Over 200,000 kilometers of free-flowing river habitat in Europe is altered due to impoundments.

European rivers are disconnected by more than one million man-made barriers that physically limit aquatic species migration and contribute to modification of freshwater habitats. Here, a Conceptual Habitat Alteration Model for Ponding is developed to aid in evaluating the effects of impoundments on fish habitats. Fish communities present in rivers with low human impact and their broad environmental settings enable classification of European rivers into 15 macrohabitat types. These classifications, together with the estimated fish sensitivity to alteration of their habitat are used for assessing the impacts of six main barrier types (dams, weirs, sluices, culverts, fords, and ramps). Our results indicate that over 200,000 km or 10% of previously free-flowing river habitat has been altered due to impoundments. Although they appear less frequently, dams, weirs and sluices cause much more habitat alteration than the other types. Their impact is regionally diverse, which is a function of barrier height, type and density, as well as biogeographical location. This work allows us to foresee what potential environmental gain or loss can be expected with planned barrier management actions in rivers, and to prioritize management actions.

The enhancement of valley water retentiveness in climate change conditions.

River system retentiveness must be enhanced to increase multidimensional environmental sustainability and thus ameliorate the effects of climate change and the occurrence of extreme hydrological events. The aim of the article is to demonstrate how ecohydrological Nature-Based Solutions can be combined with conventional infrastructure to improve WBSRCE benefits (Water, Biodiversity, ecosystem Services, Resilience, Culture, Education) by taking a holistic approach to multifunctional reservoir design. The paper proposes a new form of lateral reservoir, which is built without disturbing the meandering river and is supplied with good quality water through a monitoring and regulatory system; its design is based on thorough analysis of hydrological pulses and suspended matter and nutrients fluxes. The regulatory system also includes an innovative Sequential Sedimentation and Biofiltration System. Lateral reservoirs increase water retentiveness in the river valley by lifting ground water level and restoring surrounding wetlands, thus enhancing biodiversity, ecosystem services for society and the resilience of the river system to climate change. An integrative understanding of the interplay between hydrology and biocenosis can be used to enhance river system sustainability potential (WBSR) and harmonise societal needs with biosphere sustainability through culture and education (CE).

More than one million barriers fragment Europe's rivers.

Rivers support some of Earth's richest biodiversity1 and provide essential ecosystem services to society2, but they are often fragmented by barriers to free flow3. In Europe, attempts to quantify river connectivity have been hampered by the absence of a harmonized barrier database. Here we show that there are at least 1.2 million instream barriers in 36 European countries (with a mean density of 0.74 barriers per kilometre), 68 per cent of which are structures less than two metres in height that are often overlooked. Standardized walkover surveys along 2,715 kilometres of stream length for 147 rivers indicate that existing records underestimate barrier numbers by about 61 per cent. The highest barrier densities occur in the heavily modified rivers of central Europe and the lowest barrier densities occur in the most remote, sparsely populated alpine areas. Across Europe, the main predictors of barrier density are agricultural pressure, density of river-road crossings, extent of surface water and elevation. Relatively unfragmented rivers are still found in the Balkans, the Baltic states and parts of Scandinavia and southern Europe, but these require urgent protection from proposed dam developments. Our findings could inform the implementation of the EU Biodiversity Strategy, which aims to reconnect 25,000 kilometres of Europe's rivers by 2030, but achieving this will require a paradigm shift in river restoration that recognizes the widespread impacts caused by small barriers.