The charity making life better by water

Brent Reservoir-Welsh Harp winter works Q&A

Here you can view our most frequently asked questions.

What was the Brent Reservoir built for?

Brent Reservoir (also known as the Welsh Harp) was built in 1835 to supply water to the Grand Union Canal. Today, surrounded by buildings and fast roads, it provides valuable green open space for people and wildlife. With a mixture of open water, woodland, ancient hedgerow, scrub, marsh and grassland, it’s a protected sanctuary for wildlife, as well as a recreational space for sailing, watersports and walking. It’s a place for physical and mental health and wellbeing and for connecting with nature.

Why is it also called the Welsh Harp?

The shape of the reservoir resembles a Welsh Harp and from 1858 until the early 1970s there was a public house there called The Welsh Harp, from which it gets its popular name. The name refers to the whole 170-acre (69 hectare or 255 football pitches) area, including the land around the reservoir as well as the reservoir itself.

Who is responsible for managing Brent Reservoir/the Welsh Harp?

National charity Canal & River Trust cares for the reservoir, while the London boroughs of Brent and Barnet share responsibility for the land around the reservoir.

Why is Brent Reservoir a SSSI?

Brent Reservoir and its shoreline was declared a Site of Special Scientific Interest (SSSI) in 1950, mainly for its breeding wetland birds (including great crested grebes), over-wintering wildfowl and 16 protected plant species.

What is being done at Brent Reservoir this winter?

The Trust is carrying out a programme of essential maintenance work, including repairs to the chains and rods that operate the reservoir’s sluice gates, and repainting the Valve House Tower from where the sluice gates which control the water levels in the reservoir are operated. A series of environmental works are also underway, including a fish rescue, rubbish removal, vegetation management and installing new nesting areas for wetland birds.

Why is the work necessary?

The Canal & River Trust has a statutory duty to maintain its reservoirs for the safety of people and to maintain the network of canals and rivers under its care. The winter works at Brent Reservoir are required under the Reservoir Act 1975.

Why does the reservoir have to be drained?

The Trust needs to fully drain the reservoir to access the chains and rods that operate the reservoir’s sluice gates. The tower supporting the Valve House is normally submerged in water, therefore this needs to be out of water to enable protective paint to be applied. These gates are used to control the water levels in the reservoir and are a vital part of the reservoir’s infrastructure.

How much water is in the reservoir?

There is around 1,000 mega litres of water in the reservoir– enough to fill 400 Olympic swimming pools. The Trust has been draining the reservoir in a slow and controlled way using the sluice gates.

How will the reservoir be refilled?

The reservoir’s water supply comes from the River Brent and Silk Stream and will naturally refill when we close the sluice gates. The time it takes to re-fill the reservoir depends on the weather.

How are the works being funded?

The Trust is funding the works from its core maintenance delivery funds, with support from players of People’s Postcode Lottery. We’ve also launched a Crowd Funding campaign to support work to remove the rubbish that is sadly expected to be revealed when the reservoir is drained.

How will the water levels and flow rate in the Lower Brent be managed throughout the project?

The Trust will drain the reservoir in a steady and controlled way using the sluice gates. The process for this draw-down has been agreed with the Environment Agency (EA) and conditions have been set by the EA to ensure the removal of the water does not erode or contaminate the River Brent downstream of the reservoir.

At what rate will the water be taken out?

In total around 1,000 mega litres of water needs to be taken out. We’ll be looking to reduce the height of the reservoir by around 300mm per day. The water will only be let out during the day so that the flow and turbidity can be carefully monitored.

What are the anticipated impacts on water quality downstream?

During the draw-down, we’ve been regularly monitoring the water downstream six times a day, checking turbidity and dissolved oxygen levels. We’ve been monitoring at three sites: one immediately downstream of first silt curtain; the second 50 metres downstream; and the third a further 50 metres downstream.

How is sediment being prevented from moving downstream?

