What is a Chalk Stream?
A chalk river or stream is watercourse which flows across or is influenced by chalk bedrock. They are usually fed by underground or seasonal springs and often have 'winterbourne' stretches in their headwaters which run dry in late summer.
Why are Chalk rivers and streams important?
England has most of the chalk rivers in Europe. There are only around 35 chalk rivers between 20 to 90 km long in the whole of the UK. Chalk geology is rare worldwide. The Sussex chalk rivers and streams are therefore of global importance.
All chalk rivers are fed from groundwater aquifers which means they have clean, clear water at relatively stable water temperatures. These unique conditions along with the chalk bedrock, support an unusual diversity of wildlife including important fish populations and many specialist insect species.
What is the difference between a chalk river and a chalk stream?
A description of Chalk rivers can be found in ‘The State of England’s Chalk Rivers’ (EA & EN; 2004). Watercourses are considered to be streams when they are no further than 5 km from their spring source, and no greater than 5 metres wide unless they have been artificially widened.
Sussex chalk streams
South Downs chalk streams are different from classic chalk rivers such as Hampshire’s River Itchen. Each Sussex chalk stream has its source high in the hills, making the gradient of the streams much steeper. This sets the South Downs chalk streams apart from other streams in the UK.
Sussex chalk streams often occur in small, natural gulleys which are more wooded than most other UK chalk rivers or streams. This results in unusual features including:
- mini chalk waterfalls which form when chalk water upwellings ‘calcify’ in the air
- dense shade which means that there is naturally less wetland plant cover
- chalk river plants like Water crowfoot are often absent – which is more natural
- stream channels are diverse because of natural flow restrictions such as tree roots
- woody debris is common in the channel and it positively influences the stream flow
- the substrate (stream bed) is less frequently made up of flints and mobile gravels
Where are the Sussex chalk streams?
In Sussex we have three designated chalk rivers – The Ems and Lavant in West Sussex and the Lewes Winterbourne in East Sussex. We also have over 140 km of chalk streams spread throughout the South Downs National Park and Chichester Coastal Plain. Many of these streams have natural chalk stream characteristics better than the highly prized chalk rivers in the UK.
Chalk streams can be winterbourne (seasonally flowing) or perennial (permanently flowing) depending on the seasonal changes in their groundwater sources. Some of the characteristic plants you might expect to find in chalk Winterbournes are:
water-cress (Rorippa nasturtium-aquaticum), fool’s water-cress (Apium nodiflorum), water speedwell (Veronica anagallis-aquatica/catenata) and sweet-grass (Glyceria spp.)
Some characteristic plants you would expect to find in Perennial chalk streams are:
blunt-fruited water-starwort (Callitriche obtusangula), lesser water-parsnip (Berula erecta), brook water crowfoot (Ranunculus penicillatus subsp. pseudofluitans). Catabrosa aquatica (whorl-grass) is also associated with shallow, trampled, silty edges of perennial chalk streams. Hildenbrandia rivularis, a red encrusting alga, the moss Cratoneuron filicinum, and the liverwort Pellia endiviifolia are a sign of strong springs that rarely if ever fail.
Problems affecting Sussex chalk streams
- Ditching, straightening & removal of natural features
- Weirs, concrete training walls and man-made obstructions to flow
- Urbanisation and 'gardenisation'
- Non native invasive species
- Water abstraction
For more information on Sussex chalk streams, click here for our advice leaflet.