Assessing surface water–groundwater interactions in a complex river‐floodplain wetland‐isolated lake system

Floodplain systems are most often hydrologically complex settings characterized by highly variable surface water–groundwater interactions that are subjected to wide‐ranging wetting and drying over seasonal timeframes. This study used field methods, statistical analysis, and the Darcy's law approach to explore surface water–groundwater dynamics, interactions, and fluxes in a geographically complex river‐floodplain wetland‐isolated lake system (Poyang Lake, China). The floodplain system of Poyang Lake is affected by strongly seasonal shifts between dry and wet processes that cause marked changes in surface water and groundwater flow regimes. Results indicate that wetland groundwater is more sensitive to variations in river levels than the seasonal isolated lakes. In general, groundwater levels are lower than those of the isolated lakes but slightly higher than river levels. Statistical analysis indicates that the river hydrology plays a more significant role than the isolated lakes in controlling floodplain groundwater dynamics. Overall, the river shows gaining conditions and occasionally losing conditions with highly variable Darcy fluxes of up to +0.4 and ?0.2 m/day, respectively, whereas the isolated lakes are more likely to show slightly losing conditions (less than ?0.1 m/day). Although seasonal flux rates range from 7.5 to 48.2 m/day for surface water–groundwater interactions in the floodplain, the flux rates for river–groundwater interactions were around four to seven times higher than that of the isolated lake–groundwater interactions. The outcomes of this study have important implications for improving the understanding of the water resources, water quality, and ecosystem functioning for both the river and the lake.     

Reconstruction of the characteristics of a natural alluvial river–floodplain system and hydromorphological changes following human modifications: the Danube River (1812–1991)

Based on detailed historical surveys from 1812, the natural riverine landscape of a 10.25‐km‐long reach of the Danube River in the Austrian Machland region prior to channelization is analysed. Anthropogenically induced changes of fluvial dynamics, hydrological connectivity and aquatic habitat composition are discussed, comparing the situations following channelization (1925) and flow regulation (1991). In 1812 the alluvial river–floodplain system of the Danube River comprised a highly complex channel network, numerous gravel bars and extensive islands, with the main channel and side arms (eupotamon) representing about 97% of the entire water surface at low flow. The floodplain was characterized by relatively flat terrain and numerous natural trenches (former active channels) connected to the main channel. These hydromorphological conditions led to marked expansion/contraction of the water surface area at water level fluctuations below bankfull (‘flow pulse’). The high degree of hydrological connectivity enabled intensive exchange processes and favoured migrations of aquatic organisms between the river and floodplain habitats over a period of approximately 90 days per year. Overall in 1812, 57% of the active zone (active channels and floodplain) was inundated at bankfull water level. Channelization and construction of hydropower plants resulted in a truncated fluvial system. Consequently, eupotamal water bodies decreased by 65%, and gravel/sand bars and vegetated islands decreased by 94% and 97%, respectively, whereas the area of the various backwaters doubled. In 1991 the former ‘flow pulse’ was halved due to artificial levees and embankments, greatly diminishing hydrological connectivity and decoupling large areas of the floodplain from the main channel. Active overflow, formerly playing an important role, is now replaced by backwater flooding and seepage inflow in isolated water bodies. Copyright © 2003 John Wiley & Sons, Ltd.     

