Jökull - 01.01.2019, Blaðsíða 3

Guðmundsson et al.
The rapid glacier downwasting since the mid-
1990s has dramatically affected the geomorphology
of the glacier forelands, with the formation of many
new glacial lakes, rapid growth of existing lakes
(Schomacker, 2010) and changes in outlet locations
and the paths of glacial rivers (Björnsson and oth-
ers, 2018). Terminus lakes have now formed in
front of all glacier termini that reached the lowland
south of Vatnajökull at the end of the Little Ice Age
(LIA) late in the 19th century (Figure 1) (Hannes-
dóttir and others, 2014, 2015a,b; Guðmundsson and
others, 2017). Radio-echo sounding measurements of
the outlet glaciers of S-Vatnajökull have shown that
most of these lakes will grow along deep subglacial
troughs if the glaciers continue to retreat in a warm-
ing climate (Björnsson, 1996, 2009a; Björnsson and
others, 2001; Björnsson and Pálsson, 2008; Magnús-
son and others, 2007, 2012).
Terminus lakes are of two main types. On the one
hand, water may accumulate in depressions by glacier
termini retreating over an undulating bed topography,
sometimes containing dead ice. These lakes are typ-
ically shallow and may change their geometry from
year to year. They often become separated from the
retreating glacier, and isolated ponds without inflow
or an outlet are formed. On the other hand, glacial
lakes may form in deep troughs in the topography of
the bedrock below glacier that were carved by glaciers
during the Ice Age. The troughs were filled with sed-
iments by glacial rivers after the Ice Age but were in
many cases carved out again and filled with ice during
the LIA (H. Björnsson, 1996, 1998). This type of ter-
minus lake continues to grow until the glacier retreats
out of the upstream end of the trough, which may be
many km long.
In addition to the two types of terminus lakes,
glacial lakes exist where glaciers dam water in side
valleys, as mentioned above, and at the glacier bed
below depressions in the ice surface, typically in con-
nection with subglacial geothermal activity such as in
Grímsvötn and at the Skaftá Cauldrons (Björnsson,
1974, 1976b, 2002, 2009b). Glacier-dammed lakes
at the lateral ice margins have shrunk and the dis-
charge of jökulhlaups from them has decreased during
the 20th century because the ice dams have become
thinner due to the retreat and thinning of the glaciers.
This paper will deal only briefly with glacier-dammed
lakes in side valleys or subglacial lakes and concen-
trate mainly on lakes formed at the terminus. The
main focus is on the development of the lakes after
the year 2000, but the earlier history of some of the
terminus lakes since their formation will be summa-
rized.
Terminus lakes affect the flow of glaciers termi-
nating in the water and their mass and energy bal-
ance. For a glacier that does not terminate in a lake,
a negative mass balance perturbation will eventually
be compensated by the reduction in the ablation area
caused by the retreat of the terminus. This stabiliz-
ing negative feedback caused by variation in the ter-
minus position is partly decoupled for a glacier that
terminates in a lake because of calving and melting
of the ice front at the shore of the lake. The effect
of terminus lakes on glacier mass balance is particu-
larly important for tidewater glaciers such as Breiða-
merkurjökull because of heat exchange with the ocean
through tidal currents (Björnsson and others, 2001;
Landl and others, 2003; Björnsson, 2017). Termi-
nus lakes thus lead to more rapid glacier downwasting
than would otherwise have been observed (e.g. King
and others, 2017; Tsutaki and others, 2018).
Terminus lakes are, furthermore, associated with
hazard to settlements and travellers in the adjacent
area, as landslides on the glaciers that propagate into
terminus lakes can create tsunami waves with a high
run-up and sudden very dangerous flash floods in the
glacier forelands (Kjartansson, 1967, 1968; Komori,
2008; Ives and others, 2010; Nie and others, 2013;
Deline and others, 2014). Gylfadóttir and others
(2017) describe a recent landslide-induced tsunami in
Askja in central Iceland, which clearly demonstrates
the hazard caused by such waves. The study of ter-
minus lakes is, therefore, important for understanding
the effect of climate change on glaciers and the asso-
ciated hazards to society.
DATA AND METHODS
Information about the formation and development
of ice-marginal lakes by S-Vatnajökull is collected
from written accounts, photographs, maps, aerial im-
2 JÖKULL No. 69, 2019