Jökull - 01.01.2019, Blaðsíða 22

Terminus lakes on the south side of Vatnajökull
from Fjölsvinnsfjöll and Maríusker. This lake was at
least 24 m deep in 1951 (Eyþórsson, 1951) and 84 m
deep in 1966 (Howarth and Price, 1969). Another
lake was formed west of the moraine after 1945 and
these two lakes merged in 1954 when the river Breiðá
changed course to the west into Fjallsárlón and the
water level in Breiðárlón fell by 3.5 m (F. Björnsson,
1954, 1955).
Breiðárlón had become ∼6 km2 in 1994, but since
then the lake has been slightly reduced in size as
a consequence of accumulation of sediments in the
lake. From 1994 to 2018, the lake area increased by
0.85 km2 due to the retreat of the glacier terminus but
decreased by more than 1 km2 because of sediment
accumulation at the lake shoreline near the river en-
trance. According to a DEM of the glacier bottom
based on radio-echo sounding (Björnsson and others,
1992; Björnsson, 1996), recent water-depth soundings
and the current water level in the lake, the volume of
Breiðárlón was ∼110×106 m3 in 2018 and the maxi-
mum depth ∼40 m.
The river path of Breiðá had become ∼3.5 km
long in 2018 because of retreat of the glacier termi-
nus and filling-in of sediments in the Breiðárlón lake.
In 1994, the river fell almost directly from the outlet
into the lake at a location that is now 1.85 km from the
current lakeshore.
The Breiðárlón lake fills a long and narrow sub-
glacial trough carved out by the glacier. Continued
retreat of the glacier will eventually lead to the for-
mation of a ∼9 km long and ca. 100 m deep lake with
an area of ∼6 km2 if the glacier retreats out of the de-
pression (Björnsson, 2009a).
A new terminus lake started to form between
Breiðárlón and Jökulsárlón around the year 2000, to
the north of a gabbro outcrop exposed by the retreat
of the terminus in this area in the 1970s. Two lakes
were growing at this location in 2003, the western-
most one 0.36 km2 in area and the easternmost one
0.6 km2 (Figure 15). The easternmost lake sits in a
subglacial trench eroded by Breiðamerkurjökull (Fig-
ure 17) and may therefore be expected to continue to
grow northwards, forming a long and deep lake in the
future.
Jökulsárlón and Stemmulón
The tidal lagoon Jökulsárlón á Breiðamerkursandi is
the largest terminus lake in Iceland (Björnsson, 1996,
1998; Björnsson and others, 1999b, 2001) (Figures
15 and 18). The lagoon began to form in 1933 af-
ter the Breiðamerkurjökull glacier retreated from the
outermost terminal moraines on the sandur plain and
the glacier surface was lowered. Several small lakes
initially appeared and they became 100–300 m wide
rather quickly (F. Björnsson, 1993). Before this, sev-
eral small lakes had formed as early as 1909 between
the retreating ice margin and terminal moraines both
to the west and east of the current Jökulsárlón (Páls-
son, 1953). These earlier lakes eventually vanished or
were left as small ponds.
There were two lakes by the terminus of Breiða-
merkurjökull at the current location of Jökulsárlón
in 1945: the western one was 0.5 km2 and the east-
ern was 1.6 km2 (AMS, 1951). These lakes merged
in 1954. Jökulsárlón was at least 35 m deep in
1951 (Eyþórsson, 1951). Soundings three years later
showed 60 m depth in the western part of the lake, and
in 1958 the depth was found to be 110 m (F. Björns-
son, 1954; Kjartansson, 1957). Bathymetric surveys
in 1975 by Boulton and others (1982) showed the lake
to be 150 m deep near the ice margin and have a vol-
ume of 500×106 m3.
The neighbouring Stemmulón by Breiðamerkur-
jökull to the east of Jökulsárlón also started to form in
the early 1930s; the first signs of the lake to the north-
west of Brennhólaalda are reported in 1931 (Todt-
mann, 1960). The lake area grew rather rapidly at
first and had become 1.1 km2 in 1945 (AMS, 1951).
Around the middle of the century, there were two con-
nected lakes in this area, one of them by Brennhóla-
alda and the other by the terminus (Figure 15). The
depth of these lakes in 1966 was 9–12 m and 36 m, re-
spectively (Howarth and Price, 1969).
The area of the Stemmulón lakes reached 3.6 km2
at the end of the 1980s. Outflow from the lakes
towards the west into Jökulsárlón started abruptly
on 1 September 1990 (Imsland, 1990). The river
Stemma, which had flowed along the eastern part of
Breiðamerkursandur, disappeared and the water level
in Stemmulón dropped. Four days later, on 5 Septem-
JÖKULL No. 69, 2019 21