THE IMPORTANCE OF VARIABLE LATERAL CONNECTIVITY BETWEEN ARTIFICIAL FLOODPLAIN WATERBODIES AND RIVER CHANNELS

The rehabilitation of lowland rivers subjected to channelization and artificial levee construction should attempt to improve habitat heterogeneity and diversity of floodplain hydrological connectivity. However, rehabilitation efforts rarely consider the importance of variable lateral hydrological connectivity between floodplain waterbodies and main river channels (ranging from those permanently connected to those temporarily connected during river level rises), instead focusing on increasing individual floodplain waterbody connectivity. This study investigated the young‐of‐the‐year (YoY) fish communities in 10 artificial floodplain waterbodies of variable hydrological connectivity with the river Trent, England, between May and November 2006, inclusive. Floodplain waterbody connectivity to the main river was positively correlated with the number of species captured (alpha diversity), Shannon–Wiener diversity, Margalef's species richness index and the relative abundance of rheophilic species and negatively correlated with species turnover (beta diversity). YoY fish communities in poorly connected water bodies were most dissimilar to riverine communities. The results demonstrate the importance of variable lateral connectivity between artificial floodplain waterbodies and main river channels when rehabilitating lowland river fish communities. Copyright © 2011 John Wiley & Sons, Ltd.     

Effect of Spatial Heterogeneity on Zooplankton Diversity: A Multi‐Scale Habitat Approximation in a Floodplain Lake

Environmental heterogeneity is an essential quality of ecosystems as it has important implications in community structure. Macrophytes are a main component of environmental heterogeneity in floodplain lakes, and their dynamics are highly influenced by water level changes. In this context, we analysed at different spatial scales the relationship among hydrological variations, environmental heterogeneity associated to macrophytes and zooplankton regional diversity (γ diversity) in a South American floodplain lake adjacent to the Paraná River, and we also compared the local zooplankton diversity (α diversity) among the different environments that comprised the lake heterogeneity. At very low waters, the environmental heterogeneity was reduced as the lake was mainly limited to open water areas with low zooplankton diversity. At high waters, the profuse vegetation development (emergent and free‐floating), in mixed or homogeneous patches, determined a higher lake environmental heterogeneity with enhanced regional zooplankton diversity; littoral species increased over limnetic ones. Zooplankton α diversity was higher in environments with free‐floating macrophytes than in those without these plants. The structural complexity in the water column provided by plant roots would be closely related to the enhanced diversity found under free‐floating mats. This study contributes to the knowledge on the effects of strong water level variations on environmental heterogeneity, which is strongly associated to macrophytes and on zooplankton diversity, and highlights the role of free‐floating plants as diversity hosts and ‘key structures’ in floodplain lakes. Copyright © 2013 John Wiley & Sons, Ltd.     

Integrating remotely acquired and field data to assess effects of setback levees on riparian and aquatic habitats in glacial‐melt water rivers

Setback levees, in which levees are reconstructed at a greater distance from a river channel, are a promising restoration technique particularly for alluvial rivers with broad floodplains where river‐floodplain connectivity is essential to ecological processes. Documenting the ecological outcomes of restoration activities is essential for assessing the comparative benefits of different restoration approaches and for justifying new restoration projects. Remote sensing of aquatic habitats offers one approach for comprehensive, objective documentation of river and floodplain habitats, but is difficult in glacial rivers because of high suspended‐sediment concentrations, braiding and a lack of large, well‐differentiated channel forms such as riffles and pools. Remote imagery and field surveys were used to assess the effects of recent and planned setback levees along the Puyallup River and, more generally, the application of multispectral imagery for classifying aquatic and riparian habitats in glacial‐melt water rivers. Airborne images were acquired with a horizontal ground resolution of 0.5 m in three spectral bands (0.545–0.555, 0.665–0.675 and 0.790–0.810 µm) spanning from green to near infrared (NIR) wavelengths. Field surveys identified river and floodplain habitat features and provided the basis for a comparative hydraulic analysis. Broad categories of aquatic habitat (smooth and rough water surface), exposed sediment (sand and boulder) and vegetated surfaces (herbaceous and deciduous shrub/forest) were classified accurately using the airborne images. Other categories [e.g. conifers, boulder, large woody debris (LWD)] and subdivisions of broad categories (e.g. riffles and runs) were not successfully classified either because these features did not form large patches that could be identified on the imagery or their spectral reflectances were not distinct from those of other habitat types. Airborne imagery was critical for assessing fine‐scale aquatic habitat heterogeneity including shallow, low‐velocity regions that were not feasible or practical to map in the field in many cases due to their widespread distribution, small size and poorly defined boundaries with other habitat types. At the reach‐scale, the setback levee affected the amount and distribution of riparian and aquatic habitats: (1) the area of all habitats was greater where levees had been set back and with relatively more vegetated floodplain habitat and relatively less exposed sediment and aquatic habitat, (2) where levees confine the river, less low‐velocity aquatic habitat is present over a range of flows with a higher degree of bed instability during high flows. As river restoration proceeds in the Pacific Northwest and elsewhere, remotely acquired imagery will be important for documenting its effects on the amount and distribution of aquatic and floodplain habitats, complimenting field data as a quantitative basis for evaluating project efficacy. Copyright © 2008 John Wiley & Sons, Ltd.     

