Newport magistrates court heard last week that the company had breached its abstraction licence on 35 separate occasions between 16 May and 26 July 2011.
The court was told that Welsh Water's Prioress Mill pumping station took 683% more water than its allowance on one day and on another, when no abstraction was permitted, took over half the actual flow of the river for two consecutive hours. The water was pumped into a reservoir for public supply, although the extra was not needed.
A spokesperson for Environment Agency Wales said:
"Our licences balance the needs of people and the environment and wildlife - the Usk is a prime river for salmon and sea trout - and we have limits in place designed to make sure there is enough water for their migration upstream whilst protecting water supplies.
"In this case, these limits were ignored and the environment was put at risk - and we will not hesitate to take action against these breaches of this kind."
Welsh Water pleaded guilty and said pump operators had been retrained since the incident, which had exposed errors in the management system.
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