River widening in mountain and foothill areas during floods: Insights from a meta-analysis of 51 European Rivers.

River widening, defined as a lateral expansion of the channel, is a critical process that maintains fluvial ecosystems and is part of the regular functioning of rivers. However, in areas with high population density, channel widening can cause damage during floods. Therefore, for effective flood risk management it is essential to identify river reaches where abrupt channel widening may occur. Despite numerous efforts to predict channel widening, most studies have been limited to single rivers and single flood events, which may not be representative of other conditions. Moreover, a multi-catchment scale approach that covers various settings and flood magnitudes has been lacking. In this study, we fill this gap by compiling a large database comprising 1564 river reaches in several mountain regions in Europe affected by floods of varying magnitudes in the last six decades. By applying a meta-analysis, we aimed to identify the types of floods responsible for more extensive widening, the river reach types where intense widening is more likely to occur, and the hydraulic and morphological variables that explain widening and can aid in predicting widening. Our analysis revealed seven groups of reaches with significantly different responses to floods regarding width ratios (i.e., the ratio between channel width after and before a flood). Among these groups, the river reaches located in the Mediterranean region and affected by extreme floods triggered by short and intense precipitation events showed significantly larger widening than other river reaches in other regions. Additionally, the meta-analysis confirmed valley confinement as a critical morphological variable that controls channel widening but showed that it is not the only controlling factor. We proposed new statistical models to identify river reaches prone to widening, estimate potential channel width after a flood, and compute upper bound width ratios. These findings can inform flood hazard evaluations and the design of mitigation measures.

Twentieth-century development of floodplain forests in Polish Carpathian valleys: The by-product of transformation of river channels?

Changes in forest extent in the corridors of four rivers of the Polish Carpathians over the last 130 years and their relation to changes in planform river geometry were investigated through the analysis of 1:25000-scale maps from the 1870s and aerial images from the mid-20th century and 2009. Average proportions of river and its geomorphic units as well as floodplain and its land cover features in the total width/area of the analysed river corridors were determined and compared between the three dates. All the analysed rivers narrowed significantly over the study period. This increased considerably the proportion of floodplains in the area of the river corridors, while lateral parts of the former, wide channels became a place of forest development. In the Koszarawa and Raba valleys, forest developed also on parts of the former floodplains following a decline in agricultural and pastoral use of lands with shallow, poor soils. The proportion of forest in the total area of the river corridors increased from 0-7.5% in the 1870s to 28.5-46.5% in 2009, and the forest expansion was mainly driven by the timing and scale of channelization works that reclaimed parts of the former channels from the rivers. A reduction in flow and sediment dynamics of Carpathian rivers over the 20th century enabled development of islands in their active zones. However, channelization works eliminated islands from most river reaches and thus islands persisted only in scarce unmanaged reaches. The expansion of floodplain forests in Carpathian valleys improves functioning of the river ecosystems but the resultant increased delivery of large wood to river channels may generate flood hazard. Optimal river management should avoid removal of riparian trees to maximize the environmental benefits but enable undisturbed transfer of driftwood through bridge cross-sections to minimize the flood hazard resulting from floodplain forest development.

First insight into the macroplastic storage in a mountain river: The role of in-river vegetation cover, wood jams and channel morphology.

Macroplastic storage in mountain rivers remains unexplored and it is unknown how river morphology and different surface types of river areas modulate this process. Therefore, we sampled macroplastic debris stored on the surface of emergent river areas with different vegetation cover and on wood jams in a channelized, single-thread reach and an unmanaged, multi-thread reach of the Dunajec River in the Polish Carpathians. Total amounts of macroplastic debris retained in these reaches were then estimated on the basis of mean mass of macroplastic deposited on unit area of each surface type and the area of this surface type in a given reach. Exposed river sediments and areas covered with herbaceous vegetation stored significantly lower amounts of macroplastic debris (0.6 and 0.9 g per 1 m2 on average) than wooded islands and wood jams (respectively 6 g and 113 g per 1 m2). The amounts of macroplastic debris stored on wood jams exceeded 19, 129 and 180 times those found on wooded islands, areas covered with herbaceous vegetation and exposed river sediments. Wooded islands and wood jams covering 16.7% and 1.5% of the multi-thread reach stored 43.8% and 41.1%, respectively, of the total amount of macroplastic stored in that reach, whereas these surface types were practically absent in the channelized reach. Consequently, the unmanaged, multi-thread reach, 2.4 times wider than the neighbouring channelized reach, stored 36 times greater amount of macroplastic per 1 km of river length. Our study demonstrated that the storage of macroplastic debris in a mountain river is controlled by channel management style and resultant river morphology, which modulate river hydrodynamics and a longitudinal pattern of the zones of transport and retention of macroplastic conveyed by river flow.

