Knowing where and when rivers cease to flow provides an important basis for evaluating riverine biodiversity, biogeochemistry and ecosystem services. We present a novel modeling approach to estimate monthly time series of streamflow intermittence at high spatial resolution at the continental scale. Streamflow intermittence is quantified at more than 1.5 million river reaches in Europe as the number of no-flow days grouped into five classes (0, 1-5, 6-15, 16-29, 30-31 no-flow days) for each month from 1981 to 2019. Daily time series of observed streamflow at 3706 gauging stations were used to train and validate a two-step Random Forest modeling approach. Important predictors were derived from time series of monthly streamflow at 73 million 15 arc-sec (~500 m) grid cells that were computed by downscaling the 0.5 arc-deg (~55 km) output of the global hydrological model WaterGAP, which accounts for human water use. Of the observed perennial and intermittent station-months, 97.8% and 86.4%, respectively, are correctly predicted. Interannual variations of the number of intermittent months at intermittent reaches are satisfactorily simulated, with a median Pearson correlation of 0.5. While the spatial prevalence of intermittent reaches is underestimated, the number of intermittent months is overestimated in dry regions of Europe where artificial storage abounds. Our model estimates that 3.8% of all European reach-months and 17.2% of all reaches were intermittent during 1981-2019, predominantly with 30-31 no-flow days. Although estimation uncertainty is high, our study provides, for the first time, information on the continent-wide dynamics of intermittent rivers and streams.

Fish stranding in rivers, due to rapid shoreline dewatering, often occurs during the down-ramping phases of hydropeaks enabling peak energy production. Multiple hydrological characteristics of hydropeaking and river morphology influence stranding, but little is known about their relative effects. The goal of our study is to identify how the combination of hydropeaking characteristics and the occurrence of morphological microstructure (e.g., puddles, scour pools) influence fish stranding. For this purpose, we used an extensive dataset of fish stranding observations collected over 3 years in spring at 48 stations along a 50 km-long river reach. We aimed (1) to characterize stranding events and their associated fish assemblages, and (2) to identify the spatial and temporal determinants of stranding. We found that the occurrence of morphological microstructures of the riverbed was the main factor explaining fish stranding. Scour pools are the most impacting microstructures, followed by scour puddles, humid zones, and alluvial puddles. Then, hydropeaking characteristics interact with morphology and modulate the intensity of stranding. Low flow ranges (low peak flow, low base flow) occurring after periods without hydropeaks induce particular “salmonid” and “super-stranding” events and other flow ranges induce regular stranding events. Salmonids are particularly subject to stranding at the beginning of the sampling period. Recommendations that emerged are (1) to act in priority on stations where stranding is most likely, by morphological operations or by installing attractive structures in the perennial area, and (2) to maintain attractive, perennial habitats in the low flow range of hydropeaks.