Wiki: Chalk stream - upcScavenger

Chalk streams are rivers that rise from springs in landscapes with chalk bedrock. Since chalk is permeable, water easily percolates 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 , which provide good spawning grounds for Salmonidae fish species. Since they are primarily fed by , the flow rate, mineral content and temperature range of chalk streams shows less seasonal variation than other rivers. They are mildly and contain high levels of nitrate, phosphate, potassium and silicate. In addition to algae and , the streams provide a suitable habitat for aquatic plants (including Ranunculus) and oxygen levels are generally supportive of coarse fishing populations.

Of the 210 rivers classified as chalk streams globally, 160 are in England.

A list of chalk streams in England gives a total of 224.

Geology and hydrology

Chalk is a highly Porosity and permeable rock, and rain falling onto chalk topography directly into the ground, where the chalk layer acts as an aquifer. The groundwater flows through the chalk bedrock and reappears lower down the slope in springs. The chalk acts as a temporary reservoir by regulating the amount of water supplied to the springs. As a result, 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, rarely deviating from . 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 because rain is naturally slightly acidic. The products of chalk weathering are dissolved in rainwater and are transported in streams. Chalk streams transport little suspended material (unlike most rivers), but are considered "Mineral water-rich" due to the dissolved calcium and carbonate . 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.

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 in particular 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 characteristically clear, shallow waters make chalk rivers and streams particularly suitable for 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 stream decline

Excessive abstraction of chalk streams by private water companies in the UK has led to the drying up of chalk streams. This has caused ecological damage, stagnant sewage flowing through the rivers and rising phosphate levels.

Chalk streams of England

Terminology

Although chalk streams are generally defined as watercourses originating in chalk hills, including winterbournes, streams, and rivers, the term chalk stream is also used for larger rivers that would normally be considered too large for the term stream. The Somme in northern France is a chalk stream on a larger scale.

Winterbournes are known by different names depending on region:

– Wiltshire and Dorset.
– Kent.
– Hampshire.
Gypsey – Yorkshire Wolds.

Regions (England)

+Distribution of England's chalk streams (including discharge) !Region !Count !Discharge !Count
Yorkshire Wolds	35	North Sea	1
Yorkshire Wolds	35	Humber	34
Eastern Wolds (Lincolnshire)	19	Humber	8
		North Sea	9
		The Wash	2
East Anglia	58	North Sea	4
		The Wash	4
		The Broads	11
		River Great Ouse	39
Thames	47	River Thames and Thames Estuary	41
Thames	47	English Channel	6
Wessex	64	The Solent	17
		Christchurch Harbour	29
		Poole Harbour	14
		English Channel	4
Isle of Wight	1	English Channel	1

List (England)

Southern England

Chalk streams of the Southern England Chalk Formation in Berkshire, Hampshire, Wiltshire, Dorset and the Isle of Wight:

River Avon, Hampshire and its tributaries including the

River Frome
River Itchen and its small tributaries
River Kennet
River Lambourn
River Loddon
Letcombe Brook
River Meon
River Piddle
River Test and its tributaries
River Caul Bourne

Chalk streams of the Southern England Chalk Formation in the Chiltern Hills, Hertfordshire and Surrey (tributaries of the River Thames, River Lea and River Colne):

Yorkshire Wolds

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