We’re taking steps to control the amount of silt leaving the reservoir both upstream and downstream of the sluices. Upstream of the sluices, we are putting in a ring of one metre squared stone-filled gabions (wire baskets) wrapped in geotextile material to hold back as much silt as possible and prevent it from leaving the reservoir. Downstream of the sluices we are installing a line of concrete legato barriers 800mm high to trap any silt that makes it through the sluices, preventing it from travelling downstream. The blocks will act like a weir, giving the silt the chance to settle. A second barrier 30 metres further downstream consists of a silt curtain made up of legato blocks with geotextile between so the water can flow through.

What will the impact on fish be and how will it be mitigated?

As part of our work as a charity, we are committed to keeping our fish healthy. When draining Brent Reservoir, or any of our other canals or reservoirs for maintenance and repairs, we follow best practice to look after and rehome the fish. We carried out a fish health check before Christmas and MEM Fisheries have rescued approximately 19,000lb (8.5 tonnes) of fish, including roach, perch, bream, carp and pike. The fish are healthy, and many are extremely large, weighing up to 30lbs.

The fish, which are owned by the Canal & River Trust, have been caught in nets, placed in large containers of oxygenated water, and rehomed at various locations on our canal network, including the Grand Union Canal at Hemel Hempstead, Berkhamsted, Leighton Buzzard, Milton Keynes and Northampton, and in the River Lee Navigation at Enfield.

Some of the fish have been taken to three of our fishing lakes in the Midlands (Blythe Waters at Solihull, Naseby Reservoir in Northamptonshire, and Sulby Reservoir in Leicestershire). This decision has been made after considering multiple factors, including health status and disease risk, the existing fish population, and legal consents. While we would have liked to move the Brent Reservoir carp into canals in the south of England, we have not been granted the necessary consent from the Environment Agency as they are non native.

Will the reservoir be re-stocked with fish?

Once the works at Brent are completed, we plan to re-stock the reservoir with suitable native fish species, including roach and perch. Fish are an important part of the reservoir’s ecosystem, with many of the water birds including great crested grebes, little egrets, cormorants, and terns, relying on them as a source of food. The fish species and re-stocking densities will be agreed with Natural England. It will be in two phases: in the spring, when the reservoir has re-filled, we’ll bring fish in from nearby canals to support breeding birds during the nesting season; and we’ll do a full re-stock next winter. Some smaller fish will have remained in the reservoir in the smaller pools of water at the margins and will enter through the River Brent and Silk Stream coming in upstream of the reservoir.

Is there any action being taken to prevent downstream transfer of invasive species?

A Water Framework Directive has been carried out as part of our Flood Risk Activity Plan (FRAP) approved by the Environment Agency. The measures in place to hold back the silt will minimise the transfer of invasive species like the American Signal Crayfish and Giant Hogweed seeds. Although we rarely catch non-native crayfish during fish rescues, any that we do will be destroyed in line with environmental law. We will also be implementing biosecurity measures to prevent the spread of invasive species, including killer shrimp. It’s possible we’ll find red-eared terrapins, but they are hard to capture. They will likely stick to the muddy margins of the reservoir where access would be unsafe. In the event a terrapin is caught, we have details for a Natural England licensed rehoming sanctuary.

How much of the rubbish has been removed?

Our volunteers and contractors have spent 844 hours removing over 400 bags of rubbish, as well as tyres, e-bikes, safes, clothing and suitcases. There have been three volunteer events and we’re very grateful to all the volunteers and partners involved as community support is vital for the Trust to achieve its objectives. We can only ask the public to help us clear litter from the land around the reservoir as it’s not safe to go onto the bed of the reservoir once drained.

Why is there so much rubbish in the reservoir?

