Natural Flood Regime Research Articles

Public preference for the rewilding framework: A choice experiment in the Oder Delta

Abstract Rewilding is an emerging paradigm in restoration science and is increasingly gaining popularity as a cost‐effective ecosystem restoration option. A rewilding framework was recently proposed that contains three integral components: restoring trophic complexity, allowing for stochastic disturbances and enhancing species' potential to disperse. However, as of yet, there has been limited quantitative analysis looking at public preference for rewilding and each of its elements. We used a discrete choice experiment approach to determine public preference for rewilding in the Oder Delta. The unique geographical context of the Oder Delta, spreading evenly across two countries, allowed us to analyse differences between the German (n = 1005) and Polish (n = 1066) samples. In both countries, we found that respondents were willing to pay for rewilding interventions when compared against a status quo option. Notably, preferences were strongest for restoring trophic complexity through promoting the comeback of large mammals. In addition, we found respondents living locally to the study region had significantly different preferences than the nationwide samples, exhibiting negative willingness to pay for the restoration of natural flooding regimes and the presence of large predator species. Read the free Plain Language Summary for this article on the Journal blog.

Flood pulses and fish species coexistence in tropical rivers - a theoretical food web model

AbstractFreshwater fish diversity reaches its zenith in large tropical rivers. Although the origins of this diversity have been relatively well studied, the mechanisms that maintain high fish diversity in tropical rivers remain largely unknown. It has been hypothesized that the annual flood pulse, a perennial feature of many lowland rivers in the tropics, reduces competitive exclusion and consequently promotes species coexistence. During the high-water season, superabundant allochthonous resources and relatively low fish density may reduce intra- and interspecific competition. During the low-water season on the other hand, resource availability is low and predation pressure high, offsetting competitive differences between species and controlling fish population sizes. Here, we tested the potential role of these mechanisms for species coexistence using a food web model where fish species exhibit strong differences in competition strength and compete for finite resources. We simulated fish interactions when a regular flood pulse was either present or absent. We found that extinction rates in the simulations without an annual flood pulse were consistently higher when compared to those with a flood pulse, indicating more species could coexist when a regular annual flood pulse is present. If the flood pulse is a relevant mechanism for fish species coexistence, as our model results suggest, then flood pulse changes could result in species extinctions and lower fish diversity. Deforestation, climate change, and the construction of large hydropower dams are current drivers of hydrological change across the tropics, increasing the urgency to understand the role of natural flooding regimes for the maintenance of tropical freshwater fish diversity.

Environmental and spatial determinants of fish community structure in an Afro‐tropical river ecosystem

AbstractWe investigated the relative influence of local environmental and spatial factors in structuring the community composition of fish at 15 sampling sites along the longitudinal gradient of the Lower Niger River Basin (LNRB) in dry and rainy seasons using distance‐based redundancy analysis and variation partitioning analysis. We collected a total of 3807 fish specimens representing 42 species. Our result indicated that the fish community composition differed between the upper and lower regions of the river. The communities in the upper region is influenced by high‐nutrient concentrations, while downstream sites were characterized by high concentrations of suspended solids. Variation partitioning revealed higher contributions of spatial than environmental predictors on fish community composition, with a higher total predicted variance in dry season. The variations in the community composition between upper and lower region may be attributable to the differences in the nature of anthropogenic activities within the regions, which influenced the local conditions differently. Differences in flow dynamics between upper and lower regions as attributable to black and white floods in the LNRB modify the connectivity between sites. Dispersal among sites may be more limited downstream than in the upper region, particularly in the dry season, because damming in the upper region also interrupts the natural flood regime such that there are low water levels in the lower region, which spatially isolate fish communities at certain sampling sites. The relatively higher total predicted variance during dry season may be attributable to the temporal differences in abiotic conditions between sites, which may have influenced site level community composition and abundance differently.

