Few places in Norway have been redrawn as often. Vefsn was established as a municipality on 1 January 1838, and then the subtraction began. In 1862 the vast eastern district, 921 people, left to become Hattfjelldal. In 1876 the port town of Mosjøen, all 379 residents of it, was carved out as a town-municipality of its own - the first town on Helgeland, and the only one for nearly half a century. In 1927 the remainder was split three ways: Drevja to the north, Grane to the south, and 3,119 people left holding the name. Not until 1962 was the district reassembled, when Mosjøen, Drevja and Elsfjord were folded back in. The reunited municipality inherited Mosjøen's coat of arms - a white rooster on black, claws and comb picked out in red, designed by Arthur Gustavsson and adopted by the town in 1960. The rooster stands for watchfulness. Given the history, it seems well chosen.

German forces took Mosjøen on 11 May 1940, a month after the invasion, and made it their headquarters for Helgeland. The town itself came through the war almost untouched - one of the few in Northern Norway never bombed. The countryside around it did not. At Knutlia, in what was then Elsfjord, the occupiers ran Lager Osen, one of the camps holding Yugoslav prisoners, most of them Serbs, shipped north to build roads and railway for an army that regarded them as expendable. At least 425 of the men held at Osen were killed. They had names, homes and families a thousand miles south, and almost no records of them survived the war, because the camp administrations destroyed what they could. In 1975 Vefsn signed a friendship agreement with Gornji Milanovac in Serbia, and it is dated 28 September of that year. The tie is not decorative. It exists because of what happened in the woods above the fjord.

In South Sami the district is Vaapste, and the name is older than the municipal boundaries that keep moving. This is South Sami country, and it has produced voices that carried a long way. Inga Juuso, born in Mosjøen in 1945, spent her life as a yoiker, singer and actress, and did as much as anyone to carry the yoik from something private into something performed. Frode Fjellheim, born at Mussere in 1959, wrote a choral piece in 1996 called Eatnemen Vuelie - Song of the Earth. Disney adapted it, shortened it and stripped out its hymn elements, and it became the opening of Frozen. Several hundred million people have now heard a South Sami yoik from Helgeland without ever knowing what they were listening to.

The second industrial wave arrived after the war, and this time it stayed. Groundbreaking for the aluminium plant at Mosjøen came in 1956, with 1,100 men on the construction site alone, and production started in 1958. It remains the largest single industrial project the town has seen, and the smelter, now Alcoa's, is still the economic spine of the place. An anode plant followed in 2007, feeding carbon anodes to smelters in Norway and Iceland. Around it, Vefsn is still mostly spruce forest, mountain, lake and farmland: 1,929 square kilometres holding roughly 13,500 people, which works out at about seven per square kilometre. The highest ground is Geittinden at 1,556 metres. Most of the population lives within sight of the water.

On 27 July 2019 the thermometer in Mosjøen read 35.0 degrees Celsius - the warmest temperature ever officially recorded in Nordland or anywhere in Northern Norway. It is a useful corrective to what people expect this far up the map. The valley faces south and inland, sheltered from the coast, and it bakes in a way the outer islands never do. The rest of the year behaves more conventionally. Snow closes in, the fjord stays open, the Nordland Line and the E6 keep running through, and the town of just over 10,000 people at the river mouth goes on being the market, the hospital and the high school for a long stretch of inland Helgeland.