Chalk Streams
Most of Hertfordshire’s rivers originate as chalk streams in the valleys of the Chilterns. They fall into two catchment areas – the Lea to the south-east, which eventually reaches the Thames via east London and the Hackney Marshes, and the Colne draining towards the west and meeting the Thames near Staines.
Chalk streams are part of a globally rare and internationally important habitat, which is found only in some of the chalklands of northwest Europe and on limestone outcrops in part of New Zealand. The majority are found in the UK and we have about 10% in Hertfordshire but they have no protection and face a number of threats to their survival including over-abstraction, water pollution and global warming.
Chalk streams are the UK’s equivalent of tropical rainforests. They are important for wildlife including species like the water vole, brown trout, grayling and bullhead. In Hertfordshire, they are also of historical importance, having been used in the past for numerous water mills and watercress beds.
As they are fed mainly by chalk groundwater, they are clearer and more uniform in temperature than other rivers. However, they rely strongly on winter rainfall to maintain summer flows. Rivers whose flows are affected in this way are known as winterbournes where seasonal variations in the water table result in flow in winter and early spring but no flow in summer and autumn.
The most famous intermittent chalk stream in the county is the Bourne Gutter or Hertfordshire Bourne. When it flows, it usually rises south of Berkhamsted at various points between Hockeridge Bottom and Bottom Farm.
Flow rates are also affected by groundwater abstraction to supply us with water, the construction of weirs and ponds for fishing, river straightening, dredging and flood relief works. Hertfordshire’s chalk streams have also been heavily modified as they pass through town centres.
Almost all of Hertfordshire’s water supply comes from the Chalk aquifer. This is the same aquifer that feeds water into the chalk streams so when too much is extracted for our needs, the stream flow slows and the rivers suffer and start to dry out.
The New River, opened in 1613 to supply clean drinking water for London, draws its water from the Lea and various chalk springs in the Ware region. Its northern part is still an important link in London’s water supply.
Global warming has led to more heatwaves which also dry out streams. In the 2019 drought 67% of the chalk streams in the Chilterns dried out which in turn led to the death of the wildlife that depended on them.
Since 2015, Affinity Water and the Environmental Agency have been working together with their partners to protect and revitalise the local rivers Ver, Misbourne, Gade, Mimram, Upper Lea and Beane. For further details see Revitalising Chalk Rivers and Affinity Water’s own River Restoration page, which gives details of various projects.
The Catchment Based Approach (CaBA) is an organisation covering all 100+ river catchments in England. It describes itself as community-led, and works in partnership with government, local authorities, water companies, businesses etc. to maximise the natural value of our environment. Its Chalk Stream Restoration Group (CSRG) recently published a new Chalk Stream Strategy, setting the future direction needed to protect and enhance England’s chalk streams. This hugely informative document can be downloaded from their web page.
More detailed information on our chalk streams
The River Colne Catchment Action Network (a member of CaBA) includes action plans for the Ver, Gade, Chess, Lisborne and Colne.
The River Lea Catchment Partnership (another member of CaBA) gives similar information for the rivers of eastern Herts – Lea, Ash Beane, Mimram, Quin, Rib and Stort.
The Chiltern Chalk Streams Project covers those streams rising in the Chilterns, including some further to the west.
The Herts and Middlesex Wildlife Trust’s Living Rivers initiative includes education and environmental advice as well as river restoration projects.
Some organisations for specific chalk streams and rivers:
River Chess Association
River Beane Restoration Association
The RevIvel Association (River Ivel)
Ver Valley Society
Boxmoor Trust (River Bulbourne)
