium along the lower Ohio River, southwestern Indiana and western Kentucky, USA

The lower Ohio River valley is a terraced fluvial landscape that has been profoundly influenced by Quaternary climate change and glaciation. A modern Quaternary chronostratigraphic framework was developed for the lower Ohio River valley using optically stimulated luminescence (OSL) dating and allostratigraphic mapping to gain insights into the nature of fluvial responses to glacial–interglacial/stadial–interstadial transitions and Holocene climate change. River deposits, T0 (youngest) to T7 (oldest), were mapped along a 75 km reach of the lower Ohio River and were dated using 46 OSL and 5 radiocarbon samples. The examination of cores combined with OSL and radiocarbon dating shows that fluvial sediments older than marine oxygen isotope stage (MIS) 2 are present only in the subsurface. Aggradation during MIS 6 (Illinoian glaciation) filled the valley to within ∼7 m of the modern floodplain, and by ∼114 ka (MIS 5e/Sangamon interglacial) the Ohio River had scoured the MIS 6 sediments to ∼22 m below the modern floodplain surface. There were no fluvial sediments in the valley with ages between MIS 5e and the middle of MIS 3. The MIS 3 ages (∼39 ka) and stratigraphic position of T5 deposits suggest the Ohio River aggraded 8–14 m during MIS 4 or MIS 3. Near the end of MIS 3, the Ohio River incised the mid Last Glacial (mid-Wisconsinan) deposits ∼10 m and began aggrading again by ∼30 ka. Aggradation continued into MIS 2, with maximum MIS 2 aggradation occurring before ∼21 ka, which is coincident with the global Last Glacial Maximum (LGM). As the Ohio River adjusted to changing fluxes in sediment load and discharge following the LGM, it formed a sequence of fill-cut terraces in the MIS 2 outwash that get progressively younger with decreasing elevation, ranging in age from ∼21 ka to ∼13 ka. From ∼14 ka to ∼13 ka the Ohio River rapidly incised ∼3 m to form a new terrace, and by ∼12 ka at the onset of the Holocene, the Ohio River established a meandering channel pattern. The river formed a broad floodplain surface from ∼12 ka to ∼6 ka, and then incised ∼1 m and formed a fill-cut terrace from ∼6 ka to ∼5 ka. After ∼5 ka, likely in response to mid-Holocene drought in North America, the Ohio River incised ∼5 m, and by ∼4 ka the river began aggrading again. The Ohio River has aggraded ∼4 m since aggradation began at ∼4 ka. The chronostratigraphic framework and reconstructed history developed here suggest that the lower Ohio River is highly sensitive to glacial–interglacial transitions and abrupt Holocene climate change and responds rapidly to these allogenic forcings.

Trace metals distribution pattern in floodplain sediments of a lowland river in relation to contemporary valley bottom morphodynamics

AbstractThe distribution of trace metals in alluvial sediments depends on their natural background concentrations, and on the dynamics of contemporary depositional and erosional (mainly flood‐induced) processes. Geological and geochemical investigations were carried out in the valley of Vistula River near Magnuszew (central Poland). Sediment samples were collected from a depth of 35 cm and comprise sediments of all defined geomorphological features. Identification and geological interpretation of the morphodynamic sediment features was supported by aerial photographs and high‐resolution satellite images. These studies revealed that the distribution of trace metals is closely linked to the morphogenesis of the alluvial floodplain. The highest concentrations of Cu, Co, Zn, V, Cr and Ni were observed in crevasse‐splays deposits. By contrast, Sr, Pb and As were concentrated in deposits which fill oxbow lakes (partly infilled with organic deposits). The lowest concentrations of trace metals were detected in flood sediments deposited within erosional troughs. The geomorphological and sedimentological history of the fluvial features explains the pattern of heavy metal distribution on the current floodplain surface. Copyright © 2014 John Wiley &amp; Sons, Ltd.