Changes in the hydrodynamics of a mountain river induced by dam reservoir backwater.

Upstream from a dam reservoir, river hydrodynamics may be directly changed by temporary inundation driven by the reservoir. This triggers morphological river changes which may additionally modify the initial hydrodynamics, even at the time when backwater inundation does not occur (indirect effects of backwater). We verified these hypotheses, applying two-dimensional hydraulic modelling of flood flows to a section of the mountainous Dunajec River upstream from the Czorsztyn Reservoir. The modelling was performed for small, medium and large floods, and hydraulic conditions were compared between the scenarios with lacking and maximum backwater inundation and between the river reaches subjected to backwater inundation and unaffected by backwater fluctuations. Direct effects of reservoir level fluctuations were limited to the reach subjected to backwater inundation during floods and comprised: significantly increased water depth and decreased flow velocity and bed shear stress in the channel and on the floodplain, as well as a re-established hydrological connectivity between the channel and floodplain during small and medium floods. Indirect effects of backwater inundation reflected channel widening and bed aggradation that triggered a positive feedback with changes in hydrodynamics, mostly by reducing the velocity of flood flows in the channel zone. These latter changes occurred on a longer distance upstream from the reservoir than the backwater reach itself, and they modified the river hydrodynamics even when backwater inundation did not occur. We propose a conceptual model which indicates that changes of mountain rivers upstream from dam reservoirs are driven by modified hydrodynamics and lead to different morphological adjustments than those induced by waters underloaded with sediment downstream from dams. Changes in hydrodynamics and the associated morphological and sedimentary adjustments of mountain rivers recorded upstream from dam reservoirs may locally mitigate impacts of channelization and channel incision on riverine and riparian ecosystems of these rivers.

Aerial photo-based analysis of the hydromorphological changes of a mountain river over the last six decades: The Czarny Dunajec, Polish Carpathians.

Changes in hydromorphological conditions in the mountainous Czarny Dunajec River over the last six decades were investigated through the analysis of archival aerial photos and contemporary orthophotos. The aerial images from 7 dates between 1954 and 2009 were used to evaluate changes in selected hydromorphological parameters in six reaches delimited along the river course. As a result of channelization works and in-stream gravel mining, most of the analysed river course experienced dramatic hydromorphological alterations in the active river zone: change from the multi-thread to a single-thread channel, river narrowing associated with a reduced occurrence or elimination of channel bars and islands, transformation of the gravelly bed to a bedrock-alluvial or bedrock bed, stabilization of river banks with engineering structures, and disruption of longitudinal river continuity by transverse hydraulic structures. These changes largely reduced habitat heterogeneity in the river and disrupted or limited three-dimensional connectivity of the river ecosystem. However, in the reach that had generally avoided these human pressures, such negative changes did not occur and the number of low-flow channels and the proportion of islands in the active river zone increased during the study period. In all study reaches, positive changes occurred in the riparian and floodplain areas: the proportion of wooded channel banks and forest cover in the floodplain area increased over the last six decades. The degradation of hydromorphological conditions in the active river zone was reflected in a significant impoverishment of fish and benthic invertebrate communities as well as of ground beetles inhabiting low river benches. The study confirmed the usefulness of archival aerial photos in reconstructing temporal changes in river hydromorphology. The number of features that can be evaluated with this approach is smaller than in field-based assessments of contemporary rivers but larger than in studies based on historical maps or paleohydrological analysis.

Island development in a mountain river subjected to passive restoration: The Raba River, Polish Carpathians.

