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Hertfordshire's crystal rivers

PUBLISHED: 13:39 03 May 2016 | UPDATED: 13:51 03 May 2016

The river Mimram at Tewinbury (Tim Hill)

The river Mimram at Tewinbury (Tim Hill)

Tim Hill

Hertfordshire is home to around 10 per cent of the world's chalk rivers. David Johnson, Herts and Middlesex Wildlife Trust living rivers officer, takes a clear look into these rare water courses

Mayfly and other riverfly population success is a good indication of river's health (Jon Hawkins, Surrey Hills Photography)Mayfly and other riverfly population success is a good indication of river's health (Jon Hawkins, Surrey Hills Photography)

It is estimated there are fewer than 200 chalk rivers in the world. The vast majority of these are found in southern England. These water courses are formed in areas of underlying chalk geology.

Around 80 per cent of the water in a chalk stream comes from rainfall that has slowly percolated through the ground and into the layer of chalk beneath it before re-emerging at groundwater springs. This filtration process makes the stream characteristically clear and fairly constant in temperature, typically about 10C. It is also mineral-rich and alkaline.

Chalk rivers are beautiful and diverse ecosystems, characterised by varied plant and invertebrate communities. They tend to be shallow and moderately fast and this keeps the water well aerated, prevents accumulations of silt and brings a continual supply of nutrients and organic food to river plants and animals. Since groundwater levels in the chalk vary depending on rainfall and season, chalk streams tend to be intermittent in their flow. In some cases, the upper section of a chalk stream may dry up in summer when the water table falls due to a lack of rainfall percolating into the chalk aquifer. This upper part of the river is then known as a winterbourne.

Vital ecosystem

Kingfisher (Andrew Howe)Kingfisher (Andrew Howe)

Chalk streams are an internationally important habitat for wildlife. Characteristic aquatic plants of Hertfordshire’s chalk rivers include water crowfoot and water starwort. Where water quality is high they may also support a diverse community of aquatic invertebrates including fish such as the brown trout and bullhead. Water voles can sometimes be spotted feeding at the water’s edge and kingfishers often hunt in the crystal clear water.

Under threat

Sadly, chalk rivers face numerous threats. Over-abstraction is a particular problem in Hertfordshire and most rivers suffer from low flows. The groundwater that feeds chalk streams is a cheap and clean source of drinking water for local populations. Pollution is another problem facing chalk rivers – road and agricultural run-off and industrial and sewage effluent discharge can all cause serious problems for this delicate habitat.

River habitats may also be threatened by the loss or degradation of bankside habitats and by the spread of introduced and invasive species like Himalayan balsam on riverbanks and the North American signal crayfish. This species has an associated fungal disease that has decimated populations of the native white-clawed crayfish in the UK.

Mayflies – a heath check

The warm days of May and June bring with them an exciting spectacle along the banks of rivers throughout the country – the hatching of mayfly nymphs into mayflies. Mayflies live most of their lives as nymphs on the bed of rivers, and emerge as short-lived adult flies to breed. As well as being an important part of a river’s ecosystem, the diminutive mayfly, along with its other riverfly cousins the stoneflies and caddisflies, are a vital indicator of the health of a river. Riverfly populations are affected by factors such as water quality, habitat diversity, water level and flow rate. As such, their populations are a good indicator of a river’s health. The Riverfly Partnership, a national monitoring project, is a ‘neighbourhood watch’ for rivers which helps to identify potential habitat problems. Herts and Middlesex’ Wildlidfe Trust’s Hertfordshire and Chiltern’s Riverfly Hub is part of this process. Last Novemeber its work was recognised by the Herts Campaign to Protect Rural England Rural Living Awards, sponsored by Hertfordshire Life.

Chalk streams can be enjoyed at these HMWT reserves: Willowmead on the river Mimram, Hertford, Waterford Heath on the river Beane, Hertford, Lemsford Springs on the river Lea, Welwyn GC

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