Judge Christine Laing KC DL, told the Lewes Crown Court, the company deliberately misled the Environment Agency during their investigation.
The incident occurred in late 2017 when a equipment failure led to the release of untreated effluent into one river, which subsequently flowed into another.
Thousands of fish lost their lives as a result of the pollution.
Investigators discovered nearly 1,400 dead fish in Gatwick Stream and the River Mole, but it is believed that the actual number of casualties was much higher. Thames Water pleaded guilty to four breaches of environmental law.
During the trial, it was revealed that a pump at the company’s sewage treatment works in Crawley had been activated in error, causing sewage and rainwater to be discharged into the rivers.
Despite no significant rainfall, the storm lagoon was in operation, which should only happen during wet weather.
Thames Water initially denied any responsibility for the incident but later admitted their guilt.
The Environment Agency criticised Thames Water‘s handling of the situation, stating that the company missed multiple opportunities to prevent the pollution and failed to respond effectively when the alarm was raised.
The agency described the incident as a “reckless failure” on Thames Water’s part for not implementing and enforcing systems to prevent such pollution.