Restoration is a word that is increasingly used in conjunction with freshwaters, this is in part due to the acknowledgement that humans and freshwaters have a history of alteration, but more so as there is a growing knowledge base that a functioning catchment provides multiple environmental benefits.
Many terms are used interchangeably to describe the restoration process (e.g. intervention, catchment management, rehabilitation, rewilding, alteration, resilience, etc.), but regardless of the terminology, projects that manipulate water connectivity are now commonplace. Some activities increase connectivity (e.g. floodplain reconnection; barrier removal) while others decrease connectivity (e.g. isolation of lakes from rivers; installation of structures to increase upstream water retention; reintroduction of beavers).
As part of our Resilience & Restoration work package, we are interested in predicting and testing the effects of altered connectivity. The removal of river barriers (e.g weirs and dams) provides an excellent opportunity to do so.
Weirs or dams have been built in various forms for thousands of years in relation to human settlement, industrialisation, flood control and hydropower. Although the length, height, design and reservoir volume are highly variable, common to all is their purpose; to alter water flow.
The simplicity and effectiveness of weirs has undoubtedly contributed to their presence across the globe, with an estimated 80,000 in the United States alone. However, there are concerns with the artificial environments weirs create; accumulation of sediment and nutrients, the blockage to upstream habitats and the potentially negative implications of disregarding ageing infrastructure.
In the UK, many weirs are now being removed or altered because they act as a barrier to migrating fish. Lowering a weir by a meter or so may open up tens of kilometres of available spawning, nursery, adult holdings and over-wintering habitats, whilst decreasing predation hotspots and crowding in good quality habitats. From the wider environmental perspective increasing weir passability may increase fish recruitment, provides a greater income to the local economy via fishing, and increases transfer of marine-derived nutrients upstream and organic matter and fine sediment downstream, thereby increasing river productivity which underpins ecosystem function.
Therefore altering one barrier may have a cascade of impacts within a freshwater catchment.
The pictures below show the removal of the Creamery Weir in Dumfries and Galloway which is one of the weirs sampled as part of out project. More details can be found on on the excellent rafts news page.
In order to assess the impact of barrier alteration it is important to collect baseline data. Benthic invertebrates are an excellent indicator of environmental change as they have differing tolerances to stress and local hydrology, therefore the community found in a sample is a strong indication of the current ecological conditions. By collecting invertebrate samples annually we can detect changes in both the short-term effects of barrier disturbance (e.g. increasing sediment and nutrient loads being transported downstream) and long-term effects of fish movement potentially altering invertebrate communities.
Working with SEPA and local fishery trusts we have identified river barriers in a mix of landscapes and habitats that are affected by various pressure and connectivity issues. With baseline invertebrate and basic chemistry data collected for 10 barrier removal projects across Scotland we can begin to investigate the effects of increasing connectivity in our rivers and whether the effects are consistent across the country or depending on the type of restoration carried out.
You can keep up to date on Scottish barrier removals via the RAFTS webpage and we’ll keep you up to date on our work here.
p.s. barrier alteration doesn’t always mean the barriers are completely removed, for example, the pictures below show a weir on the Avon Water before alteration and after (with a rather exquisite fish pass constructed!)