Unfortunately, the site is frequently a victim of fly-tipping and rubbish also enters the reservoir via the River Brent and the Silk Stream. The Environment Agency is responsible for managing and maintaining the trash screen that limits the amount of rubbish entering the reservoir from the Silk Stream. Plastic pollution is a societal issue that is polluting our rivers and reservoirs. Our Plastics Challenge is raising awareness of this issue and asking people to be more responsible, as well as help to clear it. Once plastic is in our canals, rivers and reservoirs it starts to break down. It’s not possible to remove submerged plastic and there’s no proven commercial scale technology to remove micro-plastics. We do skim floating plastic where we can, for example where it collects on trash screens and we’re currently trialling the use of a Seabin autonomous plastic harvesting device at Gloucester Docks.

While the reservoir is drained, can you test the quality of the water entering the reservoir?

The quality of the water entering the reservoir from the Upper Brent and Silk Stream is being monitored for turbidity, temperature and dissolved oxygen daily throughout the works.

What are the plans for the use of the crowd funded money?

The crowd funded money will go towards our debris removal contract and the disposal of the rubbish.

Why is the Trust raising money for this work?

The Canal & River Trust is a charity that cares for and brings to life 2,000 miles of canals and rivers across England and Wales. We also have responsibility for 72 reservoirs, including Brent Reservoir. We receive government funding from the Department for Environment & Rural Affairs (Defra) which enables us to carry out vital statutory repairs to our reservoirs and waterways, but we want to be able to make further improvements to wildlife habitats and the visitor experience. As a charity, we raise funds through campaigns and working with our stakeholder partners, including local authorities, so that we can maximise the benefits of the waterways and reservoirs in our care for people and wildlife.

What other improvements are planned at Brent Reservoir?

We’ve been carrying out a series of environmental improvements for the SSSI. In September we completed a two-week programme of reed bed habitat improvements using an excavator to create alcoves in the reed beds, increasing the surface area available to water birds for nesting and shelter. We’re also doing some tree works, including the removal of some of the willows to prevent them from taking over marginal habitats on the East Marsh of the reservoir. Once the reservoir has refilled with water, with the support of the Welsh Harp Conservation Group, we plan to install 14 new island habitats for common terns, one of the species of water bird that lives on the reservoir.

What are tern rafts?

Common terns (Sterna hirundo) are one of the species of water bird that lives on the Welsh Harp (Brent) Reservoir. They are silvery grey and white birds with long tails, nick-named the ‘sea-swallow’. They are on the UK conservation Amber list, and their numbers have been significantly declining in recent years. The rafts provide useful island habitats for the terns in areas of deep or fluctuating water levels. They can improve breeding success by providing areas safe from flooding and predators. The rafts have a layer of shingle to imitate the tern’s natural nesting habitat.

How many tern rafts are there at Brent Reservoir?

There are currently around 25 tern rafts on the reservoir. Most of them are around 40 years old and in need of repair or replacement to be useable again. Local conservation groups, including the Cool Oak Group and Welsh Harp Conservation Group, have recently re-shingled some of the existing rafts. We’ve removed rafts that are beyond repair and we’ll re-shingle as many of the remaining rafts as possible.

What are the new tern rafts made of?

We’re installing 14 new tern rafts made of recycled plastic, which is a tough material with a very high impact resistance. It will not rot or degrade in water and it has UV stabilisers to increase its lifespan. As well as being more durable in the water than timber, these plastic rafts are more flexible and cheaper.

What other types of water bird live at the Welsh Harp (Brent) Reservoir?

As well as common terns, cormorant, kingfisher, great crested grebe, little grebe, moorhen, coot, common snipe, curlew, little ringed plover, lapwing, common sandpiper, green sandpiper, dunlin, gadwall, water rail, shelduck, teal, and many types of heron, egret, gull, dove, swan, duck and goose visit or breed at reservoir.

What are the protected plant species?

