Development Of Riparian Vegetation Research Articles

Soil and water bioengineering in cold rivers: A biogeomorphological perspective

Soil and Water Bioengineering (SWBE) for river management is a viable alternative to civil engineering when bank stabilization is needed. Unlike riprap, SWBE techniques support bank stabilization while promoting the development of riparian vegetation. The preservation of vegetation biodiversity on riverbanks helps maintain and create essential ecosystem services such as recreation, carbon sequestration, pollutant filtration, and the creation of ecological niches and corridors. However, the potential of SWBE remains largely underestimated. Managers are often reluctant to use these techniques as they present failure risks, particularly in rivers with severe mechanical constraints. In cold environments experiencing freezing waters, ice-related processes such as ice abrasion or ice jams are significant disturbance factors for both river morphology and riparian vegetation. The marginality of SWBE is thus exacerbated in these environments, where considerable knowledge gaps persist regarding the interactions between ice, river channel morphology, and vegetation persistence. This review article aims to discuss the insights that biogeomorphology can provide for SWBE in cold environments. Biogeomorphology, a science that studies the interactions and feedbacks between living organisms and the physical processes shaping the landscape, offers new concepts and models as tools for understanding the co-development between landforms and vegetation. In the scope of SWBE, biogeomorphology can be used to (1) provide a better understanding of a river's dynamics and biogeomorphological changes in time and space to better identify the root causes of degraded riverbanks, (2) identify assemblages of species best suited to local conditions and better understand the relationship between channel morphology, vegetation, and ice to improve SWBE structure design, and (3) develop monitoring and evaluation tools to define the biogeomorphological functions of SWBE structure and improve maintenance strategies.

Recent changes in riparian and floodplain vegetation in England and Wales and its geomorphic implications

AbstractRecent river studies in the United Kingdom have observed an increase in riparian vegetation and its potential impact on river channel change and sedimentation. Here, we quantify changes in floodplain and riparian vegetation during the first two decades of the 21st century along reaches of eight gravel‐bed rivers in England and Wales that exhibit varied active and stable, single and multi‐thread planforms. The analysis employs information drawn from open‐access sources including national LiDAR surveys and the photographic analysis of repeat aerial and satellite imagery. Most reaches show an increase in woody vegetation across their floodplains reflecting both natural colonisation and deliberate planting, and all but one of the reaches show an increase in woody riparian vegetation cover along their riverbanks. Of the six reaches that have exposed riverine sediments, five show a reduction in exposed sediment area as a result of vegetation encroachment. Contrasts in the rate, extent and location of riparian change were associated with the stability of the channel planform, with differences seen between stable and more active reaches. The significance of riparian woodland in promoting riverbank stability is demonstrated. Riparian vegetation development varies between river reaches, largely in response to differing rates of channel movement but also because of multiple and diverse local decisions regarding the management of in‐channel and channel‐adjacent vegetation by landowners rather than a single national policy. We consider the relevance of our findings and the value of remote monitoring for future river management interventions.

Hydraulic Effect of Vegetation Zones in Open Channels: An Experimental Study of the Distribution of Turbulence

The development of vegetation in riverbeds is an important part of river engineering, and an in-depth understanding of its hydraulic influence is greatly needed. Our research focuses primarily on common reed (Phragmites australis) in riverbeds. To date, little is known about the hydraulic impact of the Phragmites australis reed and both field and laboratory data are still very scarce. Consequently, the main goal of our study was to evaluate the effect of vegetation zones on the spatial distribution of turbulence. Based on laboratory measurements of local instantaneous velocities, the values of the turbulence intensity (degree) Tu were determined, and its spatial distribution was illustrated. Analysis of the results showed that the relatively dense clusters of plants (reeds) act as “openwork deflectors” of the current and very clearly shape its spatial distribution. This can also be observed in the case of the distribution of the turbulence parameter Tu. For example, in the case of the development of riparian vegetation in the form of quasi-triangular communities of common reed (Phragmites australis) located alternately, there is a channelization of the flow, but also spatial changes in its character that occur. This work only presents results for preliminary hydraulic tests for Phragmites reed. These experiments should also be continued for other species of flexible riparian vegetation such as wicker. In the laboratory, the hydraulic influence of only triangle-shaped vegetation zones has been studied. Therefore, there is also a need for further hydraulic studies on vegetation zones of shapes other than triangular, e.g., rectangular, as well as vegetation zones with irregular shapes The authors see the need for such research and have already planned its continuation. Research on the interactions between vegetation and the structure of water flow in the riverbed is a very important aspect of contemporary trends in river environment management. Conscious, planned, and model-tested locating (or removing) of vegetation in a stream allows for shaping hydraulic and morphological conditions, thus controlling the processes of erosion, transport, and accumulation of debris.

