The Riwaka River is now running clear again after turning white with cream yesterday when a truck crashed on the Takaka Hill Road, near Golden Bay.
The truck-and-trailer unit carrying cream to Takaka crashed on the Motueka side of the hill.
Up to 19,000 litres of cream spilled out and much of it flowed down the dry hillside into the river.
Fish and Game field officer Rhys Barrier said the cream could have seriously harmed the fishery by starving the water of oxygen.
Mr Barrier said tonight's rainfall would also help clear the water.
Spokesperson for Tasman District Council Chris Choat said environmental health officers would be at the river today and tomorrow assessing the extent of the pollution.