Natural Flood Events Research Articles

Contaminant mobilization from the vadose zone to groundwater during experimental river flooding events

Natural river flooding events can mobilize contaminants from the vadose zone and lead to increased concentrations in groundwater. Characterizing the mass and transport mechanisms of contaminants released from the vadose zone to groundwater during these recharge events is particularly challenging. Therefore, conducting highly-controlled in-situ experiments that simulate natural flooding events can help increase the knowledge of where contaminants can be stored and how they can move between hydrological compartments. This study specifically targets uranium pollution, which is accompanied by high sulfate levels in the vadose zone and groundwater. Two novel experimental river flooding events were conducted that utilized added non-reactive halides (bromide and iodide) and 2,6-difluorobenzoate tracers. In both experiments, about 8 m3 of traced water from a nearby contaminant-poor river was flooded in a 3-m diameter basin and infiltrated through the vadose zone and into a contaminant-rich unconfined aquifer for an average of 10 days. The aquifer contained 13 temporary wells that were monitored for solute concentration for up to 40 days. The groundwater analysis was conducted for changes in contaminant mass using the Theissen polygon method and for transport mechanisms using temporal moments. The results indicated an increase in uranium (21 and 24%), and sulfate (24 and 25%) contaminant mass transport to groundwater from the vadose zone during both experiments. These findings confirmed that the vadose zone can store and release substantial amounts of contaminants to groundwater during flooding events. Additionally, contaminants were detected earlier than the added tracers, along with higher concentrations. These results suggested that contaminant-rich pore water in the vadose zone was transported ahead of the traced flood waters and into groundwater. During the first flooding event, elevated concentrations of contaminants were sustained, and that chloride behaved similarly. The findings implied that contaminant- and chloride-rich evaporites in the vadose zone were dissolved during the first flooding event. For the second flooding event, the data suggested that the contaminant-rich evaporites continued to dissolve whereas chloride-rich evaporites were previously flushed. Overall, these findings indicated that contaminant-rich pore water and evaporites in the vadose zone can play a significant role in contaminant transport during flooding events.

Effects of sediment flushing operations versus natural floods on Chinook salmon survival

Flushing is a common measure to manage and reduce the amount of sediment stored in reservoirs. However, the sudden release of large volumes of sediment abruptly increases the suspended solids concentration and alters the riverbed composition. Similar effects can be produced also by natural flood events. Do flushing operations have more detrimental impacts than natural floods? To answer this question, we investigated the impact of flushing on the survival of the Chinook salmon (Oncorhynchus tshawytscha) in the Sandy River (OR, USA), assuming that sediment is flushed from hypothetical bottom gates of the, now decommissioned, Marmot Dam. The effects of several flushing scenarios are analyzed with a 2D morphodynamic model, together with habitat suitability curves and stress indicators. The results show that attention has to be paid to duration: the shorter the flushing operation, the lesser the stresses on fish survival and spawning habitats. Flushing causes high stress to salmon eggs and larvae, due to unbearable levels of suspended sediment concentrations. It also decreases the areas usable for spawning due to fine-sediment deposition, with up to 95% loss at peak flow. Without the dam, the corresponding natural flood event would produce similar effects, with up to 93% loss. The study shows that well-planned flushing operations could mimic a natural impact, but only partly. In the long-term, larger losses of spawning grounds can be expected, since the removal of fine sediment with the release of clear water from the reservoir is a lengthy process that may be undesirable due to water storage reduction.

Modeling Contaminant Microbes in Rivers During Both Baseflow and Stormflow.

Rivers transport contaminant microorganisms (including fecal indicator bacteria and human pathogens) long distances downstream of diffuse and point sources, posing a human health risk. We present a mobile‐immobile model that incorporates transport as well as immobilization and remobilization of contaminant microbes and other fine particles during baseflow and stormflow. During baseflow conditions, hyporheic exchange flow causes particles to accumulate in streambed sediments. Remobilization of stored particles from streambed sediments occurs slowly during baseflow via hyporheic exchange flow, while remobilization is vastly increased during stormflow. Model predictions are compared to observations over a range of artificial and natural flood events in the dairy contaminated Topehaehae Stream, New Zealand. The model outputs closely matched timing and magnitude of E. coli and turbidity observations through multiple high‐flow events. By accounting for both state‐of‐flow and hyporheic exchange processes, the model provides a valuable framework for predicting particle and contaminant microbe behavior in streams.

