The Darent

Under threat

What are the issues?

The quantity and quality of water together with the quality of the physical habitat are all key to the ecological health of chalk streams (Chalk Stream Restoration Strategy, 2021). 

The quantity of water flowing through the Darent is an ongoing concern. Although sections of the river have not dried up in recent years, water levels regularly dip below ecologically acceptable levels. The river is particularly vulnerable in periods of drought.

One of the characteristics of chalk streams is the pure, clean water that originates in underground aquifers and springs. The pristine quality of the water in the Darent is affected to varying extent by external pollutants from road and agricultural run off.

The following pages describe key issues in more detail, explaining how they can and should be addressed.

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Over Abstraction

Water held under the ground within the rocks and soil across the river catchment is pumped out in a process known as abstraction. It is used by water companies to supply drinking water to homes and businesses in London and the South East.

While chalk streams, springs and wells have been used as a source of water for centuries, it was during the second half of last century that groundwater abstraction for public water supply accelerated markedly.

By the late 1980s, the impact of this abstraction on chalk streams was becoming clear. Stretches of the Darent completely dried up and the river was identified as suffering from one of the lowest flows of any in the country.

The Darent Action Plan led to a reduction in abstraction to restore ecologically acceptable flow levels. However, these levels have not been achieved consistently. As recently as 2021, more than half the average annual recharge for the river continued to be lost to abstraction.

Abstraction on this scale, particularly in combination with the changing climate, damages the balance of our unique stream and the wildlife it supports.

What’s the solution?

Abstraction from what is acknowledged to be a precious and important habitat needs to be reduced further.

There is widespread support for the strategy to restore chalk streams – including from Thames Water which announced a plan to stop abstracting from vulnerable chalk streams by 2050.

This timescale is too slow, particularly when taken with data from the Met Office which suggests that the record-breaking heatwave experienced in 2022 will be considered an average summer by 2035.

Plans are being made to secure a resilient and sustainable water supply. DRiPS closely follows progress on this by WRSE – an alliance of the six water companies covering the region. It is a slow and lengthy process and we are not alone in thinking there is a lack of urgency and clarity in what has been presented so far (eg see the Environment Agency response). 

In addition, we are campaigning for the principle of water neutrality to be applied in the local planning process. New reservoirs and solutions to boost water supplies are desperately needed, particularly since demand for water is expected to exceed supply as early as 2030.

Pollution

We can consider ourselves lucky that the Darent largely avoids pollution from sewage outflows that afflicts watercourses across the country.

However, areas of the river are vulnerable to pollution carried in the run-off from roads and agri-businesses. This can raise levels of nutrients, sediment and chemicals, including pesticides.

 

Nitrogen and phosphorus from fertilisers used on farms and gardens can have a detrimental impact on the health of the river. Pollutants, including microplastics, may poison fish and invertebrates, and move up the food chain.

Low water flows exacerbate problems by concentrating pollution within a smaller body of water. Low flow also allows silt to settle on the river bed, disrupting the ecology. Clean river gravels are essential for many of the species typical of chalk streams, such as water crowfoot, invertebrates and fish. 

What’s the solution?

Effective drainage is needed for roads near the river to avoid rainwater cascading into watercourses. Rules to prevent water pollution from agricultural land should be followed and enforced.

As ever, nature provides some answers too. The plant habitats along the river margins and banks – known as riparian vegetation – play a role in stabilising the stream bank, controlling erosion and sediment. They filter nutrients and other pollutants and reduce the impact of floods as well as providing habitat and food for wildlife and pollinators.

Did you know?

Many external flea and tic treatments for pets contain powerful insecticides which affect insect larvae such as mayflies and dragonflies if they get into the river.

Only use flea treatments when your animal has fleas – or else use pills. Furthermore, do not let your dog swim in rivers for at least a week after treatment.

Climate change

Weather patterns that have long been relied upon to maintain our water supplies are changing.

We are seeing hotter drier summers which increase public demand for water. According to Thames Water, demand was as much as 50% above the norm in some areas during the 2022 heatwave.

In addition, rain that typically restored the aquifer levels through autumn and winter is less predictable. While groundwater resources tend to be fairly resilient in single-year droughts, those lasting two or more years are far more serious for the base flow of the river in the summer as well as for the wider natural environment.

Climate change is bringing more frequent extreme weather. This includes heavy, intense rainfall. Torrential rain tends to flash along the land and can lead to localised flooding. Equally, it does not seep into the soil or recharge aquifers as efficiently as less intense rain of longer duration. The increased run-off is also likely to wash pollutants from roads, farmland and urban areas into the river.

What’s the solution?

Our water systems need to be more resilient and designed to adapt to the changing climate.

This includes ensuring that homes are built with water stress in mind – grey water recycling, low-usage appliances and maximum capacity for sustainable drainage in the vicinity.

Hard paving, loss of natural habitats and the compaction of intensively managed agricultural soils all reduce the capacity of the landscape to capture water.

That’s why techniques that work with natural processes are increasingly important. Measures which hold water in the catchment  help to manage flood risk and increase the amount of water available to percolate into the underground aquifers.

Invasive species

Invasive, non-native species are those which have been introduced into areas outside their natural range through human action and are posing a threat to native wildlife. They can have a significant modifying impact on the river.

Problem species for the Darent include: Himalayan balsam, American signal crayfish, American mink, Japanese knotweed and giant hogweed.

Balsam bashing

Himalayan balsam

A relative of the busy lizzie, it originates from the Himalayan mountains and was introduced into Britain in 1839. By 1855 it had escaped out of gardens and started to become a problem. It forms dense, tall clumps which swamp out other plants When it dies away in the winter it leaves bare river banks vulnerable to erosion. Native plant species cannot compete with the invasive species for space to grow, resulting in the balsam creating a monoculture and reduced opportunity for native species to recolonise these areas.

What’s the solution?

Himalayan balsam is loosely rooted so it is relatively easy to pull up before seedpods form and bashed to prevent regrowth. If control is undertaken systematically, complete eradication is possible in two to three years.

American signal crayfish

Introduced by the government in the 1970s as a commercial product for export, signal crayfish escaped and spread rapidly through British waterways where they have outcompeted and infected our smaller, white-clawed crayfish with deadly crayfish plague. As a result, white-clawed crayfish which are well adapted to English chalk streams, are increasingly rare and in danger of being wiped out.

Signal crayfish are voracious predators and will eat juvenile fish, invertebrates, amphibians and plants. They also burrow into soft riverbanks causing the banks to collapse, progressively widening the stream and releasing sediment, leading macrophyte and invertebrate communities to spiral downhill.

What’s the solution?

Currently there is no obvious solution. There are restrictions on trapping and dispatching of signal crayfish. It is not clear that this approach would offer a long term solution because traps tend to catch catching large males, reducing predation of younger ones.

Note that crayfish plague is easily spread. Take care not to unwittingly spread it to other non-infected areas. If you see signal crayfish in the river, contact the Environment Agency on 0800 807060 and send us an email (darentdrips1985@gmail.com) so we can track infestations.

American Mink

Introduced for fur farming from the 1920s, mink became established in the wild as a result of escapes and deliberate introductions. They have been sighted in many places in the Darent Valley from Westerham to Dartford and have a significant impact on native wildlife, especially water voles and fish on which they predate. Mink are good swimmers and females are small enough to enter the waterline burrows of water voles and take their young.

What’s the solution?

Eradication is possible through coordinated trapping and control projects.

American mink – a non-native invasive species.

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