Spatial Variability of Topsoil Properties on a Semi-Arid Floodplain

This study relates the spatial heterogeneity (or patterning) of geochemical elements in the topsoil of a semi-arid floodplain/hillslope system in north-eastern Australia to vegetation distribution and rates of flood inundation. A total of 540 topsoil samples were collected from six flood frequency zones, ranging from a frequently flooded area (RI = 1:1–2 yrs) to two zones that have not flooded in living memory (RI > 50 yrs). Within each zone, topsoil samples were collected from both vegetated and non-vegetated surfaces, and each sample was analysed for 26 parameters. A combination of multi- and univariate analyses reveals that vegetation is an important contributor to topsoil heterogeneity. In zones subject to relatively frequent flooding, the spatial distribution of parameters in the topsoil is greatly influenced by the movement of water, with vegetation acting as a sink rather than a source. However, as floods become increasingly rare, distinct resource-rich units become evident in the topsoil beneath the vegetation. These findings indicate that topsoils in semi-arid floodplains are altered when their natural flooding regimes are reduced, beginning to approximate hillslopes when flood frequencies exceed 1-in-7 to 10 years. This points to the need for frequent flood (overbank) releases that are able to cover the 1-in-20-year floodplain to maintain the character of the soils and support vegetation growth in these environments.

Simulated effects of floodplain restoration on plant community types

AbstractAimsChannelization and artificial embankments have altered the natural flood regime of many rivers, impacting the hydrological characteristics of floodplain ecosystems and their biological communities. This study was undertaken on a floodplain meadow to assess spatial patterns of plant communities in relation to soil physical and chemical conditions, and the impacts of floodplain restoration that involved embankment‐removal.LocationRiver Glaven, Hunworth, Norfolk, UK.MethodsFine‐scale plant and soil chemistry sampling was conducted prior to embankment removal, and hydrological and climatological conditions were monitored prior to and after embankment removal. Hydrological/hydraulic modelling simulated groundwater levels for a 10‐year period to assess changes in soil aeration stresses and plant community composition following embankment‐removal.ResultsHydrology was identified as the primary driver of plant community composition. Soil fertility was also important. Unique continuous measurements of vadose dissolved oxygen concentrations using oxygen optodes indicated strong coupling between water table depth and root zone dissolved oxygen concentrations. Reinstatement of overbank flows did not substantially affect aeration stress across most of the meadow because of pre‐existing wet conditions. However, along the river‐floodplain ecotone, aeration stress increased substantially from conditions normally associated with dry grassland to those characteristic of fen communities (p < 0.05).ConclusionsThis restored water table regime may be suitable for more diverse plant assemblages. Benefits of flooding for increased species richness and transport of propagules may, however, be over‐ridden without accompanying water level management during the growing season, or hay removal to balance additional supply of nutrients from river floodwater and sediment. Our results show that hydrological/hydraulic modelling combined with quantitative measures of plant water‐requirements can provide practical and adaptive management tools to estimate the response of floodplain communities to changing water regimes.

ASSESSMENT OF IMPACTS OF THE ROAD SYSTEM ON FLOOD REGIME IN THE COASTAL FLOODING AREA OF VU GIA - THU BAN RIVER BASIN, QUANG NAM PROVINCE

Flooding is one of the dangerous types of natural disasters, which has been causing great damage to human values and even human life, leading to negative impacts on socio-economic development, specifically in the lowland areas of the Vu Gia - Thu Bon river basin. The road system contributes to promoting intra-regional and inter-regional connectivity and is an important factor for socio-economic development. However, in certain cases, the road system continues to be expanded and enhanced, turning them into "dyke systems", interfering with the natural flood regime, increasing the area and flooding time. To assess the impact of the road system on the flood regime, the paper uses the Mike Flood modeling module to simulate the change of flood regime for flood events in 1999, 2007, 2020, and the 5% flood scenario. The comparison between the current road system and the old road system shows a certain increase in both depth and flooded area, especially in the case of 5% flood, the flood depth increases by 0.1 - 0.2 m in the upper part of the basin but only a small change in the lower part of the basin from 0.01 - 0.04 m.

