Chalk streams are rivers that rise from springs in landscapes withchalkbedrock. Since chalk ispermeable, water easily percolates through the ground to thewater table and chalk streams therefore receive littlesurface runoff. As a result, the water in the streams contains littleorganic matter andsediment and is generally very clear.[c]
The beds of the rivers are generally composed of clean, compactedgravel andflints, which provide good spawning grounds forSalmonidae fish species.[3]Since they are primarily fed byaquifers, the flow rate, mineral content and temperature range of chalk streams shows less seasonal variation than other rivers.[3][4] They are mildlyalkaline[5] and contain high levels ofnitrate,phosphate,potassium andsilicate.[3] In addition toalgae anddiatoms, the streams provide a suitable habitat formacrophytes (includingwater crowfoot)[6] andoxygen levels are generally supportive ofcoarse fish populations.[3]
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]
Chalk is a highlyporous andpermeable rock, andrain falling onto chalk topographypercolates directly into the ground, where the chalk layer acts as anaquifer. Thegroundwater flows through the chalk bedrock and reappears lower down the slope insprings. The chalk acts as a temporaryreservoir 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 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 inrainwater because rain is naturally slightlyacidic.[7] The products of chalkweathering are dissolved in rainwater and are transported in streams. Chalk streams transport littlesuspended material (unlike most rivers), but are considered "mineral-rich" due to the dissolved calcium andcarbonateions. The surface water of chalk streams is commonly described as "gin clear". The channel bed consists of angularflintgravel 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 ofsand 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 forfly fishing, and in particulardry fly fishing. The chalk streams hold a good number of wild salmonid fish species such as thebrown trout (Salmo trutta), andgrayling (Thymallus thymallus). In addition to these there are also considerable numbers of stocked brown trout and stockedrainbow trout (Oncorhynchus mykiss). The richinvertebrate 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 forwatercress production, due to the constant temperature and clean,alkaline, mineral-rich spring water.[h]TheMid Hants Watercress Railway inHampshire 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 forwater abstraction by local and national water utility companies.
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.[9]
Although chalk streams are generally defined as watercourses originating in chalk hills, includingwinterbournes, streams, and rivers, the termchalk stream is also used for larger rivers that would normally be considered too large for the termstream. TheSomme in northern France is a chalk stream on a larger scale.
Winterbournes[i] are known by different names depending on region:
| Region | Count[n] | Discharge | Count |
|---|---|---|---|
| Eastern Wolds (Yorkshire) | 35 | North Sea | 1 |
| 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 andThames Estuary | 41 |
| English Channel | 6 | ||
| Wessex | 64 | The Solent | 17 |
| Christchurch Harbour | 29 | ||
| Poole Harbour | 14 | ||
| English Channel | 4 | ||
| Isle of Wight | 1 | English Channel | 1 |
| Total | 224 | 224 |
Chalk streams of theSouthern England Chalk Formation inBerkshire,Hampshire,Wiltshire,Dorset and theIsle of Wight:
Chalk streams of the Southern England Chalk Formation in theChiltern Hills,Hertfordshire andSurrey (tributaries of theRiver Thames,River Lea andRiver Colne):
Chalk streams of theYorkshire Wolds:
Chalk streams ofLincolnshire:[10]There are several chalk streams in theLincolnshire Wolds including
Chalk streams of Kent:
Chalk streams of Norfolk:[11]
Chalk streams of Suffolk: