Jökulsá á Fjöllum is the second longest river of Iceland (206 km). Its source is the Vatnajökull glacier. It flows into the Arctic Sea. Jökulsá á Fjöllum streams over the waterfalls Selfoss, Dettifoss, and Hafragilsfoss, the second of which is the most powerful waterfall in Europe, and through the canyon at Jökulsárgljúfur National Park, which was formed by the explosion of a volcano situated directly beneath the river. Consequently the course of the river changed to fit the mountains.
The gorge Ásbyrgi was formed by the river in the shape of a horseshoe. The park is one of four national parks in Iceland. The largest Jökulhlaups (glacial floods) in Iceland are known to have occurred along Jökulsá á Fjöllum between 7100 and 2000 yr BP.The source of these floods were likely an eruption of Bárðarbunga, the central volcano under the Vatnajökull glacier, in the late Holocene. The peak volume of this flood is estimated to have been 900,000m3/s of water discharge.