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Large active channels usually store more wood than channels with a narrow flow because of the availability of large unvegetated bars for wood deposition and inner functioning that usually supplies more wood through channel shifting. However, the dynamics of the wood supply (wood input, output, or stability) can vary substantially over time and the drivers are largely unknown. To explore them, we studied the temporal variability of large wood pieces and logjams along a 12-km reach of the lower Allier River using six series of aerial images of variable resolution acquired between 2009 and 2020, during which maximum river discharge fluctuated around the biannual (Q2) flood magnitude. We show that the wood budget was controlled by specific hydrological conditions. Wood output was best explained by water levels exceeding bankfull discharge (Q1.5). The duration of the highest magnitude flood (over bankfull discharge) was the best predictor of wood inputs, with shorter floods resulting in higher input rates. Finally, most of the wood remained stable when the river discharge did not exceed 60% of the bankfull discharge over a long period of time. Hydrological conditions driving jam build-up and removal were similar to those controlling individual wood piece dynamics. A succession of floods of similar (relatively low ~ Q2) magnitude and decreasing flood duration since 2016 have probably reinforced the filtering effect of wood obstacles, leading to positive feedback, which has been strengthened by riparian vegetation colonisation of the active channel.

River restoration and rehabilitation projects are widespread, but rarely include the data needed to fully evaluate if they are successful in achieving their goals or how long the process of readjustment takes before a new ‘recovered’ regime state is reached. Here we present a seven-year post-project dataset detailing the morpho-sedimentary responses of a river to the reconnection of a formerly diverted tributary, and relate observed changes to conditions in the river prior to the reconnection. We describe changes in the tributary and main-stem channels, including changes in channel planform, morphology, and the export of coarse and fine sediment from the tributary to the main-stem river. We use the data to develop a conceptual model of the system's response to the reconnection. Marked geomorphic changes occurred within the first two years after the reconnection. Changes during this ‘shock phase’ included dramatic erosion and subsequent deepening and widening of the tributary channel, rapid development of a confluence bar and an increase in fine sediment delivered to the main-stem. After this shock phase, and despite the continued occurrence of high magnitude flow events, the rate of geomorphic change in the tributary began to decrease, and the rate of growth of the confluence bar slowed. Fine sediment volumes in the main-stem also decreased steadily. After an adjustment phase lasting a total of approximately 4.5 yr (including the initial 2-yr shock phase), the tributary to mainstem system appeared to reach a new dynamic equilibrium that we consider the adjusted regime state. This new regime state was characterised by, among other things, an increase in geomorphic heterogeneity in the tributary and main-stem channels. Changes in both fluvial processes and forms indicate that within 4.5 yr the project was successful in achieving its goal of augmenting sediment and increasing geomorphic heterogeneity. Our conceptual model of adjustment mirrors that developed by Petts and Gurnell (2005), with the river passing through a complex and dynamic adjustment phase before reaching a new regime state. However, unlike the responses to impoundment represented by Petts and Gurnell, our model of river response to rehabilitation charts increases in dynamism and heterogeneity. The research presented in this paper has been supported by a number of organisations. We thank United Utilities (UU) and the Environment Agency (EA) for funding and support. In particular, we thank Kat Liney and Alice Senior from UU, and Gez Foster and Jane Atkins from EA. Part of this research benefited from the methods and outcomes of the MorphHab (PID2019-104979RB-I00/AEI/10.13039/501100011033) research project. B. Marteau's contribution was in part supported by a postdoctoral fellowship from the EUR H2O'Lyon (ANR-17-EURE-0018). D. Vericat is a Serra Húnter Fellow at the University of Lleida. We are also grateful to the reviewers and editor for their comments that improved the manuscript.

