Chalk streams are globally rare and 85% of them are to be found in England. Chalk streams are fed from groundwater aquifers that store rainwater that has soaked through the chalk. As it passes through this filtering process the water becomes alkaline and rich in nutrients. It eventually emerges through springs at a constant temperature, year round. Chalk streams therefore start their journey with high water clarity and a good chemical quality. These qualities in turn generate the characteristic gravel river beds of chalk streams, all contributing to the creation of a special habitat that fosters an abundance of insects, such as scarce species of invertebrates and a range of mayfly and damselfly species. These provide food for a variety of fish, including brown trout, and small mammals such as water voles and water shrews.
Chalk streams in rural areas are typically fringed with lush bankside vegetation. The clear and high quality of the water readily supports a characteristic range of plants, such as the white-flowering water-crowfoot and watercress. These provide shelter for the fish from predators. The meandering nature of chalk streams also creates small silted pools on the stream edges in which fish can lay their eggs. The combined effect produces some of our most beautiful English and Chiltern rivers.
Where these chalk stream characteristics are conserved, or can be recreated, these special habitats support a rich range of biodiversity, both in the river and long its riverside corridor.
Few of our Chiltern Chalk streams flow continuously through rural countryside. Stretches of them inevitable pass through a range of different settlements. Over the years, most have been modified in various places for agricultural or industrial purposes, often creating barriers to fish movement. Further damage to chalk streams has been caused by over-abstraction of water from the rivers and aquifers for domestic and industrial water supply purposes. Whilst the excesses of abstraction over recent decades have been reduced, the ever increasing demand for domestic water remains a threat to these special, but fragile, rivers.
Another characteristic of chalk streams is that most of them have ‘winterbourne’ stretches at their headwaters, which can be dry for part of the year. This arises from the variation in rainfall over the seasons. During winters when there has been a lot of rainfall, the level of groundwater stored in the chalk aquifer is well topped up. The springs that feed the chalk stream, from fissures (cracks) in the chalk, then emerge at ground level higher than they do following particularly dry winters. Hence the source of the stream might appear to vary from year to year, depending on the amount of rainfall. Following dry seasons, the water table drops and the head of the stream moves down the river valley. The length of a chalk stream’s winterbourne section can also be affected by the extent of water abstraction.
Most of the Chiltern chalk streams now have partnerships, like Revive the Wye, and volunteer groups to help conserve and manage them. Revive the Wye is also very active in working with local councils and government agencies to ensure that no opportunity is lost through local planning processes to improve the quality and sense of place of the River Wye.
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