Giving Tuesday was created in 2012 and has now grown into an international movement that is embedded on the social calendar annually, with the simple idea of doing good.
What better way is there to do that than make a donation on Giving Tuesday (28th November 2023) or instead of a physical present this festive season, to help protect rivers – our very lifeblood?
While there are many great causes, one that underpins our very existence – water – is hard to ignore.
Much of our drinking water is abstracted from rivers, supplementing what is stored in reservoirs to supply the needs of our homes and businesses.
However, in the South East we live in an area that is classed as water-stressed. This means that we are already facing a water shortage because of a growing population and climate change, which brings with it erratic weather patterns, from sudden storms from which we can’t capture all the water to drought.
All this puts huge pressure on the wildlife that thrives in rivers. Your rivers. Rivers that have been straightened, boxed in by concrete or boarding along the edges thwarting animal movement between water and land, or restricted by weirs and other barriers – all in the name of convenience for people at various times in our history.
But this has left our rivers unable to function as they should, to allow fish to migrate (some as far as the sea) to better habitats, to allow flowers to flourish to attract pollinators, or to give creatures that move between water and land the chance to do so.
The very habitats that support the wildlife that supports our existence needs help – and we’re on a mission to make that happen.
However, we can’t install fish passes or ‘rewiggle’ rivers to make them places where aquatic life can truly thrive without funding.
By the end of November, many people are already making decisions about gifts for the festive season. Many of you might be tempted by offers on Black Friday weekend (23rd to 27th November). But many of you might be thinking that a gift to nature might be better for your recipients for Christmas-time festivities this year – a year in which we have made inroads in many areas.
Some highlights from this year include:
encouraging people in the Medway to take a first step to caring for their local river by addressing their reliance on single-use plastic – 70 people signed up to become official Medway River Guardians with many of them becoming River Champions.
working with landowners in the Beult to install nature-based solutions to retain back water in the landscape for the benefit of wildlife and people. We’re now building up similar work on the Darent
The South East Rivers Trust (SERT) has been awarded £393,000 as part of Natural England’s Species Recovery Programme, which aims to support targeted action to recover the UK’s most endangered species.
On 14th September, 2023, Natural England chose SERT to host the scheme’s launch at the Hogsmill Stepping Stones in Ewell, Surrey, where the fortunes of water voles, eels and trout will be boosted by the project.
A total of 63 projects across the country have been awarded a share of £14.5 million by Natural England to help recover 150 species nationwide. Following a competitive application round, the money will be used by environmental charities, wildlife organisations, local authorities and charities to deliver the Nature Recovery Network.
The funding supports propagation, captive rearing, translocations, research and solution-trialling to find the best approaches to enable endangered wildlife to survive and flourish.
Some of the UK’s most iconic river wildlife has been in severe decline for decades, but now thanks to a generous grant awarded to SERT by Natural England, outcomes for rare and endangered wildlife and their habitat are about to improve on the Hogsmill river in South West London.
The WET Hogsmill project led by the South East Rivers Trust, will improve the habitat of the Hogsmill river, a chalk stream in South West London. There are only around 220 chalk streams worldwide meaning that this is an exceedingly rare and special habitat. The project will reintroduce Water Voles onto the river and create new habitats for both European Eel and Brown/Sea Trout. The project will run until Spring 2025.
Co-CEO of SERT, Dr Bella Davies said “We are thrilled to have been successful in our application to Natural England’s Species Recovery Grant to support the recovery of water voles, eels and native wild trout which have become near extinct on the Hogsmill river in South West London. The Hogsmill is a rare urban chalk stream meaning that it’s important for nature and an important resource for people too.
“Our project will restore river habitat and create backwaters where fish can take refuge from pollution by creating wetlands to improve water quality which will help reintroduce Water Voles which were once prevalent on the river. We are excited to see this much needed work begin on the ground to bring Water Vole, European Eel and Brown/Sea Trout back to the Hogsmill.”
The Hogsmill river is the first tributary of the non-tidal river Thames and a chalk stream making it a UK Biodiversity Action Plan Priority habitat. Despite its rarity and importance, the Hogsmill has suffered a wide range of pressures leading to decline and loss of habitats and species over the last century and beyond.
