At 588 km (365 miles) long, the river Glomma (also spelt Glåma) is Norway’s longest river, and one of its most important waterways. It takes its source in the Aursunden lake in the Røros area and runs pretty much straight south into the Oslofjord just outside Fredrikstad, Østfold. Because the Glomma crosses some of the densest forest areas in the country, it has in the past been Norway’s main river for timber floating. Today, however, it plays a more important role as a source of hydroelectric power – the Glomma alone accounts for 10% of the whole hydroelectric power produced in Norway.

In Østfold, the Glomma divides up into two separate arms, the eastern arm (the main one) going through Sarpsborg and then Fredrikstad, and the western arm running through Mingevannet in Sarpsborg, Isnesfjorden and onto Visterflo. The stretch going from Isnesfjorden to Visterflo is called Ågårdselva, and is famed locally for its salmon.

Industry in Sarpsborg has in the past been hugely reliant on the hydraulic power of the Sarpsfossen, Scandinavia’s most powerful waterfall, with a water debit of 577 m³ per second. Until 1980 timber was floated down to Fredrikstad on the Glomma – a timber tunnel was even built in nearby Eidet to facilitate the transport of the logs and avoid them having to go down the waterfall.