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Trust your bank manager: riparian zones to protect and restore rivers

May 30, 2018
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Restored river section five years after restoration with riparian vegetation consisting of grasses, shrubs and trees. Photo: Christian K. Feld

Safeguarding the banks and margins of streams and rivers has a key role to play in ensuring river health. This is the major conclusion from a new international study recently published in Water Research.

The authors of the new study synthesised the findings of more than 100 river management studies, many of which addressed the effects of riparian restoration on riverine habitat and biological conditions. It is widely acknowledged that riparian plants provide food for aquatic organisms, and can mitigate water temperature increases under climate change. Riparian zones can also provide valuable ‘buffer zones’ for run-off from farming and urban areas, preventing pollutants from reaching the river channel. Yet such riparian effects are not to be taken for granted.

Rivers are increasingly threatened by pollution, habitat modification and over-exploitation. Based on the results of the first Water Framework River Basin Management cycle, in 2012 the European Environment Agency (EEA) concluded that more than half of the European river network is damaged by severe human alterations. This picture appears even worse when individual country’s results are considered.

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Riparian vegetation is often degraded to a one-row tree line. This may provide shade during summer, yet fundamental functions of riparian vegetation such as nutrient and fine sediment retention cannot be fulfilled. Photo: Christian K. Feld

River restoration approaches generally seek to improve river health, but often struggle with the identification of suitable management options. The team of researchers behind the new study addressed this need during the EU MARS project by reviewing available scientific literature. The scientists analysed the ecological role of riparian vegetation types as well as their configuration and spatial extent. Riparian features were then related to the level of restoration success observed in individual studies.

These results can then be applied to help guide future river restoration activities. Restoration success was evaluated through measures such as the efficiency of nutrients and sediments retention or water temperature ‘dampening’ as a result of riparian vegetation. Over a longer term, the effects that riparian vegetation had on riverine plants and animals could be evaluated.

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Benthic invertebrate (caddisfly larvae) that feeds on leaves from riparian shrubs and trees. Photo: Aquatic Ecology, University of Duisburg-Essen

Based on the reviewed literature, the study found that woody riparian vegetation had consistent effects on the supply of leaf-litter and large wood to rivers. Leaves are a fundamental food source for riverine insects in upland rivers, where primary production is strongly limited, and where the food web of aquatic organisms relies on the input of terrestrial carbon.

Woody debris – such as fallen trees, tree trunks, branches and twigs – often increases habitat quality and variety in rivers. Besides the input of organic material, bankside vegetation also provides shade and can effectively ‘dampen’ water warming in spring and summer.

However, despite the observed positive effects of riparian vegetation, this review article also revealed weak and inconsistent effects on the biological quality and diversity of river organisms after riparian restoration. Unfortunately, the reasons behind this finding remained unclear, given that just a small fraction of the reviewed studies included biological effects at all.

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Large woody debris in the restored Lippe River in Germany. Image: Benjamin Kupilas | REFORM

Leader of the study, Dr. Christian Feld said, “Land management such as farming or forestry are essential for society – but can damage rivers downstream. We therefore need ways to reduce their unwanted effects, and management of the riparian zone has long been proposed as a cost-effective and local solution. Our evidence shows that riparian restoration can be effective in offsetting some problems, but not all. Larger-scale problems such as pollution from agricultural chemicals or sediments will need larger-scale solutions applied through improvements in the management of whole river catchments.”

Feld highlights the unclear effects of riparian restorations in the retention of nutrients and sediments originating from farmland on the floodplain. The study showed that only riparian plantations that combined trees, shrubs and grasses could effectively reduce nutrient and sediment pollution. However, even then, riparian restorations had only a limited effect where the catchment further upstream was intensively used by agriculture. In contrast, positive effects were more pronounced if riparian restorations took place in the upper parts of the catchment, where the aquatic environment is stronger linked to the riparian conditions.

Professor Steve Ormerod – Co-Director of Cardiff University’s Water Research Institute and co-author of the study – added, “This whole issue is one that needs more holistic ecosystem management. Fresh water is a crucial human resource that needs care, maintenance and sometimes very expensive treatment before it can be supplied to people.  Freshwater ecosystems are also losing biological diversity at an alarming rate globally because they are not well protected. We need to step up efforts to balance productive land use against these downstream costs – and our work shows that this needs a blend of local riparian solutions as well as improved large-scale thinking.”

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Overview of a restored river section five years after restoration. This riparian zone can effectively buffer the river system from adverse land use impacts on the floodplain, while the shade and organic material provided by the vegetation can promote the instream biology. Photo: Viktoria Berger.

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C.K. Feld, M.R.P. Fernandes, M. T. Ferreira, D. Hering, S.J. Ormerod, M. Venohr, C. Gutiérrez-Cánovas. (2018) Evaluating riparian solutions to multiple stressor problems in river ecosystems — A conceptual study. Water Research, 139, 381-394.

The work was funded by the EU MARS project under project No.: 603378

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