Fine sediment dynamics is an important issue in alpine rivers since it impacts both dam management (decrease of the reservoir capacity, flushing strategy) and river management (interaction between fine sediment and vegetation, gravel bar stabilisation, flood risk). Suspended Particle Matter (SPM) is generally assumed to be transported as washload although significant exchanges with the river bed have been detected in several cases. As a consequence but also due to the variable SPM sources and differences in the discharge and sediment wave velocities, one can often observe a hysteresis behaviour during hydrological events that evolves spatially along the system. In this study, we will present results from a network of hydro-sedimentary stations (measuring both water discharge and sediment concentration) along a reach of 120 km starting from the Arvan River at its confluence and the Arc and Isère rivers, that is part of the ZABR (or Rhône Basin Long Term Environmental Research Observatory). The SPM dynamics is analysed for different types of events (large flood, spring flood, flushing event, debris flow). In particular, we focus on the potential of erosion or deposition between each station by calculating the total SPM flux during each event at each station as well as the hysteresis behaviour using a hysteresis index. The evolution of these parameters is related to input parameters such as discharge and sources and grain size characteristics of the suspension. Indeed, most observed sediment exchanges (erosion or deposition) occur on the gravel bars when inundated, and depend on settling characteristics of the SPM. Also the experimental site provides some useful data for estimating advection and dispersion characteristics of various SPM events that could be used to validate numerical models.

Lakes and reservoirs are a significant source of atmospheric methane (CH4), with emissions comparable to the largest global CH4 emitters. Understanding the processes leading to such significant emissions from aquatic systems is therefore of primary importance for producing more accurate projections of emissions in a changing climate. In this work, we present the first deployment of a novel membrane inlet laser spectrometer (MILS) for fast simultaneous detection of dissolved CH4, C2H6 and d13CH4. During a 1-day field campaign, we performed 2D mapping of surface water of Lake Aiguebelette (France). In the littoral (pelagic) area, average dissolved CH4 concentrations and d13CH4 were 391.9 ± 156.3 (169.8 ± 26.6) nmol L-1 and -67.3 ± 3.4 (-61.5 ± 3.6) ‰, respectively. The dissolved CH4 concentration in the pelagic zone was fifty times larger than the concentration expected at equilibrium with the atmosphere, confirming an oversaturation of dissolved CH4 in surface waters over shallow and deep areas. The results suggest the presence of CH4 sources less enriched in 13C in the littoral zone (presumably the littoral sediments). The CH4 pool became more enriched in 13C with distance from shore, suggesting that oxidation prevailed over epilimnetic CH4 production, that was further confirmed by an isotopic mass balance technique with the high-resolution transect data. This new in situ fast response sensor allows to obtain unique high-resolution and high-spatial coverage datasets within a limited amount of survey time. This tool will be useful in the future for studying processes governing CH4 dynamics in aquatic systems.

The present dataset is related to the Arc-Isère long-term environmental research part of the Rhône Basin Long Term Environmental Research Observatory. This alpine watershed located in the French Alps is characterized by high Suspended Particulate Matter (SPM) in very anthropogenized valleys. Suspended Sediment Concentrations (SSC) naturally observed in the river are very high, ranging from a few tens of milligrams per litre at low flow to tens of grams per litre during major natural hydrological events (floods, debris flows) or river dam hydraulic flushes. One research objective related to this site aims at better understanding the SSC dynamics along the river using a system of nested watersheds (Arvan, Arc, and Isère) in order to access to both temporal and spatial dynamics. Studies using this dataset are on the quantification of fine sediment fluxes but also on the related morphological changes due to fine sediment deposition or resuspension. Additionally, the observatory database can support studies on contaminants (either dissolved or particulate contaminant). Six hydro sedimentary stations monitor SSC with high frequency via turbidity sensors associated to automatic samplers. Discharge is measured via classical water level measurements and a rating curve. The oldest station (Grenoble-campus) started recording data from 2006 while others hydro-sedimentary stations were built from 2009 to 2011. Data are available in an online data website called “Base de Données des Observatoires en Hydrologie” (Hydrological observatory database, https://bdoh.irstea.fr/ARC-ISERE/) with DOI references for each site. The hydrological and sediment transport time series are stored, managed and made available to a wide community in order to be used at their full extent. This database is used as a data exchange tool for both scientists and operational end-users and as an online tool to compute integrated fluxes.