As commonplace as they may seem to us in the south of England, chalk streams are rare and special – so much so that they are sometimes compared to the Okavango Delta, the Great Barrier Reef or even the rainforests. There are only 210 of them recorded in the world and 170 of those are in England, with most of the remainder in northern France. Their nutrient-rich waters are an important habitat for a wide range of plant, insect, fish and other species – but they are at risk.

Any river or stream you see in the area around Fairfield will have come from a chalk source. The Pix Brook, our closest, rises in Letchworth, passes through Norton Common and Standalone Farm, runs along the back of our favourite sewage works, goes under the A507 to Stotfold, then west through the fields behind Etonbury Academy to join one of the Hitchin chalk streams, the Hiz, north of Arlesey. The Ivel, which the Hiz flows into at Henlow Park, is the most important chalk stream in this area; it rises in Baldock and makes its 25-kilometre journey to join the Great Ouse near Tempsford, passing through the Stotfold Mill nature reserve along the way.

Chalk is a highly porous and permeable rock and rain falling onto it percolates directly into the ground. This means that rivers in areas dominated by chalk bedrock receive very little surface runoff. Instead, the chalk formation acts as both a sponge and a giant filter. The rock regulates the amount of water flowing through it – this is why many chalk streams in the UK have stable flow patterns which vary only slightly over the seasons and the years.

But the water doesn’t just pass through the rock. Chalk is soluble in rainwater because rain is slightly acidic, and so the water, when it eventually emerges lower down the slopes as springs, is considered mineral rich due to its dissolved calcium and carbonate ions. The surface water of chalk streams is often described as ‘gin clear’ thanks to the absence of silt – which has been filtered out by the percolation process. The bed of a healthy chalk stream consists of small angular pieces of flint gravel derived from the flint deposits embedded in the chalk geology. The temperature of the spring water is well regulated, rarely deviating from 10 degrees centigrade – which can be seen on winter mornings when water vapour from the relatively warm stream condenses in the cold air above it to form fog. All of these conditions are ideal for plant and animal life.

A well-managed chalk stream should have an abundance of insects providing food for fish, and vegetated banks in which they can shelter from predators. It should also support water voles, kingfishers, water shrew, dragonflies and white tailed crayfish. These water conditions also happen to be ideal for growing watercress. The Watercress Railway Line in Hampshire is so named because it was used for transporting watercress to London from local chalk streams.

A few English chalk streams have been extensively managed for generations – usually with a view to producing the best conditions for fly fishing. The rivers Test, Itchen, Kennet and Wylye in the counties of Hampshire, Berkshire and Wiltshire are considered to be some the greatest trout and grayling rivers in the world. They are stocked with fish, carefully managed, and cost a fortune to fish. They also look very good in photos.

Despite the obvious attractions of well managed chalk streams, only 17 per cent of those in England are considered to be in good condition. The reasons for this are easily defined, but not easily cured.

The first problem is that over extraction of water has dramatically reduced flow. Almost half of the water supplied in the south east of England is drawn from the underground chalk aquifer (which feeds the springs), and the rest comes from the rivers themselves. Some chalk streams have dried up almost entirely, destroying the whole habitat, whilst others have lost so much ground storage water that the springs have moved miles downstream, leaving dry river beds behind. The reduction in flow also allows a buildup of silt on the bed of the river, chocking off the gravel bed. The main reason for increased water extraction is the large rise in population in the last 50 years – but since research shows that a third of the water that we take from our rivers is wasted, this is a problem we can all do something about.

Secondly, farmers are under pressure to maximise crop yields and increase the size of animal herds in order to remain profitable. Current farming practices result in large quantities of nitrates, phosphates and slurry leaking into rivers. More intensive farming also results in the erosion of topsoil, which then finishes up as silt in our rivers. Farmers are now well aware of the problems they are contributing to – and most want to protect the environment – but the solutions are expensive and there is inadequate financial help from government.

Finally, water treatment and sewage works are responsible for high levels of phosphorus reentering the rivers after treatment. It is difficult and expensive to remove phosphates from water and this has not been given a high priority because they are not considered to be harmful to human health.

The good news is that there are many organisations working to improve the situation. The installation of water meters in houses has achieved a drop in demand of 16 per cent over the past five years. Water treatment works are being modernised to improve the removal of phosphorus. The World Wildlife Fund (WWF), the Wildlife Trust organisations and the Fly Fishing Conservancy, plus at least a dozen other large and small organisations, all have programmes to improve river quality. In the main it is not lack of funding but lack of coordination that has so far failed to deliver the intended result. The actions taken over the next five years are critical if we are to restore our unique waterways.

Although ‘gin clear’ might be a stretch, even the Pix Brook is relatively silt-free in places. The banks are well vegetated and audibly hum with insect life at this time of year – the dragonfly above was photographed in the Etonbury Woods, right beside the brook – and it’s not too difficult to spot fish in the clear water. We did not see water voles, kingfishers, water shrew or crayfish, but if you do, please let us know!