Physiological Responses of Typical Wetland Plants Following Flooding Process—From an Eco-Hydrological Model Perspective

Anaerobics increase resistance to gas transport and microbial activity in flooded soils. This may result in the presence of aerenchyma in the roots of some wetland plants. Increased aerenchyma airspaces enable oxygen to be transported from the above-ground plant parts to the submerged roots and rhizosphere. Nevertheless, there is still a lack of studies linking field experiments and eco-hydrological modeling to the parameterization of the physiological responses of typical wetland plant species to natural flooding events. Furthermore, from the modeling perspective, the contribution of aerenchyma was not sufficiently considered. The goal of this study was to develop and apply an eco-hydrological model capable of simulating various patterns of plant physiological responses to natural flooding events based on key processes of root oxygen diffusion and aerenchyma functioning in a variably-saturated wetland soil environment. Eco-hydrological experiments were conducted accordingly, with surface water level, root-zone soil water content, soil temperature, leaf net photosynthesis rate and root morphology monitored simultaneously in situ at a site dominated by meadow species Deyeuxia angustifolia (Kom.) Y. L. Chang and invaded shrub species Salix rosmarinifolia Linn. var. brachypoda (Trautv.et Mey.) Y.L. Chou in a typical natural floodplain wetland. The results are as follows: (1) Root oxygen respiration rates are strongly correlated with leaf net photosynthesis rates of the two plant types, particularly under flooding conditions during the growing season; (2) Meadow species with a preference for wet microhabitats has a competitive advantage over first-year invading shrub species during flooding events; and (3) an aerenchyma sub-model could improve the eco-hydrological model’s accuracy in capturing plant physiological responses. These findings have the potential to contribute to the management of wetland and its restorations.

Sediment Transport Management Using the Planned Construction of the Lower Vistula Cascade as an Example

Construction of the Włocławek Barrage on the Vistula River in 1970 became an important source of energy; however, it discontinued sediment transport in the Vistula River. This phenomenon resulted in accumulation before and erosion below the barrage; a similar problem can be expected due to the planned construction of the Lower Vistula Cascade. This study is dedicated to finding an effective and feasible solution to sediment transport management, acceptable to a wide group of users (e.g., navigation, energy producers, and ecologists). A numerical 1D model was applied to carry out analysis of hydrodynamics and sediment transport in the river section from Włocławek Barrage to the Vistula River mouth in the Baltic Sea. For calibration, historical data describing the accumulation and erosion created by the Włocławek Barrage were used. The proposed model allowed incorporation of new barrages chronologically, in accordance with the proposed time schedules. Analysis was carried out in a 50-year horizon, using a set of historical flow data. For sediment flushing, it was proposed to use natural flooding events exceeding Q10% discharge and to control sediment management of the adjacent barrages. To use this method in practice requires verification.

A synthetic multivariate exponentially weighted moving average scheme for monitoring of bivariate Gamma distributed processes

AbstractSeveral scholarly works intrinsically assume that the frequency and magnitude of events are two independent variables . However, in many practical situations, it is necessary to consider the dependence between these two characteristics of events. In this paper, two Smith–Adelfang–Tubbs (SAT) models, namely, the four‐ and the five‐parameter bivariate Gamma distributions (BGD) with a particular dependency parameter, are first reviewed. Then, a synthetic multivariate exponentially weighted moving average (synthetic MEWMA, or SMEWMA) type scheme is proposed to monitor the individual BGD observations (subgroups of size one) generated from those two specific SAT models. The Monte‐Carlo simulation method is developed to evaluate the run length () properties of the recommended SMEWMA BGD scheme in both the zero‐state and the steady‐state cases. Furthermore, we discuss the average run length (ARL) of the proposed SMEWMA BGD scheme with different design parameters, and then compare both the zero‐state and the steady‐state out‐of‐control ARL performances of the recommended SMEWMA BGD scheme with those of the classic MEWMA chart, the paired individual Gamma chart, and the MCUSUM chart. Simulation results suggest that the overall performance of the recommended SMEWMA BGD scheme is superior to the other comparative schemes. Finally, the implementation of the proposed SMEWMA BGD scheme is illustrated with a natural flood event dataset .

