Large rivers have been selected as one of the satellite topics both within WP3 and WP4, because of their particular features which could not be analysed in the case study catchments framework. Large rivers are considered rivers with a catchment larger than 10,000 km2 and > 100 m3/s. This encompasses rivers such as the Danube, Rhine, Rhône, Ebro, Vistula but also major tributaries such as the Sava, Narew, and Main rivers. Most fulfil major socio-economic functions, which will remain strongly modified and thus direct the options for rehabilitation. Because of their multifunctional use, large rivers can often only be partially rehabilitated or mitigated to achieve Good Ecological Potential according to the Water Framework Directive. This report addresses both hydrological modifications and restoration (rehabilitation, mitigation) following a DPSIR approach for six case studies that are spread across Europe
The historical trajectory of driving forces, river regulation (100 – 200 years) and rehabilitation (20 years) is used to underpin and illustrate the state-of–the-art regarding the effectiveness and potential of large river rehabilitation. For this, experiences and case studies from various large rivers in Europe are presented. For each case study the following information is given:
- General characteristics of the river (stretch);
- Description of historical state or reference condition(s) used in the rehabilitation project;
- Functions of the river (stretch): for which socio-economic functions is the river used, and what are the resulting pressures for its ecological functioning?
- The effects of identified pressures on hydromorphology and ecology;
- Mitigation and rehabilitation measures; what measures have been taken or planned to improve the hydromorphological and ecological status of these rivers?
- Ecological effects of measures.
The six case studies are representatieve of various European conditions with regard to climate, hydromorphological characteristics and catchment size. The case studies are situated in three biogeographical regions and six countries, viz. Atlantic region: River Trent (UK) and Delta Rhine (Netherlands), Continental region: Middle Vistula (Poland), Lower Danube and Po River (Italy) and Mediterranean region: Ebro (Spain). All these rivers can be characterized as large rivers (viz. catchment area larger than 10,000 km2), although they differed strongly in climatic zone, river length, catchment size, discharge, slope and river style. Large rivers can be considered as unique ecosystems and results are difficult to generalize. Still these case studies together give a good impression on the present regulation and rehabilitation of large rivers in Europe.
The case studies share but also differ substantially in drivers and associated pressures. Both flood protection and navigation are important drivers for the occurrence of many pressures. The rivers Trent, Po, Ebro and Delta Rhine have a large number of drivers and associated pressures, while the Danube Delta and middle Vistula are less impacted. For the majority no information was available regarding the extent of drivers and pressures.
There was a general pattern in the chronological sequence of the impact of drivers and associated pressures. The primal drivers for early regulation of all rivers were flood protection (embankments) and agriculture (deforestation). For most, these forms of river regulation started already centuries ago. Navigation became an important driver during the 19th century requiring further channelisations. As a result, the occurrence of highly dynamic habitats strongly declined caused by stabilisation of the river bed (by groynes, bank protection) as well as by deepening of the main channel. Of our case studies, only the river Vistula in Poland is currently not regulated for navigation purposes, and – hence – large parts of the main channel of the river have not been channelised. More recently, especially after the Second World War, many dams were constructed in the rivers, which resulted in a decreased longitudinal connectivity, thereby impeding conditions for migratory fish and other species. Additionally, the hydrological regime of rivers was strongly altered and sediment supply to downstream sections was strongly reduced. Especially the rivers Trent, Po, Ebro and Lower Danube have been severly impacted by the construction of dams.
For the majority of the case studies, only limited information was available regarding the impacts of pressures on hydromorphology and ecology. Large rivers are impacted by multiple stressors which complicate to identify the primal causes for degradation. It seems that the sequence of drivers (and associated pressures, see above) have initiated major transition points for ecological processes and biota along large rivers. We discuss the effects briefly in respect to the time line of occurrence of these drivers and pressures.
There are some striking differences in the restoration measures taken. Along the lowland stretches of large rivers, such as the Lower Danube and the Delta Rhine, measures focus on restoring lateral connectivity gradients between main channel and floodplains. Because of constraints imposed by navigation, only a limited number of measures are taken that improve conditions for lateral migration to rejuvenate riparian zones and bar and island formation, because these will affect navigational depth in the main channel. Along the river Trent and Po (and to some extent, the Delta Rhine), measures are taken that increase variation in width and depth of the main channel, which variation is an important variable for the occurrence of several hydromorphological processes. Restoring conditions for island and shoal formation will only be carried out along the river Vistula where navigation is not an important driver.
In summary, along relatively intact river stretches, such as the Vistula and Danube delta only a limited amount of measures can already improve ecological conditions. In highly regulated rivers such as the river Trent and Delta Rhine having extensive and diverse pressures a large number of measures are required and have been taken or planned. By contrast, the Mediterranean Rivers Ebro and Po are also highly regulated, but along these rivers only a small number of measures are planned at present.