In the beginning of May Hannes from the farm Núpsstöðum noticed a discharge increase in the river Núpsá. On 23 July the local postrider Björn Stefánsson was on his way west over Skeidarásandur when he noticed that the river Núpsvötn (downstream from Núpsá) had an unusually high discharge. One day later, on 24 July, the same river was impassable for horses. The neighbouring rivers Blautakvísl and Skeiðará had not changed.

The flow rate continoued to increase over night, and at the outflow of  lake Grænalón, the water started to damage the glacier. On 26 July the river Núpsvötn flowed over its banks and destroyed land west of Mt. Lomagnjúpur.

- On 27 July icebergs started to flow down to the lowlands. - On 28 July the telephone line was swept away. - On 29 July the sander area between the rivers Núpsvötn and Blautakvísl was covered almost completely with water.

The peak discharge was reached at 10 a.m. on 31 July, but the flood already had diminished by the following night. The jökulhlaup ended completely on the evening of 1 Aug.

The lake was examined after the flood and was found empty. The icebergs that had come downstream were up to several meters high and they were spread over an area that reached south of mt. Lomagnúpur. Numerous icebergs had aswell come down with river Blautukvísl but though less than with river Súla.

Hannes from Núpstöðum compared the peak discharge of that jökulahlaup to the one in the river Skeiðará in 1954 which had about 4,000 - 5,000 m³/s.