Water Vole numbers have declined sharply since the end of the 20th Century making them currently the UK’s fastest declining mammal with a 97% decrease in population. Once ubiquitous and found in their millions, they are now considered to be on the brink of extinction. Water Voles were once prolific on the Hogsmill but became locally extinct in 2017.
Partnering with Citizen Zoo, a conservation charity, SERT will release 150 Water Voles across two sites on the Hogsmill, supplementing 101 Water Voles previously released by Citizen Zoo in 2022. This will help to increase the genetic diversity of the population.
European Eel are also critically endangered with levels declining by 90-98% from historic figures. Eels migrate up rivers during their life span and recent surveys on the upper Hogsmill recorded just one eel in 2016 and three in 2022. The WET Hogsmill project will support the creation of a large wetland and backwater while also creating a more complex instream habitat which is favoured by European Eel.
Trout have been extinct on the Hogsmill since the 1900s, owing to 19 predominantly obsolete weirs barring their passage, and preventing them from reaching critical spawning grounds in the river’s headwaters. Over the past decade SERT has made 18 of these weirs passable for fish and other species by either removing them or installing technical fish passes or easements. In 2024 the final remaining weir will be made passable for multiple fish species including Brown/Sea Trout and European Eel. By spring 2025 these fish will once again be able to access and migrate throughout the whole river for the first time in over 200 years.
SERT will provide a wide range of complementary community education and engagement activities for members of the public, schools and local businesses. Planned activities include installation of an interactive nature trail, indoor and outdoor education sessions, and community talks to help local people and businesses understand how they can help protect rivers and wildlife.
There will also be several opportunities to volunteer. Those interested to learn more about the project and volunteering activities can sign up to our newsletter or bookmark our events calendar for more information.
Watch our Co-CEO Dr Bella Davies explain how the project will help species in the Hogsmill.
Luke Beckett, one of our assistant project officers, reports on our latest river restoration work on the Blackwater Restoration Project. This has improved the ability of fish to move along the river, opening up a 4.5km stretch known as the Wish Stream tributary, where it meets the River Blackwater, in the north Hampshire stretch of the River Loddon.
Helping fish move between river sections
This was my first fish passage improvement since joining the South East Rivers Trust, so it was particularly fulfilling to deliver this work, which will help fish pass between the two watercourses and access good spawning habitat.
The Wish Stream is an important semi-rural tributary of the River Blackwater, which supports a population of brown trout (Salmo trutta) and offers good spawning habitat in its lower reaches. Subsequently, improving connectivity between the Wish Stream and the River Blackwater is important for ensuring resilient fish populations in the catchment.
At the confluence of the two watercourses, we identified a concrete shelf which poses an obstruction to fish and eel movement, particularly during low-flow conditions.
Sloping notch and coping stones
The concrete shelf reduced the water depth, making it too shallow for fish to pass over. It also created a small weir between the Wish Stream and the River Blackwater during low-flow conditions, preventing fish moving up into the tributary. The smooth concrete also made it difficult for European eels to move along the river. This critically endangered species requires rougher substrates and slower flowing water to migrate along rivers.
To make this confluence more fish friendly, we created a sloping notch/ramp down the front of the concrete, allowing water to flow more naturally. The concrete was particularly hard – too hard for our machines and so we had to return with a hydraulic breaker to finish the notch.
In addition to this, we installed coping stones along the edge of the concrete to increase water depth over the shelf, enabling fish species to swim through. Finally, we improved the climbing substrate for European eels by adding an eel brush over the crest of the coping stones and safely down onto the concrete bed and vegetation upstream.
This passage improvement work was a relatively simple and straightforward delivery project. Most of it was completed in just over a day, although we had to wait until conditions were right to complete the final notch, and we were able to use coping stones which were left over from a previous project.
Great for brown trout and eels
The work enables fish, such as brown trout and European eels, to access important habitat for spawning and juvenile recruitment along the Wish Stream, which is roughly a 4.5km stretch of waterway.
It also allows mature individuals to disperse down into the River Blackwater and mix with other populations. With improved access to these habitats, fish numbers will hopefully increase and provide a greater prey source for other species such as kingfishers and herons.