International audience; Given the actual trajectory of greenhouse gases emissions, in situ actions are needed to limit rivers water warming. Riparian tree planting has been shown as a successful mitigation strategy, but most studies have been conducted on upland forested rivers. The hydraulic, geomorphic and thermal conditions of lowland rivers, subject to straightening and fragmentation, are very different from upland systems. In this study, the role of riparian cover relative to the presence of weirs was investigated in lowland slow-flowing rivers within the agricultural context of the Bresse region, France. Using airborne thermal infrared (TIR) imaging, nine reaches of 20 to 30 km each were mapped to quantify water temperature, riparian vegetation and assess weir location. Campaigns were carried out during the hot summer 2018. Results have comforted that riparian vegetation is capable of limiting increase in temperature during warm events by 2 to 3 degrees C, but with varying success depending on the rivers' thermal regimes. Rivers with a 'colder' regime (i.e., ground/karstwater-fed) are thermally less influenced by riparian vegetation but also more resilient to the presence of weirs. Rivers with the highest degree of thermal alteration showed the highest positive response to riparian shading. Riparian vegetation prevents stagnant water from extreme warming but does not prevent alteration of the thermal regime, for example thermal stratification, likely to occur on a daily basis and to large extents (> 80%) during warm and low-flow events in impounded rivers. Thermal conditions were sub-optimal for most fish species that could be expected in these streams. Although the direct relation with thermal alteration is not proved, hydrobiological scores of flow alteration are correlated with our metric of thermo-physical alteration. Management strategies to limit temperature increase should carefully consider restoring the thermal regime of rivers through weir removal when it is possible, combined with riparian tree planting as an additional mitigation measure.

Gravel augmentation has become common practice to mitigate the effects of decline in upstream sediment supply in gravel-bed rivers. However, functional aspects of river systems such as thermal functions are often left out of rehabilitation monitoring programmes. Despite temperature being a fundamental parameter determining the general health of rivers, a limited number of studies have tested whether gravel augmentation can aid restoring thermal functions. Using airborne thermal infrared (TIR) imagery, this paper explores potential feedbacks through the monitoring of gravel augmentation on 3 rivers in France. To overcome the lack of pre-rehabilitation data, we used hydromorphological indicators within a trajectory-based Before-After-Control-Impact (BACI) framework to assess the success of rehabilitation on thermal functions. This design, combining long-term geomorphic evolution with TIR-based CI strategy, indicated that restoring forms was not sufficient to restore thermal functions. Nonetheless, hydromorphological indices mesures on historical aerial photographs can be used to estimate long-term evolution of groundwater-surface water interactions. We emphasise the benefits of trajectory-based BACI assessment to identify current conditions, understand the past evolution (trajectory) of the system to define the framework within which rehabilitation can objectively be assessed.

International audience

The development of Airborne Infrared Thermal sensing (TIR) is an example of how technological advancements and the field that they focus on have fostered one another. The pace at which global change is occurring has fed the demand for better understanding of the thermal behaviour of rivers. In turn, the improvement of remote sensing and data processing techniques has provided researchers and managers with new tools to apprehend such aspects at ever larger scales. Still, recent studies have mostly focussed on rivers showing little human alteration, with a particular interest on groundwater–surface water interactions. Lowland streams are scarcely considered when it comes to the study of temperature despite their widespread occurrence, their relatively high degree of disturbance and the risks that they face in the light of temperature rising following climate change. Some of these streams already display critically high maximum summer temperatures and their state is likely to worsen in the future, putting all compartments of biota at risk.The aims of this project were twofold. We first tested the applicability of airborne TIR to study lowland, slow-flowing stream reaches draining agricultural catchments, some of which being particularly narrow and sinuous. We then sought to understand the role of different environmental factors, observed in such context, on driving river temperature during the warmest days of the year. A number of anthropogenic actions such as clear-cutting of riparian trees, stream rectification and the construction of weirs are likely to influence the longitudinal temperature profile of such streams. By choosing rivers with no or limited groundwater inputs, we were able to quantify the relative role of each of the three tested factors and identify stream sections showing critically high maximum temperature over the summer.A final step was proposed to upscale these results in order to identify sections of streams showing high risks of reaching critically high summer temperature at a regional network scale. To do so, we used a combination of high resolution land-cover data, digital elevation models and other existing databases (e.g. national inventory of weirs). Identification of the risks in relation with the relative contribution of the different factors is key to process-based river management. This type of output is valuable to river basin managers and decision makers as it can be used to implement targeted restoration initiatives or remediation actions in areas where these have higher chances of being effective.