How daily groundwater table drawdown affects the diel rhythm of hyporheic exchange

Abstract. Groundwater table dynamics extensively modify the volume of the hyporheic zone and the rate of hyporheic exchange processes. Understanding the effects of daily groundwater table fluctuations on the tightly coupled flow and heat transport within hyporheic zones is crucial for water resources management. With this aim in mind, a physically based model is used to explore hyporheic responses to varying groundwater table fluctuation scenarios. The effects of different timing and amplitude of groundwater table daily drawdowns under gaining and losing conditions are explored in hyporheic zones influenced by natural flood events and diel river temperature fluctuations. We find that both diel river temperature fluctuations and daily groundwater table drawdowns play important roles in determining the spatiotemporal variability of hyporheic exchange rates, temperature of exfiltrating hyporheic fluxes, mean residence times, and hyporheic denitrification potentials. Groundwater table dynamics present substantially distinct impacts on hyporheic exchange under gaining or losing conditions. The timing of groundwater table drawdown has a direct influence on hyporheic exchange rates and hyporheic buffering capacity on thermal disturbances. Consequently, the selection of aquifer pumping regimes has significant impacts on the dispersal of pollutants in the aquifer and thermal heterogeneity in the sediment.

Elaborate simulations and forecasting of the effects of urbanization on karst flood events using the improved Karst-Liuxihe model

Urbanization, especially land use pattern changes, has a great impact on natural flood events in a karst basin. Quantitative simulations of the effects of urbanization on karst flood events and forecasts of future evolution trends are difficult based on the current hydrological models. These models often require a large amount of data when used in karst areas due to the complex model structure and model parameters. To overcome the limitations of model applications, a physically based and fully distributed karst hydrological model, i.e., the improved Karst-Liuxihe (IKL) model, was proposed in this study to simulate and forecast karst flood events under the influence of urbanization. This IKL model was developed through the overall improvement of the Karst-Liuxihe (KL) model. The main additions were the improved runoff generation algorithm and the underground river confluence module. The karst flood simulation results of the IKL model were much better than those of the KL model: the average values of the Nash–Sutcliffe coefficient, correlation coefficient, and coefficient of the water balance increased by 23%, 23% and 26%, respectively, while the process relative error, flood peak flow relative error, and flood peak flow time error decreased by 21%, 22%, and 3 h, respectively, which confirmed that the improvements to the model were effective and feasible. Therefore, this paper used the IKL model to simulate karst flood events and annual runoff under the influence of urbanization based on 4 extreme land use scenarios. In addition, the model effectively forecasted the future runoff in 2030, 2040, and 2050. The results indicated that the critical proportion of urbanized area throughout the watershed area was 45%. When the urban proportion exceeded 45%, waterlogging could occur in the study area.

Rural households’ vulnerability and risk of flooding in Mbire District, Zimbabwe

Flood risk studies remain focused on understanding causation, impacts, perceptions and coping and adaptation measures and there remain many inappropriate measures taken to reduce household risk. Through a case study approach, this study assessed rural household vulnerability to flooding which helped to determine the level of risk to flood hazards in Mbire District of Zimbabwe. Three hundred and thirty-five (335) households were randomly selected to participate in the survey. The composite index approach was used to calculate vulnerability and risk indices. Results indicated that more than 70% of the households were highly vulnerable to flooding and about 50% were in the medium to very high categories of flood risk. Household vulnerability was not only a question of the natural flood event, but a result of the intersection of poor socio-economic conditions and the physical exposure to floods. The socio-economic determinants of vulnerability included high levels of unemployment resulting in low income and poverty; low levels of education; and over-dependence of the community on rain-fed smallholder farming. Physical exposure of the community mainly resulted from materials used to construct houses. Pole and dagga houses had a higher degree of exposure compared to houses with bricks and cement. Therefore, we recommend that instead of focusing on the whole ward/district the institutional services should first target the most vulnerable and at risk households in order to increase their capacity in dealing with flood hazards.

