Wherever glaciers and volcanoes meet, massive floods can ensue. There is perhaps no better example of this than in Iceland. In fact, the largest flood of this kind in recorded history occurred after the Katla volcano erupted in 1918. A deluge, formed from melting subglacial ice, roared across the landscape with a discharge of around 300,000 cubic meters per second.
Astoundingly, this flood was merely a baby compared to others that have occurred in Iceland in the past. A flood down Jökulsá á Fjöllum, also caused by volcanism, is thought to have occurred about 7,000 years ago with a discharge of up to 1 million cubic meters per second.