The Okavango Delta’s waterbirds – Trends and threatening processes

The Okavango Delta is renowned as an extraordinary ecosystem of high biodiversity, listed as both a Ramsar and World Heritage Site, with part protected in the Moremi Game Reserve. This extensive floodplain ecosystem has 444 recorded bird species, with just under a quarter of these waterbirds, including at least 16 breeding and 4 threatened (1 endangered, 3 vulnerable) species. Despite the global importance of this ecosystem, and its transboundary nature, there are surprisingly few long-term assessments of status of the ecosystem or waterbird communities, a key indicator of ecosystem health, with threats such as upstream water extraction, and climate change threatening its outstanding biodiversity. We compiled a comprehensive 53-year dataset comprised of citizen science and other datasets (1970–2019), on 36 waterbird species (Anhingidae, Ardeidae, Ciconiidae, Gruidae, Pelecanidae, Phalacrocoracidae, and Phoenicopteridae), including eight waterbird breeding colonies in the Okavango Delta. We investigated trends in waterbird biodiversity as well as responses to temperature, flow, flooding, and local rainfall. Waterbird breeding colonies were associated with relatively high areas of riparian woodland, and experienced moderate flooding frequencies (> 1 in 5 years). Total abundance of all 36 waterbird species was positively related to river flows. Despite increased citizen science effort over time, total abundance within the Okavango Delta significantly declined with declining average inundation. Four species led these declines (African darter Anhinga rufa, green-backed heron Butorides striata, slaty egret Egretta vinaceigula, squacco heron Ardeola ralloides) and one marabou stork Leptoptilos crumenifer, increasing (only sufficient data to analyse 15 species individually). Decreased inundation within the Delta and other internal factors (urbanisation, tourism, vegetation change) as well as external factors (habitat loss elsewhere) are likely driving these declines. Rigorous monitoring of waterbirds, including the eight breeding colonies across the Delta, is needed to explore these changes closely, providing baselines in the case of water resource developments on the rivers supplying the Okavango Delta. Long-term conservation of the magnificent Okavango Delta and its dependent biodiversity, including its waterbirds, is highly reliant on protection of river flows in three countries to ensure natural flooding regimes, alongside the conservation of neighbouring wetlands.

Amazon floodplain hydrology and implications for aquatic conservation

Abstract The amplitude, duration, frequency, and predictability of runoff and inundation of aquatic habitats are key hydrological characteristics linked to aquatic ecosystem functioning and biodiversity, but they are seldom integrated into analyses of Amazon floodplain ecology. Remote sensing approaches, measurements and modelling of floodplain hydrology provide a basis for this integration. Effective legislation to protect floodplains and other wetlands depends on operational definitions that require application of hydrological data. Extent and changes of flooded areas are linked to fish diversity and to presence and growth of flooded forests and floating plants. Dam construction reduces river system connectivity and modifies the flood pulse, with major negative implications for floodplain ecosystems adapted to and dependent on a natural flood regime. Trends and variability in climate plus deforestation are altering the Amazon's hydrological cycle, causing changes in discharge and flooded area with concomitant ecological impacts.

The legacy of management approaches and abandonment on old-growth attributes in hardwood floodplain forests in the Pannonian Ecoregion

In the absence of primeval floodplain forests, near-natural remnants are key references for close-to-nature forestry and nature conservation. Old-growth forest characteristics (OGCs) were quantified in 16 managed (rotation or selection) and abandoned semi-natural floodplain forests by taking snapshot inventories of structural features. Principal component analysis revealed a short gradient from closed-canopy managed stands dominated by large trees (mainly oaks) and hornbeams, with less deadwood, to abandoned older stands with more gaps, very large trees and different kinds of deadwood. All the studied forests are mainly in the early and late biostatic phases of forest development, with innovation and degradation phases almost completely absent. Results showed that selection forests had significantly more very large trees compared to rotation stands. The difference in structure between abandoned selection stands and abandoned rotation stands was more pronounced than in their counterparts that are still managed. Based on diameter distribution and the amount of very large trees and deadwood, abandoned selection stands were closest to the structure of natural forests. Selection forests may recover certain OGCs more rapidly than rotation forests. We concluded that after 100–150 years of homogenising rotation forestry, OGCs are slow to develop in just 25 years of non-intervention. Based on our results, we recommend: (i) an increase in the rotation cycle with selective logging in order to restore and maintain OGCs in managed rotation forests and (ii) total abandonment in certain well-preserved, near-natural forests with natural flooding regimes. However, the failure of oak to regenerate naturally remains a challenge.