Benchmarking Fluvial Dynamics for Process‐Based River Restoration: the Upper Rhine River (1816–2014)

Multi‐temporal analysis of river‐floodplain processes is a key tool for the identification of reference conditions or benchmarks and for the evaluation of deviations or deficits as a basis for process‐based river restoration in large modified rivers. This study developed a methodology for benchmarking fluvial processes at river segment level, focusing on those interrelations between morphodynamics (aggradation, erosion, channel shift) and vegetation succession (initial, colonization, transition) that condition habitat structure. Habitat maps of the free‐flowing Upper Rhine River downstream from Iffezheim dam (France–Germany border) were intersected with a geographic information system‐based approach. Patches showing trajectories of anthropization, changeless, progression and regression allowed for the identification of natural and human‐induced processes over almost 200 years. Before channelization, the riverine system was characterized by a shifting habitat mosaic with natural heterogeneity, high degree of surface water connectivity and equilibrium between progression and regression processes. On the other hand, the following 175 years of human interventions led to severe biogeomorphologic deficits evidenced by loss of natural processes and habitat heterogeneity, hydrological disconnection between the river and its floodplain and imbalance of progression versus regression dynamics. The main driving forces of change are found in hydromorphological impacts (channelization, regulation and hydropower plant construction). Regression processes are now almost absent and have to be the objective of process‐based river restoration measures for the studied river‐floodplain system. A sustainable view on water management and river restoration should aim at a more resilient riverine system by balancing the recovery of natural processes with societal needs. Copyright © 2016 John Wiley & Sons, Ltd.     

Multi‐annual contemporary flood event overbank sedimentation within the vegetated lower Orinoco floodplain,Venezuela

Multi‐annual contemporary flood event overbank sedimentation rates were quantified on the World's third largest river in terms of discharge, the tropical lower Orinoco. We discuss the role of variables at the basin and reach scales that contributed to the complexity of spatio‐temporal overbank sediment deposition patterns. Monitored in situ plots were characterized by distance to the main channel, hydroperiod, different geomorphological units, and vegetation cover. Flood event sedimentation rates showed a high spatial variability ranging from the absence of sediment deposition up to 225.46 kg m^‐2 yr^‐1. Banks and levees received relatively high amounts of sediment (39.6 kg m^‐2 yr^‐1), whereas observed mean sedimentation rates on the more distant floodplain and backswamps tended to be lower (17.7 kg m^‐2 yr^‐1). Significant differences in sedimentation rates were observed in two major vegetation types: dense herbaceous and shrubby vegetation (42.2 kg m^‐2 yr^‐1) and floodplain forest (12.7 kg m^‐2 yr^‐1). However, overbank sedimentation patterns also reflected imbricated hydrosedimentary and biogeomorphological vegetation feedbacks that co‐construct fluvial landforms. The incidence of an El Niño–Southern Oscillation–La Niña episode during the study period on sediment availability and floodplain sedimentation suggests that within whitewater rivers, where suspended sediment concentrations are naturally high, hydrological connectivity seems to be more important for floodplain sedimentation than variations in suspended sediment concentrations. These results may provide a good basis for future biogeomorphological investigation projects using complementary methodologies, in order to better anticipate global change and fluctuations in the occurrence, strength or duration of El Niño–La Niña episodes in the tropical zone and their consequences for flood discharge and sediment dynamics during channel–floodplain exchanges.     

