Why are the Rivers Cam and Rhee so murky? Experts warn of dire impact on aquatic life and human health
Precious and rare chalk streams in and around Cambridge are facing further serious threats, river experts have warned.
Aquatic life could be struggling to survive and bacteria and viruses harmful to human health may be thriving because the water in the Rivers Cam and Rhee is so murky, they say.
This murkiness - more formally known as ‘turbidity’ - will have a dire effect on biodiversity and reduce the amount of sunlight penetrating the water.
And while turbidity is known to be an issue for our chalk streams in winter, amid additional rainfall, it has increasingly become visible in summer months, according to the Cam Valley Forum.
Mike Foley, from the voluntary group, believes a combination of geological factors and the activities of an invasive species, the American signal crayfish, could be to blame.
Already suffering the effects of sewage pollution, turbidity represents a further threat to our chalk streams - sometimes described as the “rainforests of the UK” due to their rarity and the global importance of their ecosystems.
“It is a problem contributing to reduced aquatic life and biodiversity,” Mike told the Cambridge Independent. “We are also concerned that high turbidity has been shown to drastically reduce the ability of natural ultraviolet radiation from sunlight to penetrate into the water column. UV light is a natural disinfectant when it can penetrate well into the water.
“The ability of the river to self-cleanse itself of viral and bacterial gut organisms harmful to humans arising from, for instance, treated sewage, misconnected pipes and wild sources is likely to be impaired.
“This is particularly pertinent in the stretch of the Rhee and Cam downstream from the Haslingfield Sewage Treatment works to Cambridge’s Sheep’s Green Designated Bathing Water.”
After visiting the River Rhee in Barrington on Saturday (5 April), Mike described it as “horrendously murky” and the opposite of the ideal "gin-clear chalk stream”.
Visibility disappeared after 58cm in the river - at a point where its depth was 159cm, meaning “any wildlife of value trying to grow on the river bed has little or no light to help it.”
Mike continued: “The Cam flowing through Cambridge often appears ‘murky’ or turbid, sometimes horribly so. For years, Cam Valley Forum has remarked that this is a poor condition for a ‘chalk stream’ to be in.
“High turbidity is detrimental to aquatic life – to fish, invertebrates, also plants such as water crowfoot and several others that we want to see flourishing in the Cam but have been sadly declining or absent.”
Historic high levels of phosphate and nitrate add to the problem, he said.
“During periods of high rainfall we note that our larger rivers become faster, deeper and swollen as they receive extra water from innumerable inflows,” he explained. “They take on a brown colour due to suspended soil which arises from runoff from the land, erosion of the banks, and resuspension of sediment in the bed. None of the rivers are spared, but this occurs mostly during the winter months. This type of turbidity is well-known and understood.
“However, in recent years there have been changes to the seasonal pattern of turbidity. We are increasingly alarmed by long periods in summer when turbidity is high, which bodes no good for aquatic life.
“This can occur in the absence of high rainfall or indeed in the prolonged absence of any high rainfall such as in the drought period of 2022.
“Summer is a period when aquatic life is in full swing, and when it is important for chalk stream plant species growing on the river bed to receive sufficient sunlight to try to compete with tougher, over-vigorous larger plants such as unbranched bur-reed and the frequent algal mats that cover the river bed.”
And the issue has become more widespread.
“Summer turbidity used to be mostly confined to the Rhee, but recently the lower part of the Cam at Hauxton also shows similar problems,” said Mike.
“There seems to be no link to physical factors such pH, electrical conductivity, dissolved oxygen, or concentrations of the inorganic chemical nutrients orthophosphate and nitrate.”
And the output from sewage works is not the issue this time.
“Sampling treated effluent from sewage treatment works shows that its turbidity is not worse than the river, and in fact can be lower,” said Mike.
“In some rivers elsewhere in England and Wales also with problems of turbidity, algal blooms have been implicated. We’ve tested the Cam catchment with a meter designed to detect unicellular green algae and we are confident they are not the cause.”
Mike a growing body of organisations believe the cause of summer murkiness could be partly geological and partly down to invasive American signal crayfish.
“The soil of the Rhee catchment and some tributaries such the Mill River is particularly ‘clayey’, having developed on the Gault Formation. This consists of particles so fine that once in suspension they cloud the water. The Environmental Agency has come to a similar conclusion.”
While historic dredging of the Rhee may not have helped, that has been undertaken at Harston since 2005, yet at Harston the river can be particularly turbid.
Mike believes further studies are needed to explore the issue on the upper Rhee, as it is unlikely American signal crayfish are sufficiently established here to be the problem.
That is not the case elsewhere, though.
“There seems to be no end to the explosive population growth of the American signal crayfish in our rivers,” said Mike. “Crayfish are ‘habitat engineers’. They burrow into banks - the tunnels are sometimes more than 50cm deep. The banks then become more vulnerable to erosion and even collapse.
“They are active on the river bed and churn up sediment as they move about and in their quest to feed, sometimes on their own kind, as they are cannibalistic.
“They feed on fish eggs and some invertebrates and are most unwelcome in our rivers.”
Introduced by the government in the 1970 to be farmed for food, the American signal crayfish escaped fisheries and spread, outcompeting the native white-clawed crayfish.
One Barrington resident, who lives by the river, has taken to trapping and eating the invasive species. He is unlikely to go hungry.
“They are marching inexorably upriver and we are concerned but there is nothing we can do other than to put in barriers of very fine mesh, which is totally impractical, to protect uncolonised river stretches,” said Mike.
“They can even walk across land. We are unsure how many have now colonised the upper reaches of our main rivers, but lower down there may be many tens of thousands.”
There are, however, ongoing efforts to understand the problem more fully.
Dr Steve Boreham, an ecologist and geologist at Wildreach, has already helped to study the suspended solids and the issue with UV light penetration.
And The Greater Cambridge Chalk Stream Project, established to help study and protect these precious watercourses, will be monitoring the problem.
Described as “more than a conservation effort”, the project is seen as “a commitment to learning, restoration and long-term stewardship.”
This month, it is due to publish timelines and specific details for the project.
But in a pre-project report on the state of our chalk streams, the project team said: “Agricultural pollution, over-abstraction, habitat degradation, invasive species, and wastewater mismanagement all contribute to the ongoing decline of these unique ecosystems. Effective conservation and restoration efforts must address these diverse threats through sustainable land and water management practices, improved infrastructure and ongoing monitoring.”
