Jokulsa a Fjollum

The 2nd place in the list of the longest rivers in Iceland is Jökulsá á Fjöllum ('The Glacial Mountain River'). This river, which flows from the giant glacier Vatnajökull, is regarded as Iceland's second-largest river and the source of several of the country's most frequented cascades, including Dettifoss, Europe's most powerful waterfall.

Jökulsá á Fjöllum is located in the country's northeastern part, serving as the eastern border to the open lava field, dáahraun, before emptying into the Greenland Sea. Jökulsá á Fjöllum has a 7,380km catchment basin, which is one of the numerous reasons why the river has been subject to spectacular glacier flooding ("jökulhlaups") throughout history.

During the Holocene epoch, the biggest glacial floods in Iceland occurred along Jökulsá á Fjöllum between 7000 and 2000 BC. These floods would have happened as a result of an explosion beneath the Vatnajökull glacier. The subsequent discharge is estimated to be roughly 900,000 cubic metres of water per second—the Amazon River has a flow that is only one-fifth of this.

Length: 206 km (128 mi)