Spatial and temporal patterns of the twentieth-century channel changes of the Dunajec, the second largest river of the Polish Carpathians, are analysed using data from six historical maps, hand-auger ...drillings in paleochannels and hydrometric data from 10 gauging stations. In its lower course, the river was considerably narrowed, shortened and embanked between the 1880s and the 1920s. Here, bed degradation commenced in the late 19th-century and has so far resulted in 3.1
m of channel incision. In the middle river course, little change occurred in mountain reaches where the Dunajec is confined by valley sides and where bedrock exposures prevent channel incision. In the reaches within intramontane basins, channelization works carried out in the 1950s–1970s considerably narrowed the river and transformed its multi-thread channel into a single-thread, straight channel. Rapid bed degradation induced by the works resulted in up to 2
m of channel incision over the second half of the century. Upstream progression of bed degradation into the adjacent mountain reach was hindered by bedrock exposures at its downstream end. In the upper river course, reaches with the deeply incised channels (up to 3.5
m) prevail over a few past decades in response to gravel exploitation, river channelization and a reduction in catchment sediment supply. More scarce are vertically stable reaches, which owe their stability to low unit stream power in wide, unmanaged reaches and to fixing of the channel bed by drop structures in channelized reaches.
This study shows that despite the overall tendency to incision over the 20th century, considerable differences in the timing and course of channel changes occurred between particular river reaches due to variable human impact and local geological and geomorphological conditions. The main phase of channel incision took place progressively later in the upstream direction. However, upstream progression of bed degradation from the already incised downstream reaches was precluded by the Rożnów Dam and bedrock-controlled reaches. Where bed material flushed out from an incising upstream reach had been introduced into an artificially narrowed channel of high transport capacity, it was easily transferred downstream and could not restore the downstream reach to its pre-channelization vertical position.
Case studies of the Soła, San and Dunajec rivers are used to demonstrate changes in planform geometry of the rivers draining the western and eastern parts of the flysch Polish Carpathians and those ...originating in the high-mountain Tatra massif since the second half of the nineteenth century. The number and width of low-flow channels and the length of river reaches are compared between the nineteenth-century and contemporary rivers, with analysis of other features of their planform geometry prevented by differences in the cartographic presentation of rivers on Austrian military maps and Polish topographic maps. In the second half of the nineteenth century, the rivers flowed in wide channels. The Soła supported a braided channel along its entire course, the Dunajec was mostly braided, with a single-thread channel within gorge sections, whereas for the San the braided pattern was limited to the reaches within intramontane and foreland basins. Over the twentieth century, the braided channel pattern was nearly completely eliminated and substituted by a single-thread pattern; multi-thread channel segments remained only in the upper course of the Dunajec River. Average low-flow-channel width significantly decreased, even by 66%, and this was accompanied by a reduction in the variability of low-flow channel width along the rivers. The change in planform channel geometry was accompanied by channel incision, which started earlier and has resulted in greater bed degradation in the foreland and foothill reaches of the rivers than in their mountain reaches. Twentieth-century river changes also encompassed a radical change in the facies pattern of channel sediments, accompanied by their coarsening and improved sorting and development of bed armour. Moreover, transformation of alluvial channel beds to bedrock ones took place in many reaches of Carpathian streams and rivers. The degradation of physical structure of Carpathian watercourses has resulted in adverse hydraulic and hydrological effects and a marked deterioration of habitat conditions for riverine biota.
The paper presents a conceptual model of the route of macroplastic debris (>5 mm) through a fluvial system, which can support future works on the overlooked processes of macroplastic storage and ...remobilization in rivers. We divided the macroplastic route into (1) input, (2) transport, (3) storage, (4) remobilization and (5) output phases. Phase 1 is mainly controlled by humans, phases 2–4 by fluvial processes, and phase 5 by both types of controls. We hypothesize that the natural characteristics of fluvial systems and their modification by dam reservoirs and flood embankments construction are key controls on macroplastic storage and remobilization in rivers. The zone of macroplastic storage can be defined as a river floodplain inundated since the beginning of widespread disposal of plastic waste to the environment in the 1960s and the remobilization zone as a part of the storage zone influenced by floodwaters and bank erosion. The amount of macroplastic in both zones can be estimated using data on the abundance of surface- and subsurface-stored macroplastic and the lateral and vertical extent of the zones. Our model creates the framework for estimation of how much plastic has accumulated in rivers and will be present in future riverscapes.
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.
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•Effects of backwater inundation on river hydrodynamics upstream from dam reservoir were investigated.•Hydraulic modelling of flood flows in backwater and control reaches and with/without backwater inundation was performed.•Backwater inundation increases water depth and decreases velocity and bed shear stress of the channel and floodplain flows.•Backwater-induced morphological river changes additionally modify river hydrodynamics.•Conceptual model of hydrodynamics–morphology adjustments downstream and upstream of a dam reservoir was presented.
