The Niagara Falls Water Board says a worker who had been monitoring the discharge was called away to another section of the wastewater treatment plant to help with another job.
When he returned, he saw that the out-flow from the basin he was previously watching had grown darker in colour to black.
The latest statement from the board comes after the New York State Department of Environmental Conservation said the discharge violated state water quality standards.
The material discharged is listed in the report as “raw sewage”.
The board originally said last Saturday's discharge was part of routine maintenance of one of its basins, and that the discharge was within permitted limits.