Image copyright Rainbow Air Inc Image caption It was initially feared the black discharge was an oil leak

Mystery surrounding a foul-smelling black cloud that appeared in water at the base of the Niagara Falls on the US-Canadian border has been explained.

Experts say the discharge - which at one point was feared to be an oil leak - was caused by residue from black carbon filters used to clean the water.

The leak happened during maintenance work on Saturday, US officials say.

The Niagara Falls Water Board (NFWB) has since apologised for causing alarm to residents and tourists.

In a statement the board said the "inky water" was the result of "routine, necessary and short term change in the waste water treatment process" at its plant near the city of Buffalo.

Image copyright Rainbow Air Inc Image caption The black cloud remained in the water for much of Saturday

"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 odour was limited to the normal sewer water discharge smell," it added.

Officials say the plant had the correct paperwork to release the discharge - which came from one of its five sediment filtration basins and was being flushed out over the weekend in preparation for contractors to begin upgrade work.

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Among the first to notice the problem was Pat Proctor, vice president of Rainbow Air Inc, which provides helicopter tours over the falls. He said the black residue remained in the water for several hours on Saturday before it dissipated.

"I was just praying it wasn't an oil leak," Mr Proctor told the BBC. "It had spread across a half-mile area, looked very menacing and smelt terrible."

Usually such basin discharges are not carried out at peak tourism times, like last weekend, he said.

The Niagara Falls straddle the US and Canada. They are made up of three separate waterfalls and have been a popular tourist attraction for more than 200 years, in addition to being a major source of hydroelectric power.