Morphometric and morphological analysis of Populus nigra L. leaves in flooded regions

Human activity induced global changes in nature, particularly the flooding regime, which is a stress factor affecting wetland ecosystems. Knowledge of plant strategies of wetland vegetation across a range of flooding gradients is therefore very important. Natural flooding events are increasing as a consequence of constant climate changes. This research was focused on the area of Special Nature Reserve “Gornje Podunavlje” which represents a complex of peculiar marshland, originating from former vast inundated parts of the Danube basin. We have selected samples located on both sides of the embankment in the defended and in the flooded area. The main aim is the assessment of Populus nigra L. riparian tree leaf morphological traits variability (centroid size, shape, developmental instability, specific leaf area and petiole lenght) between two habitats (flooded and not flooded). The geometric morphometry methods were applied to provide visual representation of differences in the leaf shape. We employed leaf composite index indices of fluctuating asymetry as a measure of developmental instability. A statistically ­significant impact of flooding was obtained only for the leaf geometric size and shape, so these traits could be an indicator of flooding as suboptimal environmental conditions.

Channel recovery in a regulated river: Effects of an experimental and natural flood in the Snowy River, SE Australia

AbstractExperimental floods, generated downstream of dams, are used to recover specific bio‐geomorphic functions in regulated rivers. Studies of the effects of experimental floods vary in their objective, location, and the hydrological and bio‐geomorphic variables used to quantify recovery. Measurements of geomorphic change are required to guide future release strategies. The focus of this study was to determine if a large experimental flood in the Snowy River Australia, could promote geomorphic recovery of the river channel downstream of Jindabyne Dam following 35 years of flow regulation. The objectives of the release were to deepen, widen, and increase channel capacity and coarsen the riverbed substratum in the Jindabyne Gorge and Dalgety Uplands sections of the Snowy River. Data from the release were compared with that of a natural flood event that occurred after the experimental flow event. Both events showed channel adjustments and a degree of geomorphic recovery, but this varied between the two river sections. Marked channel adjustments occurred in the Dalgety Uplands reach following both the experimental and natural flood event and in the Jindabyne Gorge section following the natural flood event. Geomorphic changes were related to the hydrological character of each flood event. The number of flood peaks, the sequence of peaks, the flood duration, and the total energy expenditure differed markedly between the two events, and these four flood hydrological characteristics explained the greater geomorphic recovery associated with the natural flood event in the Jindabyne Gorge. No clear hydro‐geomorphic relationship was derived for channel change in the Dalgety Uplands where existing morphological constraints limit flood effectiveness.

Fractal characterization of sediment particle size distribution in the water-level fluctuation zone of the Three Gorges Reservoir, China

The combined effect of periodic water impoundment and seasonal natural flood events has created a 30 m high water-level fluctuation zone (WLFZ) around the Three Gorges Reservoir (TGR), China, forming a unique eco-landscape. Siltation, eutrophication, enrichment of heavy metals, and methane emissions in the WLFZ have been widely associated with sediment and soil particles generated from the upstream catchment or upland slopes. However, little attention has been paid to the complexity of sediment particle-size distributions in the WLFZ. In the present study, core samples (from a 345 cm thick sediment core from the base of the WLFZ), slope transect surface samples (across/up a WLFZ slope), and along-river/longitudinal surface samples (from the reservoir reaches) were collected. Laser granulometry and a volume-based fractal model were used to reveal the characteristics of sediment particle-size distributions. Results indicate that the alternation of coarse and fine particles in the sedimentary core profile is represented as a fluctuation of low and high values of fractal dimension (D), ranging from 2.59 to 2.77. On the WLFZ slope transect, surface sediment particles coarsen with increasing elevation, sand content increases from 3.3% to 78.5%, and D decreases from 2.76 to 2.53. Longitudinally, surface sediments demonstrate a downstream-fining trend, and D increases gradually downstream. D is significantly positively correlated with the fine particle content. We conclude that D is a useful measure for evaluating sediment particle-size distribution.

