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.

At first, it looked like a shadow to Pat Proctor, who spotted the expanding mass from a Rainbow Air Inc. helicopter around 4:15 p.m. Saturday. He soon realized that wasn’t the case.

“The first thing that came to my mind was, ‘Dear God, please don’t be an oil leak,’” Proctor, a vice president for the company that flies tourists over the falls, said today. His video and photos of the peculiar sight have been widely shared online and through social media.

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.”

Executive Director Rolfe Porter did not immediately return telephone messages left by The Associated Press today.

Wastewater flows through carbon beds as part of a seven-step purification process, according to the water board’s website. Eventually it is discharged into the river.

The Maid of the Mist tweeted video and an aerial photo of one of its tour boats packed with tourists and surrounded by murky water. It asked city and state officials, “Why the smelly black discharge into Niagara River on very busy tourist weekend?”

Proctor said the stain continued to grow through the afternoon.

“By 6:30 or 6:40 it looked like it was pitch black in the water,” he said.

A Maid of the Mist spokesman said today that its operations were not impacted. The spokesman, Kevin Keenan, referred other questions to the city and state.

The state Department of Environmental Conservation said it began investigating after receiving complaints about the discolored water and odors. The investigation was continuing today, the department said.

The discharge had dissipated by Sunday morning.