Restoring river connectivity is a global conservation priority but quantifying river fragmentation has proved difficult due to the paucity of good barrier records, duplicate entries, and other sources of biases. Here we present some tools to help overcome some of these challenges and illustrate their application with case studies drawn across different spatial scales. We begin by proposing a classification of artificial instream barriers that harmonises disparate barrier types into six functional types, and present a binary classification key for ease of use. We then introduce a method for excluding duplicate barrier records that retains most genuine barriers and illustrate its practical use. Sampling bias is a pervasive problem in barrier inventories and we show how to detect and correct for it via bootstrapping of data obtained from standardised field surveys, ad-hoc records provided by citizens, and modelling. Finally, we show how to assess fragmentation when barriers cannot be aligned with the river network, and how to estimate barrier impacts from barrier height and when information on barrier passability or permeability is not known. Collectively, our toolbox will help generate more realistic estimates of river fragmentation and help inform more efficient restoration of river connectivity. 

Rivers support some of Earth’s richest biodiversity and provide essential ecosystem services to society, but only if they flow. In Europe, attempts to quantify river connectivity have been hampered by the absence of a harmonised barrier database. We assembled ~630,000 unique barrier records from 36 European countries and surveyed 2,715 km of 147 rivers to reveal a ~61% underestimation of barrier numbers. We estimate there are at least 1.2 million instream barriers (mean density = 0.74 barriers/km), 72% of which are low-head (<2m) structures, making Europe the world’s most fragmented river landscape. The highest barrier densities occur in the heavily modified rivers of Central Europe, and the lowest in the most remote, sparsely populated alpine areas. Barrier density was predicted by agricultural pressure, road density, extent of surface water, and elevation. Relatively unfragmented rivers are still found in the Balkans, Scandinavia, the Baltic states, and parts of southern Europe, but these require urgent protection from new dam developments. Our findings can inform the implementation of the EU Biodiversity Strategy, which aims to reconnect 25,000 km of Europe’s rivers by 2030, but achieving this will require a paradigm shift in river restoration that recognises the impacts caused by small barriers.