Chiltern Chalk Streams

The Hamble Brook flows through the peaceful Hambleden Valley, Buckinghamshire in the heart of the Chilterns Area of Outstanding Natural Beauty. The comings and goings of this delightful little Chalk Stream Brook are well documented.  In recent years it has spent long periods without Flow and, most recently, was dry for 2-yrs during the severe drought of 2011/12.  Now, however, after one of the wettest years on record, the Hamble Brook has sprung back to life.  This footage was filmed for the Chilterns Chalk Streams Project on 19th February 2013.  Led by the Chilterns Conservation Board, the Chilterns Chalk Streams Project is a Partnership which aims to protect & conserve the Chalk Streams of the Chilterns AONB and to encourage enjoyment and understanding of them.  It rises in the aptly named Frog Lane in Turville and joins the River Thames at Hambleden Mill End.

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 ‘Winter-bourne’ because it only flows after the winter rains.  Winter-bourne Streams have their own special wildlife which is adapted to cope with intermittent flows.
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