Reach‐scale interactions between aquatic plants and physical habitat: River Frome,Dorset

Substrate and flow velocity are two key physical factors influencing the distribution of aquatic plants. These two controls are closely related and reflect interactions between flow regime, quantity and calibre of sediment supplied to and bounding river channels, and channel dimensions. Seasonal growth of aquatic plants has important impacts upon flow resistance, flow velocities and sediment dynamics. This paper focuses upon the reach scale and the aggregate impact of the seasonal growth of aquatic plants on the meso‐habitat characteristics of chalk rivers. It provides a contribution to the broader literature by illustrating how submerged and emergent macrophytes interact with one another at the reach scale to control flow patterns and the retention of fine sediment. The research was conducted within the River Frome, Dorset, UK. Four sets of observations from 2004 of the seasonal growth of aquatic plants, flow velocity behaviour and the storage of fine sediment are presented. Aggregate velocity behaviour over the four observation periods is classified using agglomerative, hierarchical cluster analysis. The spatial arrangement of patches exhibiting different aggregate velocity behaviour are explored and the degree to which velocity behaviour can be attributed to physical characteristics of patches and the growth of aquatic plants is discussed. Conclusions include: (i) patterns of aggregate velocity behaviour within reaches are strongly influenced by macrophyte growth; (ii) average fine sediment deposition is higher around emergents such as Sparganium erectum than submerged plants such as Ranunculus penicillatus subsp. pseudofluitans; (iii) complementary flow patterns evolve across the river channel as aquatic macrophytes grow and it appears that this allows marginal emergents to scavenge fine sediment from faster flow velocity threads that become diverted towards the channel edges. (iv) These processes are important for mesohabitat evolution and maintenance, and they also contribute to patterns of bank erosion and aggradation. Copyright © 2006 John Wiley & Sons, Ltd.     

Fast response of lake plankton and nutrients to river inundations on floodplain lakes

Key variables in ecosystems tend to operate on widely different time‐scales. These time‐scales become relevant when a disturbance rocks the ecosystem. Here we try to explain the fast dynamics of plankton and nutrients in the water column of floodplain lakes after disturbances (inundations). We take advantage of natural experiments, that is occasional massive overflow of floodplain lakes with river water. We sampled 10 lakes in two floodplains along the Dutch river Waal monthly for 3 years, capturing the impact of three inundation events. The inundations reset the plankton as well as chemical composition of most lakes to largely the same state. While biologically inert macro‐ion data reflected a large and long lasting impact of the river water, dynamics of nutrients, phytoplankton and zooplankton communities between lakes diverged in a few weeks to regimes characteristic for the different lakes. While one spring inundation synchronized plankton dynamics to let the subsequent clear water phase occur at the same moment in different lakes, winter inundations did not have the same effect and apparently dynamics quickly diverged. Our results showed that effects of inundations and other processes that affect the state of the ecosystem should be studied considering the level of the slow components such as the sediment nutrient pool, fish stock and macrophyte communities. Plankton communities and lake water nutrient status give a practically instantaneous reflection of the condition of these slow components. Copyright © 2008 John Wiley & Sons, Ltd.     