The annual global loss of organic carbon from terrestrial ecosystems into rivers is similar to the organic carbon stored in soils each year. Dissolved organic matter (DOM) flows through the food web to macroinvertebrates, but little is known about the effect of DOM increase on stream food webs and how much macroinvertebrates may contribute to the regulation of carbon fluxes in rivers. Using a before and after control impact (BACI) experimental design, we increased by 12% (+ 0.52 mg C L−1) the concentration of DOM in a stream for three weeks by adding sucrose, with a distinctive δ13C signature, to simulate a pulse of natural DOM supply from soils. We partitioned the diet of macroinvertebrates from carbon sources according to the green pathway (autotrophs) and detrital pathways (bacteria and terrestrial organic matter). Our flow food web approach based on C fluxes, with bacteria as a key node, showed the dominant contribution of the detrital pathways for macroinvertebrates in the reference stream. DOM addition induced changes in the diets of individual taxa, but did not have any strong effects on the relative overall contribution of the detrital pathways versus the green pathway. Autotrophic uptake of CO2 respired by bacteria was much larger than bacterial C flux to invertebrates (that is, the classic microbial loop) and allowed a significant fraction of natural allochthonous organic carbon to make its way to macroinvertebrates via autotrophs fixing CO2 respired by bacteria. Overall macroinvertebrates did not regulate directly to any great extent the flux of stream DOM towards downstream ecosystems.

Gravel augmentation has become common practice to mitigate the effects of decline in upstream sediment supply in gravel-bed rivers. The success of such rehabilitation schemes relies partly on the monitoring strategy and efforts. When long-term monitoring is lacking, some aspects of rehabilitation initiatives suffer more than others, such as insights into functions and functionalities of the river system. Despite temperature being a fundamental parameter determining the general health of river ecosystems, a limited number of studies have tested whether gravel-augmentation can aid restoring thermal functions. With the help of airborne thermal infrared (TIR) imagery, this paper explores the potential positive feedbacks through the monitoring of gravel augmentation actions, of different magnitude, taken on 3 rivers of the Rhône basin in France. A specific trajectory-based Before-After-Control-Impact (BACI) framework using simple indicators, combined with a TIR-based Control-Impact strategy, was designed to assess the success of thermal function restorations based on dynamic fuzzy references. Results indicate that restoring forms is not sufficient to restore thermal functions. The control-impact strategy shows limitations in the sense that two neighbouring reaches can display similar planform characteristics but different thermal functions; what is observed in a control reach should not necessarily be expected following rehabilitation. When assessing thermal processes, a before-after strategy is needed to either serve as a target or help define an adequate target in accordance with changes in the catchment and channel adjustments and responsiveness. We therefore recommend a trajectory-based BACI assessment to identify current biogeophysical conditions within which rehabilitation can be assessed. From a technical perspective, airborne TIR proved to be useful to rapidly map surface temperature over dozens of kilometres at high resolution, and can be advocated as a powerful tool to monitor and diagnose thermal functions of gravel-bed rivers. With an increasing number of rehabilitation schemes, and increasing pressure of global changes on rivers, we suggest that monitoring of water temperature, even with simple but well-designed sampling strategies, becomes a routine part of river rehabilitation projects.