Such improvement works show that sometimes it doesn’t take much to connect habitat and improve conditions for a range of species. I am looking forward to returning over the coming months to see these benefits. We hope this enhanced connectivity will strengthen fish populations in the catchment long into the future and I hope this is my first of many fish passage improvement deliveries.
The South East Rivers Trust regularly works towards removing weirs or installing fish passes. In the second of a series of blogs about the problems weirs cause to rivers, Dr Chris Gardner, Head of Science and Partnerships, writes about two big issues. These are the movements of fish (which he tracked for his PhD) and drought resilience. Here, he addresses some myths about both aspects.
‘Migratory’ fish? All fish migrate
It is often thought that weirs and other barriers that restrict the movement of fish only affects ‘migratory’ species such as salmon, sea/brown trout and eels, as their migrations are relatively easy to observe and are well known.
However, all fish migrate to some extent, and all fish have life stage specific habitat requirements affected by habitat degradation. Coarse (or freshwater) species such as barbel, chub and dace are affected by habitat fragmentation and degradation caused by weirs.
Higher water velocities on riffles encourages plants such as water crowfoot to grow. Plant growth on riffles makes it difficult for predatory fish to hunt. This also provides an abundant food supply of invertebrates and overhead cover that hides fish from predatory birds and animals.
Riffles are great places for baby trout and young salmon to live, they are also the preferred habitat for juvenile barbel. However, riffles hidden under lake-like habitats upstream of a weir lack the characteristics that make them great juvenile barbel habitats. And if there are low numbers of small barbel then there will be fewer numbers of big barbel and eventually no barbel at all.
Here, the trout and salmon fraternity are ahead of the game. The economic value of salmon (commercial and recreational) and the large declines in salmon populations since the 1980s have caused scientists and anglers alike to research and understand what the habitat requirements are for all life stages of these fish.
They have used this information to minimise potential population bottlenecks or limiting factors because of available habitat. There are many things impacting our fish populations, but in-river habitat is the one thing that is relatively easy to address and benefits all wildlife. Organisations such as the Wild Trout Trust have been encouraging progressive thinking and educating game anglers in fisheries management and river restoration.
Research shows why fish need to access the whole river
Coarse fish migrate and need to move around a river system to locate specialised habitats required at certain times and during certain conditions. Fish are streamlined, live in a near weightless and frictionless environment and need to constantly swim just to maintain a static position in the river. Hence, fish have great potential to be highly mobile.
Modern tracking studies using implanted radio or acoustic tags have revealed these migrations. For example, in 2010 Dr Karen Twine, of the Environment Agency, radio tracked 20 adult barbel (6-15lb) in an 8.2km reach (between two impassable weirs) of the Great Ouse for 18 months. She demonstrated that the barbel utilised most of the river length available to them and made seasonal movements to spawning and over wintering habitats.
Similarly, in 1993 Dr Martyn Lucas, of Durham University, radio tracked 31 adult barbel (2-6lb) over 15 months in a 7.2km reach of the River Nidd, a tributary of the Yorkshire Ouse, with open access to the Ouse.
Again, these fish were highly mobile, ranging over sections of river from 2-20km in length. Their movements were associated with seasonal shifts in habitat, upstream spawning migrations and their downstream migrations to ‘over wintering’ habitats in the lower reaches and main River Ouse.
Basically, fish will move as far as they are able to fully exploit the best available habitat/resources: the more limited the resources, the further they will travel.
Other studies on less fragmented rivers with more limited essential habitats have shown that fish have the potential to move over very long distances.
Study shows the benefits of free passage
During my PhD in 2006, I tracked the movements of 80+ adult common bream (4-7lb) over four years in a long 40km reach of the Lower River Witham, a very uniform fenland river in Lincolnshire.
My bream were tightly shoaled and relatively immobile in a deep tributary off the main river at the upstream extent of the reach during the winter, moving short distances of, on average, less than 5km a month up and downstream.
In the spring, they became highly mobile moving on average 30-40km a month, utilising the entire length of the river available, with one individual moving more than 120km in a single month!
