Hvitarvatn is a proglacial lake adjacent to Langjokull, the second largest ice cap in Iceland. The lake is 422 m a.s.l. and 36 km2 with a maximum depth of 85 m (Fig. 1). Langjokull feeds two outlet glaciers, Suðurjokull and Norðurjokull, which both terminated in the lake during the Little Ice Age (LIA; AD 1300–1900). The LIA extent of both outlet glaciers is clearly defined by moraines visible on land and in high-frequency multibeam sonar bathymetric images of the lake bottom (Geirsdottir et al. 2008). The multibeam sonar bathymetry, chirp bottom profiles and numerous sediment cores contain information on the sedimentary history of the lake and indicate that, following regional deglaciation 10 ka ago, glaciers entered the lake only during the LIA (Larsen et al. 2011, 2012). Suðurjokull remained in the lake until the mid-twentieth century, whereas Norðurjokull retreated from the lake in the summer of 2009. The multibeam sonar bathymetric data show large differences in the behaviour of Norðurjokull and Suðurjokull during the LIA that can be explained by both glacial processes and variations in the stability of the catchment. Fig. 1. Langjokull and Hvitarvatn glacier geological setting. ( a ) Topographic map of field site showing location of Hvitarvatn (HVT) adjacent to Langjokull's eastern margin (map gridlines are spaced 1 km). Lake surface is overlain with bathymetric map; maximum LIA extent (dashed lines) of outlet glaciers Norðurjokull (NJ) and Suðurjokull (SJ) are visible in lake bathymetry. Inset map highlights position of Langjokull in Iceland's central highlands. Other geographical features mentioned in text are Skriðufell (SK), Leggjarbrjotur (LB) and the Hvitarnes delta (HV). ( b ) Multibeam bathymetric map of Hvitarvatn's main basin showing detailed view of lake-floor morphology and position of seismic profile (dashed line) presented in (c). Multibeam acquisition system Seabat 8101. Frequency 240 kHz. ( c ) Chirp seismic profile (dashed line …