The River Brain was this week declared unharmed by a spill of between 2,000 and 3,000 litres of oil into its waters last month.
Environment Agency workers constructed a boom across the river to contain any possible damage after the spill was discovered on the Springwood industrial estate in Rayne Road, Braintree at the end of March.
A film of oil stretched up to four kilometres downstream, while pockets of oil were trapped among reeds, an Agency spokeswoman confirmed this week.
No fish were harmed in the incident, although some invertebrates and wildfowl may have been affected, she said. The boom was removed on Tuesday.
The spokeswoman added: "Any possible environmental damage is not now considered to be severe." She was unable to explain the circumstances leading to the spill.
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