Chalk stream

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The River Bourne at Winterbourne Gunner, a typical chalk stream

Commercial watercress production near Alresford in Hampshire

Chalk streams have characteristics which set them apart from watercourses associated with other rock types.

Aside from those with an interest in the geological and ecological disciplines, the term 'chalk stream' is most widely used among a small group of fly fishermen (who fish for trout on these rivers utilizing a specific type of artificial fly and their attendant techniques), as the ecology of the chalkstreams creates an especially entertaining variation on the general theme of fly fishing.

Chalk rock is porous and rain falling onto chalk hills soaks directly into the ground, where the chalk acts as an aquifer. The water is filtered through the chalk, re-emerging lower down the slope in springs. The chalk acts as a reservoir, regulating the amount of water supplied to the springs, so that its flow varies little day-to-day. The temperature of the water scarcely varies either, because the spring rarely deviates from 10ºC. On cold winter mornings, it can look as though steam is rising above the relatively warm river (see picture).

Chalk is soluble in rainwater because rainwater is naturally slightly acidic. Because the products of chalk weathering are dissolved in rainwater, chalk streams transport little suspended material (like most rivers) but are instead mineral rich. The water runs clear and the river bed is covered by angular flinty gravel, derived from flints found within the chalk itself.

All these characteristics of chalk streams — consistent temperature, consistent flow, clarity of water, lack of sandy debris — contribute to their very particular ecology.

The chalk streams have been intensively managed for many generations and, in the twentieth and twenty first 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 brown trout and grayling as well as stocked brown trout and stocked rainbow trout. The rich insect life and clear shallow water make the rivers 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 chalkstreams.

Chalk stream is a term generally applied to all 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.

See Somme River for a hydrological view of a chalk stream on a larger scale.

Chalk Streams of the Southern England Chalk Formation in Berkshire, Hampshire, Wiltshire, and Dorset