When you’re in the wilderness and alone at your campsite, it turns out that there’s no such thing as a small bear.

That was one memorable lesson Paul Firth learned last summer, when he transformed an unexpected gift of time — two weeks’ vacation and plans that had fallen through — into a paddling trip on Canada’s Yukon River.

For Firth, a Harvard Medical School associate professor of anesthesia at Massachusetts General Hospital and lover of the outdoors, the choice of wilderness, solitude, and a bit of adventure was easy when faced with a suddenly blank calendar.

An experienced outdoorsman who has led an expedition to Mount Everest, Firth thought of a book he had read years earlier: “Reading the River: A Voyage Down the Yukon” by John Hildebrand. Published in 1988, the book details the author’s own trip down the 2,000-mile length of the waterway, which runs through northwest Canada and Alaska.

Firth trimmed the trip to fit the time he had and, with just three weeks to spare, began planning a 460-mile paddle from Whitehorse to Dawson City in Canada’s wild Yukon Territory.