Water bosses have blasted vandals who cut through barbed-wire fencing causing livestock to be injured.
United Utilities say yobs on the moors at Anglezarke are damaging fences and leaving barbed wire which sheep are then getting hurt on.
Neville Kidd, estate manager for United Utilities, said: “Our tenant farmers around Anglezarke and White Coppice are telling us that they are finding sheep with wire tangled in their coats, and nasty cuts where they have struggled to free themselves.
“It’s distressing to see, and it’s a direct result of the vandalism.
“These fences are there for a good reason, and they don’t cause any harm to the animals until they are wilfully damaged.”
Water bosses say the fences, which help stop livestock getting into watercourses and polluting the water, have already cost around £10,000 to repair.
2,000 metres of wooden fencing have needed to be replaced so far.
Anglezarke is part of the Rivington reservoir chain, which supplies water to Wigan and Liverpool, and United Utilities also manage the land around the reservoirs.
Mr Kidd said United Utilities would be working with police to try and stop the vandalism, and encouraged anyone with information to contact them.
He said: “We actively encourage walkers and visitors to this beautiful area but everyone should follow the countryside code and leave the area as they found it.
“I’d urge anyone out and about in the area who sees suspicious activity to report it to the police.”
Anyone wanting to report criminal activity can call police on 101.