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Chalk Rivers

Chalk Rivers are in the news as an All Party Parliamentary Group has recently been established to protect this rare type of river. Rare, because throughout the world there are only 180 chalk rivers. 85% of these are in England, mostly in Buckinghamshire, Hertfordshire, Essex and Cambridgeshire. The Rivers Mel, Rhee, Granta and Cam are examples of chalk rivers in Cambridgeshire. The River Mel flows through Melbourn and Meldreth before joining the River Cam. Its source is springs in the lake in Melbourn Bury. Whilst the Mel dries up occasionally in extremely dry periods, it must have flowed well in the past given that it powered two water mills, Sheene Mill in Melbourn and Topcliffe Mill in Meldreth.

Most rivers in impervious catchments are fed largely by water running off adjacent land. This is particularly so during periods of heavy rainfall, when rivers can overtop their banks and inundate the flood plain. This can happen following heavy rainfall in the summer, when the ground is hard, and in winter, when it is saturated or as snow melts. Other sources are road run-off and treated sewage effluent.

Chalk rivers, however, function differently as they are essentially fed by water issuing from springs. This water originates as rain falling on porous catchments, sinking into the underground chalk aquifers from where it eventually runs out of the ground weeks or months later.

As most rain falls in the winter the chalk aquifers generally begin to fill up late in the year as the ground becomes saturated. For this reason chalk streams usually flow well from around Christmas to April or May, depending on the extent of rainfall and state of the aquifers. Being spring-fed, flows in chalk streams are more uniform with only slow response to rainfall. Aquifers are large and can hold varying amounts of water. If they are full, springs high up will flow, but if depleted higher springs dry up and the source of the chalk river moves down the valley where springs at lower levels continue to issue.

In the winter of 2000/01, chalk aquifers around the Chiltern Hills were over-full and springs ran which had never been seen in living memory. Accordingly, the source of chalk rivers moved much higher up valleys than usual. Towns, villages and houses flooded and roads had to be closed. Conversely, on occasions during periods of extreme drought in the 1990s, the source of chalk rivers moved further downstream than previously recorded.

Water passing through aquifers filters out impurities so chalk river water is clear, pure, mineral rich and at a constantly cool temperature. These qualities are the reason that watercress farms are usually to be found along chalk rivers. The good water quality and gravel bed also benefit fish, particularly brown trout, as well as water vole, otters, white clawed crayfish, invertebrates and flora.

The main risk to the unique habitat provided by chalk rivers is lack of water, due to the depletion of aquifers caused by climate change and abstraction for water supply. The need for abstraction is increased by leakage from distribution mains, overuse by households and demand from new housing. Water companies are being pressured to reduce abstraction by the Environment Agency and leakage by OFWAT, but new housing is subject to Government policy.

You can help to protect chalk rivers by checking your water usage. Average use in England is about 140 litres per person per day and reducing usage to under 100 litres per person per day will help to maintain healthy flows in rivers and could save you money if you have a water meter. To calculate your usage, if metered, • add the amount of water you have used in cubic metres, shown on each of your last two 6-monthly bills, to give annual usage, • multiply annual usage by 1000 to give usage in litres, • divide usage by 365 to give daily usage in litres • divide daily usage by the number in your household to give litres per person per day. • If your usage is above 100 litres per person per day, the measures below will save water, • don’t water grass and buy water resistant plants • fit water butts to down pipes • water plants from water butts or using spent washing up water • buy water efficient dish washers and washing machines and always fill them before use • jet wash patios in wet rather than dry periods • wash cars during or after rain using a bucket of water from a water butt not a hose or pressure washer • fit water saving devices to showers, taps and toilet cisterns • take shorter showers • don’t run the tap whilst washing your teeth • keep a jug of cold drinking water in the fridge rather than running the tap until cold • So do your bit to ensure good flows in Mel-Bourn and under Cam-Bridge. David Telford