The effects of weirs on fish migration

Many of our rivers have been extensively modified over the years, potentially compromising the natural habitats and benefits rivers provide us.
Weirs are man-made structures, historically constructed on rivers or becks for industrial purposes. Many were constructed to hold and divert water to mills to power machinery. Many of these mills have long since closed or disappeared altogether and their associated weirs no longer fulfill the function they were originally built for.
So what effects can weirs have on a river?
Weirs can alter the flow of a river, can be barriers to fish migration, effect temperature, transport of sediments and effect habitat for wildlife.
Status of salmon and sea trout stocks
According to the Environment Agency data on Salmonid and fisheries statistics for England and Wales 2019, the assessment of salmon stocks shows that the status of salmon populations continues to be of concern. The assessment places each river’s salmon stock into 1 of 4 categories with the strongest classed as ‘not at risk’ and the weakest as ‘at risk’.
In England-
  • 39 of 42 principal salmon rivers were assessed as being ‘at risk’ or ‘probably at risk’
  • none were categorised as ‘not at risk’
More information on the River Kent can be found using the two links below-
Salmonid and fisheries statistics for England and Wales 2019
Status of Salmon and Sea Trout Map 2018
There have been many scientific papers written on the effects that weirs have on fish migration. We have pulled together some of these resources for you below:
Gauld et al 2013: Reduced flow impacts salmonid smolt emigration in a river with low-head weirs
Newton_et_al-2018-A test of the cumulative effect of river weirs on downstream migration success, speed and mortality of Atlantic salmon (Salmo salar) smolts: An empirical study
Aarestrup et al 2003: Survival of migrating sea trout (Salmo trutta) and Atlantic Salmon (Salma salar) smolts negotiating weirs in small Danish rivers
Dam removal Europe website;
Useful Information Pages –
Weirs, dams and other river structures: Their effects on wild brown trout – Wild Trout Trust
How weirs affect fish communities – South East Rivers Trust
The Cumbria River Restoration Strategy (CRRS) was developed to help deliver the joint Natural England and Environment Agency drivers to improve the quality and function of the three Cumbrian riverine Site of Special Scientific Interest (SSSI) and Special Area of Conservation (SAC) sites; namely the Eden, Derwent and Kent catchments.
Through River Restoration interventions the ambition is to restart natural river processes and provide benefits to both people and wildlife.
SCRT are the lead partner in the CRRS for the River Kent and Tributaries SSSI/SAC. It is our ambition to look at barrier removal and other interventions that can provide cumulative benefits for the catchment.
Local weir removal examples-
Ennerdale Mill Dam was removed by West Cumbria Rivers Trust (WCRT) in 2019, as part of the Cumbria River Restoration Strategy. It was the largest weir removal in Cumbria to date and a fantastic example of the weir removal process. WCRT worked with the local community and interest groups to understand reluctance to removal and provided resources to highlight the importance of removal as well as the benefits that removal could provide to local people and wildlife.
You can view the removal video here:
There is also an information sheet explaining the removal process from start to finish: wcrt – rrc emd information
Eden Rivers Trust have also removed weirs under the Cumbria River Restoration Strategy, visit their website for more information; Eden Rivers Trust
SCRT are working to remove barriers to migration where we can. We have delivered a number of barrier removals within the last couple of years, including a weir in the Grizedale catchment, a culvert at Crosthwaite and a culvert at Town View Fields in Kendal. We are currently working up a proposal to remove a large weir on the River Kent, Bowston weir, as part of the Cumbria River Restoration Strategy.