Carbon dioxide (CO2) fluxes in regulated Alpine rivers are driven by multiple biogeochemical and anthropogenic processes, acting on different spatiotemporal scales. We quantified the relative importance of these drivers and their effects on the dynamics of CO2 concentration and atmospheric exchange fluxes in a representative Alpine river segment regulated by a cascading hydropower system with diversion, which includes two residual flow reaches and a reach subject to hydropeaking. We combined instantaneous and time-resolved water chemistry and hydraulic measurements at different times of the year identifying the main CO2 pathways through a one-dimensional transport-reaction model. The spatiotemporal distribution and drivers of CO2 fluxes depended on hydropower operations. Along the residual flow reaches, CO2 fluxes were directly affected by the upstream dams only in the first 2 km downstream of each dam, where the supply of supersaturated water from the reservoirs was predominant. Downstream of the hydropower diversion outlets, the magnitude and dynamics of CO2 fluxes were dominated by systematic sub-daily peaks in CO2 transport and evasion fluxes (‘carbopeaking’) driven by hydropeaking. The additional input of CO2 released locally into the river at the hydropower diversion outlet during hydropeaking matched the amount of CO2 transported, metabolised, and exchanged with the atmosphere along the whole upstream reach. Hydropower operational patterns and regulation approaches in Alpine rivers significantly affect CO2 fluxes and their response to biogeochemical drivers across different temporal scales. This work contributes to understanding and quantifying these processes to clarify the role of natural and anthropogenic drivers in global carbon cycling.

not-yet-known not-yet-known not-yet-known unknown The ecological effects of sediment flushings from artificial reservoirs have been widely documented, but the underlying sediment dynamics are less known. We investigated sediment dynamics associated with a long flushing event divided into two periods (2 and 1 week) in an Alpine river, each followed by a clear water release (“washing”) from the reservoir. Suspended sediment dynamics were investigated at the event and annual time scale, and at the river segment (~1000 channel widths) and reach (~100 channel widths or less) spatial scales. Analysis of suspended sediment concentration (SSC) and streamflow time series from 5 in-situ calibrated optical turbidity sensors reveals a downstream decrease in the total passing sediment fluxes, a spatial trend that is paralleled by the theoretical suspended sediment transport capacity, allowing for the estimation of the deposited fine sediment volume in different reaches. Washing events result in variable effects among reaches, with some experiencing net sediment entrainment and others net deposition. Out of 16 quantified sediment fluxes, 5 were statistically significant with p<0.05, with an average uncertainty of 23\% in fine sediment flux quantification. Georeferenced analysis of colored gravel-cobble plots before and after the two flushing events revealed partial reach-scale mobility of the coarse bed surface material, particularly in the geomorphic units located at lower elevations and more exposed to higher flows (edges of side bars nearby riffles or rapids), while local fine sediment deposition was observed at less exposed units, as side channels or point bars in river bends. Grain size distributions of surface sediments taken in the same locations before and 1 month after the flushing reveal a clear shift towards a finer sediment composition, which is partially retrieved also 1 year after the event. Event-averaged SSC values during the flushing are considerably higher compared to natural flood events in such a regulated river, with SSC-streamflow relations being highly irregular and event-dependent, especially during the flushing. The work shows the relevance of multi-scale (time and space) investigation of sediment dynamics for planning and monitoring sediment flushing from artificial reservoirs.