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Within river basin districts, socio economic drivers generate a multitude of human activities that rely on the natural system. Important activities are agriculture and forestry, urbanization and transport, flood protection, hydropower production, navigation and recreation, that all, but in different ways, add pressure to the river-floodplain ecosystem. These changes are linked to the need to provide flood protection of people and property and to the historical desire to increase agricultural area, but also navigation and mining have added to pressures. Such pressures are broadly referred to as hydromorphological pressures – pressures to the natural system from changes to both hydrology (water flow) and morphology (physical characteristics) of rivers and floodplains.

Hydromorphological pressures covers a very wide range of changes, that all have different ways of influencing survival species, habitats and even ecosystems. The changes introduced by hydromorphological pressures impact the ecology of the natural system. They have a tendency to eliminate the lateral connectivity between the river and floodplain, reducing habitat quality, and influencing the species that can thrive. For example, barriers across rivers prevent fish from migrating upstream, reducing the ability of migratory fish to reach spawning areas. In Norway, following pressures from escaped farmed salmon and salmon lice, hydromorphological pressures are seen as the largest single factor influencing the wild salmon population (Forseth et al., 2017).

Under the Water Framework Directive, hydromorphological pressures are captured as one of the quality elements supporting the achievement of good ecological status. I.e. if benthic invertebrate fauna, fish or aquatic vegetation fail to achieve good ecological status, the possible impact of hydromorphological pressures needs to be assessed. The recent overview of the results in the second River Basin Management Plans under the Water Framework Directive show that hydromorphology is a significant pressure in 45% of Europe’s rivers, with physical alterations to floodplains or the river channel accounting for 30% of those pressures, and dams, locks and weirs account for another 27% (EEA, 2018b, 2018c). The average value, however, masks large geographical variations, with some countries not reporting hydromorphological pressures and other reporting 85% of waterbodies impacted. Some examples of hydromorphological alterations linked to pressures are listed in Box 2, and the hydromorphological alterations of the Tisza river, Hungary is provided as an example of the large modifications Europes rivers have undergone (Box 3).