The August 2020 j&#246;kulhlaup from a marginal lake at Langj&#246;kull, W-Iceland:&#160;Course of events, discharge and volume estimates, future monitoring
Authors: Thorsteinsson Thorsteinn , Eythórsdóttir Kristjana G. , Jensen Esther H. , Jónsdóttir Ingibjörg , Pálsson Finnur , Gunnarsson Andri , Pálsson Hlynur Sk. , Sigurðsson Oddur , Karlsdóttir Guðbjörg H. , Þrastarson Ragnar H. , Sigurðsson Gunnar , Jóhannesson Tómas , Roberts Matthew J. 
Published: March 2021

&lt;p&gt;J&amp;#246;kulhlaups from marginal and subglacial lakes are a considerable hazard in Iceland and the rapid retreat of glaciers and ice caps is leading to hydrological changes in many locations at or near the glaciers. This calls for careful monitoring of glaciers and proglacial areas.&lt;/p&gt;&lt;p&gt;On August 17 2020, increased discharge was observed in Hv&amp;#237;t&amp;#225;, a glacial river originating in the ice cap Langj&amp;#246;kull. Sediment-laden j&amp;#246;kulhlaup waters filled a narrow gorge of the river near the farm and tourist resort H&amp;#250;safell and dead salmon were found strewn over fields 30&amp;#8211;40 km downstream.&lt;/p&gt;&lt;p&gt;Reconnaissance trips, overflights and satellite image studies revealed the following course of events:&lt;/p&gt;&lt;p&gt;A marginal glacial lake (current size: 1.3 km&lt;sup&gt;2&lt;/sup&gt;) started forming at 890 m elevation at the western margin of Langj&amp;#246;kull after the turn of the century. Sentinel-2 satellite images indicate that subglacial outflow from the lake had started in the morning of August 17. The exact path of the 2 km long subglacial water course can be inferred from a Landsat-8 image taken on November 11 2020. The image shows a narrow surface depression resulting from lowering of the glacier surface when the subglacial tunnel carrying the water was formed. The ice thickness averages 70 m along the flowpath.&lt;/p&gt;&lt;p&gt;Emerging from beneath the ice cap, the water flowed 13 km through the Svart&amp;#225; river canyon, eroding sediment from the river bed and canyon walls. Fresh colouring and sediment deposition was observed on sandur plains where Svart&amp;#225; joins the Geit&amp;#225; and Hv&amp;#237;t&amp;#225; rivers.&lt;/p&gt;&lt;p&gt;Observations of the j&amp;#246;kulhlaup (water level and flow velocity) as it passed beneath a bridge near H&amp;#250;safell help constrain discharge levels and flood volume at a location 18 km from the outlet at Langj&amp;#246;kull. In addition, real-time data on Hv&amp;#237;t&amp;#225; river water level are available from the Klj&amp;#225;foss hydrometric station 35 km further downstream, discharge started rising from a background value of 90 m&lt;sup&gt;3&lt;/sup&gt;/s on August 17 at 16:00. The flood peaked there at 260 m&lt;sup&gt;3&lt;/sup&gt;/s at 01:45 in the early morning of August 18 and had subsided again at noon on that day.&lt;/p&gt;&lt;p&gt;Using imagery from the Sentinel-2 satellites the area of the marginal lake is estimated to have diminished from 1.29 km&lt;sup&gt;2&lt;/sup&gt; to 0.46 km&lt;sup&gt;2&lt;/sup&gt; during the j&amp;#246;kulhlaup. A lowering of 4 m has been determined from aerial imagery and the total volume released was 3.4 million m&lt;sup&gt;3&lt;/sup&gt; according to preliminary estimates. We estimate an average flow velocity of 3&amp;#177;1 m/s for the entire distance from the outlet at the glacier to Klj&amp;#225;foss.&lt;/p&gt;&lt;p&gt;The glacier margin in the region has retreated by 500-1000 m and thinned by 3 m/a in the period 2004-2019 leading to the formation of the proglacial lake. Flooding events occurring in 2014 and 2017 have now been detected in hydrometric and remote sensing data. The lake is likely to become larger when retreat continues and further thinning of the ice may lead to more frequent j&amp;#246;kulhlaups in coming years. Plans to monitor the lake level and install early warning systems will be outlined in the presentation.&lt;/p&gt;