"Chalk river" redirects here. For the village in Ontario, Canada, see Chalk River.
Chalk streams are rivers that rise from springs in landscapes with chalkbedrock. Since chalk is permeable, water percolates easily through the ground to the water table and chalk streams therefore receive little surface runoff. As a result, the water in the streams contains little organic matter and sediment and is generally very clear.
[c]
Of the 210 rivers classified as chalk streams globally, 160 are in England.[5]
A list of chalk streams in England gives a total of 224.[g]
Geology and hydrology
Chalk is a highly porous and permeable rock, and rain falling onto chalk topography percolates directly into the ground, where the chalk layer 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 (50 °F). On cold winter mornings, water vapour from the relatively warm stream condenses in the cold air above to form fog.
Chalk is slightly soluble in rainwater because rain is naturally slightly acidic.[7] The products of chalk weathering are dissolved in rainwater and are transported in stream flow. Chalk streams transport little suspended material (unlike most rivers), but are considered "mineral-rich" due to the dissolved calcium and carbonateions. 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 (Salmo trutta), and grayling (Thymallus thymallus). In addition to these there are also considerable numbers of stocked brown trout and stocked rainbow trout (Oncorhynchus mykiss). The rich invertebrate 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.[h]
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 stream Decline
The over-abstracting of chalk streams by private water companies in the UK has caused the chalk streams to dry up. This has caused ecological damage and stagnant sewage that flows through the rivers and increasing phosphate levels.[9]
Chalk streams of England
Look up winterbourne in Wiktionary, the free dictionary.
Terminology
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 considered too large for the term stream. The Somme in northern France is a chalk stream on a larger scale.
Winterbournes[i] are known by different names depending on region:
^ WWF-UK (2014). . . A " chalk stream " is broadly defined as one that derives most of its flow from chalk-fed groundwater, and it exhibits – in varying degrees depending on the particular geology of a given valley – the 'classic' chalk stream characteristics of alkaline, crystal-clear water, flowing consistently and equably over clean gravelbeds.[a][b]
^State of England's Chalk Rivers (2004), published by:
^Winterbournes are rivers that only flow when groundwater levels are high.
^ WWF-UK (2014) See PDF page 9, actual page 14.
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^ WWF-UK (2014). . .The report
[d] showed 161 chalk rivers in varying degrees of health. . .Since then there has been a growing view that smaller chalk streams, chalk stream headwaters and winterbournes[e]
should also be recognised. . .224 chalk streams have been identified. . .[f]
^Flora Britannica ( Richard Mabey ). . .Water-cress grown commercially in beds has the advantage. . .of growing in water drawn directly from underground springs or bore-holes. . .[8]
^ abcdBerrie AD (1992). "The chalk-stream environment". Hydrobiologia. 248: 3–9. doi:10.1007/BF00008881.
^Casey H (1969). "The chemical composition of some southern English chalk streams and its relation to discharge". Yearbook of the Association of River Authorities: 100–103.
^Dawson FH (1979). "Ranunculus calcareus and its role in lowland streams". Annual Reports of the Freshwater Biologists' Association. 47: 60–69.
^West G, Dumbleton MJ (1972). "Some observations on swallow holes and mines in the chalk". Quarterly Journal of Engineering Geology. 5 (1–2): 171–177. doi:10.1144/GSL.QJEG.1972.005.01.16. S2CID129120488.