Damn those damn dams: Fluvial longitudinal connectivity impairment for European diadromous fish throughout the 20th century.

River longitudinal connectivity is crucial for diadromous fish species to reproduce and grow, its fragmentation by large dams may prevent these species to complete their life cycle. This work aims to evaluate the impact of large dams on the structural longitudinal connectivity at the European scale, from a Diadromous fish species perspective, since the beginning of the 20 century until the early 21 century. Based on large dam locations and completion year, a multitude of river impairment metrics were calculated at three spatial scales for six European oceanic regions and 12 time periods. The number of basins affected by large dams is overall low (0.4%), but for large river basins, that cover 78% of Europe's area, 69.5% of all basins, 55.4% of the sub-basins and 68.4% of river length are impaired. River network connectivity impairment became increasingly significant during the second half of the 20 century and is nowadays spatially widespread across Europe. Except for the North Atlantic, all oceanic regions have over 50% of impacted river length. Considering large river basins, the Mediterranean (95.2%) and West Atlantic (84.6%) regions are the most affected, while the Black (92.1%) and Caspian (96.0%) regions stand out as those with most compromised river length. In 60 years, Europe has gone from reduced impairment to over two-thirds of its large rivers with structural connectivity problems due to large dams. The number of such barriers increased significantly in the second half of the 20 century, especially main stem dams with decreasing distance to the river mouth. Currently, the structural longitudinal connectivity of European river networks is severely impacted. This concerns all regions considered, and those in southern Europe will face even higher challenges, given that this will be a future hot spot for hydropower development and predictably more affected by climate change.