This paper is the first of a pair that report the findings of a river rehabilitation project centred on the reconnection of a formerly diverted headwater tributary (Ben Gill) to its main‐stem river (the River Ehen). The present paper describes the geomorphic evolution of the tributary in the 2 years following its reconnection, with a particular focus on assessing the volumes of sediment now being supplied to the main‐stem Ehen. Structure‐from‐Motion photogrammetry was used to produce Digital Elevation Models (DEMs) of the newly connected tributary, with successive DEMs compared to assess topographic changes in the channel and quantify volumes of material exported. 3D errors in the DEMs were small relative to the scour and fill observed in the channel (error 0.016–0.056 m compared to up to 1.7 m vertical change between consecutive surveys). Erosion was the dominant process in the tributary channel, though this varied spatially and temporally. Over the 2‐year period, an estimated minimum of 384 m3 of coarse sediment was exported from Ben Gill and delivered to the confluence zone, where a new bar feature developed as a result. This estimate is twice as high as earlier ones. Analysis of the growth of this bar suggested that much of the material supplied by Ben Gill remains here temporarily, with onward conveyance constrained by the competence of the regulated main‐stem. The work shows that, thanks to the reconnection, this small (0.55 km2) ephemeral tributary (flowing for only around 20% of the time) has become a key source of sediment for the main‐stem Ehen. The second in the pair of papers focuses on the geomorphic responses of the main‐stem to this renewed supply of sediment. This study was funded as part of a PhD grant by the Environment Agency UK and United Utilities. DV was funded by a Ramon y Cajal fellowship (RYC‐2010‐06264) at the time the project was developed, and is now employed as a Serra Húnter Fellow at the University of Lleida. Authors acknowledge the support from the Economy and Knowledge department of the Catalan Government through the Consolidated Research Group “Fluvial Dynamics Research Group”‐RIUS (2017‐SGR‐459), and the additional support provided by the CERCA Programme, also from the Catalan Government.

This paper describes changes in bed morpho‐dynamics and topography in the River Ehen, a regulated river in NW England (i.e., temperate climate) following a rehabilitation project that reconnected a formerly diverted headwater sub‐catchment back to its main‐stem. Sediment grain‐size distributions in the Ehen changed subtly and in rather complex ways following the reconnection. Changes were most evident in the riffle morphological unit, where gravel‐sized material accumulated in the first 2 years after the reconnection. All morphological units initially experienced an addition of fine sediment (size

This research is funded by the Environment Agency and United Utilities whose support is gratefully acknowledged. Some of the methods employed in this work have been tested on the background of the results obtained in MorphSed (www.morphsed.es), a research project funded by the Spanish Ministry of Economy and Competiveness and the European Regional Development Fund Scheme (FEDER; CGL2012-36394). The second author is funded by a Ramon y Cajal Fellowship (RYC-2010-06264). Authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through the Consolidated Research Group ‘Fluvial Dynamics Research Group’ (2014 SGR 645). The authors thank Manel Llena from the University of Lleida for his help and contribution to the camera calibration experiments. We are also grateful to the three anonymous reviewers and the editors for their comments that greatly improved the manuscript.

Studies of ephemeral streams have focused mainly in arid and semi‐arid regions. Such streams also occur widely in temperate regions, but much less is known about their influence on fluvial processes in main‐stem rivers here. In this paper, we present evidence of the importance of a small ephemeral temperate stream for main‐stem fine sediment dynamics. The paper focuses on a restoration project (River Ehen, North West England) which involved the reconnection of a headwater tributary to the main‐stem river. We present data on suspended sediment transport 2 years prior to and 2 years following the reconnection. Despite the small size and non‐perennial flow of the tributary, its reconnection resulted in an increase of 65% in the main‐stem sediment yield. During both the pre‐reconnection and post‐reconnection periods, a higher proportion of the annual yield was conveyed during short events with relatively high suspended sediment concentrations. Following the reconnection, the magnitude and frequency of such events increased, primarily due to sediment being delivered from the tributary at times when main‐stem flows were not elevated. Overall, the main‐stem remains supply limited and so is highly dependent on sediment delivered from the tributary. The study helps stress that even non‐perennial tributaries yielding only a small increase in catchment size (+1.2% in this case) can have a major influence on main‐stem fluvial dynamics. Their role as sediment sources may be especially important where, as in the case of the Ehen, the main‐stem is regulated and the system is otherwise starved of sediment. Damià Vericat is funded by a Ramon y Cajal Fellowship (RYC‐2010‐06264). Authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through the Consolidated Research Group “Fluvial Dynamics Research Group”—RIUS (2014 SGR 645), and the additional support provided by the CERCA Programme, also from the Catalan Government.

Considéré comme étant actuellement le plus grand projet urbain européen, le Grand Paris Express est un vaste projet de réseau de transport public. Visant à limiter l’usage de la voiture dans Paris, à faciliter les déplacements entre les banlieues et le cœur de la métropole, ce projet améliorera aussi le développement économique en reliant différents pôles d’activités en facilitant l’accès à l’emploi.