The Flåmselvi runs the length of the valley, and the train follows it most of the way from Flåm to Kjosfossen. It's a protected river - no hydroelectric development is allowed here, which is why the waterfalls still flow freely while so many others in Norway have been tapped for power.
This was once a famous salmon river. The English came here in the 1800s, as they did to rivers across Norway, and the fishing was so good that locals joked you had to eat salmon three times a day. The river still holds salmon up to 20 kilos, as well as sea trout to 8 kilos. The old fishing platforms are still visible along the banks - stone structures built to give anglers the best positions. A major rockslide in 1965 took out part of the railway track and changed some of the legendary fishing pools.
In October 2014, after days of torrential rain, the river burst its banks in the worst flood for 200 years. Flåm was cut off entirely. Three houses were swept away, bridges destroyed, and 22 people had to be rescued by helicopter. The valley school closed, and it took months to re-establish the river's course. One researcher later said it was "sheer luck" that nobody died.