The j&#246;kulhlaup from the subglacial lake Gr&#237;msv&#246;tn, beneath Vatnaj&#246;kull ice cap, in November-December 2021, revealing new insight in to slowly rising j&#246;kulhlaups
Authors: Magnússon Eyjólfur , Drouin Vincent , Pálsson Finnur , Hannesdóttir Krista , Belart Joaquín M. C. , Sigurðsson Gunnar , Wuite Jan , Jóhanneson Tómas , Ófeigsson Benedikt G. , Nagler Thomas , Gudmundsson Magnús T. , Högnadóttir Thórdís , Parks Michelle , Roberts Matthew J. , Berthier Etienne 
Published: March 2022

&lt;p&gt;The subglacial lake Gr&amp;#237;msv&amp;#246;tn, beneath Vatnaj&amp;#246;kull ice cap, has been an important study area since the first attempts to explain the physics of j&amp;#246;kulhlaups. The lake, covered by an up to 300 m thick ice shelf, is situated within a caldera of an active central volcano. It collects meltwater produced by geothermal and volcanic activity, in addition to meltwater from the glacier surface. During most of the 20&lt;sup&gt;th&lt;/sup&gt; century the period of water accumulation was typically 4-6 years, collecting 1-3 km&lt;sup&gt;3&lt;/sup&gt; of water. The j&amp;#246;kulhlaups, as&amp;#160; observed in the river at the glacier terminus ~50 km south of the lake, typically reached peak discharge of 2,000-10,000 m&lt;sup&gt;3&lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt; after approximately exponential increase over 2-3 weeks. In October 1996, 3.2 km&lt;sup&gt;3&lt;/sup&gt; of meltwater from an eruption north of Gr&amp;#237;msv&amp;#246;tn were collected in the lake. This resulted in hydrostatic uplift of the lake ice dam and sudden release of the accumulated water, reaching a peak flow of ~50,000 m&lt;sup&gt;3&lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt; at the glacier terminus in less than a day. Due to damage to the lake ice dam during the 1996 j&amp;#246;kulhlaup and further undermining from geothermal activity near the dam, the water accumulation and release has been different after this event. Between 1996 and 2018, smaller j&amp;#246;kulhlaups typically occurred at 1-2 year intervals with the largest volume of ~0.6 km&lt;sup&gt;3&lt;/sup&gt; in 2004 and 2010. The j&amp;#246;kulhlaup discharge still resembled an exponentially rising discharge, but faster, reaching a peak discharge at the glacier front in 3-5 days after detection of flood water in the river. In 2004 and 2010 the peak discharge was ~3,000 m&lt;sup&gt;3&lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt;. From autumn 2018 until November 2021, ~1 km&lt;sup&gt;3&lt;/sup&gt; of water accumulated in Gr&amp;#237;msv&amp;#246;tn. The lake level has been monitored since the 1990s, but now with increased accuracy using online GNSS stations located on the floating ice shelf and repeated glacier surface mapping using Pl&amp;#233;iades stereo images. Around mid-November 2021 the GNSS instruments started subsiding, revealing that the lake had started draining. In 3 weeks, the discharge from the lake, estimated from the subsidence rate and the lake hypsometry, gradually increased from a few m&lt;sup&gt;3&lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt; to a peak discharge of ~3500 m&lt;sup&gt;3&lt;/sup&gt;s&lt;sup&gt;-1&lt;/sup&gt; on 4 December. A few days later, the lake had drained completely. We present the data showing the development at the lake prior to and during the j&amp;#246;kulhlaup, and we report on: a) discharge measurements near the glacier front, which combined with the lake discharge allows for an estimate of the temporal subglacial floodwater storage, b) horizontal and vertical ice motion in the vicinity and above the subglacial flood route during the j&amp;#246;kulhlaup, from ICEYE and Sentinel-1 radar images obtained with InSAR (24 hour repeat) analysis and amplitude offset tracking, showing the distribution of flood water and the widespread effect of the j&amp;#246;kulhlaup on the horizontal ice motion, c) ice surface motion measured by a GNSS station located half-way between the lake and the glacier margin, spanning the entire j&amp;#246;kulhlaup. All this provides new insight into the physical processes occurring during a slow, exponentially rising j&amp;#246;kulhlaup from Gr&amp;#237;msv&amp;#246;tn.&lt;/p&gt;