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Chalk streams are rivers that rise from springs in landscapes with chalk bedrock. 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. The beds of the rivers are generally composed of clean, compacted gravel and flints, which are good spawning areas for Salmonidae fish species.
Since they are fed primarily by aquifers, the flow rate, mineral content and temperature range of chalk streams exhibit less seasonal variation than other rivers. They are mildly alkaline and contain high levels of nitrate, phosphate, potassium and silicate. In addition to algae and diatoms, the streams provide a suitable habitat for macrophytes (including water crowfoot) and oxygen levels are generally supportive of coarse fish populations.
Of the 210 rivers classified as chalk streams globally, 160 are in England.
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. 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 carbonate ions. The surface water of chalk streams is commonly described as "gin clear". The channel bed consists of angular flint gravel 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.
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), Atlantic salmon (Salmo salar) 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. 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 considered too large for the term stream. The Somme in northern France is a chalk stream on a larger scale.
- River Avon and its tributaries including the
- River Frome
- River Itchen and its small tributaries
- River Kennet
- River Lambourn
- Letcombe Brook
- River Meon
- River Piddle
- River Test and its tributaries
- River Ash
- Barton Springs
- River Beane
- River Bulbourne
- River Chess
- River Gade
- Hambleden Brook
- Hughenden Stream
- River Lea
- River Mimram
- River Misbourne
- River Pang
- River Quin
- River Rib
- River Stort
- River Ver
Chalk streams of the Yorkshire Wolds:
- Driffield Beck, which is a tributary of the River Hull
- Gypsey Race, the most northerly chalk stream in Europe, runs east to the sea at Bridlington
- Settrington Beck, which is a tributary of the River Derwent
Chalk streams of Kent:
Chalk streams of Norfolk:
- Weybourne Beck
Chalk streams of Suffolk:
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- 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.
- Pearce, Fred (24 July 2014). "The threat to chalk streams, our unique contribution to global ecology". The Guardian. London. Retrieved 19 August 2020.
- 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. S2CID 129120488.
- "The Lincolnshire chalk streams project". Lincolnshire Wolds Area of Outstanding Natural Beauty. Archived from the original on 2013-07-27. Retrieved 30 March 2015.
- "Rivers". Norfolk Rivers Trust. Archived from the original on 12 June 2018. Retrieved 7 June 2018.