DOG-WALKERS raised the alarm after diesel was seen spilling into the river Asker at a Bridport beauty spot.
Environment Agency workers were called to the river where it passed through Happy Island.
They deployed a boom to try to stop the fuel spreading down the river.
A woman who was walking her dog said she had seen what was clearly fuel pouring into the river in the area of open space near the Shoe Lane industrial estate off St Andrew’s Road.
The woman, who did not want to be named, said: “It appeared to be diesel and was coming from a drainage pipe from the estate.
Some people’s dogs had just been swimming in the river there.
“I contacted the Environment Agency and reported it.”
An Environment Agency spokesman said: “We had eight reports of pollution in the river over an 18 hour period.
“We sent a team to the site to investigate and they installed a boom to capture what was being spilled.
We are working with Wessex Water, which owns the drains concerned to trace the source of what appears to be fresh diesel.
“We take any pollution incident seriously and this involved diesel in sufficient quantity to give us cause for concern.”