In the twentieth century the heavily channelized Raba River incised deeply in its mountain course. Abandonment of channelization structures in a 2.3-km-long reach within a forested corridor was followed by considerable channel widening during floods of 30- and 35-year recurrence interval, re-establishment of a multi-thread channel pattern and island development. Morphological and botanical surveys were conducted annually between 2011 and 2017 to determine the processes and patterns governing development of islands and their floristic complexity. Hydraulic conditions promoting establishment and persistence of islands were determined with one-dimensional hydraulic modelling of flood flows for 8 unmanaged river cross-sections with islands and 8 cross-sections in the adjacent channelized reaches. Average age, number of islands and their average and total area in the reach markedly increased over the study period. However, the increase was not steady but moderated by island erosion by flood flows, island establishment shortly after major floods and island coalescence in the years without such floods. Hydraulic modelling indicated that river cross-sections with islands are typified by significantly lower values of mean water depth, flow velocity, unit stream power and bed shear stress at flood flows than cross-sections in the adjacent, channelized reaches. Such conditions promote deposition of living driftwood on channel bars, initiating island development, and reduce the probability of erosion of existing islands. The total number of plant species on islands varied highly and either exceeded or was similar to that recorded on riparian forest plots in particular years. This study indicates that (i) island re-establishment in the river was initiated by substantial channel widening, (ii) variation in flood magnitudes exerts a considerable influence on the trajectory of island development, and (iii) the contribution of islands to the overall species richness of plant communities in the river corridor at early stages of island re-establishment may be highly varied.

Ground beetle communities in a mountain river subjected to restoration: The Raba River, Polish Carpathians.

Effects of passive restoration of mountain rivers on the organisms inhabiting exposed riverine sediments are considerably less understood than those concerning aquatic biota. Thus, the effects of a recovery of the Raba River after abandonment of maintenance of its channelization scheme on ground beetle (Coleoptera: Carabidae) communities were investigated by comparing 6 unmanaged cross-sections and 6 cross-sections from adjacent channelized reaches. In each cross-section, ground beetles were collected from 12 sampling sites in spring, summer, and autumn, and 8 habitat parameters characterizing the cross-sections and sampling sites were determined. Within a few years after abandonment of the Raba River channelization scheme, the width of this gravel-bed river increased up to three times and its multi-thread pattern became re-established. Consequently, unmanaged river cross-sections had significantly larger channel width and more low-flow channels and eroding cutbanks than channelized cross-sections. Moreover, sampling sites in the unmanaged cross-sections were typified by significantly steeper average surface slope and larger average distance from low-flow channels than the sites in channelized cross-sections. In total, 3992 individuals from 78 taxa were collected during the study. The ground beetle assemblages were significantly more abundant and richer in species in the unmanaged than in the channelized cross-sections but no significant differences in carabid diversity indices between the two cross-section types were recorded. Redundancy Analysis indicated active river zone width as the only variable explaining differences in abundance and species richness among the cross-sections. Multiple regression analysis indicated species diversity to predominantly depend on the degree of plant cover and substrate grain size. The study showed that increased availability of exposed sediments in the widened river reaches allowed ground beetles to increase their abundance and species richness within a few years after the onset of river restoration, but more time may be needed for development of more diverse carabid communities.

Ecological state of a mountain river before and after a large flood: Implications for river status assessment.

Assessment of the ecological status of rivers is key to monitoring the achievement of the environmental goal of the EU Water Framework Directive and the success of restoration projects. In summer of 2009 and 2010, repeated assessments of physical habitat conditions and of fish and benthic invertebrate communities were performed at low-flow conditions in 10 unmanaged and 10 channelized cross-sections of the Biala River, Polish Carpathians. Between the two surveys, an 80-year flood occurred, significantly affecting habitat characteristics and river communities. In unmanaged cross-sections, active channel width increased, whereas the degree of cross-sectional variation of flow velocity decreased. In channelized cross-sections, the increase in active channel width and the cross-sectional variation of flow velocity was accompanied by a decrease in bed-material grain size. Before the flood, the unmanaged cross-sections hosted 2.3 times more benthic invertebrate taxa than the channelized ones, whereas after the flood, the number of taxa they supported was so reduced that the taxonomic richness of benthic invertebrate assemblages in both cross-section types became similar. In comparison to pre-flood conditions, the abundance of fish juveniles (YOY) in unmanaged cross-sections was reduced nearly by half; before the flood they hosted 5 times more juvenile individuals than channelized cross-sections and only twice as many after the flood. Finally, a differing assessment of flood impact on the ecological river quality was obtained with the invertebrate-based BMWP-PL index and the European Fish Index, with the former indicating a significant reduction of the quality in unmanaged cross-sections and the latter pointing to no such change. The results indicate that assessments performed before or after a major flood may yield significantly different results for the quality of abiotic and biotic elements of the river ecosystem. Final assessment should thus be based on repeated surveys to balance the effect of extreme hydrological events.

