Environmental campaigners have raised the alarm after ‘runoff’ water in River Chess.

Eagle-eyed residents may have seen the water in parts of River Chess turn into a brown sandy colour after heavy rain.

Campaigners for the precious chalk stream have raised fears the runoff water from the roads and fields includes sediment, fertiliser and pesticides harmful to the river ecosystem.

Chairman of the River Chess Association Paul Jennings said what he saw on New Year’s Eve was the “highest concentration” of sediment in the river he has seen “for a long time.”  

The runoff appeared downstream from where the Vale Brook meets River Chess.

Bucks Free Press: River Chess near Duck Alley, Chesham, on January 2.River Chess near Duck Alley, Chesham, on January 2. (Image: River Chess Association)

During wet weather, Chesham “becomes like a sink” as majority of the roads leading into the town and towards the Chess go downward, he explained.

READ MORE: Thames Water discharges sewage into the Chess and Misbourne

The campaigner claimed the runoff water carries with it sediment from the road, and fertiliser and pesticides.

He said: “With the Vale Brook you’ve got a number of drains that are connected. The Vale Brook does not flow at all really, but when you have heavy rain there is a lot of stuff going in from the roads. The sandy colour of the material in the picture is a normal indicator it’s coming from an arable source on a field.

“With heavy rain it runs into the river.”

Although the water will eventually clear, some of the fine sediment will fall onto the river bed, covering the chalk and trout eggs, which the female fish have laid over the past couple of weeks, he explained.

“That sediment will cover those eggs and the eggs require clear water to take on the oxygen." 

"Runoff could risk failing the eggs. Birds including king fishers eat them and when they get bigger otters feed on them. It’s all part of that food chain,” Paul said.

The runoff risks causing eutrophication of the river, a process in which a nutrient enriched water begins accumulating and changing the chemical balance of the river.

“It encourages algae growth. In the spring and summer you get algae bloom and that smothers normal natural plant life and disturbs the chemicals of the water and affect the vertebrates.”

The sediment runoff was spotted upstream of the Chesham Sewage Treatment Works, which began to discharge storm overflow on January 2.