by Various obstacles are dotted along Britain’s rivers – some as large as dams, others as small as weirs (which divide a river like steps) – stopping creatures, sediments and plants from moving along the watercourse. Only 1% of the UK’s rivers are free of artificial barriers.
Movements are underway in many countries to remove such obstacles and allow rivers to “renaturalize” and follow their own paths. But while many scientists agree that river barriers must also enter the UK, others are hesitant – worried about creating unpredictable water flows in already flood-prone regions.
“Our rivers and lakes are the most damaged ecosystems,” says Paul Kemp, professor of ecological engineering at the University of Southampton. Globally, about a third of the planet’s vertebrate species are in freshwater ecosystems, and these are declining at twice the rate of marine and land-based animals. The situation is bad in England, with around 15% of rivers achieving good ecological status, according to the Rivers Trust.
Removing barriers “results in the fastest impact with the least amount of money,” says Kemp.
In England, there are more than 50,000 barriers affecting the passage of the country’s beleaguered rivers, according to the database of the Environment Agency. Scientists within the organization suspect there are many more. The vast majority of these are relatively small: about 27,500 culverts, usually round concrete pipes, and about 16,300 weirs, which adjust the water level and effectively create a small dam.
These barriers are not only expensive to maintain – they also cause avoidable environmental damage.
“A few decades ago, we thought it was only necessary to move migratory species,” says Dr Perikles Karageorgopoulos, a senior technical expert at the Environment Agency. “But we understand more than ever that all species have to move, from the smallest that will migrate locally to the others that migrate for many kilometers to winter or to reach their spawning grounds .” So many barriers effectively create a network of dams or small lakes, which provide habitats for plants and animals very different from those found in free-flowing rivers, and are often unsuitable for riverine species.
Sediment must also move. Barriers trap soil and geomorphological materials upstream, removing downstream areas of sand and gravel, vital for spawning creatures and many plants.
“Removing the weirs is the most effective way to restore a river,” says Karageorgopoulos. This is what happened in 2010, when a disused mill weir on the Ouse in East Sussex failed and had to be removed. That stretch of the river was a “river of ponds with lilies”, he says. Now “there is a huge physical diversity that supports a huge variety of species” – making the river and its creatures more resilient to climate change.
“A diverse habitat is much more resilient to high or low [water] extreme flows and temperatures, and can provide refuge for many species,” he explains. “In the past, the same stretch of river would have warmed up and become deoxygenated during low flows and warm weather. The warm water flowing downstream would also affect the ecology downstream.”
We cannot continue to build huge concrete walls and structures in our rivers to control the water – that is impossible
Professor Hannah Cloke, University of Reading
In addition to that particular section, the removal of the weir had a positive effect on the river more than a kilometer upstream, says Karageorgopoulos. Within two years, the river stretch was reclassified, moving from poor to good.
“It’s a good technique to focus your efforts on restoration,” says Jesse O’Hanley, an environmental systems specialist and current associate dean of research and innovation at the University of Kent. “A lot of restoration efforts happen on a very small scale like planting some trees, bending the river in a small way or putting in rocks. It’s a very localized solution that’s not really scaling and it’s expensive. It’s usually easier to let the river take care of itself.” This is what removing river barriers does: it allows rivers to regenerate.
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But letting a river choose its own path is a dangerous business, especially in flooded parts of England.
The Environment Agency estimates that around 3.4m properties in England are in areas at risk of surface flooding. Would removing river barriers make the situation worse, or improve it?
“Flooding is a natural process,” says Karageorgopoulos. “When you get really big floods, there’s physically no room in the river to hold the water.”
Rivers run in three dimensions: the stream we usually see is where water follows the river channel; but rivers also run laterally, breaching their banks and spreading laterally on floodplains, as well as vertically, connecting the riverbed to the water table below. Obstacles and obstructions in the course of a river have different consequences in each of those three directions, upstream and downstream from the obstruction.
