Help us identify all South East chalk streams

The South East Rivers Trust has launched a Chalk Streams Review, to ensure that all rivers and streams which qualify across our catchments are identified and mapped. Dr Chris Gardner, our Head of Science and Partnerships, sets out the plan and how the public can help.

Identification has been overlooked

While a lot is known about the “classic” chalk streams of Hampshire, the smaller chalk springs and streams of the North and South Downs have been overlooked. There is still not a definitive list of them.

At the foot of the Downs scarp (steep) slopes, in particular, there is a sequence of numerous small springs feeding small streams.

Their flow is derived from the chalk aquifer, either directly or indirectly, after percolating through greensand or other geological formations.

It is the uppermost sections of these small streams – sometimes only the first few 100 metres of them – for which documentation is lacking.

While they might be very small in size (as little as 30cm wide) they could be of surprising ecological significance. Large brown and sea trout might enter them very briefly to spawn in the winter, as they provide ideal spawning and nursery habitat for eggs and newly hatched fish.

The urban River Dour chalk stream in Dover

We need your help mapping all chalk streams

Are you an angler, river lover, member of a conservation group, landowner or farmer or simply a walker who loves rivers? For our Chalk Steams Review, we are asking for your help to develop a full inventory of these overlooked chalk streams.

Some of these watercourses will be naturally winterbournes (dry through summer), or perennial. Little data exists on their status and we know little about the need for restoration or enhancement.

We have developed a mapping portal to capture your knowledge. We will use this information to update the national dataset of chalk streams – and to inform our own work delivering healthy, thriving rivers for people and wildlife in the south east.

Visit our web-based portal to contribute your information on where these streams are and their current status. The portal shows you data on geology, catchment and local authority boundaries, the Detailed River Network of all known rivers and streams – and known chalk streams. It identifies both what the national dataset is confident are chalk stream and those it is less confident about.

We want to collate information by 15th December 2022, in order to present a comprehensive list in the new year.

Visit our web portal  to contribute!

Our mapping portal showing all known rivers and streams (blue), those we have confidence in being chalk streams (purple) and those with less confidence (pink)

What makes a river?

All rivers are different. They are a product of their individual local geology and flow rate. Rivers are essentially erosion channels carved into the landscape by water draining the landscape and/or flowing from underground aquifers.

The laws of physics govern this movement, which is why we see the remnants of river channels on the surface of Mars, where there was once water – much like the river channels we see on Earth.

The same laws of physics dictate their form, water flowing over an erodible surface, but each source or amount of water and the nature of the surface – both gradient and the type of substrate material – give each river a unique character.

In the south east, there are two extremes of geology, clay and chalk, which produce very different types of rivers.

The difference in the number of river channels resulting on the chalk landscapes of the North Downs (yellow) and clay landscapes of the Weald (green), due to the different permeability of the two geologies.

Differences between clay and chalk streams

On clay landscapes, soils are heavy, the ground is largely impermeable and rainfall runs off, picking up impurities, which lowers the water’s quality. Rivers in these landscapes are very reactive to rainfall, rising quickly and producing high energy erosive forces that drive erosion and deposition, moving rivers and changing their course as they migrate across the floodplain. The whole landscape is covered in river channels even right up into the headwaters, created during rainfall events so water can drain downhill.

On chalk landscapes, soils are thin, the ground is highly permeable and rainwater soaks into the ground, resulting in much less reactive rivers and streams, and much fewer river channels in the landscape (see graphic).

This results in little surface run-off and rivers are formed instead where water springs from the ground. It overflows from the rocks beneath our feet and it flows downhill to the sea. Due to the filtering nature of the rocks, this water is very high quality and has high alkalinity (it is very ‘hard’) because of the dissolved minerals from the rocks it passes through.

A dry section of river © South East Rivers Trust

The definition of a chalk stream

Chalk streams are any stream or river that has a flow regime dominated by natural discharges from a chalk aquifer. In the South East of England, we have many rivers and streams that rise from greensand aquifers, as well as chalk (pictured).

We’re including these in our definition of chalk streams. This is because these greensand aquifers are often linked to the chalk, providing high alkalinity water. It is also because they also exhibit the other key characteristics of chalk streams, such as low energy, high water quality with low fine sediment inputs, stable base flow and stable temperature regime, which extends growing seasons.

These are base flow-dominated watercourses with high alkalinities. These characteristics are fundamental in shaping their biodiversity. More information can be found in the guidance to stakeholders on proposing local refinements to chalk river maps.

Chalk in the landscape

Water flows slowly through rocks

Water flows through these rocks very slowly through the rock matrix, but also quite quickly through tears and fractures in the rocks.

The amount of water in the aquifer determines the amount of water discharging to form the river.

This amount of water varies seasonally, with flows at their peak in late winter and spring after winter rainfall has ‘recharged’ the aquifer, and lower in summer and autumn.

Only winter rainfall effectively recharges our aquifers, because summer rainfall is taken up by vegetation before it can soak down into the ground.

Our Chalk Stream Strategy aims to map all chalk streams in the south east

Winter-only areas appear

In a natural situation, the main springs that form chalk streams flow 12 months of the year. However, there can be other more uphill springs that only flow when the aquifer is very full, and these can form ‘winterbourne’ headwaters that only flow for part of the year. Water abstraction from underground aquifers for drinking water supply can negatively impact the amount of water in the aquifer and therefore the river.

