Jökull - 01.01.2019, Blaðsíða 29

Guðmundsson et al.
A lake started to form in front of Svínafellsjök-
ull, the western tongue of Hoffellsjökull, around the
year 1900 (Figures 21 and 22). The DGS (1905c)
mapped two lakes by the terminus in 1903, when the
glacier had retreated 0.3–0.5 km from LIA terminal
moraines, with a combined area in excess of 0.04 km2.
There was one rapidly growing lake by the terminus in
the 1930s. When the terminus was again mapped by
the DGS (1944a) in 1937, the glacier had retreated an
additional 0.8 km, and the total retreat from the LIA
maximum extent was 1.1–1.3 km, The lake area was
approximately 1.5 km2 around the middle of the 20th
century. The lake grew slowly to 1.7 km2 in the pe-
riod 1945–1973 (∼0.006 km2 a−1), to 2 km2 in the
period 1973–2000 (∼0.013 km2 a−1) and to 2.5 km2
in 2000–2018 (∼0.028 km2 a−1).
A lake started forming in front of the eastern
glacier tongue, Hoffellsjökull, in the 1930s. The
glacier terminus still extended to the LIA moraine
at this time and the lake was located by the outlet
of Austurfljót river, north of Svínafellsgöltur. The
river outlet had moved to this location earlier in the
20th century, but before that time the outlet was lo-
cated by Geitafellsbjörg (AMS, 1951; Þrúðmar Þrúð-
marsson, pers. comm. May 2018). A lake filled with
calved icebergs was first formed in the eastern part of
the terminus at Geitafellsbjörg in the 1950s (Eyþórs-
son, 1962; Rist, 1984b). This lake grew at a rate of
∼0.012 km2 a−1 to 0.37 km2 in 1973–2000.
The growth of the eastern lake by Hoffellsjökull
intensified around the turn of the century. The river
Austurfljót from the eastern part of the glacier gradu-
ally dried up from the autumn of 2006 to 2008 (Páls-
son and Björnsson, 2007; Þrúðmar Sigurðsson, pers.
comm. November 2018), the water level in the lake
was gradually lowered and the outermost part of the
tongue of the glacier started to break up. The lake
grew by ∼0.1 km2 a−1 in the period 2000–2018 and
the area had then reached > 2 km2. In 2016–2017,
the glacier tongue south of Öldutangi was completely
broken up and the lakes by Svínafellsjökull and Hof-
fellsjökull had merged into one large lake, which had
a combined area of 4.7 km2 in 2018. Further retreat
of Hoffellsjökull will eventually lead to the formation
of a ∼7 km long and > 250 m deep lake with an area
> 11 km2 if the glacier retreats out of the subglacial
depression (Pálsson and Björnsson, 2007; Björnsson,
2009a), (Figure 22). The current volume of the lake is
∼220×106 m3 according to the radio-echo sounding
map of the bed.
DISCUSSION
The formation and continued growth of terminus lakes
in front of most termini of outlet glaciers from S-
Vatnajökull, is one of the most important changes in
the natural environment in Iceland due to recent and
possible future warming of the climate. These lakes
affect the ice flow and the mass and energy balance of
the outlet glaciers and dramatically change the visual
appearance of the landscape in the neighbourhood of
the glaciers, many of which are popular tourist attrac-
tions. The lakes are, furthermore, associated with haz-
ard to settlements and travellers in the adjacent area,
as landslides on the glaciers that propagate into the
lakes can create very dangerous flash floods in the
glacier forelands.
The formation of the terminus lakes is one of
many changes caused by glacier variations that the
neighbouring settlements have had to adapt to since
the settlement of Iceland in the 9th century. Before
the Little Ice Age, many valleys that glaciers later ad-
vanced into, were forested and used as grazing areas.
These were destroyed by advancing glaciers, which
also overran farmsteads in several cases (Þórarins-
son, 1974). If the outlet glaciers of S-Vatnajökull
retreat into the bottom of the valleys in the future,
which seems likely, the newly formed lakes will in
many cases extend across the valley, and the inner-
most parts of the valleys will be most easily accessed
by boat. Several of the lakes will be among the largest
lakes in Iceland, Jökulsárlón will eventually become
∼80 km2 and the largest lake, at Skeiðarárjökull will
be ∼130 km2.
Landl and others (2003) estimated heat flow into
Jökulsárlón due to atmospheric radiation and sensible
heat flux as 80 W m−2, and a similar result is used in
the analysis by Bergsdóttir (2012) of the energy bal-
ance of Jökulsárlón for the year 2011. In lakes where
the availability of calved ice fragments is sufficient,
one may assume that similar energy flux conditions to
those in Jökulsárlón would be encountered and the ad-
28 JÖKULL No. 69, 2019