Riparian vegetation and geomorphological interactions in anabranching rivers: A global review

AbstractAnabranching rivers are characterized by multiple active channels. Vegetation, flow regime and sediment retention are critical components for the formation, evolution and stability of these rivers; however, the connections relating river dynamics with riparian vegetation development and distribution are still not well known. This review discusses the relationship between anabranching rivers and riparian vegetation, showing examples of how vegetation affects anabranching rivers and how it is also affected by river dynamics. Consequences of poor management and human activities that can disrupt river–vegetation systems are also explored, presenting some examples of modifications to the stability and longevity of anabranching rivers. Challenges to design and implement appropriate policy and management practices in anabranching rivers are most frequently driven by poor experimental evidence due to difficulties in collecting data in these often remote and large river systems. Remote sensing technologies that account for key interactions between riparian vegetation, river flow and morphology changes can provide means to overcome these challenges.

The local responses of aquatic ecosystems to adjacent grassland conservation: Can streams of dreams exist in a degraded riverscape?

Abstract Landscape homogenization and the removal of riparian areas have altered stream ecosystems worldwide. Numerous conservation programmes attempt to improve water quality and increase instream habitat heterogeneity to elicit desired biological responses. However, the effectiveness of many conservation efforts on isolated stream fragments remains unknown, especially in grassland regions. The effects of grassland conservation practices and the re‐establishment of riparian corridors in the James River basin, South Dakota (USA) on stream water quality, habitat availability and aquatic macroinvertebrate and fish assemblages were studied in an agriculturally dominated prairie landscape. Grassland conservation efforts may have repaired riparian condition, reduced turbidity and created more diverse instream habitat complexes at conservation sites based on comparisons with paired reference reaches. Reference sites were relatively homogeneous, with prevalent siltation, bank erosion and disturbances to the riparian vegetation. Owing to significant riparian vegetation development, overhanging and aquatic vegetation, benthic detritus and woody materials were significantly more common at conservation reaches. Restoration efforts that assume ‘if you (re‐)build it, they will come’ (i.e. the ‘field of dreams’ hypothesis) underestimate other important barriers to biodiversity restoration in dynamic, grassland riverscapes. Although aquatic organisms in grassland ecosystems are adapted to rapidly inhabit available habitats, the development of niche space at conservation reaches did not directly result in colonization by aquatic life. Grassland management actions did not address stream connectivity issues or overcome land use influences elsewhere in the riverscape that may govern the responses of aquatic macroinvertebrates and fish. Stream fragmentation and continuing, damaging land use patterns seemed to exceed the positive effects of restoring isolated stream reaches in these heavily degraded catchments. Catchment‐scale management strategies that combine reach‐level restoration actions with efforts to improve connectivity are likely to be more successful in degraded riverscapes.

Different microbial functioning in natural versus man-made Mediterranean coastal lagoons in relation to season

