The river starts high, at the lake Simskardvatnet in the mountains of Børgefjell National Park, and runs north for 163 kilometres through Hattfjelldal, Grane and Vefsn before discharging into the Vefsnfjorden at Mosjøen. Its upper reaches are often called the Susna. Its catchment - well over four thousand square kilometres, the largest in the county - reaches close enough to the frontier that some minor tributaries begin on the Swedish side, which is why the river carries three names: Vefsna in Norwegian, Vapstälven in Swedish, and Vaapstenjeanoe in South Sami. That last one is the oldest, and the district it names, Vaapste, is the same word the municipality of Vefsn eventually inherited. Average flow runs to roughly 200 cubic metres a second.

About twenty kilometres north of Trofors, in Grane, the whole river drops seventeen metres over a rock lip in a single broad fall. Laksforsen sits close enough to the E6 that people pull off, walk down and stand in the spray without planning to; some 80,000 did so in 2004 alone. At high water the fall carries several hundred cubic metres a second, and you feel it in the rock underfoot before you hear anything else. The name means the salmon fall, and it is literal: salmon running upstream have to get past this, and a fish ladder was cut to help them do it. What happened to that ladder is the rest of the story.

The Vefsna was one of Norway's serious salmon rivers, a fishery that mattered to the people who lived along it. Then, in the 1970s, Gyrodactylus salaris got in. The parasite arrived in Norway from outside and spread river by river with catastrophic effect - by 2001 some 41 Norwegian rivers had been effectively wiped out as salmon fisheries. It breeds fast enough to be almost unopposable: the young are nearly the size of the adults, and the next generation is already forming inside them at birth. In the Vefsna it meant not only the loss of the runs but a standing threat to every watercourse nearby. In 1992 the fish ladder at Laksforsen was closed deliberately, sealing the upper river off, so that infected fish could not carry the parasite further into the system.

The eradication campaign was extensive and it was brutal in the way such campaigns have to be. Rotenone does not distinguish between a parasite's host and anything else that breathes through gills; treating a catchment of this size meant dosing the main river and its tributaries and accepting what that costs. It worked. The Vefsna was declared free of Gyrodactylus salaris in 2018, and the ladder at Laksforsen had already reopened the year before, in 2017, after twenty-five years shut. Newer methods now exist - aluminium salts dosed at low concentration can kill the parasite while leaving the fish alive - but the Vefsna belongs to the older, blunter chapter.

A river of this size, dropping from Børgefjell to sea level, was an obvious hydropower prospect, and for decades it was studied as one. In 2009 the Norwegian parliament placed the Vefsna under permanent protection in the national watercourse conservation plan, and the development case closed for good. That decision, taken while the river was still infected and its salmon still gone, was a bet on a future the river had not yet earned. The bet came in. Below the falls the water still runs out past Mosjøen into the fjord, where the old Nordland boats used to work - and salmon can climb past Laksforsen again for the first time in a quarter of a century.