Thames pleaded guilty to breaching section 85(3) of the Water Resources Act 1991 at Hemel Hempstead magistrates court on 20 February. It was ordered to pay £2,010 in costs on top of the fine.
The incident came to light on 5 April last year when a member of the public reported smelling sewage along the Hartsbourne river near Watford. However, Environment Agency officers were initially unable to find the pollution source and further reports of pollution came in on following days.
Officers found 12 dead fish and low dissolved oxygen levels in the river and eventually tracked down the source to a blocked sewer on 10 April.
By that time the pollution affected a two-mile stretch of the river down to the junction with the river Colne. Thames Water said that it had been aware of the blockage since 7 April but did not alert the Agency due to "a failure in reporting procedures".
The offence was the company's first in 2004. Last year, it appeared in court three times and was fined a total of £47,000 for seven offences.