Coastal marsh lagoons are of high ecological relevance playing a key role in the carbon cycle but are threatened to disappear due to global change effects. Restoration practices can counteract this process. This study compares the microbial heterotrophic functioning in three “new” man-made lagoons (created in 2016) to that in three “old” lagoons (two natural plus one created in 2002) from a Mediterranean coastal marsh. The activity of a range of microbial extracellular enzymes, the functional diversity, and the dissolved organic matter (DOM) quality, together with microbial biomass and water physic-chemistry were analysed seasonally in the water column of the six lagoons during 2017. Seasonality was a key driver for the lagoons microbial functioning mainly characterized by lowest microbial activities in winter when DOM was highly aromatic after storm events and an increase in organic matter degradation capabilities from winter to summer probably due to a rise in temperature and DOM input from primary production. Significant differences among lagoons appeared when they were less connected (summer and autumn), and old lagoons showed a greater utilization of proteinaceous and polysaccharidic compounds than new lagoons probably linked to their greater algal biomass (chlorophyll content), which may be supported by their larger phosphorus content. In autumn, there was also a greater use of allochthonous plant material in the old lagoons (higher XYL, and XYL/GLU ratio) probably related to their greater development of riparian vegetation. The functional diversity was the lowest in autumn when the lagoons showed distinct functional fingerprints and the lagoon created in 2002 was grouped with the new ones and distinguished from the natural ones, suggesting that it did not achieve complete restoration. Results indicate that microbial functional parameters related to organic matter use are a relevant and sensitive tool to study lagoon restoration processes, reflecting whole ecosystem nutrient and carbon cycling.

Insights from long-term variations in gravel riverbed topography as an indicator of vegetation–morphology interactions

Flooding shapes the fluvial landforms by interacting with the development of riparian vegetation. However, the quantitative evaluation of the effects of flooding on fluvial landscapes often relies on snapshot physical parameters regarding hydrological processes, as typified by shear stress on the bed surface. Flooding events over time regulate riparian vegetation, and introducing a long-term perspective into river environmental assessment could complement understanding the vegetation–morphology interactions. In this study, we estimated the long-term variations in channel topography (geomorphological variations) of a gravel riverbed of the Chikuma River. We used cross-sectional profiles of the river channel from 1959 to 2010 as an indicator of vegetation–morphology interactions and examined the effects of geomorphological variations on the fluvial landscape. Our results show that boreal and herbaceous vegetated areas had significant smaller geomorphological variations than those with active channels and gravel bars in the river. Temporal changes in the elevation of the river channel reduced after riparian vegetation was established and riverbed stabilisation could enable riparian forests to develop. We, therefore, identify geomorphological variations as an explanatory factor for fluvial landforms. The index could support river management with a long-term view of channel evolution with vegetation–morphology interaction.

Avaliação e Proposta de Novas Obras de Engenharia Natural para Estabilização de Taludes Fluviais do Rio Taquari no Município de Estrela, RS

O Parque da Lagoa, localizado no baixo curso do Rio Taquari, tem sido um sítio experimental para aplicação de modelos biotécnicos, que visam estabilizar e revegetar taludes fluviais susceptíveis aos processos erosivos. O presente trabalho busca avaliar atributos geotécnicos, ecológicos, econômicos e estéticos de técnicas de engenharia natural implantadas na área do parque no ano de 2010. A partir do diagnóstico destas intervenções biotécnicas, é proposto um projeto de revegetação e estabilização de taludes fluviais com uso das técnicas de bioengenharia, visando mitigar a erosão e ausência de vegetação ciliar que coloca em risco o rompimento da Lagoa Maria Dresch. No decorrer dos anos os modelos biotécnicos aplicados na área do parque adquiriram maturidade ao estabelecer uma vegetação, garantindo estabilidade e proteção às margens do rio. A proposta técnica sugere o uso do material dragado do canal de navegação do rio para remodelagem dos taludes fluviais e a aplicação das técnicas de enrocamento, esteira viva e banqueta vegetada, por serem soluções viáveis com baixo custo e menor impacto ambiental valorizando fatores ecológicos e paisagísticos. A engenharia natural se apresenta como uma alternativa ambientalmente sustentável e os resultados comprovaram sua eficiência como facilitadora para desenvolvimento da vegetação ciliar, e como ferramenta para correção e diminuição dos processos erosivos ao longo dos cursos d’água.

