The salmon were once so plentiful in the river that old-timers talk about having been able to cross on the backs of fish so thick they were like steppingstones. Such was the renown of the Cowichan River, flowing east on Canada’s Vancouver Island, that its fly-fishing conditions were posted in fishing clubs in London. John Wayne and Bing Crosby were regulars in Cowichan Bay.

So when hundreds of young salmon and trout were found dead in the river last month, even as record wildfires burned across Canada, the news made the front page of the local newspaper. The die-off, the biggest in living memory, quickly led to an investigation.

It remains a mystery. Government officials found partially treated wastewater in the river a couple of weeks after the fish were found, but they have yet to draw conclusions about its impact. Local scientists suspect the bigger culprit is climate change, which has contributed to the decline of salmon populations in British Columbia by increasing droughts and heat waves.

In a summer of global catastrophes for Canada, climate change has been felt across this vast country — from Cowichan Valley on the Pacific Coast to Halifax on the Atlantic, from the long border with the United States to the remotest towns above the Arctic Circle. But if the world has been consumed with the fires raging across Canada’s forests, turned into tinderboxes from the effects of climate change, the plight of the river has hit close to home in Cowichan Valley.

A biologist, swimming in a wet suit for miles downriver from where the juvenile fish, or fry, had been found, discovered hundreds more dead inside pools at the bottom of the river. Further downstream, past eerily “barren zones” with no fish at all, he found dozens of dead adults inside larger, deeper pools — foot-long rainbow trout and even bigger brown ones.

“It was the first time not just in my career, but the first time in my life, that I had seen anything like that,” said the biologist, Tim Kulchyski, 50, who said he “basically grew up in the river” as a member of Cowichan Tribes, where he now works as a natural resources expert.

The mass death of the cold-water fish has occurred during another summer of extreme drought and heat on Vancouver Island, a region known for its temperate climate. Wildfires cut off access to some of the island’s western communities for more than two weeks during the tourist season, leading to losses estimated by a local chamber of commerce at around $30 million.

The country has experienced a summer of extreme weather events and record-shattering temperatures. Inuit communities, some above the Arctic Circle, have broken records with temperatures above 90 degrees Fahrenheit.

With at least a month left in the wildfire season, fires have burned the equivalent of the area of the state of Georgia, about 38 million acres of forests, more than seven times the annual average. The fires have forced nearly 200,000 Canadians to evacuate from their homes this year and led to the deployment of thousands of foreign firefighters to help, as experts have called for a fundamental rethinking of Canada’s forest management and firefighting.

In Cowichan Valley, the effects of the provincewide drought have been most visible in the river that has sustained Indigenous communities for centuries and helped grow local industry and tourism. Recognized as a Canadian Heritage River, the Cowichan’s ecosystem can no longer survive without direct human intervention, experts and local groups say.

“There’s a lot of talk about climate change, but living here, it’s undeniable,’’ said Tom Rutherford, a salmon biologist and executive director of the Cowichan Watershed Board.

“We’ve never had a significant fish kill like this in the Cowichan River, or at least in living memory,’’ Mr. Rutherford said. “The event is still under investigation. But if there was more water in the river, if it wasn’t this hot, the impacts would have been less. Salmon are cold-water species. Things may not have in the past tipped them over the edge. Now they do.’’

Government investigators found partially treated wastewater from a local treatment facility in the river 14 days after the dead fry were first discovered, but have not reached any conclusions yet about its “toxicity’’ or “impacts on fish,” according to a spokeswoman for Environment and Climate Change Canada, a federal department.

In recent years, the government and other experts have warned that increasing droughts, heat waves and heavy rains exacerbated by climate change are leading to the sharp decline of British Columbia’s salmon population, especially of species that spend more time in rivers. Thousands of salmon have been found dead in rivers and creeks on the province’s Pacific Coast amid severe drought in the past two years. The stresses from a changing habitat also weaken the fish and make them more likely to die from other causes, experts say.

From its source at Cowichan Lake, the river flows for 30 miles across southeastern Vancouver Island, in one of the most fertile areas in Canada, past forests once full of towering cedars and Douglas firs, before draining into the Salish Sea. The Cowichan was the perfect habitat for chinook, chum and coho salmon, which could gorge on insects and swim in cool water shaded by trees.

The local Indigenous communities, according to their cosmology, are the people who descended from the sky to earth where they found a river full of salmon. The river and the salmon were central to their way of life and spirituality, said Lydia Hwitsum, the chief of Cowichan Tribes.

“The river and everything within the river are considered part of our family,’’ Chief Hwitsum said. “And it’s our corresponding responsibility to look out for and take care of it.’’

Logging began in Cowichan Valley after the arrival of European settlers in the mid-19th century, and continues to this day. In the 1950s, a weir was built at Cowichan Lake to provide water storage for a paper mill, storing and releasing water during the dry months.

Residents in their 60s and older recall seasons of steady rain that fed the Cowichan and its tributaries, and cool, often cloudy summer months that kept the waters favorable for young salmon and trout. Some remember jumping off an old railway bridge nicknamed “Black Bridge’’ into the river — at a spot where the water might now be a foot deep.

Logging has felled many old-growth giant trees that kept the river and valley cool and that helped absorb rainfall that was gradually released into the river, experts say. Now rains have become irregular, often dumping huge amounts of water that cannot be absorbed into the soil. Snowpacks are melting sooner because of warming weather, leaving less water for the river during summer.

Joe Saysell, 75, a fishing guide who has lived his entire life along the river, said that the Cowichan’s shape has morphed in his lifetime, becoming wider and shallower, its bottom covered increasingly with gravel and less with the medium-sized rocks under which fry can feast on insects and hide from predators.

As a heat wave in mid-August brought days with temperatures in the mid-80s to the region, Mr. Saysell said, “The poor fish are just baking.”

Mr. Saysell, a retired logger and founder of the Friends of the Cowichan, a private organization formed to protect the river, was one of the first to see the dead fry last month after he was alerted by a friend swimming in the river with his daughter.

“This river is in the emergency room with a pile of doctors trying everything they can to keep that patient alive,” he said.

This summer, to conserve water amid severe drought, water release from Lake Cowichan was restricted to the lowest level possible. About 10 days before the dead fry were found, the flow of water in the river was reduced by more than a third.

The decades-old weir is incapable of providing sufficient water in the era of climate change, said Mr. Rutherford of the Cowichan Watershed Board.

The Cowichan Watershed Board is pressing for the construction of a bigger weir that would store more water for the dry months, Mr. Rutherford said. With the local government’s climate projections predicting hotter, drier summers and warmer winters, more human intervention will be needed to keep the Cowichan alive, experts say.

In the past, the Cowichan River went through periods of drought but was always able to regenerate. Today, that is no longer possible, said David Anderson, who served as a federal minister of the environment two decades ago and is a member of the board.

“Nature does correct itself, but it can’t correct itself where man is substituting himself for nature and making decisions inimical to any possible recovery,” Mr. Anderson said. “We’re in a different world. We’re simply taking too much out of the environment worldwide.”