Connectivity but not recruitment: Response of the fish community to a large‐scale flood on a heavily regulated floodplain

AbstractRecent environmental flow management in the Murray–Darling basin, south‐eastern Australia, has centred on restoring natural flooding regimes to meet ecological requirements, including the promotion of native fish recruitment, despite uncertainty about the effectiveness of managed environmental flooding. This study investigated recruitment responses of small‐bodied native and non‐native fish species to a large natural flood, supported by managed environmental flows in the Macquarie Marshes of the Murray–Darling basin. We surveyed the fish community within the heavily regulated, dryland floodplain system of the marshes, the year after breaking of the Millennium drought (2002–2009). During the spring–summer of 2010, the Macquarie Marshes experienced a one‐in‐10‐year flood, sustained by two environmental flow allocations, flooding 174,000 ha of floodplain. Five native species were collected across the nine study sites. Two non‐native species were particularly abundant during the flood, outnumbering the five native species in a ratio of 32:1, with Gambusia holbrooki the most abundant species, comprising 87% of all native and non‐native individuals captured. There was no marked effect of flood stage on the total abundance of small‐bodied native or non‐native fishes, but the composition of the fish community varied between surveys of the rising and falling phases of the flood. We did not capture any larval or post‐larval fish despite using a variety of suitable sampling methods. The lack of larvae in the Macquarie Marshes, following extensive flooding, most likely reflected the overall poor condition of native fish communities and low standing stocks of mature fish, resulting from deleterious long‐term effects of flow regulation exacerbated by drought. A gradual rebuilding of ecosystem health, fish health, and resilient fish populations by active management of environmental flows to impose more natural boom and bust dynamics may be needed to restore fish reproduction and recruitment in this regulated river.

Shifts in characteristics of the plant-soil system associated with flooding and revegetation in the riparian zone of Three Gorges Reservoir, China

The operation of the Three Gorges Reservoir (TGR), the largest hydropower dam in the world, has triggered a dramatic shift in the flooding regimes of sites upstream of the reservoir. Little is known about how disrupted flooding regimes and consequent management approaches might affect the ecological and biogeochemical characteristics of riparian ecosystems. In this study, we evaluated the effects of disruptions to natural flooding regime on basic soil properties, soil nutrient and heavy metal levels, and key characteristics of riparian plant and soil microbial communities. To do this, we used an elevational gradient that encompassed four flooding duration zones (0 (i.e., control), 169, 237, 286 days of flooding per year on average). The disrupted flooding regimes were associated with levels of soil total N and P that were on average 17% and 24% lower, respectively, than those in the non-flooded areas. On the other hand, the concentrations of heavy metals (Hg, Pb, Cu, Zn and Mn) were higher in flood-affected areas than in the non-flooded areas. Increased flooding frequency was also associated with lower plant diversity and species richness relative to non-flooded areas. Thus, disruption of the natural flooding regime had strong and often negative consequences for the ecological and biogeochemical properties of the riparian ecosystems in our study. There was some evidence that riparian plant communities were able to partially recover from prior flooding during a single growing season, even after nine years of repeated flooding, and these recovery trajectories were associated with shifts in soil chemical properties during the same period. However, revegetation efforts had few effects on ecosystem properties or their recovery trajectories following flooding events, suggesting that natural regeneration could be a useful option for the management of these sites. We conclude that the unnatural flooding regimes associated with large scale reservoir development are likely to have profound impacts on the structure and functioning of riparian ecosystems, and these will pose a considerable challenge for environmental management and biodiversity conservation.