Morphodynamics of alternate bars in the presence of riparian vegetation

AbstractAlpine gravel‐bed rivers are dynamic systems that have been subjected to many anthropic alterations in the past centuries. Riparian vegetation development on previously bare sediment bedforms has been a common adjustment, raising important management issues in terms of flood risks and biodiversity. Many of these rivers are also channelized, and as a result present a pattern of alternate bars. Considering recent advances in numerical biomorphodynamic modeling, this study aims at exploring numerically the morphodynamics of alternate bars in the presence of riparian vegetation. To this end, a dynamic vegetation module has been implemented on top of an existing morphodynamic model, accounting for ecological processes of seed dispersal, seedling recruitment, growth, and mortality. Numerical simulations have been performed on a simplified reach of a gravel‐bed river with free migrating alternate bars at initial state. In this work 96 scenarios have been simulated, each representing 50 years of channel evolution, with different flood regimes characterized by various peak discharges and flood durations. Yearly peak discharge variability is explicitly modeled in 48 scenarios. Model outcomes present two possible equilibrium biomorphodynamic behaviors: stationary vegetated bars, or free migrating bars in the case of frequent vegetation removal during floods. This binary behavior holds true when the stochasticity of annual peak discharges is represented, and for a wide range of parameter values included in vegetation dynamic modeling. Transient mobility of vegetated bars is observed in specific configurations where large sediment deposits deflect the flow field, eroding bar heads. Modeled bar wavelengths are in the range of values predicted for free bars by linear bar theory, and remain far from the theoretical values of hybrid, steady bars. The shift from unvegetated migrating bars to steady vegetated bars seems to show that in these simulations vegetation constitutes a hydraulic forcing, leading to a shift from free bars to forced bars, with a final configuration largely inherited from the initial state. © 2019 John Wiley & Sons, Ltd.

Limiting the development of riparian vegetation in the Isère River: A physical modelling study

Physical modelling experiments are conducted to investigate if a modification of the Isère River (French Alps) hydrology by changing dams management is able to foster riverbed morphodynamic and, thus limiting riparian plant development. The experimental setup is a 1:35 scale, undistorted movable bed designed to ensure the Froude number similarity and initial conditions for sediment particle motion. The physical model is 35 m long, 2.6 m wide, with a sand mixture composed of three grain size classes. Two runs with different flow and bed load conditions are simulated. Preliminary results show an intense riverbed activity when the system reaches a dynamic equilibrium state. Under these conditions, bar mobility is strong enough to limit vegetation encroachment only when water discharges are higher than the discharge of a 5-years flood during more than 10 days. These results indicate that the hydrological characteristics of the Isère River and the actual configuration of the hydropower structures could be not able to release annually the flow conditions needed to control riparian plant development.

Importance of considering riparian vegetation requirements for the long-term efficiency of environmental flows in aquatic microhabitats

Abstract. Environmental flows remain biased toward the traditional biological group of fish species. Consequently, these flows ignore the inter-annual flow variability that rules species with longer lifecycles and therefore disregard the long-term perspective of the riverine ecosystem. We analyzed the importance of considering riparian requirements for the long-term efficiency of environmental flows. For that analysis, we modeled the riparian vegetation development for a decade facing different environmental flows in two case studies. Next, we assessed the corresponding fish habitat availability of three common fish species in each of the resulting riparian landscape scenarios. Modeling results demonstrated that the environmental flows disregarding riparian vegetation requirements promoted riparian degradation, particularly vegetation encroachment. Such circumstance altered the hydraulic characteristics of the river channel where flow depths and velocities underwent local changes of up to 10 cm and 40 cm s−1, respectively. Accordingly, after a decade of this flow regime, the available habitat area for the considered fish species experienced modifications of up to 110 % when compared to the natural habitat. In turn, environmental flows regarding riparian vegetation requirements were able to maintain riparian vegetation near natural standards, thereby preserving the hydraulic characteristics of the river channel and sustaining the fish habitat close to the natural condition. As a result, fish habitat availability never changed more than 17 % from the natural habitat.

Spatial patterns of water-dispersed seed deposition along stream riparian gradients.

