Subglacial sediments are a large component of the sediment budget of glacierized catchments but insights into the subglacial origin of sediments (bedload, in particular) linked to proglacial runoff dynamics remain scarce. In this study, we use a tracer-based approach to quantify melt water proportions related to sediment transport at two proglacial streams, draining glaciers (named debris-covered and clean glacier) of different size, aspect and elevation range with contrasting distribution and thickness of debris cover and lithology of the subglacial sediments (i.e., metamorphic vs. sedimentary), in the Sulden/Solda catchment (Italian Alps). Results indicate that the glacier melt component (75 to 80 %) was associated with bedload concentrations of 1 to 10 kg m−3 at the debris-covered glacier and much lower concentrations of 0.01 to 1 kg m−3 at the clean ice glacier. At the seasonal scale, bedload and suspended sediment concentrations at both sites strongly varied with discharge. While daily bedload concentrations varied by up to two orders of magnitude obscured the seasonal development of bedload concentrations at both sites, a clear seasonality for suspended sediment concentrations was found. At the daily scale, the relationship of discharge, bedload, and suspended sediment was more complex because discharge and sediment transport did not always follow the daily variation of air temperature, or similar daily air temperatures resulted in different discharge and sediment transport responses and vice versa. Glacier size, presence of debris cover, and substrate were identified as the main drivers of melt dynamics and sediment transport at both glaciers. This study adds further insights into the interplay of meltwater contributions and sediment transport, which are essential to better assess the impact of climate warming on sediment supply in glacierized catchments.
A 1:20,000-scale geomorphologic map of the valley floor system of the Biferno River (Molise Region, Italy) is presented in this paper. Map analysis and aerial photography interpretation, in addition to field surveys, have allowed digital mapping of fluvial landforms and associated coastal, slope and anthropogenic forms which have characterised the valley floor in different periods. Comparative multi-temporal analysis performed in a GIS for the time-period 1869–2010 allowed the reconstruction of spatial-temporal relationships between most of the mapped landforms and acquisition of quantitative data on channel adjustments. The geomorphologic map provides an overall large-scale view of major channel adjustments over the past 150 years as well as the current morphological setting and dynamics of the Biferno valley floor. It provides a basic map for land planning policy with the observed distribution of active geomorphic processes to be used for landslide and erosion susceptibility assessment and the data obtained on channel adjustments essential in investigating land cover change over the same period. As the Biferno valley floor system is characteristic of major river systems in Central-Southern Italy, the reconstructed framework of valley floor transformations turns out to be useful when assessing a representative channel evolution model and potential scenarios concerning near future dynamics of similar river systems.
The map is aimed at illustrating the relationships between landscape evolution and human occupation in the Isernia basin since the Middle Pleistocene. We carried out a detail scale geological–geomorphological investigation integrated with archaeological data. Overall data suggest enhanced landscape modification related to the long-term evolution of the Isernia basin. Moreover, during the Middle Pleistocene an alluvial plain environment was present, as testified also by the famous Lower Palaeolithic site of Isernia La Pineta dated to ca. 600 ky. From 600 ky onwards, extensional tectonics and related valley incision reshaped the Isernia basin, with the formation of terraced surfaces and the deposition of a travertine plateau. Archaeological findings from the Lower Palaeolithic up to the Chalcolithic Age testify to pre- and protohistoric settlements on these surfaces. In historical times, Romans settled on the Isernia terrace ridge taking advantage of the outcropping travertines to support wall foundations and to extract construction material.
In mountain environments, the coupling of hillslopes processes with the channel network during extreme events is of great importance for rivers dynamics, as debris flows and landslides are among the most important sources of sediments. The Stolla Creek (40 km2 drainage area, South Tyrol, Italy) is a confined and partly confined mountain channel that was affected by an extreme flood in August 2017, followed by a smaller event in August 2020. The geomorphic effects of the two floods were investigated both in the main channel and over the entire basin with the aim to assess the impacts of the lateral sediment connectivity to the channel response and to the event-scale sediment export. An integrated approach was applied, including radar rainfall estimation, hydrologic-hydraulic analysis, analysis of morphological changes and sediment delivery to the stream network. Hillslope and channel processes were mapped and characterized by using geomorphological analysis of multitemporal orthophotos and Digital Terrain Models. Debris-flow connectivity to the main channel was derived by combining field evidence and GIS-based analysis. The 2017 flood was caused by rainfall with a short duration (6 h) and a rainfall intensity exceeding 45 mm/h. More than 600 debris flows were triggered in the Stolla basin, and the main channel experienced widening (width ratio between 1.3 and 4.9) through bank erosion and overbank deposition. Widening was accompanied by aggradation in the river corridor up to 1.2 m or incision down to −2.2 m. The 2020 flood was characterized by lower rainfall intensity (max 17 mm/h) and a longer duration (48 h), and debris flows were not triggered. The moderate magnitude of the 2020 flood peak did not lead to channel widening, but marked bed incision (up to −1.4 m) occurred in the reaches where aggradation took place during the 2017 event. In both flood events, limited volumes of sediments were exported from the catchment outlet. Overall, our results highlight how structural connectivity at the basin scale determines the potential sediment cascades linking hillslopes to channels but time-varying functional connectivity – driven by hydrological drivers as rainfall intensities and durations – eventually control the actual sediment transport effectiveness both on hillslopes and along the channel.
