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This paper discusses Howard et al. (2012) who reconstruct the peak discharge of a glacial outburst flood, or ‘jökulhlaup’, for part of the Jökulsá á Fjöllum in north-central Iceland. They propose that this flood was the largest on Earth. We consider that the magnitude of the jökulhlaup proposed by Howard et al. (2012) warrants much more robust field evidence and demands more carefully parameterised hydraulic modelling. For these reasons we firstly (i) present their study in the context of previous research (ii) highlight issues with attributing landforms and sediments to jökulhlaups, and (iii) consider uncertainty regarding the timing and magnitude of jökulhlaups along the Jökulsá á Fjöllum. We argue herein that whilst a range of landforms and sediments that are attributable to jökulhlaups can be observed along the Jökulsá á Fjöllum, these are not necessarily diagnostic of jökulhlaups. Secondly, we critically discuss (iv) the major underlying assumptions of their study, and (v) their calculations and subsequent interpretations. These assessments lead us to consider that the proposal by Howard et al. (2012) of the largest flood on Earth is highly unrealistic, especially when due consideration is given to a possible source area and a trigger mechanism. [ABSTRACT FROM AUTHOR]

The 2010 Eyjafjallajökull eruption in Iceland provided a unique opportunity to quantify the evolution of proglacial geomorphology during a series of volcanogenic jökulhlaups (glacial outburst floods) (>140 events). Time-lapse imagery and repeat terrestrial laser scans before and directly after the eruption show that the jökulhlaup of 14 April 2010 composed 61% of the 57 x 106 m³ total discharge of the combined events, and had the highest peak discharge for the two main flood events, but only deposited 18% of the total volume of sediment in front of Gigjökull glacier. The majority of sediments (67% of a total volume of 17.12 x 106 m³) were deposited by the 15 April 2010 jökulhlaup, and this was followed by extensive reworking by a series of smaller jökulhlaups over the following 29 days that deposited 15% of the total sediment. The geomorphological and sedimentary signatures of the two largest jökulhlaups associated with the onset of the eruption have either been reworked by later floods or are buried by later flood deposits. Consequently, the ice-proximal, posteruption landscape cannot be used to reconstruct the characteristics or magnitudes of either of the two largest jökulhlaups. The findings support a complex-response model in which peak discharge and the bulk of the sediment transported is decoupled by changing routing mechanisms and water:sediment ratios during the eruption. [ABSTRACT FROM AUTHOR]