Decadal-Scale Variations of Thalweg Morphology and Riffle–Pool Sequences in Response to Flow Regulation in the Lowermost Mississippi River

The lowermost Mississippi River (LMR) is one of the largest deltaic systems in North America and one of the heavily human-manipulated fluvial river systems. Historic hydrographic surveys from the mid-1900s to the early 2010s were used to document the thalweg morphology adjustments, as well as the riffle–pool sequences. Extensive aggradation was observed during 1950s to 1960s, as the Atchafalaya River was enlarging before the completion of the Old River Control Structure (ORCS). Following the completion of the ORCS, reductions in sediment input to the LMR resulted in net degradation of the thalweg profile patterns since the mid-1960s except for the 1992–2004 period. Different flood events that supplied sediment might be the cause of upstream aggradation from 1963–1975 and net aggradation along the entire reach from 1992–2004. Furthermore, the change pattern of thalweg profiles appear to be controlled by backwater effects, as well as the Bonnet Carré spillway opening. Results from riffle–pool sequences reveal that the averaging Ws ratios (length to channel width) are 6–7, similar to numerous previous studies. Temporal variations of the same riffles and pools reveal that aggradation and degradation might be heavily controlled by similar factors to the thalweg variations (i.e., sediment supply, backwater effects). In sum, this study examines decadal-scale geomorphic responses in a low-lying large river system subject to different human interventions, as well as natural flood events. Future management strategies of this and similar river systems should consider recent riverbed changes in dredging, sediment management, and river engineering.

Sediment replenishment combined with an artificial flood improves river habitats downstream of a dam

River reaches downstream dams where a constant residual flow discharge is imposed, often lack sediment supply and periodic inundation due to the absence of natural flood events. In this study, a two-year return flood was released from an upstream reservoir and combined with sediment replenishment to enhance instream habitat conditions downstream of Rossens hydropower dam on the Sarine River in western Switzerland. Sediment replenishment consisted of four sediment deposits distributed as alternate bars along the river banks, a solution which was previously tested in laboratory. The morphological evolution of the replenishment and of the downstream riverbed were surveyed including pre- and post-flood topography. A hydro-morphological index to evaluate the quality of riverine habitats, based on the variability of flow depth and flow velocity in the analyzed reach, was investigated. The combination of the artificial flood with sediment replenishment proved to be a robust measure to supply a river with sediment and to enhance hydraulic habitat suitability.

A dam in the drylands: Soil geomorphic treatments facilitate recruitment of the endangered Santa Ana River woolly star

AbstractThe long‐term management of aridland riparian ecosystems impacted by dams is crucial to reduce losses of biodiversity, reduce extinction risks for species, and restore ecosystem services. When dams preclude natural flow, safeguarding aridland riparian ecosystems adapted to infrequent, catastrophic floods poses additional challenges owing to the need to consider biological and pattern legacies. Seven Oaks Dam (California, USA) eliminated the occurrence of flooding, scouring, and deposition across the rare plant community downstream. This Riversidian alluvial fan sage scrub or alluvial scrub includes one of the most endangered plants in California, Eriastrum densifolium spp. sanctorum (Santa Ana River woolly star, hereafter called woolly star). In this study, we evaluated the impact of six soil geomorphic treatments on alluvial scrub and woolly star re‐establishment after 5, 7.5, and 13 yr. We implemented a complete randomized block design, with each block incorporating six treatments: cleared, diked, cut, filled‐10 (10 cm soil), filled‐20 (20 cm), and filled‐30 (30 cm), mimicking one or more physical disturbances (pattern legacy) occurring after a natural flood event. We performed plant community surveys (cover, abundance, maturity, invasibility, diversity) on full plots in 2006, representing 7.5 yr of response from the original 1999 treatment, and on half‐plots in 2012, representing 5 yr of response following re‐disturbance in 2007, and 13 yr of response on half‐plots left intact since 1999. We found very limited recruitment of woolly star into control plots (1.2% cover). By contrast, the cut treatment showed consistently higher cover of woolly star (25.3%, 53.4%, 14.3%), after 5, 7.5, and 13 yr, respectively. Other treatments showed responses ranging between these extremes. Similar results were found for total native cover and diversity. Woolly star cover was inversely related to alien grass cover, suggesting that exotic grass invasions inhibit early recovery of the perennial. The re‐establishment of base flows and flood pulses or soil geomorphic disturbances that mimic pattern legacies associated with infrequent, catastrophic flooding could be implemented to sustain downstream ecosystems that include woolly star populations. In the absence of such actions in the Santa Ana River floodplain, the persistence of woolly star as a species appears unlikely.

