All Change: The Climate and our Chalk Streams

Rain on window © Peter Cairns/2020VISION

As the threat of climate change looms large, what could the consequences be for some of our most sensitive freshwater habitats?

If you happened upon last month's Met Office report, 'State of the UK Climate 2020', you likely found it a sobering read. Far from being some distant future, the effects of climate change are happening all around us. Last year was the first on record to make the top ten for warmth, sunshine, and rainfall, continuing a decades-long trend of rising temperatures and declining snowfall.

When we think of how our climate is shifting, especially in regards to global warming, we often assume that the future will be drier. In reality, the more likely scenario is one of greater extremes, with a higher chance of droughts in the summer and flash flooding in the winter. This trajectory is troubling for all of our wild places, but especially so for Hampshire's beautiful chalk streams.

These cool, clear, mineral-rich waters enrich the landscape for people and wildlife alike. A unique relationship with our county's chalk aquifer lets them flow steadily all year round - the underground water store keeping them supplied even in the warmer months. These features are due to a lucky alignment of geology, geography, and weather that occurs almost nowhere else on Earth.

This rarity makes the streams immensely precious, but also extremely vulnerable: should the delicate balance be broken, a crucial habitat type could be lost entirely. With other environmental pressures mounting, climate change could be the final nail in the proverbial coffin. For a glimpse into this possible future, we need only look at how severe weather already affects the streams.

Dry chalk stream bed © HIWWT

Dry chalk stream bed in a winterbourne stretch © HIWWT

Times of drought

Where our chalk streams are concerned, 'dry' isn't always a worrying word. Some of their upper reaches are winterbournes, meaning they can run low or even vanish for months at a time. This natural phenomenon is limited to certain stretches where the resident wildlife is suitably adapted, but when it happens over larger areas - or for longer periods - the results can be catastrophic. 

Low stream levels set in motion a vicious cycle of interrelated issues. The water flows more slowly, causing it to warm up in the sunshine. The increased heat encourages eutrophication - a process where excess nutrients in the water cause a overgrowth of algae. These algal 'blooms' expand rapidly, and then proceed to die and rot when the nutrients are exhausted.

This process, combined with the rising temperature, depletes the water's oxygen and leaves it more acidic. Fish and aquatic insects can suffocate, and their eggs may fail to develop. Plants are less able to grow in or along the stream channels, leaving juvenile fish and water voles vulnerable to predators. The channel sides, no longer secured by vegetation, start to wear away.

Now wider than they should be, the streams flow even more slowly. Sediment from the eroding banks settles on the gravelly beds instead of being swept off, decimating the resident invertebrate populations. The channels become ever shallower; the water continues to heat up and slow down. The streams become pressure cookers, desperately waiting for rain.

Urban flooding © Scott Petrek

Urban flooding © Scott Petrek

Times of flood

When rain does return to the chalk streams, it isn't always a welcome respite. Regular and moderate rain serves to 'top up' the aquifer, giving the streams a continuous supply, but intensely wet weather after a drought brings its own problems. Soil that has been dry for long periods can actually repel water, and even unaffected ground can only take in so much rain.

In these situations, any surplus water flows across the landscape, picking up fertilisers, chemicals, and organic matter. It washes over roads, collecting pollutants from fuel, tyres, and brakes that have concentrated there during the dry spell. When the deluge finally reaches the streams, their banks can collapse under the onslaught, unleashing floods on homes, roads, and businesses.

Water that arrives indirectly can be just as damaging. In urban areas, it may first run down storm drains that connect with local sewer systems. But if the rain is heavy enough these systems can overflow, causing sewage to back up into streets and buildings. To avoid this, water companies may open pipes that flush untreated waste into streams, rivers, and the sea.

The impact of all this lasts long after the rain subsides. Sediment from collapsed banks smothers the stream beds. Fertilisers and sewage boost eutrophication. Chemicals and heavy metals poison wildlife. If another drought comes, the streams are less able to cope with its effects, and so the cycle continues.

Volunteers implementing natural flood management measures

Volunteers implementing natural flood management measures

Time for action

So how can we spare our streams a future of drought and downpour? People across the UK are tackling the climate crisis in various ways, such as planting trees, reducing household waste, and cutting carbon emissions. It's important that we all take responsibility for our own impact, but our climate is already shifting and will likely continue to do so until the problem is addressed at a global level. This is a story not about prevention, but about mitigation and adaptation.

When it comes to the fate of our chalk streams, the Wildlife Trust is helping to deliver Watercress and Winterbournes. This scheme is working with local communities to reduce future impact on the Test and Itchen headwaters, but it's also bolstering these streams so they have a better chance of coping with issues like pollution and low flow. The healthier we can make these habitats now, the more likely they are to survive what the future throws at them.

It's worth remembering that, while these efforts will benefit wildlife, it's certainly not a one-way street. Chalk streams have the potential to help their human neighbours deal with climate change by lessening flooding, cooling the air, and trapping carbon, but only when they're in good condition. In other words: if we take the time to look after our streams, they will look after us in return.

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