At this time, they were visiting shallow tributaries off the main river, before using these for spawning in late May/early June. Once they had spawned, they spread out and spent the rest of the summer in the main river foraging, again moving on average 20-30km a month up and downstream. In the autumn, they moved back upstream to the deep tributary for the winter. This same yearly pattern was observed throughout the study.
These studies demonstrate that adult fish use different habitats at different times of the year and require free passage between them.
Habitat requirements are different for adult and juvenile fish. So, during a fish’s life it will have many different habitat requirements. These requirements will be more crucial for juvenile fishes because of their vulnerability to predators and therefore their need to find safe cover.
If any single habitat type is lacking, limiting or inaccessible, there will be consequences for individual survival and therefore the population as a whole. Weirs often restrict populations to those reaches that have sufficient habitats to enable life-cycle completion.
The “weirs” thing about drought resilience …
A common misconception is that weirs delay river discharge and therefore make the river more resilient to drought. Weirs do hold back a quantity of water in the upstream section, which is “impounded” – leaving the river more like a still canal or pond. However, a weir just stores water in the upstream area – and once the river is full, river flows over the weir at the same rate it enters the impoundment.
Imagine an impounded section of river as being like a kettle being filled from a tap. Once full, the kettle overflows at exactly the same rate as the tap runs and the water bill ticks up exactly the same.
In the event of drought, rivers tend to dry from their upstream end first. Impoundments upstream of weirs can and do provide refuge areas for fish in such an event. However, these areas are likely to be heavily silted because of the lack of flow, and will quickly deoxygenate because of biological processes in the silt, leading to fish deaths.
Fish will move downstream naturally in response to a drying river using the river’s flow to navigate.
However, if the fish encounter an impoundment upstream of a weir, and there is no flow going over the weir (because of the drought), there will be no flow cues to navigate by and the fish will simply be unable to move any further and become trapped where they will die as the water in the impoundment deoxygenates.
If no weirs exist fish will move downstream, seeking out deeper, fresher water in the river’s lower reaches. Once the drought has broken, they will then move back upstream.
So, perhaps counterintuitively, weirs actually reduce a river’s resilience to drought.
In conclusion, the impacts caused by weirs are problems for freshwater fish as well as salmon and trout: the principles might not be as well understood or as popular, but they are real. If our rivers are to fulfil their ecological potential, we need to address this and other factors that are limiting fish populations.
In the first of two blogs, Dr Chris Gardner, head of our Science and Partnerships team, outlines some of the problems caused by weirs on habitats and fish. This has a particular focus on the River Darent in Kent, where we have recently carried out several pieces of work as we try to help fish species move along the whole river catchment.
Restoring rivers to help wildlife
The term “river restoration” describes a set of activities that help improve the environmental health of a river or stream. These activities aim to restore the natural state and functionality of the river system to promote improved fish populations, biodiversity, recreation, flood management and development.
Restoration tends to focus on increasing habitat quality and diversity. A popular first step to achieving this is to overcome barriers such as weirs, increasing the access animals have to the existing range of habitats available in the catchment.
Over the centuries, we have adapted rivers for our own use, modifying them to facilitate land use for agriculture and development, navigation, water supply, power generation and other priorities.
Fish and other aquatic organisms evolved in rivers long before humans had this influence, and so they have not adapted to the modifications we have made, such as concrete banks, deep dredged sections, straight uniform channels and weirs, all of which block their passage.
Weirs impact rivers in three main ways
This is frequently caused by human activities which disrupt the continuity of habitats used by wildlife and is a land conservation issue as well as an aquatic one. Habitats which were once continuous become divided into separate fragments, restricting the movements of organisms (for example fish) and separating them from habitats, natural resources and other fish required for their survival or the completion of their life-cycle.
Fragmented habitats are also less resilient, preventing re-colonisation after pollution incidents and lowering genetic variability because of the restricted, effective population size, potentially placing populations at an evolutionary disadvantage.
This is degraded in quality because an enclosed – or “impounded” – area is created upstream. In other words, river-like habitats become lake-like – stiller, with less water flow than a natural river should be. This drowns out natural features such as riffles (a shallow place in a river where water flows quickly past rocks) causing the loss of important spawning and nursery habitats for river fishes, thus lowering the numbers of fish and breeding success.
