REFORM - D3.4 Guidance on how to identify impacts of hydromorphological degradation on riparian ecosystems

Large river regulation and rehabilitation in Europe – six selected case studies

tom.buijse@deltares.nl — Fri, 04 Dec 2015 12:08:11 +0000

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 19^th 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.

3.5_Satellite topic Large Rivers 09 Nov 2015-def.pdf

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Final

Policy Brief:

Guidance to detect impact of HyMo degradation on riparian ecosystems

tom.buijse@deltares.nl — Mon, 24 Aug 2015 10:19:29 +0000

The aim of this deliverable is to address the impact of hydromorphological degradation on floodplain and riparian ecosystems, with specific focus on vegetation, fish and invertebrate responses and to provide guidance on how to identify those impacts.

An introductory chapter summarises the research context and reviews the lessons for managers and stakeholders. Based on the results of the analyses, and the river styles typology developed in Work Package 2 of REFORM, a generic process is recommended for assessing the impact on floodplain and riparian ecosystems, incorporating our key findings. It also highlights the usefulness and limitations of existing EU Directives in providing a suitable legislative framework.

Assessments of instream impacts on riverine ecosystems make use of multi-site datasets, riparian and floodplain ecosystems are not subject to this type of extensive monitoring; hence, the results presented here are based primarily on case studies from across Europe.

A key finding is that impacts to hydromorphological processes and that these impacts can take years to fully manifest themselves. The results can be dramatic with changes in river style and loss of riparian forest as unpredicted outcomes of human intervention.

We spotlight vegetation, unlike other biota, it has a very direct influence on fluvial geomorphological and hydrological processes, by stabilising sediments and influencing flood conveyance. A number of case study contributions address various forms of this key interaction. A short summary chapter is provided to link these findings to this particular theme. Use of the vegetation-process model developed in WP2 is recommended to compare the post impact role of the physical processes of vegetation with the ‘natural’ conditions. This model is used in all the vegetation case studies and helps explain why impacts can take time to become fully manifested.

In three Italian case studies, on the Magra, Panaro and Aurino rivers, the channels narrowed and the beds encised following a variety of impacts including gravel mining and catchment scale deforestation. This altered the patterns of riparian vegetation and tree growth. The three case studies also highlight the complexity of the relationships occuring between riparian vegetation and river hydromorphology in impacted rivers and how human disturbances may become dominant in structuring such relationships. The case study examples confirm that plant diversity alone cannot be considered a valid and exhaustive indicator to assess the health of a river system and its functioning.

For the two Spanish case studies aerial photographic data was available before and after damming of the rivers. After damming, vegetation encroachment on downstream gravel bars stabilised the banks and the channel changed from braided to single threaded. Similar effects of flow regulation have been reported on other rivers. In the River Porma, the vegetation composition and structure changed from one dominated by young pioneer species to a mature forest with a dense overstory of late-seral species near the channel banks. In the case of the River Guadalete, the flow regulation reduced the recruitment potential of native species and favoured the exotic species Eucalyptus camaldulensis. As a direct consequence of flow regulation, areas affected by fluvial disturbances under pre-dam conditions have turned into areas dominated by hydrologic processes with negligible sediment dynamics during inundation. Based on these results key indicators of change are proposed.

In Austria, the River Traun is regulated via a flood protection dam that cuts off the river from its floodplain and side arms and wet areas of the riparian forest have dried up. The river is deeply encised, and this has caused a significant lowering of the groundwater table with a consequent loss of riparian forests. The River Traun study site is representative of a large number of European rivers where the typical shruby pioneer vgetation and softwood riparian forests have disappeared and been replaced by mainly hardwood riparian forests that constitute the largest part of the remaining European riparian forests.

In Poland, the low energy river Narew is one of the few remaining anastomising river systems in northern Europe. This river type was once common and widespread but is now confined and regulated across much of its historic distribution. The Narew has been subject to flow regulation in parts of its catchment. The analysis of inundation duration for the period 1978-2009 shows that the vulnerability to changes in the flood regime, induced by damming upstream, is habitat dependent and related to the duration of flooding. In the case of wetlands sedge Phalaris and Carex-Phalaris communities, their natural inundation periods are relatively short. These communities were affected by the change in the flood frequency, while other communities were unaffected. The study demonstrated that natural (or semi-natural) lowland river valleys can be quite resistant to a single pressure, in this case flood frequency changes.

We also provide primary research on invertebrates and fish responses to riparian degradation. In Scotland, three rivers subject to varying degrees of flow regulation were studied, two of which are Special Areas of Conservation. The response of riparian invertebrates to flood inundation on mid-channel islands was studied. Overall, the insensitivity of these riparian invertebrate assemblages to flow peak or intensity floods suggests that the community structure is resilient. High abundance of Carabidae indicates a system unaffected by floods, suggesting that the system is hydrologically impaired. More important environmental factors were the size and habitat structure of the riparian habitat. The semi-natural habitat in the surrounding landscape provided a source of colonists.

Swedish streams in catchments with natural (forest) and degraded (agriculture) riparian zones were compared. Degradation of the riparian zones had important effects on in-stream hydromorphology (riffle and pool sections) and instream invertebrate communities (changes in species traits composition). The agricultural streams characterised by long stretches of pool habitat are less likely to support insects with traits favouring greater dispersal than forested streams with a higher abundance of riffles. This in turn affects the subsidy of energy and nutrients to the riparian zone, in the form of aquatic insects emerging as adults and dispersing into the riparian zone. This implies that the few short riffle habitats in the agricultural landscape are important for the transfer of high quality food to terrestrial/riparian consumers.

In the Danube Delta, Romania, fish communities have been significantly affected, locally, by loss of connectivity between the main stems of the river and floodplain lakes. The lakes have high species diversity due to the co-occurrence of rheophilic, eurytopic and limnophilic forms. Analysis of long-term data on commercial fishing and the history of hydrotechnical works indicates negative changes in the catch, which correlates well with the blocking of canals to alleviate siltation and nutrient inputs. Alternative solutions should now be considered. Reliable long-term commercial fishery data on migratory anadromous and potamodromous fish species can be used to indicate and explain effects of historical changes in the lateral or longitudinal connectivity of river systems. From a management point of view, maintaining the existing connectivity gradient in the delta lakes is vital for biodiversity conservation and economic needs.

3.4 Guidance to detect impact of HyMo degradation on riparian ecosystems.pdf

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D3.4 Guidance on how to identify impacts of hydromorphological degradation on riparian ecosystems