Katla is an active volcano on the south coast of Iceland. It erupts about two times in every century and has done so at least since Iceland was settled (about 900 AD) and because there is a big glacier on the top, there isa big flood when it erupts.
The local residents near Katla say that a witch called Katla causes the volcanic eruptions. In an Icelandic book about legends after Jón Árnason, is the story told like this.
Once when there was an abbey in Þykkvabæ (SW. of the glacier), there was a house- keeper called Katla. She was a witch and everybody including the prior where afraid of her. Katla had a pair of trousers that had the power to let evryone run as much as he liked without getting tired. In the abbey there was a shepherd called Barði. He often got severely rebuked by Katla if some of the sheep were missing when he got home. Once at autumn time the prior and Katla went to a party, and when they came back Barði should have brought home all the sheep. But Barði couldn't find them all, so he took Katla´s trousers, ran like hell without getting tired and found them. When Katla came home she soon saw that Barði had taken her trousers, so she drowned him in a tub with sour whey in it, and hid the body in it so that nobody knew where he was. In the spring time when the level of the sour whey in the tub had got lower, the people heard her say: "Soon will Barði be seen". But when she thought that her wickedness would be found out, she put on her trousers and ran to the Mýrdalsjökull glacier and jumped down a crack in the ice. Shortly after this there was a flood from the glacier that headed directly towards the abbey. Since then people around the glacier have believed that floods are the result of her magic.