Patch age since disturbance drives patch dynamics for flycatchers breeding in both reservoir and riverine habitat

AbstractSpecies dependent upon early‐successional landscapes often occupy patches at different stages of recovery after disturbance. The demographic processes that drive patch dynamics in these systems have rarely been described but are important for developing effective conservation and management plans, especially when humans have modified the timing and intensity of disturbances that drive regeneration. In riparian systems, disturbance by floods historically initiated plant regeneration, but many rivers are now regulated and stream flows disrupted by dams and reservoirs. We studied the demography and patch dynamics of an endangered, neotropical migrant bird dependent on remnant riparian patches for breeding, the southwestern willow flycatcher (Empidonax trailli extimus), over 9 yr at both a riverine and reservoir site in central Arizona. We found that at both sites, number of territories/ha within patches increased for 2–4 yr after colonization and then declined, with several patches abandoned after 6–10 yr. Age of birds increased with patch age, with younger birds in colonizing patches and older, site‐faithful birds in older patches, while mean per capita reproductive success did not differ with patch age. Natal dispersal and breeding dispersal were primarily from intermediate‐aged patches into either young‐ or other intermediate‐aged patches. At both riverine and reservoir sites, both the number of patches and the number of territorial birds increased over time, with the percentage of territories shifting into younger and younger patches. The type of disturbance driving patch regeneration differed between riverine and reservoir sites (seasonal flooding vs. falling lake levels due to drought), but the demographic patterns did not, indicating that reservoirs can generate patch dynamics similar to those on rivers. Managing stream flows and reservoir levels to maintain disturbance cycles sufficient to generate riparian patches at different stages of regeneration through time would benefit succession‐dependent species like the endangered flycatcher we studied, whether those disturbances arise from natural flooding events along free‐flowing rivers or through changes in reservoir levels.

Fish age and size distributions and species composition in a large, hydropeaking Prairie River

AbstractFluctuations in river flows result from diverse natural and/or anthropogenic causes. Hydropeaking, an important anthropogenic flow alteration, results from the rapid increase or decrease of water releases from reservoirs at hydroelectric power stations to meet variable demand for electrical power, thereby altering the flow regime of the river downstream of the hydroelectric power station. Hydropeaking causes short‐term, artificial fluctuations in flow on an hourly, daily, and/or weekly basis. The frequent and regular occurrences of these high and low flow events are fundamentally different from natural flood and drought events and may affect fish fauna. We compared the fish species composition and fish age and size distributions in the Saskatchewan River (Saskatchewan, Canada) downstream of a hydropeaking facility with results from an unaffected Reference Site situated upstream of the reservoir. Lower fish diversity was observed in the 2 downstream sites (Sites 1 and 2, number of species = 11 and 9, respectively) closest to Generating Station in comparison to Site 3 (n = 15) situated further downstream and the upstream reference site (n = 13). There was no difference in the age–length relationship of any of the fish species above and below the Generating Station suggesting that fish grew at the same rates. However, lower numbers of small‐bodied and juvenile fish were caught downstream of the Generating Station suggesting the possibility of increased mortality, decreased habitat suitability, or altered behaviour of small fish downstream of the dam. These data illustrate potential impacts of hydropeaking power stations and has management implications.

