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4101 CK Culemborg
the Netherlands
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Dood hout in de rivier voor de Kaderrichtlijn Water, KRW
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Reintroduction of large woody debris in navigable rivers

The Dutch Government has carried out a pilot study to place large woody debris in rivers was carried out, which aims to improve biodiversity and to increase scores under the Water Framework Directive.

Dead trees were placed at three loctions in the River Nederrijn-Lek by Blaauwendraat landschapsverzorging. The oaks we being removed from the area and were anchored underwater complete with branches and roots. Large chains and steel beams were used to secure the trees to ensure that they could not break free during periods with high water levels.

Improving biodiversity and the Water Framework Directive score

The main goal of reintroducing large woody debris into the river system was to enhance ecological diversity, especially for fish and macroinvertebrates. In smaller rivers, there is often sufficient dead wood along the banks. However, in larger, navigable rivers dead trees are removed to prevent them being hazards to bridges or shipping.

Dead trees create unique habitats under the water for a range of species of macrofauna. Fish also profit from these habitats as they can hide amongst the branches and feed on plants and animals on the trees. It is expected that the reintroduction of large woody debris will improve the score under the Water Framework Directive. This study aims to investigate this.

Cost effective measures alongside creation of lower banks

Foto Rijkswaterstaat. Bron: ©Blik onder water

Despite various measures being taken during recent years, WFD scores have shown no clear improvement. Based on the results of monitoring along the Nederrijn-Lek, Bureau Waardenburg has advised that more measures are needed, such as the creation of gullies and the lowering of banks. The banks of the Nederrijn-Lek are largely made up of groynes and artificially strengthened banks. The reintroduction of large woody debris is expected to increase biodoversity at relatively low cost.

The initial locations for the trees were chosen to be away from the main channel, such as near fish stairs, side channels and bays. The anchoring on these trees will be monitored and if successful the method can be applied along the main channel.

Monitoring the effects of trees in the river

Bureau Waardenburg is undertaking biological monitoring together with Hydrobiologisch adviesbureau Klink. Because of the special structure of the trees new monitoring methods have been developed. During the macrofauna sampling the trees are lifted out f the water in their entirety and rinsed. It will soon be known whether these tees have had the desired effects towards meeting Water Framework Directive targets.

Outcome

The results of the pilot on wood in rivers have been published in a Dutch report and in the peer-reviewed journal Aquatic Invasions. In addition, we produced a set of guidelines for Rijkswaterstaat to aid the process of decision-making on when and where to add wood to rivers.

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Staff from Blaauwendraat landschapsverzorging place an oak in the River Lek. This provides habitat for aquatic life and increases the biodiversity of fish and macrofauna.