Swansea University scientists say the proliferation of weirs, dams and culverts is now creating a threat to wildlife

Near the mouth of the River Afan in Port Talbot, south Wales, a pair of seagulls were to be seen last week pecking in a leisurely way at a dead salmon lying on a gravel bank. It was an unusual sight. Salmon are rarely found in the Afan these days.

The scene may have been unexpected, but it nevertheless illustrates a growing problem, say researchers – one that already affects rivers across Europe and could pose even greater threats to habitats and wildlife in future. Increasing numbers of dams, weirs, sluices and other barriers built in rivers over the past 200 years are, they say, fragmenting waterways, isolating habitats and weakening wildlife populations.

The salmon on the Afan provides a good example, according to Peter Jones, a postdoctoral researcher at Swansea University. “It was lying on gravel in front of a weir. It would have been trying to get upriver, and although there are fish steps at the side of the weir it would still have had its progress slowed, and been left vulnerable to predators such as otters. The gulls are probably just having an opportunistic snack.”

Jones is a member of a Swansea-led project called Amber (Adaptive Management of Barriers in European Rivers), which aims to find ways to halt the fragmentation of European rivers and the dangerous isolation of their habitats. “Flowing rivers are healthy rivers – and by flowing we don’t mean just water, we mean sediment, energy, nutrients and organisms,” said the project leader, Professor Carlos Garcia de Leaniz.

The trouble is that very few rivers today have flowing, healthy waters. Apart from abstraction by water companies and farmers, which lowers river levels, more and more barriers are being revealed by the Amber project, which is funded by the EU.

“In Europe we have detected 460,000 barriers, and by the end of the year we estimate that we’ll have 600,000,” said de Leaniz, director of Swansea’s Centre for Sustainable Aquatic Research. These dams and weirs pepper Europe’s rivers and prevent migratory fish from swimming upstream to reach their spawning grounds in river headwaters. They also stop vital nutrients flowing downstream to the sea, to the detriment of coastal marine ecosystems.

Facebook Twitter Pinterest An Atlantic salmon jumps a small waterfall. Photograph: Alamy

Amber is intended to provide a European overview of the problem. In addition, a second, very detailed survey – called Reconnecting Our River – has been established to study the nearby River Afan. The lessons learned here could be applied to other UK rivers.

“Technically the Afan is considered a free-flowing river because there are no big dams on it,” said Josh Jones, another researcher with the Swansea team. “In fact, our work shows there are about 600 barriers inside its catchment area, including weirs, culverts, pipes, channels and sluices.

“In many cases, no one can remember why these were built. But each one contributes in some way to the fragmenting of the river environment.”

And it is a national problem, added Jones. “Only about 1% of rivers in the UK are free-flowing. The rest have all sorts of manmade barriers in them, and these limit the movement of wildlife and nutrients. It is becoming a worrying issue. Some fish like fast-flowing water and some like slow-moving water. By constantly adding structures you interfere with their habitats and weaken them.”

It is also thought that the dams, weirs and culverts built on the Afan over the centuries may be playing a part in slowing its restoration as a salmon and trout river.

In the 19th century coalmine waste and other industrial pollutants killed most fish in the Afan; it was not until the middle of the 20th century that its waters were cleaned and restored.

However, salmon and trout have been slow to return and the researchers point to the hundreds of barriers that have been placed across the Afan and may well be delaying the restoration of fish stocks in the river.

Scientists also warn that, as the climate crisis increases its grip on the planet, river fragmentation will become an even greater problem. Pools of still water created by dams and barriers will become vulnerable to rising temperatures, for example. “That risks triggering major changes in wildlife,” added Richard O’Rorke, another Swansea researcher.

The crucial point is that river barriers will need to be removed – which is likely to be costly. A major aim of Amber, and also of the Reconnecting Our River project, is to highlight ways of alleviating problems of river flow without spending vast sums.

“It may be that, instead of knocking down a large dam to improve water quality in a river, we can interfere with smaller structures and make less expensive changes that will still bring significant improvements,” said O’Rorke. “That is a specific goal of this research.”

Facebook Twitter Pinterest The Green Park weir on the Afan. Photograph: Roger Donovan/Alamy

Troubled waters

All major rivers in Europe are now disconnected from the sea, and this has had a catastrophic impact on many species, according to campaigning organisation Amber.

• Salmon and eel have become extinct or are threatened with extinction in many UK rivers, and the presence of dams and weirs is partly to blame.

• In the Netherlands and Germany, barriers along the River Rhine prevent the migration of sturgeon and shad.

• In Spain and Portugal, salmon have disappeared from all but a handful of rivers.