Rivers are naturally dynamic with erosion and deposition occurring in balance, creating a highly varied mosaic of temporal micro-habitats for all life-stages of fishes.
Weirs stop this natural tendency for change, creating a uniform, static environment. Upstream, an over deep river channel, similar to a lowland river, is formed in the impounded area, which might be inappropriate for the fish community (for instance, barbel habitat may become bream habitat). Impoundments also alter the temperature regime, oxygen content and cause sediment build up (siltation) in the upstream impounded area.
Natural processes, such as moving sediment along a river, are prevented by weirs and instead the sediment stays in one place, covering the riverbed. This inhibits the riverbed’s function as a feeding and breeding ground. In a natural river, sediment (for example gravel) is shaped and sorted by water flow patterns. This creates a large diversity of ever-changing habitat types that support a rich diversity of wildlife.
Weirs stop natural processes and impact river channels in two main ways:
Upstream – Sediment transport is interrupted by the weir. Instead, it accumulates upstream. The lack of energy in the impounded area causes the sediment to not be shaped and sorted by the water flow and therefore creates a uniform habitat that supports less diversity of wildlife.
Downstream – Sediment transport is interrupted by the weir, reducing the supply of sediment (for example gravel) to the downstream reach, which is vital for creating habitat features for wildlife. This lack of sediment from upstream leads to increased erosion of riverbanks and riverbed, leading to “channel incision”. This is when the river begins at one height and cuts downward (incises) through its bed while leaving its floodplain behind [higher up] throughout its course.
Incised channels have knock-on impacts for:
The incised channel only connects to its floodplain in extreme flood events, when higher than normal water velocity is maintained in-channel during small to medium flood events. Aquatic wildlife, such as juvenile fishes, may become swept downstream during high flow events. The steep banks also cause a lack of marginal transitional habitats which provide a refuge for wildlife in flood conditions.
Flood risk may increase downstream, again caused because the incised channel cannot connect to its floodplain other than during extreme flood events. The problem is simply shifted downstream.
Addressing the impacts of weirs
Removal of the weir should always be the considered as the preferred option, which solves all the issues described above. However, total removal is often not possible because of the way the landscape has developed since the weir was built.
Other factors that need to be considered include the wishes of landowners and river users, such as anglers, who might value the weir and its effect on the river.
The next best option might be a partial removal (lowering of the weir) and/or the implementation of a fish passage, which solves part of one problem (reducing the impounded reach) and all of another (connecting the upstream and downstream habitats). Fish passage solutions include natural bypass channels (which are preferred as they create additional habitat) rock ramps and baffles, as installed in 2023 by SERT at the Quester weir.
Essentially, any “solution” needs to provide fish with free movement and suit a range of fish sizes and flow speeds and depths that might be experienced at various times of the year.
It also need to be delivered with usually tight budgets available.
Modelling, using an existing locally specific Environment Agency flood model is used to satisfy these needs and ensures no increase in flood risk for any nearby residential properties.
This approach is best practice and gives all interested parties the confidence to implement solutions with the simplest, and most cost effective design.
The South East Rivers Trust (SERT) installed a “baffle” fish pass on the weir at the Questor industrial estate in Dartford in late April. Project officer Jack Hogan reports.
Over the past few years, we have been working to improve the lives of fish which need to move up and down the River Darent in Kent.
This began in 2021 when we completed our Acacia Hall River Restoration project in Central Park, Dartford. That project removed a large weir – a barrier to fish moving up and down the waterway – and transformed 600 metres of a silty backwater into a flowing natural river, improving both the ecology and beauty of the park.
The Acacia weir removal also opened up the way for migratory fish to undertake their journeys upstream – and salmon have already been spotted up the river beyond the project.
Our latest efforts have been at a weir a little way upstream from Central Park. Known as the “Questor weir”, it was an impassable obstacle for fish. It is long, wide, and gently sloping. This means that, for most of the time, the water that passed down it was just a skinny, shallow sheet in which no fish could have swum.
To help the fish overcome this, we designed, and have now installed, what is known as a baffle fish pass.
What are baffles and what do they do?
A baffle is defined as a device which regulates flow or passage. In this case, essentially it holds back water, becalming it and making it easier for fish to navigate. In doing so, the baffles increase the depth of water on the weir along each line of the section of river.