Flood risk is always context specific, says Carlos Garcia de Leaniz, professor of aquatic biological sciences at Swansea University. Weirs and culverts can make flooding worse upstream, because they slow down the water in the river, collecting it in small ponds and stopping it from flowing downstream. It’s also quite easy for gutters to get blocked by trees and debris.
In addition, the barriers prevent the sediment from moving down the river, causing it to collect in specific areas. This substrate acts as a blanket on the riverbed, cutting off the connection between the river and the water table, as the water cannot filter through. “That means less water is able to reach the water table and more water has to be transported by the river channel,” says Garcia de Leaniz. “That means the risk of flooding could actually increase.”
The reality is that floods are necessary and will happen – it’s a matter of deciding where that water will go.
All the scientists Observer The contact agreed that the results of removing a culvert or weir can be predicted. “There will be local changes in terms of flooding, and that is one of the consequences [of barrier removal],” says Hannah Cloke, professor of hydrology and co-director of Water@Reading at the University of Reading. “But you can predict where those will be, and those are the kinds of things we should be doing anyway – making room for water on the floodplain.”
In a 2019 report, the Environment Agency warned that if current development on floodplains continues, the number of properties at risk of flooding could double in the next 50 years.
“We can only work our way out of some level of flooding… We cannot continue to build walls and huge concrete structures in our rivers to control the water – that is impossible. “Working with the landscape is a much more sensible way of managing our rivers, and it also has benefits for ecosystems and water quality,” says Cloke.
Europe and the US are leading the global charge on barrier removal, especially when it comes to dam removal. (Britain has about 2,800 dams.)
The European Union has decided that it wants 25,000 kilometers of rivers in Europe to be free-flowing by 2030, says Garcia de Leaniz. He led the EU-funded Amber (Adaptive Management of Barriers in European Rivers) project, which found that there are more than 1m barriers dividing rivers in Europe and the UK. “Europe is making great progress; the UK, it’s not much,” he says.
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A significant reason for this is that the UK, particularly England, is much more densely populated. “It’s much easier to restore rivers when there aren’t many people living around,” says Garcia de Leaniz. He also has a pragmatic approach to removing barriers. “There are so many obstacles to choose from; let’s start with those that are obsolete and those that are at risk of flooding or a hazard,” he says. “And in the bigger scheme of things, some people would argue that you can remove 100 or more barriers for the cost of removing one large dam, which will be much more beneficial.”
In England, the Environment Agency tends to work on a more ad hoc basis, says Karageorgopoulos. Although he has a list of priority obstacles to remove – such as those that cost money to maintain and have no function – he must move when opportunities arise. Although most of these barriers are owned or maintained by the Environment Agency, many are on or affect private land.
A major challenge, says Kemp, is land and river ownership. “You can’t just go and strategize on a river because you have multiple owners on that land and you have to work with them collegially, and try to find a solution,” he says.
Often, the Environment Agency “will be able to do things when they find a landowner who accepts the idea, so it’s very opportunistic,” says Kemp.
Even if the owner is on board, the process still takes time. It takes years of planning and consultation with stakeholders, says Karageorgopoulos. “The obvious thing is money [obstacle],” to remove barriers, but communities and anglers are often vociferously opposed. “We’re human – we like routine. There was even opposition from people who would like to feed the swans and ducks in a particular place,” says Karageorgopoulos. But when there are no land restrictions and “everyone is on board, it’s the easiest way” to improve river health.
The rivers of England are not “fixed”, however, warns Cloke. “It’s always a good idea to naturalize rivers because natural flows are what rivers are designed for,” she says. “In the long term, it’s probably going to be helpful and it’s one of the lowest hanging fruits.”
However, none of this will address the major threats to Britain’s rivers – which are huge quantities of pollution in the form of sewage and agricultural and industrial waste, and development. “Should we be building on floodplains? Should we be polluting our rivers? No. Those are the huge questions that the government needs to think about carefully,” she says.
