The river had trouble being called anything consistently. Admiralty charts labelled it River Courtenay. Dr. Robert Brown of the Vancouver Island Exploring Expedition wrote it down as Courtney River. One early reference dismisses it entirely as the slough known as Tsalum. The spelling Tzolum turned up on a BC Lands map in 1895, reappeared in 1905, and only in 1922 did the present form, Tsolum River, become official. Surveyors could not agree even on where it ended: the Tsolum runs down the Comox Valley and joins the Puntledge at Courtenay, and the two together become the Courtenay River for the last short run to the sea. It is a small river with a complicated paper trail, which makes what happened to it easier to overlook.

The Mount Washington Copper Mining Company moved into the headwaters in 1964 and started a small open pit. By 1967 it was finished and gone, its Japanese backers bankrupt after less than three money-losing years of operation. Mining had lasted barely two full seasons, but the arithmetic of the aftermath is what matters: 940,000 tonnes of sulphide-bearing waste rock, left uncovered on a mountainside in one of the wettest regions in Canada. Rain and air working on that rock generate acid, and acid strips copper out of the stone and carries it downhill in solution. Dissolved copper does not have to be abundant to be devastating. At a few parts per billion, it destroys the sense of smell young salmon use to find their way home and kills the insect life the fry depend on. The river looked perfectly clear the whole time.

In 1997, more than two hundred residents of the Comox Valley formed the Tsolum River Task Force. They were farmers, anglers, retired biologists, and people who had simply grown up swimming in the thing. The company that caused the problem no longer existed, so there was nobody to sue and nothing to recover. What the group had instead was persistence: monitoring water chemistry year after year, publishing numbers nobody in government especially wanted, and refusing to let the file close. It took them a decade to force the question of who would pay to cover a mountainside that a bankrupt company had left open.

The answer, funded with $4.5 million the province's Treasury Board approved, was almost absurdly simple in concept and difficult in execution. If rain and oxygen make the acid, keep rain and oxygen out. Starting in 2009, crews sealed the waste rock under a waterproof membrane of bitumen-impregnated polyester and buried it beneath 128,000 tonnes of gravel. The response took a few years to show in full. In 2013, roughly three years after the pit was capped, about 61,800 pink salmon returned to spawn in the Tsolum - the largest run since the 1950s. Two years later, in 2015, 129,000 pinks came back, a record since counts began in 1953.

The Tsolum is now one of the more encouraging environmental stories on Vancouver Island, and also one of the more sobering. Three years of extraction produced more than forty years of damage and a cleanup bill carried by taxpayers rather than by anyone who profited. The fish are returning, though a run rebuilt from fourteen animals takes generations to recover its depth. On a clear day in the Comox Valley the river reads as pure pastoral: alders leaning over the banks, gravel bars, the white cone of Mount Washington visible upstream where the whole thing began. Both facts are true at once, and the people who spent twenty years in meetings to get here would tell you that neither cancels the other.