The efficacy of stream power and flow duration on geomorphic responses to catastrophic flooding

Geomorphologists have long studied the impacts of extreme floods, yet the association between the magnitude of flow parameters (discharge, velocity, shear stress, or stream power) and resulting geomorphic effectiveness remains vague and non-deterministic. Attempts have been made to include flow duration and total expenditure of stream power, in combination with peak unit stream power, as important variables, but there has been minimal exploration of this hydraulic combination. Taking advantage of Tropical Storm Irene's rapid track through eastern Vermont (USA) in late summer 2011, this paper presents the array of geomorphic responses to a short duration (time to peak of <8h) but high magnitude flood that was the twentieth century flood of record for numerous watersheds. We present herein the geomorphic imprint of Tropical Storm Irene flooding within a larger context of fluvial theory concerning the role of, and trade-off between, the magnitude of energy expenditure during a flood and its duration. Focusing on a detailed field effort within the 187-km2 Saxtons River basin in southeastern VT, augmented by select sites along the adjacent lower gradient Williams River (291-km2), we elucidate (1) the geomorphic effects of a short duration flood in a humid, well-vegetated landscape; (2) the relationship between geomorphic response and (a) peak stream power, (b) total stream power, and (c) flow duration of stream power above a critical threshold; and (3) the spatial variation of geomorphic effects relative to reach-scale geologic and geomorphic controls. Flooding associated with Tropical Storm Irene ranged from the 1000year recurrence interval (RI) flood (based on Weibull flood frequency analysis) to the 300year RI flood (log Pearson Type III). Discharges spawned a peak unit stream power of 712W/m2 (Saxtons River) and 361W/m2 (Williams River), with total energy expenditure throughout the event of ~16,000×103 and 15,000×103J, respectively. For the Saxtons River, channel widening was spatially infrequent and limited in magnitude; however, other geomorphic effects were profound (1) the entrainment, transport, and deposition of extremely coarse material; (2) stripping of floodplain surfaces; (3) channel avulsions and incision into Pleistocene-aged material; and (4) deposition of coarse material across floodplains. Based on our extensive field data and hydrologic/hydraulic analyses, we contend that short duration, high energy flows can have profound sedimentological effects but have limited erosive, channel widening impacts. Gravel entrainment and deposition of a catastrophic nature can certainly occur under these flow regimes, but the impacts of these extreme flows on channel geometry may have limited expression.

(Before and) After the Flood: A multiproxy approach to past floodplain usage in the middle Wadi el-Hasa, Jordan

Floodplains are an important feature of arid landscapes, enabling intensive agricultural activity by providing a locale with a consistent and largely predictable water source that is accessible without costly infrastructural modifications. Floodplain agriculture, although likely an important part of ancient agricultural systems in the Near East, is notoriously difficult to detect, as the dynamic environments in which floodplains are situated means that these geomorphic features are rarely preserved. However, recent survey in the Wadi el-Hasa, Jordan has revealed a preserved fragment of floodplain surface indicating past floodplain usage during the 7th–8th century cal AD. A multi-proxy analysis that incorporates geomorphic, geochemical, and paleobotanical analyses of this exposure has revealed a process of floodplain aggradation and incision modified by human activities and anthropogenic deposition. Analysis of the anthropogenic sediments suggests that the Wadi el-Hasa floodplain has not been adequately considered as a component of economic and subsistence activity during the early Islamic period.

Floodplain ecohydrology: Climatic, anthropogenic, and local physical controls on partitioning of water sources to riparian trees.

Seasonal and annual partitioning of water within river floodplains has important implications for ecohydrologic links between the water cycle and tree growth. Climatic and hydrologic shifts alter water distribution between floodplain storage reservoirs (e.g., vadose, phreatic), affecting water availability to tree roots. Water partitioning is also dependent on the physical conditions that control tree rooting depth (e.g., gravel layers that impede root growth), the sources of contributing water, the rate of water drainage, and water residence times within particular storage reservoirs. We employ instrumental climate records alongside oxygen isotopes within tree rings and regional source waters, as well as topographic data and soil depth measurements, to infer the water sources used over several decades by two co-occurring tree species within a riparian floodplain along the Rhône River in France. We find that water partitioning to riparian trees is influenced by annual (wet versus dry years) and seasonal (spring snowmelt versus spring rainfall) fluctuations in climate. This influence depends strongly on local (tree level) conditions including floodplain surface elevation and subsurface gravel layer elevation. The latter represents the upper limit of the phreatic zone and therefore controls access to shallow groundwater. The difference between them, the thickness of the vadose zone, controls total soil moisture retention capacity. These factors thus modulate the climatic influence on tree ring isotopes. Additionally, we identified growth signatures and tree ring isotope changes associated with recent restoration of minimum streamflows in the Rhône, which made new phreatic water sources available to some trees in otherwise dry years.Key PointsWater shifts due to climatic fluctuations between floodplain storage reservoirsAnthropogenic changes to hydrology directly impact water available to treesEcohydrologic approaches to integration of hydrology afford new possibilities