Progressive changes in phytoplankton community structure in urbanized lowland river floodplains—a multivariate approach

Phytoplankton communities along the river Yamuna in Delhi stretch and its associated floodplain lakes showed considerable spatio‐temporal variations. Most of the species identified were cosmopolitan and typical of highly polluted rivers in tropical ecosystems. All the groups of phytoplankton with predominance of Cyanophyceae and some species of Chlorophyceae were distributed within the river stretch though with varying diversity and density. Relatively cleaner sites supported larger phytoplankton assemblages compared to those subjected to sewage outfalls and industrial effluents. Species elimination was observed mainly in the mid‐stretch and downstream of Okhla subjected to high loads of pollution. Co‐inertia analysis indicated significant co‐structures between water quality and species richness and density indicating water quality as the governing factor for phytoplankton distributions in the river stretch. Floodplain plays an important role in regenerating water quality thereby enhancing species richness and density. Floods play an important role in seasonal dynamics through dilution effects and promoting connectivity between river channel and floodplain lakes thereby facilitating species exchanges. Copyright © 2008 John Wiley & Sons, Ltd.     

A 12‐month sediment and nutrient budget in a floodplain reach of the River Adour,southwest France

The sediment and nutrient budget in the floodplain reach of the River Adour in southwest France has been examined over a 1‐year period (1991–1992). Outputs of total suspended matter, dissolved inorganic nitrogen (i.e. nitrate+ammonium ions) and dissolved inorganic phosphorus (DIP) and particulate phosphorus (PP) have been compared with inputs. Alternating phases of mobilization and retention are observed for all the elements considered. Low water periods in summer allowed retention of both soluble material (24–26% for inorganic nitrogen) and suspended matter (9–19%). However, mobilization of PP associated with suspended matter transport during these periods. Elevated discharges associated with increased rainfall during the autumn period induced significant mobilization for all elements. In contrast, the winter flood event gave rise to a retention of particulate material (i.e. suspended matter and PP, 11% and 26%, respectively), but a loss of soluble materials (5% for inorganic nitrogen and 186% for DIP). The spring flood event produced retention for particulate materials (26%) while inducing losses of inorganic nitrogen (9%) and gains of DIP (7.4%). These results are related to riparian vegetation and the dynamics of nutrient cycling within the floodplain. Copyright © 2000 John Wiley & Sons, Ltd.     

沉水植物苦草对上覆水各形态磷浓度的影响

沉水植物是影响湖泊磷营养状态的重要因素。在室内模拟了"水-苦草-沉积物"生态系统,在沉水植物苦草生长过程中分析上覆水中的总磷(TP)、溶解性总磷(TDP)、溶解性有机磷(DOP)、溶解性活性磷(SRP)、颗粒态总磷(PP)的变化,探讨了苦草对上覆水磷赋存形态的影响。结果表明,苦草在生长过程中减缓了沉积物磷的相对释放速率使上覆水中TP、SRP、DOP、PP均呈现不同程度的下降。上覆水TP、SRP、DOP、PP分别下降了37.5%、74.6%、42.0%、19.5%;苦草主要通过吸收上覆水和间隙水中的磷及对环境因子的影响,使得上覆水中各形态磷浓度保持在较低的水平,降低沉积物向水体释放磷的速率。     

THE RELATIONSHIP BETWEEN FINE SEDIMENT AND MACROPHYTES IN RIVERS

The interplay between erosion and deposition are fundamental characteristics of river basins. These processes result in the delivery, retention and conveyance of sediment through river systems. Although the delivery of sediment to rivers is a natural phenomenon, in recent years there has been increasing concern about the enhancement of sediment loadings as a result of anthropogenic activities. The presence of macrophytes in river channels tends to increase the retention of fine sediment leading to changes in bed composition. However, a complex relationship exists between macrophytes and fine sediment: macrophytes affect the conveyance of fine sediment and are, in turn, affected by the sediment loading. This review deals with these two reciprocal effects and, in particular, summarizes the available evidence base on the impact of fine sediment on macrophytes. Increased inputs of fine sediment appear to have both direct and indirect impacts on the macrophyte community, altering light availability, and the structure and quality of the river bed. The nature of these impacts depends largely on the rate of deposition and the nature of the material deposited. Changes in macrophyte community composition may ensue where the depositing material is more nutrient rich than the natural river bed. Many of the changes in macrophyte flora that occur with increased fine sediment inputs are likely to closely parallel those that occur with increased dissolved nutrient availability. If attempts to manage nutrient inputs to rivers are to achieve their goals, it is critical that fine sediment‐associated nutrient dynamics and transfers are considered. Copyright © 2011 John Wiley & Sons, Ltd.     

