There's never a good time to discharge black sewer gunk into Niagara Falls. But doing it on a beautiful Saturday afternoon at the height of tourist season may be the worst timing ever.

Niagara Falls State Park was packed with visitors around 4 p.m. Saturday when the foul-smelling, black discharge came billowing out into the Niagara River within sight of the base of the American Falls, alarming city businesses and tourists as it engulfed the shoreline from the Rainbow Bridge to the Maid of the Mist boat dock.

Pat Proctor, vice president for Rainbow Air Inc., which provides helicopter tours over the falls, was among the first to notice what he initially assumed was a shadow cast by the skyline of Niagara Falls, Ont. Then he realized the growing black stain was no shadow at all. He picked up his phone and called the mayor.

"I was just praying it wasn't an oil leak," Proctor said, adding, "It looked like something out of a sci-fi movie."

It turns out the discharge came from the Niagara Falls Waste Water Treatment Plant. The plant had the proper sewer discharge paperwork to release the volume of sewage and sediment that spewed into the river Saturday, according to the Niagara Falls Water Board.