A 10KM stretch of waterway has been left devastated after a pollution spill left hundreds of fish dead.
The incident at Aldborough Beck has been described as the most serious of its kind by the Environment Agency.
The spill was first detected at Aldborough St John, near Darlington, yesterday (Wednesday) morning, but dead fish have been found several kilometres upstream at Hutton Magna, in Teesdale.
Environment Agency officials are focusing their investigation into the source of the pollution around Hutton Magna, as live fish have been found upstream of the village.
Staff are carrying out numerous tests of the water in the beck to determine the exact make-up of the pollutant and to assess the damage done to the watercourse.
It is thought that the pollution has now dispersed and the waterway is running clear.
No dead fish have been found downstream of Aldborough St John and invertebrates living in the beck appear to be unaffected by the pollution.
Tests of the River Tees, of which the beck is a tributary, have come back clear and there is said to be no danger to the main river.
The Northern Echo reported the spill yesterday (Wednesday), which was discovered when Environment Agency staff arrived at Aldborough St John to release hundreds of young grayling into the water to boost fish stocks.
Those fish were taken elsewhere for release as the investigation into the pollution spill was launched.
It is thought it could take several years for fish stocks in the beck to recover from the damage caused by this incident.
Jon Shelley, environment management team leader at the Environment Agency, said: “This incident has been confirmed as a Category 1 incident – the most severe kind, with hundreds of fish having been killed.
“Our officers have been taking water quality samples, assessing the full impact on the environment and investigating the source of the pollution. It appears the watercourse is now running clear, and there is no ongoing pollution.
“Reports suggest that the pollutant entered the water on Tuesday evening.”
The Environment Agency has warned that anyone found to have been dumping waste into the watercourse will be prosecuted.
Anyone with information about the spill is urged to call the incident hotline on 0800-807060.