River Network Connectivity in Europe ^†

Introduction: Fragmentation is potentially the most significant pressure affecting European rivers. With the projected number of barriers exceeding 1 million for the EU alone, the quantification of river network connectivity is of extreme practical importance. The Nature Restoration Regulation aims to restore connectivity by increasing the length of free-flowing rivers by 25,000 km across Europe. In this work, we quantify connectivity for European river basins, quantify connectivity affectations and account for ecological status while projecting potential restoration outcomes. Methodology: We used RivTool and RivConnect to manage data across European river basins and to quantify connectivity using graph-based metrics. For this, we validated natural and artificial barrier position and used European-wide datasets for navigation routes, railways and highways to account for additional connectivity impacts. Results: River connectivity in Europe is severely impaired by barrier placement and infrastructure development. The integration of ecological status shows a generalized decrease in potential connectivity. Conclusions: River connectivity quantification needs to be an integrative process and must account for natural and artificial connectivity as well as habitat quality and quantity. It is clear that single-use measures are ineffective. If we enhance connectivity without improving ecological status, the outcomes will lack effectiveness, as will the reverse solution. Basin-wide, data-diverse approaches to connectivity enhancement planning should be the norm and not the exception. Nevertheless, this is, to our knowledge, the first integrated assessment of river network connectivity of European river basins that integrates natural and artificial barriers, additional connectivity affections and ecological quality.