Reduced flows result in lower quality aquatic habitats, less available habitat quantity, because of reduced river size, and less geomorphological activity – natural processes of erosion and deposition, that are required to shape and refresh niche habitats needed by fish and invertebrates to complete their life cycle. For example, low summer flows in chalk streams, and other rivers, resulting from the 2022 drought, will reduce the quality and quantity of clean gravel habitats that are so important for salmon and trout spawning. This will potentially lower recruitment of fish this winter.

Chalk heathland © South East Rivers Trust

Why are chalk streams unique freshwater habitats?

It is estimated that there are only about 210 chalk streams in the world – and 160 of them are in the UK. This rarity makes them very valuable habitats.

Chalk streams are also special because they have very stable flows. Also, the fact that water comes out of the ground at about 10ºC all year round means they have very stable temperature regimes, giving them longer growing seasons because they are warmer than other rivers in the winter months.

Their low energy, resulting from little surface run-off, also makes them very stable. The fact that the water flowing through them has been filtered by underground rock means it is very pure and has very low fine-sediment inputs.

Life likes things stable. These four characteristics have made chalk streams very productive habitats rich in aquatic life, ecology and biodiverse.

In the UK, our rivers were reshaped and covered by ice sheets 10-12,000 years ago during the last ice age. After the ice sheets retreated, our rivers were recolonised by fish and invertebrates. These all evolved to suit the individual characteristics of the rivers they found their way into, being effectively isolated in these environments.

This has led to these animals adapting locally. For example, trout and salmon from chalk streams are genetically distinct, having adapted to these unique habitats more than their counterparts in more ‘normal’ river types. This local adaptation has taken 10-12,000 years of evolution. If these populations are lost, this progression will be lost forever. This loss of genetic variability in the whole population means they are less able to adapt to future pressures, such as climate change.

The River Darent chalk stream near Shoreham

The pressures on chalk streams: 1 Water Quantity and asbstraction

Groundwater abstractions lower water tables and reduce flows in chalk streams. This affects the quality and quantity of habitats available for aquatic ecology.

Groundwater aquifers have been used preferentially for drinking water supply, because the water needs less treatment. Abstraction reform and reductions from priority aquifers are required to safeguard and restore our chalk streams.

Water neutrality measures should be implemented, so abstractions don’t increase in the future, to service house building and development needs.

Many current abstraction licences have ‘headroom’ – the difference between licenced amounts and actual abstracted quantities – where water companies can legally increase abstraction amounts, potentially increasing this pressure on these precious habitats.

The Darent Chalk Stream © South East Rivers Trust

Pressures on chalk streams: 2 Water Quality

Water quality in chalk streams is impacted by a number of things, but two important aspects are:

  • nutrients, that fuel eutrophication (an excessive richness of nutrients) and reduce the diversity of plants and animals;
  • Fine sediment, that clog the spaces between riverbed gravels reducing their permeability, so riverbeds become less oxygenated and less able to support invertebrates and fish early life stages.

Sources of these are from sewage treatment works discharges, both continuous discharges that contain nutrients, specifically phosphorus, and intermittent Combined Sewer Overflows (CSOs) that contain nutrients, but also fine sediment. Other sources include rural and urban diffuse pollution, that can be sources of again both nutrients and fine sediment.

Urban run-off, from roads and pavements, contains a high amount of fine sediments. This form of pollution also carries potentially toxic chemicals from human activities, such as copper from car brake pads and zinc from car tyre wear.

These sources can be reduced by Nature-based-Solutions, such as treatment wetlands, land management and land use change in high-risk areas and improved agricultural practices.

Chalk streams have not been fully charted

Pressures on chalk streams: 3 Weirs and old watermills

Human modification of river channels has reduced the diversity and connectivity of our rivers, including chalk streams. Due to their consistent flow, chalk streams were prime sites for watermills, pre-industrial revolution. However, while these mills no longer provide us with energy, their damaging infrastructure remains.

Weirs and old watermills impact rivers in three main ways:

  • Fragmentation of habitats used by wildlife. Habitats which were once continuous become divided into separate fragments restricting the movements of wildlife such as fish and separating them from habitats and resources, other fish required for their survival or the completion of their life cycle. Fragmented habitats are also less resilient, preventing recolonisation after pollution incidents and lowering genetic variability.
A brown trout Image by Christopher Cutler from Pixabay

Pressures on chalk streams: Natural processes inhibited

  • River habitat is degraded by the creation of an impounded (enclosed) reach upstream (river-like habitats become like lakes) drowning out natural features like riffles, causing important spawning and nursery habitats for river fishes to be lost, thus lowering recruitment and breeding success.
  • Natural processes such as the transport of sediment  are prevented when weirs or mills break up the river. Where there are no barriers, gravel flows down a river, much like water – just a bit slower! Rivers are naturally dynamic, with erosion and deposition occurring in balance, creating a highly varied mosaic of micro habitats for aquatic ecology. Weirs stop this natural tendency for change, creating a uniform static environment, unlike the one animals have evolved to utilise.
Abingdon Weir, the channel on the far side feeds the Sandford Brook and the nearest channel feeds the River Ock © South East Rivers Trust

Help us identify the gaps in listings

While there are datasets of which rivers are classified as chalk streams, these lists are incomplete and many smaller streams and tributaries are missing. We need your local knowledge to help identify these precious habitats so they can be restored and enhanced.

Visit our web portal  to contribute!

The information you submit will be reviewed by our team and verified as we put forward changes to national mapping lists.

The River Wey, Bentley, is among chalk streams in the South East Rivers Trust area