This results in a series of steps, or a series of rectangular ‘pools’. Breaks in the line of each baffle – known as notches – allow fish to pass between these areas of calmer water, from one ‘pool’ to another. The shapes of the baffles are also designed to reduce turbulence in the water, which helps ease the way upstream for even small fish.
The concrete structure of the weir also had to be modified to allow the fish pass to function properly. At the downstream end, modifications had to be made to the bed of the weir in order for the final three rows of baffles to the installed.
At the upstream end, the crest of the weir was also notched. This was done to ensure clear passage through the old steel and concrete crest, and to ensure that enough water flows down through the fish pass. A fish pass without water would be like a ladder without rungs!
What we have installed will be used to pass both up and downstream by everything from baby eels – or elvers – to larger fish such as salmon: and the weir, or rather the newly installed baffles, can serve as their ladder to the next step up the catchment. This work builds on the eel pass that we installed on the Darent in March 2022 and we are currently looking at future projects to further improve fish passage on this river.
SERT is developing other projects on sites all along the Darent which will, step by step, restore and reconnect fragmented habitats across the whole river catchment. Some sections of the river have become “impounded”, meaning they become stand-alone water bodies that are hemmed in by man-made objects such as weirs or dams.
Our video below shows how the flow of this section of river has been dramatically transformed from a rushing, single sheet of water into one that will help the survival of migratory fish.
We would like to thank WL West Sawmill in Petworth, who hand-picked some magnificent pieces of oak from which they cut our beautiful custom baffles for the lower end of the design. The rest of the baffles, made of recycled plastic, were donated by the Environment Agency.
Special thanks must also go to Chris at JLL for facilitating the work. Without cooperation from landowners and managers we couldn’t do good work for nature such as this.
We are also very grateful to everyone at Questor – in particular, the people at Daikin and Kent Powder Coating. They offered us forklift support on demand, saving our backs, and secure overnight storage for our equipment, proving that successful river restoration work always is a collaborative effort by many different groups.
To paraphrase Monty Python’s Life of Brian, what have sea trout ever done for us? They haven’t built any aqueducts, roads or made any wine…. but scientific research has demonstrated just how important they are in maintaining freshwater resident brown trout populations.
Enabling migratory sea trout to return to their spawning areas is just one reason freshwater conservationists work to improve fish passage and the ecological connectivity of rivers.
The South East Rivers Trust’s head of Science and Partnerships, Dr Chris Gardner, illustrates below an example of the trout to underline the importance of overcoming barriers to fish migration.
We’re eel-y excited to announce that Thames Rivers Trust in partnership with the South East Rivers Trust, Action for the River Kennet, and Thames21, have been successful in gaining funding to aid the long-term survival of the European eel.
Eels have a spectacular and complex life cycle! European eels spend most of their lives living in Europe’s rivers, including here in the UK. When they are ready to spawn they migrate more than 6,000km across the Atlantic to the Sargasso Sea, where their lifecycle begins again.
Once hatched, the larvae make the incredible journey back across the ocean to our rivers, and develop into young eels, also known as elvers, before swimming upstream.
In September 2020, we successfully installed two fish passes on a weir on the River Ock in Abingdon. The River Ock is a small tributary of the Thames and owes its name to the pre-Saxon word “Ock” meaning young salmon. Salmon were a common sight on the Ock and a staple part of local diets in the middle ages. So why are these migratory majesties no longer present on the river?
Aside from other more global issues, barriers to migration such as the weir at Abingdon would present challenges to upstream migration of all fish species, not just salmon. The weir results in fragmented habitat which in turn can create bottlenecks at varying life stages. As a result, the survival and success of fish is compromised. It was therefore in the river’s best interest to implement fish passage on this weir.
The weir is a standard crump gauging weir, used by the Environment Agency’s Hydrometry and Telemetry’s team to monitor flows in order to help predict downstream flooding and manage abstraction licenses. It is 5.7m long and split into two channels. The left hand channel (as you look downstream) feeds the River Ock and the right hand channel feeds the Sandford Brook. The design of the weir, coupled with the fact that it was a gauging station, added several complexities.