Conservation status and bio-ecology of Brycon orbignyanus (Characiformes: Bryconidae), an endemic fish species from the Paraná River basin (Brazil) threatened with extinction

ABSTRACT Brycon orbignyanus is an endemic species from La Plata basin whose stocks have been presenting significant reductions throughout the Paraná River. Brycon orbignyanus is categorized as an endangered species. This study evaluated aspects of the bio-ecology of this species that may be related to this threat, highlighting its distribution, abundance, and diet as well as the corresponding relationships between its recruitment and flood regimes. Data were obtained from different parts of the upper Paraná River (stretches free and regulated by dams) from 1986 to 2010 with more detailed data collected from the free remnant of this basin. The results indicate that no records for species exist at more than half of the sampling points located in dam-regulated sections of the Paraná River, whereas specimens were collected from 75% sites in the free plain remnant. We observed a remarkable effect of the hydrological regime on recruitment as well as distinct food demands during ontogenetic development, with adults almost exclusively consuming fruits and seeds, revealing that these individuals are supported by riparian vegetation. Thus, it is concluded that changes in the natural flood regime as well as riparian vegetation removal threaten B. orbignyanus populations in the Paraná River basin.

A functional group approach reveals important fish recruitments driven by flood pulses in floodplain ecosystem

The periodical flood pulses, assisting by the connectivity between rivers and lakes in floodplain ecosystems, are of paramount importance in underpins fish meta-communities. Regardless of the unprecedented anthropogenic disturbances on natural flood regimes, there is limited understanding for the response of fish assemblages to flood pulses. Most studies documented the issue sorely on floodplain rivers or floodplain lakes, but the dynamics of fish communities in the transitional floodplains accounting for both lentic and lotic environments are rarely reported. Here we measure temporal (high and low water seasons) and spatial (lotic and lentic environments) patterns of fish assemblages in the transitional floodplain between the Yangtze River and Lake Caizi using taxonomic and functional group metrics. Our results demonstrated significantly variations in fish community composition among different environments between high and low water seasons. During flooding season in high water phase, we observed significantly patchy population recruitments of riverine fish species. Then in low water season, widespread lake residents dominated the lentic environments due to their higher environment tolerance. Benefitting from the functional group measurement, we identified three flooding related functional groups i.e., G2, G9 and G10 (consisting of riverine catfishes, gudgeons, and loaches), that may bear the brunt of natural flood pulses alterations. Despite the significant variations of richness in taxonomic diversity, we detected higher sensitivity of functional diversity, since three functional diversity indices were significantly higher in high water season. This findings demonstrated important functional recruitments triggered by flood pulses, highlighting a pressing need to sustain natural flood regime with hydrological connectivity for better conservation of fish assemblages in floodplain ecosystem.

Artificially Induced Floods to Manage Forest Habitats Under Climate Change

Global change is affecting agroforestry and its inherent ecosystems in Sweden. Here we examine the benefits of ecologically adjusted dam regulations to conserve biodiversity under climate change in floodplain habitats, including meadows and riparian mixed forests. The natural flood regime in snow-dominated regions has changed significantly during the last decades, in line with the projections for climate change. The ecosystems of temporary flooded forests show high biodiversity but are dependent on river high flows with long duration. These events are rare in the new climate scenario, but on the other hand, snow-fed rivers are also affected by hydropower dams and regulations. In this study we explored the potential of using reservoir regulation to artificially induce flood events; water management would then be a method to conserve biodiversity in forest habitats and adapt management to climate change. We made detailed calculations in lower Dalalven River, central Sweden, using observed time-series of river flow and dynamic scenario modelling for highly valuable Natura 2000 habitats. Here we show that long-term flooding is less frequent since extensive hydropower was introduced during the 1920s, and moreover, since the 1990’s the spring floods are low due to low snow storage and short winter seasons. Sustainable management of 50% of the riparian forest requires flooding by 25 continuous days of 800 m3 s-1. We found that artificial floods using new ecological regulation regime of upstream hydropower reservoirs would help, but not be enough, to achieve this goal. The new regulation routines would correspond to a loss of 50-200 GWh in hydropower production for each artificial flood. Sustainable ecosystems in the study site do not request flooding every year, but some every fifth year. For practical implementation, the County Board is currently driving the process locally and we discuss the relevant social features, such as legal and funding aspects, of this adaptive management of water and forests. A smaller part of the forest could probably be rescued and costs could potentially be lowered by using only the most snow rich years and seasonal forecasting of river flow for optimal timing of water release from dams to induce flooding.