Seasonal variation in nutrient retention during inundation of a short‐hydroperiod floodplain

Floodplains are generally considered to be important locations for nutrient retention or inorganic‐to‐organic nutrient conversions in riverine ecosystems. However, little is known about nutrient processing in short‐hydroperiod floodplains or seasonal variation in floodplain nutrient retention. Therefore, we quantified the net uptake, release or transformation of nitrogen (N), phosphorus (P) and suspended sediment species during brief periods (1–2 days) of overbank flooding through a 250‐m floodplain flowpath on the fourth‐order Mattawoman Creek, Maryland U.S.A. Sampling occurred during a winter, two spring and a summer flood in this largely forested watershed with low nutrient and sediment loading. Concentrations of NO increased significantly in surface water flowing over the floodplain in three of the four floods, suggesting the floodplain was a source of NO. The upper portion of the floodplain flowpath consistently exported NH, most likely due to the hyporheic flushing of floodplain soil NH, which was then likely nitrified to NO in floodwaters. The floodplain was a sink for particulate organic P (POP) during two floods and particulate organic N and inorganic suspended sediment (ISS) during one flood. Large releases of all dissolved inorganic N and P species occurred following a snowmelt and subsequent cold winter flood. Although there was little consistency in most patterns of nutrient processing among the different floods, this floodplain, characterized by brief inundation, low residence time and low nutrient loading, behaved oppositely from the conceptual model for most floodplains in that it generally exported inorganic nutrients and imported organic nutrients. Published in 2007 by John Wiley & Sons, Ltd.     

Fatty Acid Profiles Distinguish Channel Catfish from Three Reaches of the Lower Kaskaskia River and its Floodplain Lakes

Despite the increasing use of fatty acids (FAs) as biomarkers in aquatic food web analysis, little information is available regarding differences in FA profiles of fish among habitat types in river–floodplain ecosystems. The objectives of this study were to (i) test whether the FA profiles of channel catfish (Ictalurus punctatus) differed among three reaches of the lower Kaskaskia River and its floodplain lakes, and (ii) to compare FA profiles among muscle, liver, and adipose fin tissues collected from these fish. Profiles differed significantly among sites, especially between upper and lower river sites, and between river channel and oxbow lake sites, suggesting differences in FA availability for channel catfish occupying different habitats and river reaches in the Kaskaskia River system. Specifically, the essential FAs 18:2n‐6 and 18:3n‐3 increased in catfish tissues from upstream to downstream reaches, which could reflect increased floodplain connectivity and decreasing impoundment effects downstream. Ratios of n‐3 to n‐6 FAs were higher in fish from oxbow lakes, perhaps suggesting increased use of autochthonous production in the floodplain relative to the main river channel. Muscle and adipose fin FA profiles exhibited similar location‐related trends, whereas liver FA profiles were markedly different from the other tissue types. These results suggest that adipose fin tissue samples may be a viable, less‐invasive alternative to muscle tissue for analysis of FA profiles in channel catfish. Our study supports the use of tissue FA profiles in identifying habitat utilization by channel catfish, and perhaps habitat‐specific energy contributions to riverine consumers. Furthermore, our work highlights floodplain habitat as a potential source of essential n‐3 FA and the associated importance of maintaining river–floodplain connectivity to support aquatic food webs. Copyright © 2014 John Wiley & Sons, Ltd.     

