In the immortal words of Sir David Attenborough, an absolute hero of mine, “Freshwater is the lifeblood of the natural world”. Like the veins in our bodies, the network of rivers flow throughout the country bringing life-giving water. Over 200,000 km of rivers and watercourses connect our landscape, providing a lifeline for a multitude of habitats and species. However, many of the rivers in the UK are in less than prime condition, and less than half of our rivers are in a good state for nature.

Let’s take a little more of an 'in-depth dive' (excuse the pun) into the underwater lives of a few species from this week’s episode of Wild Isles and our chalk streams, the global rarity that we are lucky to have in Dorset. 

Chalk streams found in Dorset are amongst the world’s rarest and most exquisite rivers. They are globally unique, representing one of the UK’s most important contributions to global biodiversity. Only around 260 of these unique rivers exist in the world, with 224 (~85%) of them found in the UK; mostly located in the south of England. They provide a rich biodiverse ecosystem, supporting important salmonid fish species including Atlantic salmon (Salmo salar) and brown trout (Salmo trutta). 

In Dorset the River Stour provides spawning ground for salmon and trout, and The River Tarrant, is known to be one of England’s most productive brown trout locations.

Chalk Streams are largely spring fed, arising from aquifers, stored in Dorset’s central cretaceous chalk geology. This means that they maintain relatively stable emergence temperatures of around 10—11°C all year round. Maintaining warmer water during the winter, and cooler water supply during the summer, compared to air temperature. Chalk streams in their natural state remain beautifully clear, with low amounts of sediment as rainwater is filtered into the aquifer slowly, with limited surface run-off.  

Despite how rare these streams are, many are in poor condition. They are under pressure from the spread of non-native, invasive species, and habitat degradation from siltation, canalisation, and human made barriers such as culverts. These rivers are important for our domestic and agricultural demand for water supply and over abstraction is of increasing concern. Over abstraction from chalk aquifers can denude the natural flow downstream, for perennial streams and winterbournes, as well as reducing the flow available for dilution of pollutants. Pollution can come in many forms but notably sewage, farming and roads.  

Farmers are continually improving farming practices to maintain soil and limit fertiliser and waste run-off. The adoption of riparian buffer strips to further limit soil and run-off getting into the watercourse are slowly being adopted throughout the county as a land management technique to limit what is entering the watercourse. We increasingly need to make space for water to protect this precious resource. 

The seasonal nature of the winterbournes, rivers that only hold water during periods of high rainwater and high groundwater, mean that some headwaters and winterbournes naturally run dry by late summer. They can still provide vital habitat and support temporary stream specialist species such as the nationally rare winterbourne stonefly (Nemoura lacustris). 

Chalk streams are characteristically beautiful. This is partially due to the amount of vegetation surrounding and within the water. Water-crowfoots help create that picturesque image of country streams, flowering on the surface and underneath the water. At the beginning of the freshwater episode of Wild Isles, Sir David is sat amidst this idyllic scenery, second to none in creating the ‘Wind in the Willows’ imagery. 

Our chalk streams provide habitat for some firm favourites: sea trout, grayling, white-clawed crayfish, European otter, water vole and kingfisher. Clean gravels also provide breeding habitat for bullhead, Atlantic salmon and brook lamprey.  

UK freshwaters are home to around 40 native fish species. One of our most loved freshwater fish has a very impressive lifecycle. The Atlantic salmon spends the majority of its life at sea, but returns to freshwater, where it hatched, to spawn. When hatched a juvenile can stay in freshwater for up to six years, before entering the sea and undergoing morphological changes to survive in salt water. 

Between November and February, after about four years at sea, they will enter freshwater and travel upstream, crossing weirs and waterfalls where needed, overcoming obstacles up to three metres in height. No other fish can clear waterfalls as high as this.  

Salmon ‘nests’ or redds, are <30cm deep and excavated into the base of the river, requiring gravel substrates and cold clear oxygenated freshwater. 

This year the River Frome saw its earliest returning adult salmon in 20 years, on 14 February 2023. When they return to freshwater, it is vital that they preserve energy by staying in cool, deep oxygenated sections of river. Here they wait until November-December to spawn.  

