How do chalk streams work?
Did you know that sections or sometimes whole chalk streams can dry up altogether? Watch our film to learn more about how these fascinating ecosystems work.
Where does the water come from?
Chalk streams are fed from groundwater held in the chalk that makes up the Chiltern Hills, and this gives them some unusual features. Chalk is an aquifer, which means that it is able to soak up and hold water – a bit like a sponge. Water can move through the chalk in cracks called fissures. The water emerges at ground level in the form of springs that feed the chalk streams. Since groundwater levels in the chalk vary according to rainfall and season, chalk streams are naturally intermittent in their flow.
Why do chalk streams sometimes dry up?
During the winter, when rainfall is heavy and able to percolate through the chalk, the aquifer will be well topped up. The head of the stream moves up the valley as the water table rises. In summer, little rainfall percolates into the chalk as it is mostly taken up by plants and lost through evaporation. The water table drops and the head of the stream moves down the valley, leaving the top section of the stream dry. This section is called a ‘winterbourne’ because it only flows after the winter rains.
Winterbourne streams have their own special wildlife which is adapted to cope with intermittent flows.
Further information
Geology of the Chiltern Chalk aquifer, Haydon Bailey, 2023
Where does our water come from?
We take our water from the aquifer too. Every time we turn on the tap, we take water which could be flowing in our chalk streams. By using water wisely, we can reduce our impact on the streams and help to maintain natural flows.
Chalk streams under threat
Despite the rarity and value of chalk streams, these precious and unique freshwater ecosystems are at risk. Watch our film Chalk Streams in Crisis to find out more about the threats they face.
Saving water is one thing we can can all do to help, but there are plenty of other ways to get involved too!
The Hamble Brook: A Chalk Stream Reborn
The Hamble Brook is a winterbourne river, which seasonally dries along its entire length. This film shows the flow returning after one of the wettest years on record.
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