Historical changes in hydrology,geomorphology, and floodplain vegetation of the Willamette River,Oregon

Historical trends in hydrology, geomorphology, and floodplain vegetation provide fundamental contexts for designing future management of large rivers, an area of fluvial research extensively informed by studies of historical channel dynamics. Changes in hydrology, channel structure, floodplain forests, and large wood were documented for the 273‐km main stem of the Willamette River from 1850 to present. Reduced sediment supply and frequency and magnitude of floods have decreased channel mobility and incised channels, leading to fewer gravel bars, islands, and side channels. Human alteration of channel morphology, vegetation, and bank hardening has exacerbated channel simplification caused by reductions in floods, sediment supply, and inputs of wood. A substantial number of floodplain channels reoccupied remnants of previous active channels inundated during recent floods, demonstrating functional but often forgotten role of historical geomorphic structure in modern floodplains and flood processes. In most reaches, area of floodplain forests in 1990 was only 10% to 25% of the area of forests in 1850. Abundance of wood in the wetted channel was generally greater in reaches with higher abundances of floodplain forests. Future trajectories will be influenced by legacies of the historical river but increasingly will reflect evolution of a new river shaped by human development, changing climate, and emerging hydrogeomorphic and vegetation processes. Understanding historical characteristics and anticipating future rates and patterns of ecosystem change provide fundamental contexts for restoring biophysical processes and structure in a large floodplain river.     

Sedimentology of New Fluvial Deposits on the Elwha River,Washington, USA,Formed During Large‐Scale Dam Removal

Removal of two dams 32 m and 64 m high on the Elwha River, Washington, USA, provided the first opportunity to examine river response to a dam removal and controlled sediment influx on such a large scale. Although many recent river‐restoration efforts have included dam removal, large dam removals have been rare enough that their physical and ecological effects remain poorly understood. New sedimentary deposits that formed during this multi‐stage dam removal result from a unique, artificially created imbalance between fluvial sediment supply and transport capacity. River flows during dam removal were essentially natural and included no large floods in the first two years, while draining of the two reservoirs greatly increased the sediment supply available for fluvial transport. The resulting sedimentary deposits exhibited substantial spatial heterogeneity in thickness, stratal‐formation patterns, grain size and organic content. Initial mud deposition in the first year of dam removal filled pore spaces in the pre‐dam‐removal cobble bed, potentially causing ecological disturbance but not aggrading the bed substantially at first. During the second winter of dam removal, thicker and in some cases coarser deposits replaced the early mud deposits. By 18 months into dam removal, channel‐margin and floodplain deposits were commonly >0.5 m thick and, contrary to pre‐dam‐removal predictions that silt and clay would bypass the river system, included average mud content around 20%. Large wood and lenses of smaller organic particles were common in the new deposits, presumably contributing additional carbon and nutrients to the ecosystem downstream of the dam sites. Understanding initial sedimentary response to the Elwha River dam removals will inform subsequent analyses of longer‐term sedimentary, geomorphic and ecosystem changes in this fluvial and coastal system, and will provide important lessons for other river‐restoration efforts where large dam removal is planned or proposed. Published 2013. This article is a U.S. Government work and is in the public domain in the USA.     

Hydrologic scaling for hydrogeomorphic floodplain mapping: Insights into human‐induced floodplain disconnectivity

Hydrogeomorphic approaches for floodplain modelling are valuable tools for water resource and flood hazard management and mapping, especially as the global availability and accuracy of terrain data increases. Digital terrain models implicitly contain information about floodplain landscape morphology that was produced by hydrologic processes over long time periods, as well as recent anthropogenic modifications to floodplain features and processes. The increased availability of terrain data and distributed hydrologic datasets provide an opportunity to develop hydrogeomorphic floodplain delineation models that can quickly be applied at large spatial scales. This research investigates the performance of a hydrogeomorphic floodplain model in two large urbanized and gauged river basins in the United States, the Susquehanna and the Wabash basins. The models were calibrated by a hydrologic data scaling technique, implemented through regression analyses of USGS peak flow data to estimate floodplain flow levels across multiple spatial scales. Floodplain model performance was assessed through comparison with 100‐year Federal Emergency Management Agency flood hazard maps. Results show that the hydrogeomorphic floodplain maps are generally consistent with standard flood maps, even when significantly and systematically varying scaling parameters within physically feasible ranges, with major differences that are likely due to infrastructure (levees, bridges, etc.) in highly urbanized areas and other locations where the geomorphic signature of fluvial processes has been altered. This study demonstrates the value of geomorphic information for large‐scale floodplain mapping and the potential use of hydrogeomorphic models for evaluating human‐made impacts to floodplain ecosystems and patterns of disconnectivity in urbanized catchments.     

