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, said Saturday’s discharge was part of routine maintenance of one of its basins.

Video taken from a helicopter showed 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.

The Niagara Falls Water Board apologized for alarming residents and tourists.

“The blackish water contained some accumulated solids and carbon residue within permitted limits and did not include any organic type oils or solvents,” the statement said. “The unfortunate odor would be limited to the normal sewer water discharge smell.”

The discharge had dissipated by Sunday morning.