Subglacial canyons in the path of jökulhlaups from the subglacial lake Grímsvötn, Vatnajökull ice cap, Iceland

The subglacial lake Grímsvötn in central Vatnajökull ice cap (Iceland), collects surface meltwater as well as meltwater produced by volcanic and powerful geothermal activity. The meltwater is released in jökulhlaups almost annually in recent times. The flood route out of Grímsvötn beneath its seal has been at similar location since 1996 when a massive jökulhlaup (net volume of 3.6 km³ and ~40,000 m³ s^-1 peak discharge) caused by an eruption north of the lake drained out via a new flood route. Here we present subglacial bedrock map of 6 km² area adjoining Grímsvötn and spanning both the current drainage route out of Grímsvötn as well as the route prior to 1996. The bedrock map, based on field surveys in 2021–2024, was created from traced bed reflections in 3D migrated radio echo sounding profiles, measured only 20 m apart. It reveals a rare example of a subglacial landforms most likely carved by large floods. This includes canyons with near-vertical walls and cataracts at the upstream ends. The most prominent canyon is over 100 m deep and ~200 m wide and is located where the flood route from the lake was before 1996. The timing of the canyon formations is unknown. The record of jökulhlaups from Grímsvötn with estimated magnitudes dates back to the 1930s, when two eruption-related jökulhlaups (in 1934 and 1938) of similar magnitude as in 1996 drained from Grímsvötn; other jökulhlaups from Grímsvötn in the past ~100 years have been smaller. Since there is no clear evidence of significant flood erosion in the path of the 1996 jökulhlaup, the canyons were likely formed by significantly larger jökulhlaups, ruling out formation by the jökulhlaups in the 1930s, as well as other smaller jökulhlaups since then. Glacier is required to produce large floods from Grímsvötn, both for damming the lake and as a source of meltwater. Canyons adjacent to Grímsvötn at same elevation and bent towards them, are therefore most likely eroded subglacially by jökulhlaups from the lake.