Community responses to dam removal in a subtropical mountainous stream

Dam removal has the potential to efficiently solve the problems caused by fragmented stream habitats but may simultaneously cause negative impacts on biotic communities. To conserve the critically endangered Formosan landlocked salmon (Oncorhynchus masou formosanus), a 15-m-tall check dam was partially removed from the Chichiawan Stream at the end of May 2011, before the flood season. Using this dam removal experience, we aimed to cast dam removal as an action comparable to a natural flood event. We applied a before-after-control-impact (BACI) design and quantified the environmental factors and major biotic communities at four sampling sites in the stream bimonthly before (2010) and after (2012 and 2013) the dam removal and monthly in the year of the dam removal (2011). After the dam removal, a faster current velocity and more turbid water were observed at the downstream sites, and the area’s deposition consisted of small-grained sediments. Despite this, our results show that the dam removal was performed during a suitable period. There was no obvious influence on tadpoles as they metamorphosed into adult frogs and left the stream before the dam removal. Fish exhibited a greater resistance to the alteration in flow resulting from the dam removal. An increase in fish abundance at the upstream sites after the dam removal suggests that the corridors created by the dam removal allowed access to more habitats for the fish. In particular, the periphyton biomass and aquatic insect densities decreased markedly at the downstream sites after the dam removal, but they recovered within a year, demonstrating the resilience of these taxa. Coleoptera, Plecoptera and Trichoptera were more resistant than the periphyton, Diptera and Ephemeroptera after the dam removal and an extreme flood event. In conclusion, the responses of stream communities to dam removal were similar to the responses to an extreme flood event. To mitigate the impacts caused by dam removal, our results suggest that stream communities may respond to dam removal as a natural flow alteration if the timing of the dam removal occurs just before the flood season.

Waterbirds in a floodplain: influence of spatial and environmental factors through time

Wetlands are rapidly being lost and fragmented around the world, making it imperative to seek an understanding of the drivers of their diversity. Among aquatic assemblages, birds constitute a conspicuous group that provides many ecosystem services. Here, we use a metacommunity approach to understand the influence of spatial (distance among patches) and environmental factors (local characteristics) in wading bird (Pelecaniformes and Ciconiiformes) assemblages in a river-floodplain system through time. We tested the hypothesis that, due to the small scale of our study, spatial factors have no determinant role in waterbird assemblages, and, due to the annual occurrence of flood pulses, the importance of environmental factors varies through time, according to the hydrological cycle. We tested this hypothesis using Partial Redundancy Analysis (pRDA). We used abundance data for the birds in 20 lagoons, sampled quarterly during two years. The spatial factors did not explain variation in community structure in any sampled month, whereas environmental factors explained variation in the assemblages only in two months. Due to high waterbird mobility, the non-significance of the spatial factor is expected among lagoons in the same floodplain. Environmental factors are important in determining the community structure only in two sampled months, evidencing that their importance varies through time in the floodplain, partially agreeing with our hypothesis. The non-conformity between the influence of environmental factors on assemblages and the hydrological cycle may be due to human impacts caused by the operation of upstream reservoirs, which alter the natural flood events, and caused a long drought period previous to this study. A multiscale approach is fundamental to the understanding on how anthropogenic impacts on wetlands affect waterbird assemblages. Thus, this study contributes to the understanding of how seasonality, environmental conditions of lakes, and a local spatial scale act in structuring waterbird assemblages.

Listen to the sound of moving sediment in a small gravel-bed river

In order to assess the dynamics of rivers, a reliable characterization of bedload transport particularly during unsteady flow regimes is required. In contrast to highly energetic cases in hillslope areas, we aim to answer the question whether the usage of acoustic measurements can improve the characterization of bedload in small rivers draining low land mountains with comparatively low water discharge and bedload. In addition to the investigation of natural flood events, controlled floods were generated by releasing water from a reservoir into a small gravel-bed stream. The controlled releases allow for an evaluation of bedload solely from channel storage or bank erosion. For acoustical in-situ characterization of bedload transport, hydrophones were mounted onto the bottom side of steel plates, thus recording the impacts of sediments via the acoustic vibrations on the surface of the plates while at the same time minimizing the disturbing noise resulting from water turbulence. Corresponding bedload traps are removable boxes with open lids fixed in the riverbed so that bedload material registered by the hydrophone is trapped. The acoustic signals correlate well with the quantity of the transported material. During summer flood events the highest transport rates occur at the beginning of the rising limb featuring clockwise hysteresis. This is due to the rising transport energy of the flow and the presence of loose, unconsolidated material. During typical winter flood events bedload shows anticlockwise loops. The intensification of bedload conveyance after the runoff peak can be explained by a decreasing stability of the bed material from the beginning to the end of a transport event. Anticlockwise behavior also results from a combination of bedload exhaustion in the vicinity of the monitoring station with a delayed arrival of new material from distal sources later in the hydrograph.