The Wild Isles camera team managed to capture some unique footage of salmon leaping in Scotland. It was great to watch how they did it in the insights section at the end of the episode. The impressive shots of salmon leaping really shows how they go about calculating and putting their all into the upstream ventures. 

After spawning, as many as 90-95% of the salmon die. Sir David mentioned that in the last 25 years returning salmon numbers have fallen by 70% and “at the current rate of decline, they may disappear from all of our rivers in as little as 20-year’s time. The grandchildren of these fish may be the last to make this astonishing journey”. A sobering fact to say the least. 

In November 2022 a Chief Scientists group report by the Environment Agency modelled river water temperature projections for English Chalk Streams. They predicted that by 2080 it is likely that 85% of chalk streams will exceed the 12°C temperature threshold for salmonid egg survival during the winter spawning period. And the high summer temperatures, exacerbated by climate change, are likely to continue to threaten adult brown trout, with all sites exceeding the species’ upper critical temperature range of 19.5 °C by 2080. Work for and on behalf of our wildlife must continue to grow on landscape scale project. Rivers do not respect political or county boundaries. What happens in its headwaters will potentially become an issue downstream, in another town or county. Partnerships working together across disciplines, organisations, counties and charities is absolutely vital for river health. 

So, what can we do?

Across the county we need to leave more space for nature to thrive along our watery corridors, giving space back to the rivers and fencing off riparian buffer strips. This helps limit poaching (disturbance of soil and vegetation by feet), erosion (sediment entering the watercourse) from livestock entering the river’s edge and allows vegetation to grow up and enhance the habitat. Vegetation on the land helps to slow the flow and filter out pollutants, whilst vegetation in the marginal edges of the water help to create important habitat and cover for fish and invertebrates.  

Perhaps counter intuitively, wood is also beneficial in water. Leaving woody debris in places, such as Devils Brook, enhances the diversity of habitat seen in the river. It is still often thought that we need to remove any woody debris found in rivers, but if it doesn’t pose a risk to humans or infrastructure then it should be left, as it has such a positive effect for nature. The wood itself creates perches, areas for latrines and spraint, and habitat for invertebrates. The woods physical characteristics, such as size and shape, change the flow of the water around it, collecting sediment in some areas for marginal plants to thrive in. And creating riffles and waterfalls where water cascades over the top of it, aerating the water and carving out different depths of water, providing different substrate patches. The more diverse the flow, depth, temperature and substrate the better for biodiversity. 

Another nature-based solution we could implement is to enable water to stay in the landscape for longer, improving our wetland habitats and alleviating flooding pressure. The work of a beaver is an excellent example of this.

The Eurasian beaver (Castor fiber) works within ~30 metres of the watercourses, creating dams in small tributaries and river headwaters where needed to access food. The dams help to filter out sediment, pollutants and debris to improve the water quality as nutrients are taken up by wetland plants. They also help to alleviate peak flow events, limiting flooding, whilst also holding water in the landscape during periods of drought. These ecosystem engineers know how to maintain an equilibrium for the wetland ecosystem and all the species that depend on the waterbodies. 

In summer 2022 we welcomed the first beaver kits to be born in Dorset in 400 years at Dorset Wildlife Trust's enclosed beaver site. It has been incredible to observe them, along with their parents, as they slowly transform the habitat around them, creating precious wetland habitat for a variety of species. You can learn more about the Dorset Beaver Project here. 

Dorset Wildlife Trust is working on a number of partnership projects to improve water quality and fish habitat, including ‘slow the flow’ techniques and using natural processes. You can read about our Water Guardians Community Project here. 

Rivers shape the landscape and in a more psychological sense, have shaped me. To protect our rivers and wetlands that support our wildlife is not only my work, career and my passion, it is a must. For how can our landscape, the body of the UK, be in good health if the veins feeding it are not?

Some of my favourite nature reserves for river watching include Nunnery Mead, Kingcombe Meadow, Bracketts Coppice and Girdlers Coppice. Sneak away for some solitude, take a mindful minute, sit with the water and enjoy the sound of the river. There’s nothing quite like it. 

We will be back next week with another blog covering episode five of Wild Isles and Dorset's spectacular marine habitats. Catch up on the BBC Wild Isles series on BBC iPlayer.