Hundreds of fish have died in a pollution incident in the Ravensworth, Gilling West and Skeeby area.
Holme Beck, Gilling Beck and Skeeby Beck have all been affected, with concern that the release of farm slurry has reached the River Swale.
The slurry came from the AWSM Farming site at Hutton Magna with bosses blaming vandals for the release.
Edward Bennett, environmental manager at AWSM, said: “Despite having robust controls in place, we discovered that one of our digestate stores had been vandalised on Thursday, leading to the loss of digestate contained within the store.
“This was promptly reported by us, to the Environment Agency and the police yesterday morning.
“We are continuing to work very hard to clean up and mitigate the impact of the digestate, while working closely with the Environment Agency and Police to assist them with their ongoing investigations.”
Olly Shepherd runs Fly Fishing Yorkshire and has fishing rights on Skeeby Beck,
He said the beck had been “running black” for around 24 hours and had counted dozens of dead fish
“This is a classic example of slurry pollution at a fish-kill level which is the highest it can be.
“It’s absolutely devastating. It’s the whole ecosystem which will have been damaged as everything relies on oxygen which is the one thing that the slurry takes out.
“Over the weeks we will assess the full damage but from what I’ve seen so far it doesn’t look good.”
Mr Shepherd, who says he has spent three and a half years improving the habitat around Skeeby Beck, said the pollution was a huge financial blow to his business.
Gayles resident Ben Lamb said: “I have spoken to quite a lot of people yesterday and there’s a great deal of sadness and dismay at what’s happened, and rightly so.
“This kind of thing shouldn’t be happening in this day and age.”
He added: “It will have reached the Swale given the volume we have seen.”
Richmondshire Today has contacted the Environment Agency for an update.