The Niagara Falls straddles the U.S and Canadian border. They are made up of three separate waterfalls and have been a popular tourist attraction for more than 200 years, as well as being a major source of hydroelectric power.

However, the water near the base of the Horseshoe Falls turned an alarming shade of black on Saturday afternoon following a foul-smelling discharge from a wastewater treatment plant on the U.S. side.

The Niagara Falls Water Board, in Niagara Falls, N.Y., said the discharge was part of routine maintenance of one of its wastewater sedimentation basins.

The Niagara Falls Water Board has since apologised for causing alarm to residents and tourists. In a statement the board said the "inky water" was the result of a "routine, necessary and short term change in the waste treatment process" at its plant near the city of Buffalo. The dark water contained accumulated solids and carbon residue within permitted limits and did not include any organic oils or solvents according to the board.

The release said that "the unfortunate odour was limited to the normal sewer water discharge smell."

The black water had mostly dissipated by Sunday.

Niagara Parks Commission chairwoman Janice Thomson said the agency is looking into the incident to determine what measures are in place to keep stakeholders informed regarding future drainage occurrences.