Case Study Number: 7 Dam name: Various (12 in Denmark, 10 in the UK) River name: Various Built date: Various AMBER TOOLS: Habitat surveys (D3.1)
Small barriers, such as weirs and water gauging stations, are highly abundant worldwide, though perhaps especially in Europe. Smaller barriers are often deemed to have fewer impacts on fish populations though their abundance and cumulative impact makes for widespread effects. Too often, these barriers are left unnoticed and simply grow old, unsafe and obsolete whilst still limiting fish movements. We would argue that their removal provides a cost-effective way of reinstating connectivity and promoting sustainability of freshwater ecosystems. Small barriers are, by their nature, most common on smaller, low stream order watercourses and the extent of their effects on biota depends primarily on their effects on habitat and connectivity.
- High barrier density
- High number of obsolete barriers
- Increased river fragmentation with reduction in natural habitat
WHAT WE DID
In Denmark, A total of 12 weirs were removed across the Jutland peninsula during the lifetime of AMBER. Changes in habitat (including depth, substrate and flow) and fish density were monitored in the 50m directly upstream of the barriers and a total of 311km of river was reconnected.Fish density was evaluated at all sites, once prior to removal, and twice after removal except for Clasonsborg which was added to the study later one, and thus only has one set of post-removal measurements. Rivers and streams across the UK contain numerous old weirs, culverts and barriers which were built for historic reasons such as powering mills, raising riverbed levels or redirecting water courses. Recent estimates suggest that in England there are 0.75 barriers river-km-1 and across Great Britain only 1% of rivers are free of artificial barriers. During the lifetime of AMBER there has been increased attention and efforts focussed on the removal of low head barriers often initiated and coordinated by local rivers trusts and this case study focuses on the removal of such barriers at 10 sites across northern England.
- Habitat surveys (D3.1)
The short-term impacts of barrier removal on the fish community at the Danish and northern English sites studied were quite different. Whilst a rapid response of the fish community to barrier removal was observed at the Danish sites, the impact of barrier removal at sites in northern England was variable with no uniform pattern across all sites. There were several differences in the sites studied in Denmark and England which appear to have contributed to the observed differences in response. The barriers removed in Denmark were larger (mean height of weir 1.53m) compared to the English sites (mean 0.49m) with a longer ponded length (Mean length of ponded zone: Denmark 931m; Northern England 47m) and lower gradient. At the one site studied in northern England that did have a barrier >1m and a large ponded zone a clear response of the fish community was recorded following barrier removal.
WHO WAS INVOLVED?
AMBER partner Technical University of Denmark (DTU)
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