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.

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.