The discharge of black smelly wastewater at the base of Niagara Falls has prompted calls for a criminal inquiry.
The Niagara County Legislature voted in favour of asking New York's attorney general to investigate on Thursday.
The local water authority said the colour was caused by residue from black carbon filters used to clean the water.
The water authority says July's treatment process was necessary and the discharge within "permitted limits".
An inky and foul-smelling cloud of water appeared at the base of Niagara Falls on 29 July 2017.
Some initially suspected an oil spill, but the Niagara Falls Water Board (NFWB) said it was caused by a "routine" water treatment and apologised for causing alarm to residents and tourists.
In a statement the board said the "inky water" was the result of a "necessary and short term change in the waste water treatment process" at its plant near the city of Buffalo.
Officials say the plant had the correct paperwork to release the discharge - which came from one of its five sediment filtration basins and was being flushed out over the weekend in preparation for contractors to begin upgrade work.