Early vegetation succession on gravel bars of Czech Carpathian streams

Rivers with a natural flooding regime and gravel accumulations are an important natural habitat endangered by regulations and other types of human impact. Succession after disturbances by floods creates a mosaic of different vegetation types, some of them containing rare specialist species. We studied vegetation succession and changes in plant diversity on river gravel bars of four streams in the Western Carpathians and their foothills in the eastern Czech Republic. This area experienced extreme 50-year flood event in May 2010. Gravel bar vegetation was destroyed, some of the former bars were covered by sediments, and some new bars arose. We sampled gravel bar vegetation two months after the floods and repeated the sampling on each site during the next three years. Initial vegetation has developed through a sparse and species-rich stage into denser stands with more shade-tolerant species. In the fourth year, tall herbs, such as Urtica dioica, Phalaris arundinacea and the alien Impatiens glandulifera, dominated the communities, but shrub vegetation started to develop only in a few places. Species capable of vegetative dispersal prevailed over species dispersed by seeds only. Altitude and size of gravel/stone particles were identified as important factors affecting vegetation succession. The succession ran faster on gravelly substrates at lower altitudes than on stony substrates at higher altitudes. Although the studied streams are partly influenced by human interventions and host only few gravel bar specialists, they are of considerable conservation importance.

To dam, or not to dam? Abolishment of further flooding impedes the natural revegetation processes after long‐term fluvial deposition of copper tailings

AbstractLong‐term potentials and constraints of nature to spontaneously recover after severe degradation by toxic mine waste, and general principles of mutual modifications of spontaneous vegetation and soils during this process, have remained widely unknown. This study investigates the effect of flooding on natural restoration of a model locality in Eastern Serbia, complexly degraded by 70‐years fluvial deposition of sulfidic copper (Cu) mine tailings in a floodplain along 80 km of the polluted river flow. We analyzed multivariate response of forest vegetation (floristic and structural parameters) together with physical and chemical properties of concomitant soils and sediments. Floods strongly modify the interactions between soil and vegetation. Three very different types of forest vegetation constitute the response of the nature to key soil adverse factors (excessive Cu availability, low nutrients, and low pH); combined with drought, these constraints completely inhibit revegetation. Continual flooding after mine closure, despite the pollution it still brings in, fosters a faster development of highly specialized vegetation and most importantly faster buildup of soil organic matter necessary for phytostabilization of substantial amounts of Cu tailings present in the floodplain. We show that enhanced nutrient fluxes facilitated by natural flooding regime might overrun the constraining effect of deposited Cu also for natural vegetation.

Conservation of the Mekong Delta wetlands through hydrological management

AbstractThe Vietnamese Mekong Delta (VMD) comprises extensive river and canal networks, both natural and man‐made, and has a history of extensive land use change and development. The delta's wetlands are under considerable ecological stress because of hydrological changes, agricultural and aquaculture development, urban and industrial pollution, climate change, and upstream water resource development. In this paper, we review the current threats and challenges to the conservation and management of the wetlands in the VMD. We recommend that the current water management practices of the Ramsar‐listed Lang Sen Wetland Reserve are changed so that the natural flood regimes are reinstated and the risks to the community from forest fires are managed. Ongoing investment is needed to support further research, set up long‐term monitoring, and to develop hydrodynamic models for the Lang Sen Wetland Reserve so that management and conservation efforts can achieve the specific objectives for the wetland. This approach may be useful for managing other wetlands across the VMD.

