Williston Lake in northern British Columbia is the seventh-largest artificial reservoir by volume in the world, and the largest body of fresh water in the Canadian province. The T-shaped reservoir has covered slightly more than 650 square miles since it was filled up in 1972.
All of its islands are mapped and well-known to boaters.
And yet in video footage circulating among Canadian social media users this summer, there was a never-before-seen island, green and woody, estimated to be nearly two-and-a-half acres in size and purported to be floating in the middle of the reservoir.
Bob Gammer, a spokesman for BC Hydro, the government-owned utility that operates the reservoir and a nearby dam, saw the video in mid-August. He could not, and would not, believe The footage was real. Probably A.I., he thought. There were no signs of the island reported by anyone in recent weeks, so Mr. Gammer dismissed it and went on as usual.
But then an engineer for the company obtained satellite imagery of the reservoir from a few weeks earlier.
There was the island.
It was real.
“This is a first for us, for this reservoir,” Mr. Gammer said in an interview on Monday.
The satellite image taken on July 21 shows that the island is about 150 yards long and 75 yards wide. Where could it have come from?
“One possible explanation that seems fairly credible is that it formed from logs and woody debris,” Mr. Gammer said. “The logs and woody debris accumulated over many years in a sheltered shoreline area.” As the material gradually decomposes, more plant life bursts out of it.
You can imagine that over time, even there, trees could be growing on that floating surface of logs,” he said. “Eventually it starts to sort of knit itself together.”
The reservoir, he added, is higher than it has been in 14 years.
“We could easily see that a mass of woody debris with live vegetation on top of it rises up, floats and breaks free from the shoreline,” he said. “Then the winds in the reservoir carry it out into open water. So that’s our best guess at how this all came to be.”
Adding to the mystery is that there is only a single video of the island, which was recorded by a boater this month. Rocky Avery, a friend of the boater and a longtime resident of the area, posted the clip to social media.
Mr. Avery told the CBC that he posted the video to warn boaters during an upcoming fishing derby. He also wanted BC Hydro to be aware, in case the island got on a path that could have sent it toward the W.A.C. Bennett Dam.
There is no danger to the dam, Mr. Gammer said, and boaters have always been aware of the risks from floating trunks and debris. He said boaters like Mr. Avery’s friend help BC Hydro to monitor the colossal lake.
But there is another mystery: How could more than two acres of island vegetation appear, then vanish?
Mr. Gammer said the authorities might never know what happened to the phantom island.
“Did it sink to the bottom? Did it break up? Are there small bits of it floating around?” he asked. “Or did the winds nestle it up against another part of the shoreline?”
Kitty Bennett contributed research.
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