An aerial view of the Welsh village of Llandogo and the River Wye. Photo: Observer Design/Michael Roberts/Getty Images
Scattered along the length of Britain's rivers are a variety of obstacles - some as large as dams, others as small as weirs (which divide a river in two like steps) - that prevent creatures, sediment and plants from moving along the watercourse. Only 1% of UK rivers are free of man-made barriers.
In many countries, steps are underway to remove such obstacles and allow rivers to "re-naturalize" and follow their own path. But while many scientists agree that river barriers should also disappear in Britain, others are hesitant - concerned about creating unpredictable water flows in areas that are already prone to flooding.
"Our rivers and lakes are the most damaged of all ecosystems," says Paul Kemp, professor of ecological engineering at the University of Southampton. Globally, freshwater ecosystems are home to about a third of the Earth's vertebrate species, and these have been declining at twice the rate of the number of marine and terrestrial animals. The situation in England is dire: according to the Rivers Trust, around 15% of rivers are achieving good ecological status.
Removing barriers "results in the biggest impact, the fastest, and for the least amount of money," Kemp says.
According to the Environment Agency database, there are more than 50,000 barriers in England disrupting the passage of the country's beleaguered rivers. Scientists within the organization suspect that there are many more. The vast majority of these are relatively small: about 27,500 culverts, mostly round concrete tubes, 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 damage to the environment.
"A few decades ago we thought only migratory species needed to move," says Dr Perikles Karageorgopoulos, a senior technical specialist at the Environment Agency. "But we increasingly understand that all species need to move, from the smallest species that will undergo local migrations, to the others that migrate many kilometers to overwinter or reach their spawning grounds." So many barriers essentially create a network of small dams or lakes, which provide very different habitats for plants and animals than those found in free-flowing rivers, and are often unsuitable for riverine species.
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Sediments also need to move. Barriers trap soil and geomorphic material upstream, depriving downstream areas of sand and gravel, which is crucial for spawning creatures and many plants.
"Removing dams is the most effective way to restore a river," says Karageorgopoulos. This is what happened in 2010, when a disused flood defense on the River Ouse in East Sussex failed and had to be removed. That stretch of river had become a "pond river of lilies," he says. Now "there is enormous physical diversity supporting a wide variety of species" - which also makes the river and its creatures more resilient to climate change.
"A diverse habitat is much more resilient to high or low conditions [water] currents and extreme temperatures, and can provide a refuge for many species," he explains. "In the past, the same river area would have warmed and become deoxygenated during low flows and warm weather. The warm water flowing downstream would also have affected the ecology downstream."
We cannot continue to build gigantic concrete walls and structures in our rivers to control the water - that is impossible
Prof. Hannah Cloke, University of Reading
In addition to that specific section, the removal of the weir also had a positive effect on the river more than a kilometer upstream, Karageorgopoulos says. Within two years the stretch of river was reclassified and went from bad to good.
"It's a good technique to focus your efforts on recovery," says Jesse O'Hanley, an environmental systems specialist and current associate dean of research and innovation at the University of Kent. "Many restoration efforts are done on a very small scale, such as planting a few trees, bending the river a little or placing some stones. It's a very hyper-local solution that doesn't really scale and is expensive. Usually it's easier to let the river do it itself." This is what removing river barriers does: it renaturalizes rivers.
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But letting a river choose its own path is a risky business, especially in flood-ravaged parts of England.
The Environment Agency estimates that around 3.4 million properties in England are in areas at risk of flooding. Would removing river barriers worsen or improve the situation?
"Floods are a natural process," says Karageorgopoulos. "If you get really big flooding, there is simply no space physically in the river to hold the water."
Rivers have three dimensions: there is the current we usually see, where water follows the channel of the river; but rivers also run laterally, exceeding their banks and spreading laterally to floodplains, as well as vertically, connecting the riverbed to the water table below. Obstacles and barriers have different effects on the passage of a river in all three of these directions, both upstream and downstream of the obstruction.
The risk of flooding is always context-specific, says Carlos Garcia de Leaniz, professor of aquatic life sciences at Swansea University. Weirs and culverts can worsen flooding upstream because they slow the water in the river, collecting it in mini-ponds and preventing it from flowing downstream. It is also quite easy for divers to become blocked by trees and debris.
In addition, the barriers prevent sediment from flowing down the river, causing it to collect in specific places. 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 can reach the groundwater table and more water has to be transported through the river channel," says Garcia de Leaniz. "This means that the risk of flooding may actually increase."
The reality is that flooding is necessary and will happen. It's about deciding where that water goes.
All scientists the Observer Those contacted agreed that it is possible to predict the outcomes of removing a culvert or weir. "There will be local changes in the 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 they will be, and we should be doing things like that anyway: making room for water on the floodplains."
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 over the next fifty years.
"We can only fight our way out of a certain level of flooding... We cannot continue to build gigantic concrete walls and structures in our rivers to control the water - that is impossible. Working with the landscape is a much wiser way to manage our rivers, and also has benefits for ecosystems and water quality," says Cloke.
Europe and the US are leading the global push for 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 1 million barriers fragmenting rivers in Europe and Britain. "Europe is making great progress; Britain not so much," he says.
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A major reason for this is that Britain, and England in particular, is much more densely populated. "It is much easier to restore rivers when few people live nearby," says Garcia de Leaniz. He also has a pragmatic approach to removing barriers. "We have so many obstacles to choose from; let's start with the outdated and the flood risks," he says. "And in the bigger picture, some people would argue that for the cost of removing one large dam, you can remove a hundred or more barriers, which will be much more cost-effective."
In England, the Environment Agency tends to work on a more ad hoc basis, says Karageorgopoulos. While there is a list of priority barriers that need to be removed - such as those that cost money to maintain and serve no purpose - it must move when opportunities arise. Although the Environment Agency owns or maintains the vast majority of these barriers, many are on or impact private land.
A major challenge, Kemp says, is ownership of land and rivers. "You can't just go and implement a strategy on a river because you have multiple owners of that land and you have to work collegially with them and try to find a solution," he says.
Often the Environment Agency "will be able to do things if they find a landowner who is receptive to the idea, so it's very opportunistic," says Kemp.
Even if the owner participates, the process still takes time. It will take many years of planning and consultation with stakeholders, Karageorgopoulos says. "Money is self-evident [obstacle]", against removing barriers, but communities and fishermen are often outspoken opponents. "We are human - we like routine. We have even had opposition from people who like to feed the swans and ducks in a specific location," says Karageorgopoulos. But if there are no land restrictions and "everyone is on board, this is the easiest way" to improve the health of the rivers.
However, it will not 'fix' England's rivers, Cloke warns. "Naturalizing rivers is always a good idea because rivers are meant for natural flows," she says. "In the long run, it's probably helpful and it's one of the low-hanging fruits."
However, none of this will tackle the biggest threats to Britain's rivers - namely huge amounts of pollution in the form of sewage and agricultural and industrial waste, and development. "Should we build on floodplains? No. Should we pollute our rivers? No. These are the big questions that the government must consider carefully," she says.