Effects of flooding regime upon the decomposition of Eichhornia azurea (Sw.) Kunth measured on a tropical,flow‐regulated floodplain (Paraná River,Brazil)

Decomposition of macrophytes is an important process in river‐floodplain systems, especially in the Upper Paraná River floodplain, given that this ecosystem receives high inputs of detritus from this vegetation. Release of nutrients by decomposition is essential in this floodplain because it is located downstream from a reservoir chain where nutrients are being trapped. Water level fluctuations are considered one of the most important aspects that affect macrophyte decomposition. Anthropogenic alterations, such as the control of flooding regimes, observed in this floodplain, could change the dynamic of this process. To evaluate the influence of the hydrological cycle upon the decomposition of Eichhornia azurea (an aquatic macrophyte that has high biomass values in this ecosystem), litter bags with senescent leaves and petioles of this plant were submitted to four different flooding treatments, which differed in time of flooding and exposure to dry conditions. The decomposition rates and the detritus chemical composition (nitrogen and phosphorus concentrations) were measured over 113 days. There were significant effects of the flooding treatments and time upon all parameters. The materials that decomposed with flood conditions showed the greatest decomposition rates. The quality of the detritus seems to be highly linked with the flooding regime, showing highest releases of phosphorus in the submerged treatments. It was shown that floods, even the short duration ones, increase the decomposition velocity and the nutrient cycling relative to dry conditions. Thus, investigations that assess the nutrient budgets on the Upper Paraná River floodplain and the role of nutrients in its productivity should consider the detritus compartment and the effects of flood regimes upon its dynamics. Copyright © 2006 John Wiley & Sons, Ltd.     

Patterns in groundwater nitrogen concentration in the riparian zone of a large semi‐arid river (River Murray,Australia)

Lateral exchanges of surface water between river channels and their floodplains are important for vegetation health and aquatic food‐web productivity in semi‐arid ecosystems. However, the significance of the lateral connectivity via sub‐surface pathways in these systems is not as well understood. Patterns in nitrogen concentration in groundwater and in the unsaturated zone were used to infer the sub‐surface biogeochemistry of N in the riparian zone of a large semi‐arid floodplain (Hattah‐Kulkyne National Park) of the River Murray, Australia. The riparian zone plays a special role in this system as it is an area of transition between fresh surface waters and saline floodplain groundwater. The river was losing water to the floodplain during baseflow conditions but gradients were temporarily reversed following floods. In general, the redox conditions were sub‐oxic to anoxic in riparian groundwater and the main forms of N present were NH and dissolved organic N. There was a gradient in NH concentration from the river to the floodplain, suggesting that the main source of NH was from the decomposition of organic matter in fluvial sediments. Elevated concentrations of NO were occasionally found in shallow groundwater away from the river following floods but tended not to persist. The source of the NO appeared to be unsaturated‐zone NO displaced to the water table during floods. Assuming that denitrification was the main attenuation process, this displacement of unsaturated zone NO to anoxic groundwater could be a significant N removal process from the ecosystem (estimated at 18 kg N ha^?1 for the largest flood during the study). Understanding the impact of river regulation on floodplain nutrient cycles in River Murray floodplains will be challenging because the changes in floodplain hydrology are complex and coincide with salinization of soils and groundwater. Copyright © 2005 John Wiley & Sons, Ltd.