Riparian ecosystems along streams naturally harbour a high plant diversity with many increasingly endangered species. In our current heavily modified and fragmented catchments, many of these species are sensitive to dispersal limitation. Better understanding of riparian plant dispersal pathways is required to predict species (re-)colonization potential and improve success rates of stream and riparian zone conservation and restoration. Dispersal by water (hydrochory) is an important mechanism for longitudinal and lateral dispersal of riparian species. Crucially for recruitment potential, it also influences the elevation along the riparian hydrological gradient where seeds become deposited. Due to the complex interplay between abiotic and biotic factors, however, it remains unclear how exactly patterns in seed deposition are formed. We compared hydrochorous and non-hydrochorous seed deposition, and quantified patterns of seed deposition along the bare substrate of newly created stream riparian gradients. Water levels were monitored and seed deposition was measured with seed traps along the full range of riparian hydrological conditions (from permanently flooded to never flooded). Average seed numbers and species richness were significantly higher in flooded than in non-flooded seed traps (5.7 and 1.5 times higher, respectively). Community-weighted trait means indicated that typically water-dispersed seeds were more dominant in flooded than in non-flooded seed traps and gradually decreased in concentration from the channel to the upland. Moreover, highly buoyant seeds accumulated at the average water line, and clear elevational sorting of non-buoyant seeds occurred within the floodplain. These results establish a critical role of flooding in shaping patterns of seed deposition along the riparian gradient, delivering many seeds of typical riparian species to riparian zones and depositing them at species-specific elevations as influenced by seed traits, suggesting species-specific dispersal pathways. This shows that hydrochory likely has important consequences for riparian vegetation development and that flooding forms a key process for successful restoration.

Riparian woody vegetation history in the campos region, southeastern South America, during two time windows: late Pleistocene and late Holocene

A detailed palynological record from Laguna Formosa (northeastern campos region, 31°S; 54°W) documents the dynamic balance between grasslands and riparian forests during the late Pleistocene (14,570 to 13,500 cal yr BP) and late Holocene (3280 cal yr BP to the present). Modern pollen-vegetation relationships and the woody pollen dispersal capacity analyses were used to improve the vegetation reconstruction. Grasslands were regionally dominant throughout the record. However, at 14,570 cal yr BP hydrophilous taxa reflect the development of riparian hydrophilous shrublands along freshwater bodies, promoting the fixation of the riverbanks, maintaining shallow, calm and clear water conditions under a relatively wet and not so cool climate. This is the first evidence of woody riparian vegetation development along freshwater bodies for the lowlands of the northern campos during the late glacial period. At 3280 cal yr BP riparian forests consisted of both hydrophilous and mesophilous woody taxa. Since 2270 cal yr BP woody vegetation gradually increased, accompanied by the incorporation of other taxa by 940 cal yr BP, and achieving a composition similar to that of the contemporary time at ca. 540 cal yr BP. The increased woody vegetation since ca. 2270 cal yr BP, and the more frequent and intense flooding events between 1800 and 1200 cal yr BP, could be related to higher precipitation over La Plata Drainage Basin, related with the high ENSO amplitude. In addition, pollen from taxa that currently no longer develops in the study area suggests connections between southern Brazil and Uruguay, and between the campos and the Chaco phytogeographic province.

Sensitivity Analysis of a Riparian Vegetation Growth Model

The paper presents a sensitivity analysis of two main parameters used in a mathematic model able to evaluate the effects of changing hydrology on the growth of riparian vegetation along rivers and its effects on the cross-section width. Due to a lack of data in existing literature, in a past study the schematization proposed here was applied only to two large rivers, assuming steady conditions for the vegetational carrying capacity and coupling the vegetal model with a 1D description of the river morphology. In this paper, the limitation set by steady conditions is overcome, imposing the vegetational evolution dependent upon the initial plant population and the growth rate, which represents the potential growth of the overall vegetation along the watercourse. The sensitivity analysis shows that, regardless of the initial population density, the growth rate can be considered the main parameter defining the development of riparian vegetation, but it results site-specific effects, with significant differences for large and small rivers. Despite the numerous simplifications adopted and the small database analyzed, the comparison between measured and computed river widths shows a quite good capability of the model in representing the typical interactions between riparian vegetation and water flow occurring along watercourses. After a thorough calibration, the relatively simple structure of the code permits further developments and applications to a wide range of alluvial rivers.

