Header image cc Malcolm Brown

There are just 240 chalk streams in the entire world and in Hertfordshire we are fortunate to be the custodians of around 10% of these precious habitats. These unique river systems are so rare, and support some of our most endangered species – in habitat terms, they are the UK’s equivalent to tropical rainforests. 

On a mid-winter's day a river murmurs to Willows, which whisper in return as it glides by. Sunlight sparkles on gravel and vapour gives meadows a mystical atmosphere. A surprise awaits those who brave the river’s stone bed, a cool 10℃ feels warm compared to the surrounding air. For this is a chalk stream where in childhood we chase Minnows darting through miniature emerald forests in glinting gravelly brooks. To learn that these valleys were forged by the power of glacial meltwaters, this river born by rainfall seeping down through ancient sea beds once brimming with life stirs a deep-rooted sense of reverence for the nature of this landscape.

Life begins in the depths of the chalk aquifer where the oldest molluscs and phytoplankton in Britain have laid to rest for millions of years. As seasons progress springs bubble up and life gets to work. Disguised as small bundles of woody gravel, Caddisfly larvae move in to forage on algae while Mud Beetles move out to the river’s edges, both making use of their short-lived home in the winterbourne. Water-crowfoot returns to the winterbourne, threading the sky-blue river with a crown of delicate white flowers.

cc: Peter Tatton, River Mimram

Downstream the beautiful hair of Water Starwort graces the river all year round, allowing the river to weave and scour through sub-channels, filtering it to crystal clarity, revealing amber pebbles. Swathes of foliage between two worlds provide landing strips for pollinators unaware that below them Blackfly larvae attach themselves to stems and stones with threads to filter food in the watery depths. Glittering blue Banded Demoiselle damselflies chase across swaying metallic green carpets and along margins Green Tortoise Beetles mimic their namesake on fragrant Water Mint.

There is an occasional glimpse of ‘The Lady of the River’, the silvery ghost-like Grayling lays her eggs in shallow gravel nests, paying fleeting visits to the surface. Above, a wraithlike newcomer drifts along the valley, the long-legged Little Egret could be partial to a few young. Breast emblazoned with the setting sun, the Kingfisher makes its deadly dive. Sadly, the White-clawed Crayfish is absent from the river, largely due to the fungus carrying red-clawed Signal Crayfish.

A chalk stream is home to the most curious and ancient species. The ‘nine-eyed eel’ or endangered Brook Lamprey hoovers up debris for years before it constructs a gravel nest then starves to death as its blind larvae make home in marginal silt. Without adequate mouthparts Mayflies too meet their end within hours of emergence, perhaps their sole purpose is to perpetuate generations but abundance is essential for the health of this ecosystem - dragonflies survey river territories, Bullheads forage ferociously in gravel and, as summer ends, House Martins swoop in mid-air over wetlands to prepare for mighty migrations - all seek protein from diverse riverflies.

Humans have managed river systems for centuries and often to their detriment. Traditional mill streams have degraded chalk rivers by perching them and stealing gradient, creating slow silt laden sections separated from groundwater. The brutal encroachment of modern development and habitat degradation present yet more challenges for these rare rivers. Wash off from both urban and agricultural sources and storm discharge from sewage systems find their way into these mildly alkaline rivers therefore increasing pollution and acidity. Abstraction decreases seasonal flow so natural scouring and oxygenation are also reduced and silt deposition smothers precious gravel habitat. A less obvious consequence is an increase from the constant 10℃. All disrupt this precious habitat. It is autumn, springs ceased months ago but there is an air of expectation that winter rains will replenish the aquifer to recharge our rivers next spring. Tinged with concern we wait with bated breath for the rebirth of the winterbourne chalk stream.

Find out more about more about The Trust’s Living Rivers Project and the action that is being taken to protect these rare habitats on our Living Rivers web page. You can also see how we are fundraising to buy and protect Archers Green, a new nature reserve, which flanks the River Mimram and a fine stretch of chalk stream - learn more about the site and the appeal on our website.