According to the Icelandic Meteorological Office, the water level of the Skaftá river at Sveinstindur (the closest gauging station at 28 km downstream from the ice margin) and electrical conductivity both rose on 29 September, indicating the beginning of a glacial outburst flood (jökulhlaup), originating from Grímsvötn’s Eastern Skaftá ice cauldron. GPS measurements indicated that the ice surface above the lake began to subside late on 27 September; the rate progressively increased reflecting increased discharge from the lake.
At 0330 on 1 October the discharge rate detected at Sveinstindur was higher than 1,300 m³/s, the highest rate recorded since the station was established in 1971. At around 1000, floodwater was also detected in Skaftárdalur at a discharge rate of ~400 m³/s and was rising quickly. GPS data from the eastern ice cauldron showed over 66 m of subsidence since 1800 on 27 September. IMO warned that hydrogen sulfide released from the floodwater as it drains is particularly potent at the river outlet from the ice margin, where concentrations may reach poisonous levels. The cauldrons drain every two years on average, producing floods of up to 1,500 cubic meters per second. During fieldwork later that day volcanologists observed where the jökulhlaup had burst through the glacier at several locations 1-2 km from the terminus. Ice fragments a few tens of centimeters in diameter were scattered near the terminus; ice blocks 3-5 m high and 10 m long were deposited close to the outflow points.
On 2 October IMO noted that the jökulhlaup was possibly the largest to have occurred from the Skaftá cauldrons. The discharge rate peaked at 0200, just short of 2,100 m³/s, however true discharge rate was thought to have been considerably greater (3,000 m³/s) since water flooded outside of the gauged area. The discharge rate peaked at 1300 at Eldvatn near Ásar at an approximate rate of 2,200 m³/s. According to a news article, the high waters in the Skaftá River damaged the bridge over Eldvatn prompting authorities to close the bridge during 4-5 October.
October 08, 2015. Grímsvötn
64.42 ° N, 17.33 ° W
Elevation 1725 m
Sources: Icelandic Met Office, Islande Magazine
Photo : Rax / Ragnar Axelsson via IMO