Multi-thread rivers are typified by the presence of interconnected channels separated by mid-channel bars or the islands forming as a result of vegetation encroachment on mid-channel bars. High ...gradients of channels and valley floors, high discharge variability, relatively low content of fine, cohesive sediments in the alluvial plain, coarse bed material and high sediment supply are environmental conditions that favour the development of braided channel pattern. Under natural conditions of the Holocene, island-braided channel pattern was typical of the rivers in the foreland of the Tatra Mountains and most likely also those draining the parts of the flysch Outer Carpathians underlain by thick-bedded sandstones complexes. These braided channels formed in the rivers flowing through mostly non-cohesive floodplains and fed with coarse-grained material exceeding competence of the rivers. Progressive deforestation of mountain catchments with the increasing area of cultivated hillslopes resulted in higher flood peaks and substantially increased sediment supply to river channels. Coupled with greater humidity of the final stages of the Little Ice Age, this led to the transformation of the island-braided rivers into bar-braided ones. Throughout the nineteenth century, the braided channel morphology expanded downstream to the foreland river reaches. In turn, over the twentieth century the braided morphology nearly disappeared from Polish Carpathian rivers, mostly as a result of their channelization. The effectiveness of these works was, however, strengthened by a decrease in sediment supply to the rivers caused by land use changes and by the reduced availability of bed material for fluvial transport resulting from in-channel gravel mining and partitioning of rivers with dam reservoirs. Environmental changes which have since occurred in the river catchments and disruption of the continuity of sediment transport by dams limit the feasibility of the restoration of braided channel pattern in Polish Carpathian rivers to their reaches supporting such morphology under natural conditions of the Holocene.
As with bedload transport, wood transport in rivers is governed by several factors such as flow regime, geomorphic configuration of the channel and floodplain, or wood size and shape. Because ...large-wood tends to be transported during floods, safety and logistical constraints make field measurements difficult. As a result, direct observation and measurements of the conditions of wood transport are scarce. This lack of direct observations and the complexity of the processes involved in wood transport may result in an incomplete understanding of wood transport processes. Numerical modelling provides an alternative approach to addressing some of the unknowns in the dynamics of large-wood in rivers. The aim of this study is to improve the understanding of controls governing wood transport in mountain rivers, combining numerical modelling and direct field observations. By defining different scenarios, we illustrate relationships between the rate of wood transport and discharge, wood size, and river morphology. We test these relationships for a wide, multithread reach and a narrower, partially channelized single-thread reach of the Czarny Dunajec River in the Polish Carpathians. Results indicate that a wide range of quantitative information about wood transport can be obtained from a combination of numerical modelling and field observations and from document contrasting patterns of wood transport in single- and multithread river reaches. On the one hand, log diameter seems to have a greater importance for wood transport in the multithread channel because of shallower flow, lower flow velocity, and lower stream power. Hydrodynamic conditions in the single-thread channel allow transport of large-wood pieces, whereas in the multithread reach, logs with diameters similar to water depth are not being moved. On the other hand, log length also exerts strong control on wood transport, more so in the single-thread than in the multithread reach. In any case, wood transport strongly decreases with increasing piece volume, although this relation is not linear. We also document a nonlinear relationship between wood transport and flood magnitude. A threshold discharge was identified below which wood transport is negligible. This threshold is higher in the multithread reach, while in the single-thread reach floods of lower magnitude are able to transport wood downstream. Wood transport ratio increases with discharge until it reaches an upper threshold or tipping point, and then decreases or increases much more slowly. This threshold is clearly related to bankfull discharge, but it is much higher for the multithread reach than for the single-thread one. Although modelling input and field observations were taken from a specific river, our findings and conclusions are likely to be applicable to a much larger suite of (mountain) rivers.
•Numerical modeling and direct field observations are used to understand controls on wood transport in mountain rivers•Patterns of wood transport differ between single- and multi-thread river reaches•Log diameter controls wood transport in multithread channel, whereas log length is key in single-thread channels•We also document a nonlinear relationship between wood transport and flood magnitude•Our observations are likely to be applicable to a much larger suite of mountain rivers
Our research introduces the river regulation effects on three sections of the upper and middle Odra River (south-western Poland), with differently channelized parts. In the upper and lower reaches, ...the river was straightened, narrowed, and trained with groins, whereas in the middle section, it was also impounded by numerous barrages. The discharge duration (DD) and water stage duration (WSD) curves for water-gauge stations from these river sections were analyzed to recognize changes in river flows and channel morphology since the mid-20th century. This analysis is supplemented by an examination of repeated surveys of the gauge cross sections of the river, annual precipitation totals in its catchment, and their relationship to the variation of the North Atlantic Oscillation (NAO) index. Our findings provide new hydrological insights for the region. The three river sections exhibited different patterns of the adjustment of the channel morphology to the river channelization: upper section was typified by channel incision, middle section by channel stability, and lower section by channel incision in its upper part and vertical stability of the channel bed in the lower part. Barrages in the middle section stabilized water stages in a wide range of hydrological conditions. Annual precipitation totals and river run-off did not change systematically over the study period. The variation in precipitation totals was inversely related to annual values of the NAO index. The study confirms the usefulness of DD/WSD curves to analyze changes in river run-off and the vertical position of the channel bed.
To verify whether the variability in benthic invertebrate communities along the mountainous Czarny Dunajec River is mainly driven by the variation in hydromorphological or water quality, diversity of ...the communities was determined for 18 cross-sections with 1–5 low-flow channels and compared with the complexity of physical habitat conditions and with physico-chemical water quality. An increase in the complexity of flow pattern in the river was associated with increasing cross-sectional variability in physical habitat parameters. Distinct hydromorphological characteristics of the cross-sections with a given number of low-flow channels were especially pronounced if the analysis was limited to the parameters measured directly, whereas calculated complex hydraulic and sedimentary variables represented information overload. Taxonomic richness of the invertebrate communities was unrelated to physico-chemical water parameters, which consistently pointed to the high water quality. Instead, the richness positively correlated with a degree of variation in physical habitat parameters and was best predicted by the number of low-flow channels in a river cross-section. This study indicates that physical habitat complexity in a mountain river can be considered a proxy to the diversity of its invertebrate communities and that restoration of such complexity will be necessary for future recovery of invertebrate communities in impacted river sections.
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.
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