This #WeirRemovalWeek, we thought it would be useful to share why West Wales Rivers Trust carry out so many weir removals around West Wales, the benefits it provides, and to answer our most commonly-asked questions.
Firstly, what is a weir?
A weir is a barrier across the entire width of a river that alters the flow characteristics of water and usually results in a change in the height of the upstream river level. There are many weir designs, but commonly water flows freely over the top of the weir crest before cascading down to a lower level.
Why are weirs built in rivers?
The majority of weirs that we come across in West Wales were built to control the flow of water for the following reasons:
- To hold and divert water to corn, flour and other mills to power machinery. Many of these mills have long since closed or disappeared altogether and their associated weirs no longer fulfil the function they were originally built for.
- For aesthetic reasons. Many historic wealthy landowners and their architects, particularly in the late 18th Century, constructed weirs to form lakes by holding water back and widening the river upstream. Lakes were considered ornaments in the landscape and commonly associated with stately homes. Similarly, weirs were also constructed for their appearance as artificial waterfalls.
- To create increased depth to allow boat navigation.
- Situated downstream of bridges, to help with the control of erosion.
What are the impacts of weirs?
The most commonly known reason for the removal of weirs is because they act as a barrier to the migration of fish. Species such as the Atlantic salmon, sea trout, European eel and Sea lamprey all require movement between the sea and our rivers to complete their lifecycles. When this movement is restricted or delayed, it has big impacts for the survival of these species. Other fish which are classed as ‘non-migratory’ also benefit from being able to move up and down rivers – to find food, cover from predation, and to escape pollution or extreme low or high flows.
A question we regularly get when discussing this issue, is, “But weirs were there when there were still plenty of fish?”. This is a good question. We are of course well-aware that there is no single factor to blame for freshwater biodiversity declines, and there are a lot of other issues that also need to be addressed. These other issues lead directly to the reason why we must now address these weirs – yes, these weirs were not a problem in the hey-day of abundant fish runs, and back then we could be quite relaxed – if a ‘fish could get over’ then all was good. Now with reduced population sizes, this is no longer the case and we need make every effort to get each fish to its spawning grounds, to achieve our ambitions of returning numbers back to abundance. If you are interested in learning more, Westcountry Rivers Trust sum this up brilliantly in their article, ‘Old weirs, new problems’ and in their image below: https://wrt.org.uk/old-weirs-new-problems/
But it’s not just about the fish! Weirs and other in-stream barriers have a number of impacts on river habitat, which affect all species in or near the river. Barriers hold back (impound) water behind the structure. This increased depth drowns out shallower, faster flows and different depths, and in turn creates still lake-like environments of uniform depth. In this new slow flowing section of river, sediment that enters the water is not flushed through the system or scoured out of gravels and instead comes to a rest on the riverbed, smothering gravels and with it the river invertebrates, refuges for smaller fish and also fish eggs.
What are the solutions?
The most beneficial solution for rivers is almost always weir removals, as this removes the impacting habitat modification and enables free flowing rivers. This is also often the most cost effective option.
However, this is not always possible due to various constraints at the site, such as the presence of utility pipes. In these locations, we construct fish passes, which are structures built on, in, or alongside a river barrier that help fish move up and down river blocking barriers. This could be in the form of a rock ramp or pre-barrage, which involves the addition of rocks and boulders downstream of a weir which change the height, waterflow and gradients of barriers, allowing fish to climb the weir with an improved water flow and resting places.
However, sometimes we need a more complex solution. In this case we will make a fish ladder from concrete and/or metal that includes multiple stepped pools that go up the side of the weir. Fish jump from pool to pool and can also use the pools to rest in. Both of the above are commonly only suitable to certain fish species, and do not address the habitat issues caused by the presence of a weir.
If the barrier cannot be modified because, for example, there are utilities running close to it, then a bypass channel can be made. This is a new channel, which bypasses the existing one containing the barrier, and rejoins downstream.
In all of the above solutions, we must we consider multiple factors when designing schemes, including heritage, flood risk, erosion risk, nearby assets, ecological impacts, stakeholder opinions, consenting and many others! If you are a landowner with a weir or other barrier and are interested n working with us to improve rivers, or would like to know more, please contact info@westwalesriverstrust.org