Abstract The combined analysis of past evolutionary trajectories of channel morphology and temporal patterns of driving factors is fundamental to understanding present river conditions, supporting river management and evaluating future changes. Rivers in Europe underwent important channel changes during the Anthropocene in response to changing natural drivers and anthropogenic pressures. A considerable number of papers have been published on this topic, in the last decades. In this study, a comprehensive meta‐analysis on channel changes during the last 200 years in Europe was performed, aiming to provide quantitative information on the intensity of changes, to highlight regional scale similarities and dissimilarities in evolutionary morphological trajectories and to discuss the main causes of such changes. Based on a review, 102 papers were selected, addressing 145 channel reaches flowing through five main mountain ranges (Iberians, Alps, Apennines, Balkans and Carpathians) in the southern and eastern parts of Europe. The results show that active channel narrowing (between 26% and 36% on average) and incision (between 1 and 2 m) prevailed in most rivers between the 1800s and the 1950s, although widening was documented in some rivers of the Alps and the Apennines. Most multi‐thread reaches maintained their pattern until the mid‐20th century. Active channel changes accelerated during the 1950s–1990s (or 2000s) period, with channel narrowing up to 60% and channel incision up to 14 m. Multi‐thread patterns strongly decreased in frequency, with anabranching channels disappearing and single‐thread patterns becoming predominant. The cumulative effect of multiple and concomitant human pressures (gravel mining, channelisation and damming) was identified as the main driving factor for these accelerated changes. These findings must feed the public debate about preventing alterations of river ecosystems—exerted by anthropic disturbances—in a context of rapid economic development, especially in river systems still poorly altered and thus preserving wide, active and heterogeneous fluvial corridors.
Abstract In mountain basins, sediment supply to the fluvial system occurs mainly through episodic geomorphic processes—such as debris flows and other landslide types—whose effectiveness is strongly influenced by the structural connectivity within a catchment. This paper presents a novel data‐driven approach to identify and map areas that are simultaneously susceptible to debris flow initiation and structurally connected to the main channel network (i.e. relevant sediment source areas for predicting and mitigating flood hazards in the river channels). The presented approach comprises: (i) the visual interpretation, delineation, mapping and classification of event‐specific connected and disconnected debris flow areas in three catchments of the Italian Alps; (ii) the development of data‐driven debris flow release susceptibility models that are combined with quantitatively classified index of connectivity (IC) maps; and (iii) a thorough evaluation of the approach, including an assessment of its spatial transferability across the catchments. The main results show: (i) quantitative IC thresholds to discriminate connected from disconnected debris flow release areas; (ii) statistically well‐performing and geomorphically plausible debris flow release susceptibility models for the three basins; (iii) diverse joint debris flow connectivity–susceptibility maps that allow identifying zones which are differently relevant in terms of debris flow connectivity. This work also highlights the spatial transferability of the approach, associated benefits and potential drawbacks, as well as the utmost importance of a thorough combined quantitative and qualitative (i.e. geomorphic plausibility) evaluation of the underlying results. The proposed approach is rather simple and requires few basic input data, and can therefore be applied over vast areas (e.g. to support regional‐scale hazard assessments or sediment management plans).
Abstract Channel incision and narrowing have occurred in the 20th century in most Alpine rivers. However, the causal links between sediment‐related human engineering and exploitation and morphological changes in rivers are mostly unclear. This study presents an analysis of the evolutionary trajectories of the main active channels in the upper Etsch/Adige River basin (Eastern European Alps) coupled with their modifications in terms of coarse sediment transport. Channel planform variations were quantified in 15 rivers (total length of 630 km) using multi‐temporal analysis of historical maps and orthophotos. Sediment volumes excavated from river channels or trapped by hydraulic structures (dams and retention basins) were retrieved from historical records, along with geospatial information regarding the presence of lateral and longitudinal consolidation works and land use variations. Results indicate that most rivers underwent slight narrowing and some of them experienced widening, from the mid‐19th century to the 1950s. From the 1950s to the late 1990s, severe variations in terms of narrowing and morphological simplification took place in all rivers. The analysis of channel changes in relation to human activities shows that gravel mining carried out in the period 1970s–1990s appears to have been the main cause of sediment imbalance in the rivers which narrowed the most. Since the 2000s, when gravel mining was banned by law, channel adjustments have become negligible throughout study area. Nevertheless, the trapping of a large share of coarse sediment fluxes—at the river basin scale—by retention check dams and hydropower dams has impeded rivers from recovering to their original conditions.
