The water near the base of Niagara Falls turned an alarming shade of black before tourists’ eyes following a foul-smelling discharge from a wastewater treatment plant.

The water board for the city of Niagara Falls, New York, says Saturday’s discharge was part of routine maintenance of one of its basins.

Video taken from a helicopter shows black-colored water along the Niagara River’s U.S. shoreline below the falls that border the country and Canada. The inky water enveloped the dock for the popular Maid of the Mist tour boats.

Officials say the blackish water contained accumulated solids and carbon residue but no organic oils or solvents. They described the odor as “normal sewer water discharge smell.” The board says the discharge was within permitted limits.

The black water dissipated by Sunday.

The Associated Press contributed to this article.