Guidance to detect impact of HyMo degradation on riparian ecosystems

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.

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