Yesterday morning the River Kennet had changed from its normal ‘gin clear’ chalk stream flow to a cloudy brown ‘splurge’. With all the recent issues regarding sewage discharge, many feared the worst. But thankfully not.
ARK were quickly on the case, alerting the Environment Agency and Thames Water. The concern was that this could be a malfunction at Fyfield Sewage treatment works, but the live Thames Water map (see right) suggested everything was working fine. The Environment Agency duty officer came out to test for river pollution and found the levels of ammonia and dissolved oxygen usually associated with pollution incidents were healthy, but the suspended solid load in the river was huge. ARK members then started to track the source of the dirty water up the river to solve the mystery.
After conducting the necessary detective work ARK were able to explain why the Kennet became so discoloured so quickly. The river Kennet is a ‘winterbourne’, which means that its flow depends on the level of groundwater: the river gets longer when the groundwater is high (usually in winter), and shrinks when the groundwater is low (usually in summer).
Charlotte Hitchmough, Director of ARK explains: “When the upper reaches of the river begin to flow we talk about ‘the springs breaking’. The time this happens varies from year to year, most often it happens between November and February, when the winter rains have begun to refill the natural aquifer. This year it was yesterday. The graph (left) shows the groundwater level on the downs at Rockley over the past 5 years. The coloured bands show a rolling average, and the black line is the actual level. We are definitely having a wet year, with groundwater well above average for autumn, which is why the springs broke so early.
“So, why was the flow so full of mud?
“In a natural winterbourne, the bed of the river has a nice stony bottom and the river vegetation has been allowed to gently die back to be replaced with wild grasses and flowers more associated with land. When the flows return as groundwater rises the protection provided by the gravel and plants holds the river bed and banks together and the water runs clear.
“Some of the upper reaches of the winterbournes are managed as ditches, using diggers to regularly clear them out, removing all the river’s natural protection and leaving bare mud and soil, which is easily washed downstream when the springs come back. (see pic – right). This wave of silt is hugely problematic. It clogs up the downstream clean gravels, which in a few months’ time will be full of wild fish nests. Each nest relies on the gaps between the gravel to allow oxygenated water to flush through, keeping the fish eggs alive. A muddy river bottom causes the eggs to suffocate, reducing the wild fish population in the river. The upper reaches of the winterbourne are magical habitats in themselves, supporting a wider range of species than the main river, because they function as both wet and dry environments through the year. As a community we need to restore them to their more natural state and manage them more sympathetically. This will benefit both the winterbourne and the river downstream.”