According to the Icelandic news an glacier flood started around midnight in Múlakvísl (link should work, let me know if it doesn’t) glacier river that runs from Mýrdalsjökull glacier. Last time this type of event did happen was in the year 1955, when an minor eruption is believed to have taken place under the Mýrdalsjökull glacier. I do not know if that is the case now. As this might just be hydrothermal water being released due to increased activity in hydrothermal areas that are in Mýrdalsjökull glacier.
Following the flood there is an strong smell of sulphur and there might be dangerous gases in it. At current time the flood appears to be growing and according to the news (written at 01:54 UTC) the bridge was still safe. But there was just 1 meter from the river and up to the floor of the bridge.
Some minor earthquakes have been taking place in the past few hours, they do not appear on the automatic earthquake list on Icelandic Met Office web page. Some of them appear on my Hekla webicorder.
Currently the harmonic tremor is dropping. But that might not mean anything in the long run. So far everything suggests that an eruption has not yet started in Katla volcano. But given the current status that might not last long as it is. The best thing that can be done is to wait and see what happens. But no fear scaremongering please!
Added: When I just finished writing this blog post an new earthquake just started to appear on my geophone at Hekla volcano. IT is from Katla volcano for sure. It was in the ML1.5 to ML2.5 size range it seems. It was an long period earthquake as they have been so far. I think it is from the main caldera, not Goðabunga area.
Icelandic News about the glacier flood.
Hlaup hafið í Múlakvísl (mbl.is, Icelandic)