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.
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.
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.
For tips on how to save water, go to:
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