Chalk streams are streams that flow through chalk hills towards the sea. They are typically wide and shallow, and due to the filtering effect of the chalk their waters are alkaline and very clear. Chalk streams are popular with fly fishermen who fish for trout on these rivers.
Geology and hydrology
is a highly porous and permeable rock and rain falling onto chalk topography percolates directly into the ground, where the chalk acts as an aquifer. The groundwater flows through the chalk bedrock, re-emerging lower down the slope in springs. The chalk acts as a temporary reservoir by regulating the amount of water supplied to the springs. This is why many chalk streams in the UK have stable flow regimes that vary only slightly over time. The temperature of the emerging surface water is fairly stable and rarely deviates from 10 °C. On cold winter mornings, water vapour from the relatively warm stream condenses in the cold air above to form fog. Chalk is soluble in rainwater because rain is naturally slightly acidic. The products of chalk weathering are dissolved in rainwater and are transported in stream flow. Chalk streams transport little suspended material, but are considered "mineral-rich" due to the dissolved calcium and carbonate ions. The surface water of chalk streams is commonly described as "gin clear". The channel bed consists of angular flintgravel derived from the natural flint deposits found embedded within the chalk geology that contains relatively low amounts of clay and silt deposits. The unique characteristics of chalk stream ecology are due to stable temperature and flow regimes combined with highly transparent water and lack of sand grade sediment particles.
Ecology
The chalk streams have been intensively managed for many generations. In the 20th and 21st centuries, much of that management has been aimed at producing the best conditions for fly fishing, and most specifically, dry fly fishing. The chalk streams hold a good number of wild salmonid fish species such as the brown trout, Atlantic salmon and grayling. In addition to these there are also considerable numbers of stocked brown trout and stocked rainbow trout. The richinvertebrate life and characteristic transparent shallow water make chalk rivers and streams particularly suited to fly fishing. Many of the chalk stream springs are also used as sites for watercress production, due to the constant temperature and clean, alkaline, mineral-rich spring water. The Mid Hants Watercress Railway in Hampshire is so named on account of its use for transporting watercress to London from local chalk streams. A number of the chalk aquifers and associated groundwater sources related to chalk streams and rivers are used for water abstraction by local and national water utility companies.
Chalk streams of England
Although chalk streams are generally watercourses originating from chalk hills, including winterbournes, streams and rivers, the term chalk stream is used even for larger rivers, which would normally be too large for the term stream. The Somme in Northern France is a chalk stream on a larger scale. Chalk streams of the Southern England Chalk Formation in Berkshire, Hampshire, Wiltshire, and Dorset:
