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.

Instream large wood drives both form and function of forested gravel-bed rivers. The beneficial effects of wood in rivers, largely overlooked, are now widely recognized, as well as that together with the flow and sediment regimes, the wood regime controls both the physical and ecological integrity of rivers. Yet, large quantities of wood transported during floods can pose additional hazards, potentially damaging infrastructures like bridges or dams and exacerbating flooding. However, unlike the water and sediment regimes intensively studied over the past decades, the instream wood regime or budgeting has been only recently defined, and thus is still rarely quantified. The instream wood budget describes the different cascading processes from supply or recruitment, entrainment and transport, deposition and storage to decay. These processes show a highly spatial and temporal variability, but they can be characterized in terms of magnitude, frequency, timing, duration and mode. Instream wood budgeting is challenging, mostly because of the lack of data, monitoring stations and standardized protocols. This contribution reviews the most important recent advances made to quantify the different instream wood budget components, notably the wood recruitment and flux. Case studies showing applications of videography, artificial intelligence, numerical modelling, tracking or biogeochemistry illustrate the current progress. Still, important challenges remain, we identify and describe some of them, and discuss how wood in riverine sciences may develop in the near future.

Accumulated driftwood (i.e., floating trunks, downed trees, branches, and roots) upstream from dams may create obstructions, reduce spillway capacity, and cause undesired higher water levels in the reservoir. To minimize these issues, some dam managers must remove the driftwood mechanically regularly. This is the case at the Génissiat dam in the Rhone River, where all wood coming from upstream and the main tributaries is trapped. Besides the management issues, Génissiat brings an excellent opportunity to explore wood dynamics: where the wood comes from? when does it arrive? and how much?. These simple questions are key to designing proper management measures but are also very challenging to answer, as data is usually scarce. In this work, we analyzed the wood extracted at the Génissiat dam and used macromorphologic, taxonomic, and dendrochemical indicators to infer their sources and origin. In addition, hydrological data series provided detailed information about the flood conditions transporting the wood. Preliminary results showed that as expected, larger floods contributed to the larger wood volume, but similar hydraulic conditions resulted in a wide range of wood volumes. Peak discharge seems not to be the best predictor and other variables such as the flood origin, duration, volume, and frequency need to be considered as well as inter-flood period length or characters. Our results showed that wood quantity arriving at the reservoir varies according to the contribution from each tributary, the wood availability related to antecedent floods, and the forest stand characteristics

Integrating flow-sediment and wood regimes in the design of e-flows (i.e., environmental or experimental flows, floods, or releases) is of great importance, particularly in forested rivers with unregulated tributaries and /or active hillslopes affected by mass movements processes. In such cases, large quantities of sediment and wood can be supplied to the main regulated channel. This is the case of the Spöl River in the Swiss Alps. The Spöl river is regulated by two dams but undergoes a restoration program based on the release of annual experimental floods since the year 2000. Despite these e-flows, the river is facing significant aggradation and other associated processes, such as bank erosion, channel widening, or vegetation dieback. Instream wood is supplied to the river from the forested slopes affected by snow avalanches, landslides, debris flows and windstorms, eroded riverbanks, and vegetated islands. In 2018, we enhanced the existing monitoring framework to include observations of sediment and instream wood transport. Before and after flows, sediment was tagged, the grain size was measured at different locations, and wood stored within the river was tagged, measured, and georeferenced. We surveyed topographical cross-sections and performed aerial surveillance with a drone. Moreover, during the e-flows, a video camera was installed at a bridge to film the events. This contribution summarizes preliminary results from these surveys. Results will be key for the design of future river restoration in the Spöl, but also for the management of regulated mountain rivers in general.

Large wood (LW) has earned increased attention as a component of fluvial systems as its ecological and physical benefits, as well as its contributions to damages during flood events, have been realized. As LW found in river networks had originated from outside of the channel corridor, significant efforts have been made to identify recruitment processes that supply LW to channels. Evidence has proved treefall, landslides, bank erosion, debris flows, and fluvial entrainment contribute to LW recruitment. Prediction and identification of the areas prone to these processes are very challenging but could serve to better understand wood dynamics. Therefore, identifying areas prone to recruitment processes, estimating available LW, and determining LW connections in a watershed will help design management strategies aimed at mitigating LW's impacts as well as provide insight on the movement and recruitment of LW in fluvial systems. Analogous challenges exist when dealing with sediment dynamics. We applied the graph theory (GT) to instream LW supply and transfer. A GT is a set of nodes representing different entities (i.e., wood sources) with edges connecting nodes based on determined relationships (i.e., wood recruitment processes). The GT proves useful in exploring landscape connectivity with the capability of identifying critical nodes or regions, measuring properties of connectivity, identifying process coupling based on spatial patterns, and defining related geomorphological processes such as that of sediment cascades in which landscape components are coupled based on properties effecting sediment transfer. GT proves capable of defining connections between LW recruitment from hillslopes to the channel and from channel segment to channel segment. Currently, the fuzzy logic toolbox presented by Ruiz-Villanueva and Stoffel (2018) has been utilized to delineate the connected, recruitment process prone areas for landslides, debris flows, and bank erosion in the study area of Vallon de Nant, Canton of Vaud, Switzerland. The delineated areas have been used in ArcPro in coalition with vegetation data to extract hillslope-to-channel connections and channel-to-channel connections. The channel or fluvial network has been segmented based on the presence of features which reduce downstream transfer of LW such as channel widening and presence of obstructions. The determined connections will be applied in the R package, igraph, to extract network properties of the constructed, instream LW GT model. GT aids complex network analyses by providing a technique which retains only the critical information. Therefore, following the rigorous work of determining the system components,