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The jökulhlaup, or glacial flood, is a regular feature of this area of Iceland. Earliest accounts date back to the 12th Century, and large jökulhlaup's have been recurring every 5 – 15 years since 1900 – but the flood of November 1996 was to be the most catastrophic since 1938. This map clearly indicates the area of glacial runoff between Vatnajökull and the Atlantic Ocean. It is an area about 20 - 30 km wide.

The level of water in the Grímsvötn caldera had sunk to 1330 m after a jökulhlaup in April of 1996. After the eruption an enormous quantity of water continued to pour into the lake. By 10 October it had risen to 1485 m. Two days later a GPS instrument was placed on the ice, allowing continuous and exact measurements. By the next day the level was 1500 m, increasing at the rate of 25 cm per day. The jökulhlaup normally begins when the level reaches 1430 m, with the water finding its way slowly through the ice. At over 1510 m the water pressure was sufficient to lift the glacier ice off the ground, causing the sudden runoff of the water underneath. Icelandic scientists were preparing for some 4.5 km³ of water to strike Skeidarasandur.

Using the BDDN (Broad Data Dissemination Network) system, it was possible to obtain images from the receiving station of Kiruna (Sweden) in real-time. Thus the images were processed while the event was happening, and put online on the internet within one day. The two images below are examples of imagery obtained for the purposes of flood monitoring.

The image below was acquired on 22 October, before the floods burst out, and shows the normal situation. It is clearly an area of high water flow, but the image suggests a pattern of deposition from the meltwater outflows, leaving the coarser deposits with their higher backscatter close to the glacier, while towards the sea are smooth beds of finer gravel and sand.

The image below, acquired on 7 November, shows the difference after the flood. Backscatter remains high, from the mouths of the meltwater streams all the way to the coast, indicating that a large quantity of coarse material has been deposited. Given the quantity of water and the high speed with which it is travelling, the rivers in the main are too turbulent to give the normal low backscatter that one associates with river water.

On 5 November the volume of the Skeidera river had begun to increase rapidly. The water was dark with volcanic ash and tephra. By mid-morning icebergs broken off the glacier were being carried down on the flood, sweeping away several of the bridges across the river. During the jökulhlaup, the water in the caldera fell 165 m, and up to 100 million tons of volcanic material and clay were carried out to sea as far as 15 km from the coast. The plain was strewn with icebergs weighing up to 1000 tons.