These include five species of orchid. Broad-leaved Helleborine grows in damp woodland near the eastern marsh, whilst Common Spotted-orchid occurs in grassland on the northern and southern shores. Southern Marsh-orchid can be found in the northern and southern marshes; a Pyramidal Orchid was discovered in the eastern marsh in the 1980s. Bee Orchid was discovered on rough ground in 1994.

What other wildlife lives at the Welsh Harp (Brent) Reservoir?

As well as being a SSSI, the reservoir and significant areas of its surroundings have been designated as a Local Nature Reserve and a Site of Metropolitan Importance for Nature Conservation. The Welsh Harp Conservation Group (WHCG) carries out regular wildlife surveys. Their recent bat survey identified six species, including Common Pipistrelle, Common Noctule, Soprano Pipistrelle, Nathusius’s Pipiestrelle, Daubenton’s Bat and Serotine. They have recorded sightings of a number of butterfly species, including small skipper, small heath, ringlet, green-veined white, painted lady, small copper, purple hairstreak, white-letter hairstreak, brown argus and common blue.

As well as water birds, owls, woodpeckers, mistle thrush, warblers, chiffchaff, pipits, wagtails, redstrarts, blackbirds, robins, wrens, jays, reed bunting, sparrows, crows, greenfinch, goldfinch, red kites and kestrels make use of the habitats at the reservoir.
The reservoir is also home to a wide variety of fish, reptiles, amphibians, invertebrates, fungi, trees and plants.

What will the impact on over wintering birds be and how will it be mitigated?

We share the concerns of local conservation groups who care about the SSSI and its designated species, including wetland breeding birds and we’ve been working closely with relevant agencies to protect all the different aspects of the natural environment as much as possible throughout the project. We are currently in discussions with Natural England to agree the best way forward to secure formal permission for the works beyond the current assent date of 29 February 2024. During a gradual, managed refill, after the works are completed, we will include sensitive and proportionate mitigation measures to discourage birds from nesting on the reservoir margins until it has refilled.

What will the impact on aquatic invertebrates be and how will it be mitigated?

We are draining the reservoir at the least impactful time of year for aquatic invertebrates, as most of them are inactive throughout the winter. There’s likely to be a temporary impact upon aquatic invertebrates, but they are predicted to recover to their current levels within one season. And in the longer term there will be a greater diversity of aquatic invertebrates as a result of the habitat restoration works we are doing within the north and eastern marsh.

Can partners assist with the habitat improvements?

All the works are being undertaken by contactors, but we welcome involvement in maintaining the enhancements delivered in the future e.g. work party to remove willow regrowth and monitoring the tern rafts.

How long will the tree works and vegetation management work take?

Our framework contractors began a programme of tree works and vegetation management at the Welsh Harp Brent Reservoir 22 January. The work is designed to increase biodiversity at the site, and is part of a restoration management plan the Trust is creating with its partners to ensure the long future of the Brent Reservoir SSSI.

In what area of the reservoir are the tree works happening?

The main tree works are on the East Marsh area and will include coppicing willow trees and the removal of large boughs that have fallen across the River Brent. Branches obstructing the view from the North West Marsh viewing platform will also be cut back.

Why are the trees being cut down?

The trees are encroaching into the reed beds and they tend to shade out the habitat needed by wetland birds for foraging and nesting. There are lots of self-set willow trees that are crowding out other species and reducing the diversity within the SSSI. Without the tree works, over time the reed beds would be lost to natural succession. The SSSI is designated for wetland breeding birds so restoring these habitats will enhance the SSSI for the designated species.

What will happen to the waste wood?

The light brash will be chipped and the larger branches will be stacked on site to create habitat piles for terrestrial invertebrates, amphibians and reptiles, in turn benefiting other species including bats.

For more information about the project, go to our webpage call the Canal & River Trust on 03030 404040 or email [email protected]

Last Edited: 09 February 2024

photo of a location on the canals
newsletter logo

Stay connected

Sign up to our monthly newsletter and be the first to hear about campaigns, upcoming events and fundraising inspiration