The term 'chalk stream' is generally applied to all watercourses originating from chalk hills, including rivers as well as streams and also winterbournes, which are streams that can dry out in summer months.
Chalk streams have many characteristics which set them apart from watercourses associated with other rock types. The main ones being consistent temperature, consistent flow, clarity of water, and lack of sandy debris.
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 10°C. On cold winter mornings, it can look as though steam is rising above the relatively warm river.
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 (unlike 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 contribute to their very particular ecology. The rich insect life and clear shallow water make the rivers particularly suited to fly fishing and in particular provide ideal conditions for the dry fly enthusiast, especially in the May fly season.
River Avon at Salisbury
The chalk streams hold a good number of wild brown trout and grayling as well as stocked brown trout and the odd rainbow trout. Some of the larger rivers also have salmon and other species in their lower reaches.