Time and Intensity Weighted Indices of Fluvial Processes: a Case Study from the Kootenai River, USA

AbstractWithin riparian landscapes, river flows and stages determine habitat gradients from less to more dynamic, and these support different plant species and their life history stages that are adapted to specific positions along these gradients. The gradients are characterized by physical processes that vary in magnitude and duration, and these shape the riparian vegetation communities. Consequently, natural riparian ecosystems are very dynamic, and the river disturbance regime is essential for sustaining ecosystem health. However, although the importance of disturbance is well accepted, disturbance regimes are poorly understood. This study was undertaken to develop indices capable of characterizing riparian habitats by considering flood magnitude and the elapsed time after flood disturbance, that is, the history that influenced the present vegetation composition. The indices were tested along two reaches of the Kootenai River in Idaho, USA, with braided versus meandering channel forms. The case study spanned a 31‐year period and emphasized two major disturbance components, the morphodynamic influence of velocity and shear stress and the flood or inundation duration. Computed indices were tested for consistency and then used to characterize different riparian vegetation development and succession phases. The statistical analysis revealed high correspondence among the calculated indices and differences across the different successional stages and between the two reaches. This demonstrated the utility of the time and intensity weighted indices to analyse the fluvial patterns that support different riparian vegetation communities, and this could be applicable for riparian management, mitigation, conservation and restoration. Copyright © 2016 John Wiley & Sons, Ltd.

Irrigation canals as tools for climate change adaptation and fish biodiversity management in Southern France

This paper is based on the interdisciplinary research conducted in the south of France that analyses the different economic, social and environmental roles played by agricultural irrigation canals. We argue that beyond their productive role, which is to supply farmers with water, they fulfil other environmental services and play an important role in the context of future climate change to face challenges of adaptation. We point up several ecosystem services provided by such canals, e.g. replenishing the groundwater table, the development of riparian vegetation and wet areas in the Mediterranean zone, tools for regulating flooding and drought, the bases for new cultural approaches to nature. Moreover, they play an important role in the maintenance of an ichthyological biodiversity that is indispensable for the persistence of natural ecosystem. Functioning as an ecological corridor, they display interesting capacities as refuges for certain fish species under stress. Indeed, they can potentially connect upstream and downstream zones over a continuum of more than 300 km and thus covering very contrasted climatic zones (alpine versus Mediterranean). For now, most of these services remain largely unknown and underestimated. However, they serve as assets for territorial development since they combine economic, ecological and social factors whose remodelling is becoming increasingly necessary in the face of climate change.

Habitat associations of birds at Manton Dam, Northern Territory

Manton Dam is an impoundment of the Manton River approximately 50 km south-south-east of Darwin, Northern Territory. Major habitats associated with the dam (open water, water edge, riparian monsoon forest and savanna woodland) were searched to determine the bird species associated with each. A total of 84 avifauna species were recorded. Diversity (22 species) and abundance of waterbirds were low in comparison with other wetlands of northern Australia - only 11 bird species were recorded using the open water habitat. The paucity of waterbirds may be due to the lack of shallow foraging areas. Bushbirds included 50 species that used riparian monsoon forest habitat and 45 species that used savanna habitat; 34 species were recorded in both habitats. Nine waterbird species were also recorded using riparian monsoon forest habitat. Further development of riparian vegetation around the fringes of the dam may encourage colonisation by additional forest bird species. Increasing the extent of shallow water areas and prohibition of motorboats may enhance habitat availability and quality for waterbirds and shorebirds.

THE INFLUENCE OF CHECK DAMS ON FLUVIAL PROCESSES AND RIPARIAN VEGETATION IN MOUNTAIN REACHES OF TORRENTS

The complex hydrogeomorphological processes within the active channel of rivers strongly influence riparian vegetation development and organization, particularly in mountain streams where such processes can be remarkably impacted by engineering control works. In four mountain reaches of Calabrian fiumaras we analyze, through previously arranged methods (integrated by a multivariate statistic analysis), the relationships among hydrogeomorphological river characteristics and structure and the development of riparian vegetation within the active channel in transects located in proximity of check dams and in less disturbed sites. The results of this study demonstrate clear and relevant contrasts, due to the presence of check dams, in the physical and vegetation properties of upstream, downstream and intermediate sites around check dams. The multivariate statistical approach through the Principal Component Analysis (PCA) highlighted evident relationships in all transects between groups of physical and vegetation properties. The regression analysis performed between the vegetation properties and the width:depth ratio or the specific discharge showed very different relationships between groups of transects, due to evident changes in channel morphology and in flow regime locally induced by check dams. Overall we have shown that check dams have far reaching effects in the extent and development of riparian vegetation of mountain torrent reaches, which extend far beyond physical adjustments to changed morphological, hydraulic and sedimentary conditions.

