Cambridge environmental groups fight for rights of chalk streams to exist

The recent absence of water at Coldham’s Brook, one of the globally (200 in total) rare chalk streams providing water for the entire region, has shocked the community – and what happens next will be crucial for its long-term survival, say local environmental groups.

The brook’s return to life has been only partial. It isn’t flowing – it’s not going over obstacles – suggesting rain rather than the correct functioning of the complex underground water basin is at play.

The Cam Valley Forum, as per a recent newsletter, describe a chalk stream’s dynamics as “the relationship between geology, surface relief, and hydrology”.

“A spring,” the group explains, “occurs where the water table intersects the surface, towards which the ground water (under the water table) seeps laterally.”

It continues: “All of Cambridge Water’s supply is pumped from ground water which results in the lowering of the water table. Hydrostatic pressure is thus reduced and spring discharges decrease. The chalk streams suffer.”

The sheer relentlessness of this pumping – along with the knowledge that it will have to be increased to service new homes developments – is a cause for concern. And so is the scale of our consumption – Cambridge Water points out that we use 140 litres per day where our great-grandparents used 18 litres per day.

In 2019 Katie Thornburrow, currently executive councillor for planning policy and infrastructure at Cambridge City Council, said of water: “We have got to conserve as much as we can, we need to sort out leaks, we need to build as efficiently as possible, we need to use less, we need to really appreciate the water and biology and ecology we have got and preserve what we’ve got. My worry is if this continues next year the Cam is going to be dead. It will be absolutely terrible, it will be an ecological disaster.”

The Cam’s not dead, but overuse and drought have stretched the ecology of the water in the region to breaking point. Arguably the situation has now worsened.

Friends of the River Cam, the community organisation set up to protect the Cam, said: “Friends of the Cam are concerned about the condition of Coldham’s Brook, and the Cam river system more broadly. The Environment Agency (EA) continues to class the flow in the Cam system as ‘exceptionally low’ following below average rainfall for nearly eight months. On top of that, the water company has taken no action to conserve water levels in the aquifer supplying the Cam catchment.

“The EA estimates from its River Cam monitoring site that the river is ‘showing early signs of deterioration’. Even if some flow is regained when it rains, as it has in Coldham’s Brook, spasmodic flow is extremely damaging to river ecology.”

That spasmodic flow into the Cam was identified in a 2006 Cambridge City Council report titled ‘Cambridge City Strategic Flood Risk Assessment’, in which the authors state: “Coldham’s Brook, which flows as the higher of two parallel watercourses on Coldham’s Common, possess a significantly low gradient and suffers as a result from inconsistent flows.”

David Brooks, editor of the Cam Valley Forum newsletter, says that another factor is at play: a deliberate stymying of the flow. He told the Cambridge Independent: “In a nutshell the Coldham’s Brook loses ALL its water because it seeps down into the lower neighbouring Cambridge East Main Drain and never reaches the Cam as the Brook.

“This summer it looks much worse than usual because it has lost its supply of water from the Cherry Hinton Brook whose flow from the Chalk spring at Giant’s Grave, Cherry Hinton, has been reduced due to a combination of over-abstraction by Cambridge Water from the chalk aquifer and low rainfall.

“Fundamentally, the predicament of the Coldham’s Brook is caused by the 19th century measure to protect east Cambridge from being flooded.”

That measure was the digging of the Cambridge East Main Drain which runs parallel to Coldham’s Brook, between the brook and the Barnwell Road but at a lower level. The Drain is lower than the brook.

“​​In effect the Drain ‘captures’ the water of the Brook,” concludes David.

Cambridge Water was approached for comment.

The upshot of all this, says Ruth Hawksley, Wildlife Trust wildlife officer, is that while we know there is very little or no water coming up from the ground to the brook, we don’t exactly know why.

She says: “​​We know that the hydrology around Coldham’s Brook, Cherry Hinton Brook and the East Main Drain is complicated so it’s hard to be sure exactly how water moves between them and through the soil. It’s likely that recent rain was enough to add a bit of water to Coldham’s Brook.

“It’s really saddening to see so little water – the Friends of Coldham’s Common and Friends of Cherry Hinton Brook do a huge amount of work improve habitat, so it’s a cause of dismay for everyone to see no water flowing.

“These streams should have water in them all year round – if we want to have healthy chalk streams flowing through and around Cambridge we need to find ways to collectively address stewardship and use of water resources.”