Differential responses by stream and riparian biodiversity to in‐stream restoration of forestry‐impacted streams

Summary Forestry can have detrimental impacts on stream ecosystems, particularly via excessive sedimentation. A key challenge to stream management is therefore to identify the best restoration practices to mitigate the harmful impacts of fine sediments on stream biodiversity and ecosystem functioning. We studied the effects of restoration of sediment‐impacted headwater streams on the habitat structure, hydrologic retention, biodiversity (microbes, bryophytes, benthic macroinvertebrates, riparian plants) and ecosystem functions (periphyton accrual rate and leaf breakdown) by comparing four treatments: wood‐restored, boulder‐restored, impacted (by fine sediments) and near‐natural streams. The restored streams were sampled 3–7 years post‐restoration. Restoration by wooden or boulder structures aimed to reduce deposited sediments and increase channel heterogeneity and hydraulic retention. Wooden structures were ineffective in removing fine bed sediments and did not induce positive responses in aquatic biota. Boulder additions reduced substrate limitation and thereby proved beneficial for aquatic bryophytes. Benthic macroinvertebrates were clearly impaired by sedimentation but responded weakly to restoration. Leaf‐decomposing microbes and ecosystem functions were unresponsive to restoration but neither did they differ between near‐natural and impacted streams, suggesting that they were little harmed by sedimentation. Wood addition enhanced hydraulic retention, and riparian plant assemblages along wood‐restored streams resembled those in near‐natural streams, suggesting that increased retention re‐established a more natural flood regime. By contrast, riparian plant assemblages in boulder‐restored streams did not differ from those in impacted streams. Synthesis and applications. Restoration improved several aspects of stream and/or riparian biodiversity, but had limited effects on ecosystem functions. Different restoration measures resulted in differing biodiversity outcomes: boulder addition was more effective at restoring in‐stream heterogeneity and aquatic biodiversity, whereas wooden structures helped restore channel hydrology and retentiveness, and, consequently, riparian vegetation. Therefore applying both measures in the restoration of forested headwater streams with naturally stony substrates enhances stream habitat variability at the watershed scale, providing the most promising scenario for biodiversity benefits in broad‐scale restoration designs. In‐stream restoration that increases hydraulic retention has impacts that extend beyond ecosystem boundaries, reinforcing the need to restore, manage and protect streams and their riparian forests in an integrated effort.

Operational regulation of water replenishment to reduce supersaturated total dissolved gas in riverine wetlands

Natural flood regimes have been altered with an increasing number of high dams constructed in the upstream stretches of rivers. The voluntary and involuntary flood discharges of reservoirs have become the primary water replenishment source for riverine wetlands. Supersaturation of total dissolved gas (TDG) in the flood discharge from high dams may seriously affect the survival of fish and aquatic organisms in riverine wetlands. Flood discharge regulation is an effective method for improving the distribution of supersaturated TDG in riverine wetlands, thereby minimizing the negative effects on aquatic organisms. A one-dimensional unsteady TDG model is developed in this paper to predict the transportation and dissipation of supersaturated TDG downstream of a high dam under different discharge patterns. The effects of TDG supersaturation on fish under different discharge patterns were evaluated based on tolerance research results for native fish. The results indicate that the transportation of TDG is affected primarily by the discharge pattern and flow characteristics. The temporal duration of TDG levels in the river, which is a major factor determining the negative effects of TDG on fish, increase with increasing discharge time. The simulation results indicate that a discontinuous discharge pattern, rather than continuous discharge pattern, is recommended to mitigate TDG supersaturation.

Interactions between flooding and upland disturbance drives species diversity in large river floodplains

Understanding and predicting vegetation patterns in floodplains are essential for conservation and/or restoration of river floodplains subject to hydrological alterations. We propose a conceptual hydroecological model to explain the disturbance mechanisms driving species diversity across large river floodplains. These ecosystems harbor a unique set of flood-tolerant species different from the surrounding upland vegetation. In elevation gradients across pristine floodplains, the greater the flooding, the fewer the number of plant species. As terrain elevation increases, flood depth and duration decrease and it is more likely that species composition is influenced by external natural or human-driven disturbances. The spatial interaction between the natural flood regime and upland factors creates patterns of disturbance gradients that influence how floodplain vegetation establishes. In regions where upland conditions are subject to strong external disturbances, species diversity peaks at intermediate stages along the disturbance gradient. We demonstrate this concept with observations from the Central Amazon and Pantanal in Brazil, the Mekong’s Tonle Sap in Cambodia, and the Okavango Delta in Botswana. We discuss how this model could be further elaborated and validated to inform management of large river basins under the impact of upstream-induced flood pulse alterations.