Impacts of dam-regulated flows on channel morphology and riparian vegetation: a longitudinal analysis of Satsunai River, Japan

We examined the impacts of the Satsunai River Dam on the hydrology and development of riparian vegetation along the upper and lower reaches of the Satsunai River downstream from the dam. We estimated frequency curves of the flood discharge during the pre-dam (1976–1996) and post-dam (1997–2006) periods and simulated the flood frequency at sampling points within sites under pre-dam, post-dam and dam-removal (using the pre-dam flood discharge and post-dam cross-sections) scenarios. Changes in channel morphology and land cover were investigated by analyzing aerial photographs. Our results indicate that the 20-year flood at the upper site decreased substantially (from 599 to 271 m3/s) after dam operation, while that of the lower site decreased slightly (from 1025 to 977 m3/s). Within the upper site, the proportion of >20-year return periods increased considerably (from 31.0 to 48.6%) while the proportion of 1- to 20-year return periods decreased (from 30.5 to 8.9%) after dam operation. Flood frequency results for the dam-removal scenario were similar to those for the pre-dam period, suggesting that a return to pre-dam discharge rates would restore the pre-dam distribution of flood frequency at the upper site. Within the lower site, however, the distribution of flood frequency varied little between the pre- and post-dam scenarios, because tributary inflows between the sites mitigated the impacts of dam-regulated flows. Land cover types were associated with flood frequency at both sites. The reduced flood frequency of the upper site resulted in increased area of riparian vegetation and decreased area of active channel.

Adjustments in channel form, sediment calibre and vegetation around check‐dams in the headwater reaches of mountain torrents, Calabria, Italy

AbstractThe structure and dynamics of vegetation in valley bottoms are both strongly associated with fluvial processes and landform dynamics. All of these associations are disrupted by the installation of engineering control works. We use survey and analysis methods developed previously to investigate the impact of the installation of check‐dams within the confined headwaters of steep seasonally‐flowing streams (fiumaras) in Calabria, southern Italy, on active channel form, sediment calibre, and the richness, cover and development of riparian vegetation. Based on detailed field measurements along transects across the active channel, estimates of indices of vegetation extent (GCC), development (WCH) and their cross‐sectional variability (coefficients of variation of both indices at each survey site CVGCC, CVWCH), the number of species present (Ns), channel shape (w/d – the width/depth ratio), cross‐sectional area (CSA), downstream gradient (slope), surface bed sediment calibre (D50) and subsurface fine sediment content (percentage less than 250 µm by weight) were obtained for 60 transects located immediately upstream (U), downstream (D) and at intermediate sites (I) around 20 check‐dams located in four different headwater catchments. Analysis of this data set suggests that statistically significant changes in channel form and sediment calibre upstream of check‐dams are associated with more consistent vegetation development across the active channel, including an increase in species richness relative to other transects, but notable increases in vegetation cover and development only arise where the physical characteristics of the channel are notably different from intermediate and downstream channels. Because of the naturally steep profile of the study torrents, intermediate sections between check‐dams tend to be more similar in form to channels located immediately downstream of check‐dams than those located upstream, leading to similar structural properties in the riparian vegetation. The intermediate transects support considerably more species than downstream reaches, but the conditions upstream of the check‐dams appear to be so favourable for riparian vegetation development that species richness exceeds that found in intermediate reaches. Despite the confined headwater locations, these contrasts in form, sediment and vegetation development around check‐dams are strong and consistent across the study catchments, over‐riding more subtle contrasts in species richness and sediment calibre between catchments. Copyright © 2009 John Wiley & Sons, Ltd.