Dry valleys and the chalk stream at Hughenden
The chalk rock that forms the Chiltern Hills is highly permeable, so rainwater doesn’t usually flow across the surface to form streams, instead it percolates through the thin soil into the rock below where it collects as groundwater. Here it can remain for months and years, slowly flowing through the chalk before emerging at springs, often miles from where the rain originally fell. And so almost all valleys in the Chiltern Hills contain no streams or rivers.
Although they are dry today, during the coldest parts of the Ice Age, valleys like Echo Valley on the west side of Hughenden Manor were once sites of surface streams. Water came from melting snow in the spring and rainfall in the summer. However, it was unable to penetrate the permanently frozen ground (permafrost). Unable to enter the natural pores and fractures in the chalk rock, the water was forced to flow over the surface, forming river channels and allowing valleys to be eroded rapidly into the chalk. When the climate warmed and the permanently frozen ground thawed the normal drainage system of the chalk resumed. With most water percolating beneath the surface once again, the newly formed valleys were left dry.
A handful of Chiltern valleys, including Hughenden Valley, have been cut so deeply into the chalk hills that groundwater ‘leaks’ out of the rock at springs to form small chalk streams. Chalk is an aquifer, so it is able to soak up and hold huge volumes of water. Rainwater moves down through the thin soil into pores, along bedding planes and through cracks (fissures), often taking several months before it re-emerges at the surface at low points in valleys to form the springs that feed the chalk streams.
Since groundwater levels in the chalk rock vary according to rainfall and season, chalk streams are naturally intermittent in their flow. Sometimes, if there is insufficient winter rain, the streams will dry up completely for many months.
Streams and winterbournes
During a normal winter, when rainfall is heavy and able to percolate through the chalk, the aquifer will be topped-up. The head of the stream may move 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 valley dry. This section is called a winterbourne because it usually flows after the winter rains. Many winterbourne streams have their own special wildlife which is adapted to cope with intermittent flows.
The time between a period of rainfall and a stream rising in response to the rain is called the lag-time. For most normal small rivers and streams, the lag-time is usually a matter of hours. However, for chalk streams, the long, slow journey of the rainwater through the chalk rock can extend the lag-time after a period of heavy winter rain to weeks and often months. This can lead to surprising patterns in which a stream remains dry during a wet winter, only to start flowing months later in the spring or summer.
Falling water table
If there is less winter rain than expected, the water table can drop further and the stream bed will remain dry for longer. The abstraction of groundwater from the chalk aquifer through boreholes, to meet the growing demand for domestic drinking water, has artificially lowered the water table further, leading to some chalk streams flowing less frequently, even in the winter months.
Chalk streams are globally rare habitats confined to North West Europe and to the UK in particular. Of the 200 or so chalk streams in the world, over 160 are found in England. Chalk streams are important habitats for wildlife and support a broad range of plants and animals. They are home to some of our most threatened plants and animals, such as the water vole and the brown trout.
The Hughenden stream
The Hughenden stream rises from springs in the Hughenden Valley and flows 3.5 km (2.2 miles) through Hughenden Park. It then heads through culverts to join the River Wye in the centre of High Wycombe. The stream is classified as a ‘misfit’ stream as it is currently far too small to have eroded the Hughenden Valley, although small amounts of chalk are still removed in solution by the stream. Insoluble sand, silt and flinty gravels are deposited in the valley bottom by the chalk streams. These deposits are referred to as alluvium or fluvial deposits.
Weirs and trout ponds
During the Disraeli era, the chalk stream was modified by the addition of weirs to provide pools for trout fishing. The pools look attractive to many people, and they provide homes for water birds such as ducks and moorhens, however, they cause the flow of the water to slow, which causes the pools to steadily silt up, changing the ecology of the stream. Some people have argued that the weirs should be removed, returning the stream to its natural state, meandering through the grassland without banks. What do you think?