Jökull - 01.01.2019, Blaðsíða 10

Terminus lakes on the south side of Vatnajökull
2019, the volume of the two main lakes in (Figure 4)
was ∼37 and ∼113×106 m3 in 2018.
Grænalón and Norðurdalur in Skaftafellsfjöll
The maps of the DGS from 1904, and of the Dan-
ish Geodætisk Institut based on aerial images from
1937–1938, show four dammed lakes in side valleys
to the west of Skeiðarárjökull. Grænalón was the
largest, > 17 km2 in area (Figures 6 and 18). The
other three, to the south of Grænalón, were 0.5, 0.4
and 0.08 km2. Three lakes were located by the east-
ern margin: Langagilslón, 0.1 km2 in area, and two
lakes in Norðurdalur in Skaftafellsfjöll, Norðurdals-
lón, 0.4 km2 in area, and a lake without a name by
Norðurfell, 0.3 km2 in area. All these lakes released
jökulhlaups onto the Skeiðarársandur plain (Björns-
son, 1976a; Björnsson and Pálsson, 1989; Roberts
and others, 2005), but since the turn of the century
these floods have been insignificant. The lakes in
Norðurdalur were located farther upvalley than now,
with water level at 630 and 640 m a.s.l., during the
early 20th century, because the glacier was thicker,
reaching an elevation of 660 m a.s.l. close to the lakes.
After the middle of the 20th century, Grænalón
grew to a maximum area of ∼15 km2 between jökul-
hlaups, which were released once or twice per year
(Björnsson and Pálsson, 1989). After 1986, the floods
became smaller as a result of the lowering of the
glacier surface (Roberts and others, 2005) and, cur-
rently, Grænalón has an area less than ∼0.2 km2 and
jökulhlaups are no longer released from the lake.
Norðurdalslón, on the other hand, developed into sev-
eral small lakes that might eventually merge into a
single lake as a consequence of further lowering of
the glacier that dams the lakes. The water level in the
Norðurdalur lakes was close to 475 m a.s.l. in 2010
and the elevation of the glacier damming them ap-
proximately 500 m a.s.l. according to the lidar DEM
of that year.
Glaciers on the west side of Öræfajökull
Terminus lakes have formed by all outlet glaciers of
Öræfajökull that extend down to the lowland. The
largest lakes on the western side are by Morsárjök-
ull, Skaftafellsjökull, Svínafellsjökull and Virkisjök-
ull/Falljökull (Figures 5 and 7), which have all re-
treated 0.8–2 km from their LIA maximum extent
(Hannesdóttir and others, 2014, 2015a,b). Radio-echo
sounding measurements (Björnsson, 2009a; Magnús-
son and others, 2012) show that three of these glaciers
have carved troughs that reach below sea level.
Morsárjökull
Lakes started forming in front of Morsárjökull after
1938 and no later than 1943, probably in 1941 or
1942, and the lake area in 1945 was close to 0.01 km2
(Stötter and others, 2016) (Figures 5 and 8). Ten years
later, they had grown to 0.1 km2 (Ives, 1956). There
were two lakes by the terminus in 1960, the northern-
most one of them quite shallow. The deeper lake was
∼20 m deep by Svarthamrar at this time (Jón Eyþórs-
son, 1960, 1961). Morsárjökull retreated steadily,
by 1500 m in total, in the period 1904–1970. The
ice margin fluctuated or advanced slightly in the last
three decades of the 20th century, and the glacier front
stood approximately 200 m farther into the lake in
1994 (Landsat 5) than in 1975 (NLSI aerial images).
The bathymetry of the lake was measured in 1986 by
Stötter (1990) who found a maximum depth of 62 m
and estimated the lake volume to be ∼8×106 m3 at
this time.
The ice margin started retreating again in the early
1990s, and the rate of retreat accelerated around 2000
according to terminus measurements of the Iceland
Glaciological Society. By 2018, it had in total re-
treated 600 m since the 1990s. The lake grew rapidly
during this period, from 0.3 to 0.64 km2 in 1994–
2018, or on average by ∼0.015 km2 a−1.
The main lake may be expected to continue to
grow, as there is a ∼3 km long subglacial trough
reaching below sea level under the glacier with a
maximum width of approximately 500 m (Björnsson,
2009a; Magnússon and others, 2012). The lake area
might eventually become ∼2 km2 if the glacier re-
treats out of the trough. Based on the radio-echo
sounding map of the glacier bed (Magnússon and oth-
ers, 2012) and the water level in the lake in 2012
according to the lidar DEM, the volume of the lake
was ∼39×106 m3 in 2018. In 2018, the terminus was
close to the deepest part of the subglacial topography
where the water depth is > 200 m.
JÖKULL No. 69, 2019 9