Comprehensive approach to the reduction of river flood risk: Case study of the Upper Vistula Basin.

The paper examines options for river flood risk reduction in the Upper Vistula Basin located partly in the Carpathian Mountains in Poland. Projections of high-flow indices for the periods 2021-2050 and 2071-2100 generally indicate small future increases, although the projected flow changes vary highly both across the study basin as well as among climate models. An overview of twentieth-century catchment and channel changes indicates that some of them decreased and others increased the rapidity of runoff but they largely reduced availability of sediment for fluvial transport, hence inducing bed incision and bank erosion that create risk to roads and bridges. Traditional methods of flood protection in the basin encompassed large structural defences such as river channelization and flood embankments. These have limited floodwater retention within floodplains and accelerated flood runoff, shifting flood hazard downstream rather than reducing it. A range of alternative approaches to reducing future flood risk are thus proposed and examples of their application in southern Poland are described. These approaches include river restoration measures aimed to reduce erosional potential of flood flows and increase channel and floodplain retention of floodwater, as well as grassroots initiatives promoting preparedness for flooding at the community level. There is an increasing need to change the existing paradigm that flood-control measures should be based on fast evacuation of floodwater that, in turn, was associated with a significant reduction in floodwater retention on the valley floors. Alternative approaches discussed in this paper extend the roster of flood risk reduction strategies and contribute to a gradual paradigm change.

Influence of academic education on the perception of wood in watercourses.

Human perception of in-channel wood can influence decision-making about wood reintroduction in watercourses for restoration purposes. A questionnaire survey was conducted among first-year and advanced Polish students and professionals involved in the management and protection of watercourses in Poland. The purpose was to assess their perception of river scenes with and without wood in terms of naturalness, aesthetics, danger and need for improvement. Generally, students perceived riverscapes with wood to be less aesthetic, more dangerous and needing more improvement than riverscapes without wood. However, many aspects of the riverscape perception are subject to changes as the student progresses in studying. The culturally conditioned, negative perception of in-channel wood is reduced during the education of geography and biology students but enhanced in the course of water engineering studies. The improved perception of watercourses containing wood was most pronounced for the advanced geography students from a university in which some tutors conducted research on in-channel wood and could have transferred knowledge about and positive attitudes toward wood to their students. A comparison of the riverscape evaluation by biology students and national park managers indicates that education in the discipline facilitates understanding of the environmental significance of in-channel wood although its effects are highly insufficient to form the positive attitude toward wood-containing watercourses typical of the managers. In turn, the education of water engineering students about in-channel wood apparently dominantly emphasizes threats caused by its presence in watercourses, so that the students' highly negative attitude toward wood needs to be significantly moderated when the graduates begin working as water authority managers.

Public perception as a barrier to introducing wood in rivers for restoration purposes.

Reintroduction of wood in rivers for restoration purposes is now recognized in a positive way by scientists. Nevertheless, the perception of wood in riverscapes is strongly affected by the socio-cultural environment. This cultural influence might explain why wood reintroduction is accepted and promoted in some regions of the world but not in others, despite the demonstrated ecological benefits. From an extensive student perception survey, we show that most of the groups from nine countries in the world considered riverscapes with wood to be less aesthetic, more dangerous, and needing more improvement than riverscapes without wood. By contrast, this way of thinking was not observed in Germany, Sweden, and Oregon (USA), where the first instances of wood reintroduction occurred.