fish passes Archives - South East Rivers Trust

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

Habitat fragmentation

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.

River habitat

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.

An impounded, or enclosed, section of river — An impounded area of river above a large sluice/weir on the River Medway. The river becomes still, affecting vegetation and animals

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.

Sediment transport

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:

1. 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.
2. 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:

Free flowing with riffles — A section of free-flowing River Darent at Shoreham. Shallow gravel riffles supporting abundant growth of Water Crowfoot, ideal spawning and nursey habitat for native brown trout.

Ecology:

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.

Flooding downstream:

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

The Brasted Lower Weir being removed by drill

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.

Natural bypass around a weir — A natural bypass channel around an impassable weir on the River Darent, at Sundridge, Kent.

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.

In the past few years, we have carried out several projects to improve passage for fish along the River Darent. This started with the removal of a large weir as part of the Acacia Hall River Restoration, completed in 2021. We have more recently installed an eel pass at the Questor weir in March 2022 and, in spring 2023, we constructed a fish pass using staggered baffles on the same estate.

Have you ever wondered what it’s like to be a catchment officer? What is it exactly they do? The South East Rivers Trust’s catchment officer for the Loddon River catchment, Lou Sykes, takes us through a typical week; from admin tasks to going out and assessing the state of sections of rivers with partners, to plan improvements.

Hopefully Lou’s insights below will give you a flavour of what to expect in our two current vacancies for Catchment Officer and Catchment Manager. These roles have various aspects to them and have at their heart a desire to help rivers thrive by improving biodiversity, for both wildlife and people.

Lou says: “My job in its simplest form? I host Catchment Partnerships, meaning I chair meetings of different stakeholders with a geographical common interest in a particular collection of rivers which make up one named network. In my case, one of these is the River Loddon and another the Cuckmere & Pevensey Levels.

“The external partners in a partnership could be water companies, the Environment Agency and Natural England, environmental NGOs, community flood groups, local councils, fishing groups. All of them have an interest in improving the health of the river. Collectively, we want to improve and maintain the health of the river network as a whole, not just treat sections in isolation.

“As detailed on our Catchment Partnerships page, this method is guided by the national Catchment Based Approach (CaBA). Every catchment partnership is different, and this is why they work so well: you can tailor them to your patch and work on the specific needs of that river system.”

Monday

Typically, this is a working from home day. I start with a morning catch up with the Science and Partnerships team before going through my emails. I complete admin tasks such as writing minutes from a Catchment Partnership meeting I hosted recently, then send off a few emails for events and activities planned for later in the week.

Tuesday

River Lyde, Water Quality — Assessing water quality on the River Lyde

Today I was involved producing a statement piece that provides a holistic view on the water quality of the river.

This included information gathering on legal and illegal discharges into rivers, drinking water standard breaches and citizen scientist data collection.

We have been working with local MPs who have a keen interest in their local river and want to work with citizen scientists – members of the public who volunteer for tasks in their spare time.

Through them we want to understand the current state of the water quality and investigate ways that we can monitor and make improvements.

Wednesday

Meeting the catchment partnership members out on site

Catchment Partnership meeting day! These happen quarterly and it was exciting to have our first one in-person, post-COVID. Typically, our meetings have themes – this one focused on looking at emerging water company plans as they go through their next phase of business planning for both their Water Industry National Environment Programme (known as WINEPs) and their Water Resources Management Plans (WRMPs).

Other topics at meetings can vary from looking at biodiversity and wildlife (both native and invasive species) to investigating ideas for tearing up a weir that is acting as a barrier to fish who need to move between different sections of river to find the best feeding and breeding grounds.

By engaging many different parties in this collaborative format, we are providing the opportunity for community groups, government bodies and water companies to come together and form common goals to help rivers thrive. This is particularly important as it allows us to look at the area in a ‘big picture’ format because in every river system, what happens upstream impacts the downstream area.

Thursday

Backwater exploration — Thursday was spent exploring funding for a potential backwater

This was a strategic day of investigating new funding opportunities that could be available for research, project development and delivery – the on-the-ground restoration work that will improve rivers for wildlife and as places people can enjoy. Being a project-funded organisation means we are constantly looking for funding opportunities. We might be looking to “rewiggle” – or remeander – a straightened river. We might want to help fish connect to different parts of a river network by removing man-made, historical objects such as weirs, as we have done at Acacia Hall, Dartford, or by installing eel or fish passes, like we have done further along the Darent. We might be wanting to look to create backwaters to increase biodiversity and mitigate against potential flooding. For one such example, read about Charvil Meadows, on our Loddon Storymap (in the “Action plan” section under “case studies”).

All this work requires funding, and the co-operation of landowners and other interested groups to make it happen, so the over-riding strategy has to look at prioritisation to improve the whole catchment. On this particular week, I also got out to meet and continue to develop a relationship with one of the key stakeholders on my patch. These meetings are some of the most vital parts of my job. Time and resource in every environmental job is a stretch and collaborative working is the key to keeping everything on track.

I finished off the day with a catch up of our Catchment Officers team at SERT. We are constantly evolving and working out ways that we can work more efficiently, working together so that we are not reinventing the wheel every time something new crops up.

Friday

A large part of my job also involves putting on a pair of wellies, getting out on site and identifying opportunities in the catchment for improving stretches of river or their surrounding land.

On this particular Friday, I walked three miles up the River Lyde – one of the tributaries of the River Loddon – with members of a local fishing club. They were looking at how they could possibly manage the river for both their fishing club members and to keep the waterway as healthy as possible. This stretch of river is a chalk stream, so we would like the water to be clear, with clean gravel, plenty of underwater plants and a decent buffer of green on the banks. Chalk streams are globally rare habitat with only about 200 of them left in the world, 160 of which are in the UK.

Pleasingly on this day, on this river, a healthy amount of aquatic plant growth was evident and there was a good amount of shade and light getting to the channel. However, it could do with some habitat or restoration work to help mitigate erosions on some stretches.

Not every week looks exactly like this, but this is an average one. There will be weeks where there is more home working, the odd day in our HQ office in Leatherhead in Surrey, or you could end up hosting a week of volunteer events that put you in a pair of waders all week, such as during Loddon Rivers Week held annually in September.

This is the beauty of being a catchment officer or manager – it’s varied nature keeps you on your toes and gives very little opportunity for burn out from doing the same thing over and over again.

The South East Rivers Trust currently has two positions available, one for Catchment Officer and another for Catchment Manager, which has line management and team development responsibilities on top. Take a look at our job opportunities page for full details and apply by 26th June.

How weirs affect rivers

Tag: fish passes