Jökull - 01.01.2019, Side 27

Guðmundsson et al.
flow over the ridge between the valleys (Thorarins-
son, 1946). In 1898, a large jökulhlaup was released
from the glacier that inflicted considerable damage
on farmland south of Heinabergsjökull (Benediktsson,
1999; Kristjánsdóttir, 1999).
Jökulhlaups were released annually from Vatns-
dalslón from 1898 until 1953 but became less fre-
quent after that (Gíslason, 1954; Björnsson, 1976a;
Benediktsson, 1999; Kristjánsdóttir, 1999). The jök-
ulhlaups from Vatnsdalslón not only burst out from
under the terminus but sometimes also flowed along
the eastern glacier margin, according to Gísli Arason
(pers. comm. July 2016).
It is likely that the first jökulhlaup from Vatns-
dalslón triggered the last outburst flood from Dalvatn
(Benediktsson, 1999; Kristjánsdóttir, 1999), which
for this reason was more abrupt and caused much
more damage than later floods. Kristjánsdóttir (1999)
claims that Dalvatn existed for 20 years after 1898
while Heinabergsjökull dammed the valley. This
is inconsistent with the DGS (1905c) map of the
area, which shows Dalá flowing unimpeded along the
valley.
The area of Vatnsdalslón is estimated to have been
1.9 km2 at the end of the 19th century when there was
overflow into Heinabergsdalur. The area was down to
0.8 km2 in 1945 according to the AMS (1951) map of
the area. There has been slight accumulation of wa-
ter in the Vatnsdalur valley since 2000 and stranded
icebergs on the valley bottom show that the lake is
occasionally emptied, but the jökulhlaups are small
and do not produce noticeable floods in the Kolgríma
river that issues from the glacier. The volume of
the 1898 jökulhlaup was estimated as 120×106 m3 by
Thorarinsson (1946). According to Gíslason (1954),
the lake was 60–80 m deep when the regular, annual
jökulhlaups were released. The elevation difference
between the valley bottom and the overflow thresh-
old into Heinabergsdalur is 160 m and the lake must,
therefore, have been approximately twice as deep at
its highest water level during the 19th century.
Fláajökull
Shallow lakes had started to form by Fláajökull
around 1900. A lake then formed north of Heina-
bergsöldur, near Grænafell, when the glacier retreated
from the LIA maximum terminal moraines in this area
(Figures 20 and 21). This lake existed until the 1940s
(Guðmundsson and Björnsson, 2017). It was a little
less than 0.07 km2 in 1903. When H.H. Eiríksson
(1932) surveyed the glaciers in the Mýrar district in
1930, it was ∼0.27 km2. The lake area may have been
similar in 1937 when Sigurður Þórarinsson inspected
this area (Sigurðsson, 2004). The water depth was ap-
proximately 15 m around this time, but the lake may
have become shallower with time because the water
level gradually fell as the glacier terminus retreated.
This lake is again similar on the DGS (1944a) map,
which was revised based on information from 1937,
but it had disappeared in 1945 according to the AMS
(1951) map of the area. This map doesn’t depict an
ice-dammed lake at Múlagil, but a lake existed there
in 1945.
Another lake had formed at the ice margin below
Merkifell before 1930 (Eiríksson, 1932), and a third
lake formed on top of Miðfell in the late 1930s. More
such lakes were formed along both arms of the Fláa-
jökull terminus in the following decades (Kristjáns-
dóttir, 1993). Most were of variable size and became
separated from the retreating terminus in the undu-
lating terrain of the glacier foreland. New lakes of-
ten formed and others disappeared or became isolated
ponds. The total lake area fluctuated with time.
The main lake increased in size by 0.4 km2 after
a protective dam was built to control the rivercourse
of Hólmsá in 2002. The lake level has maintained the
same elevation since then and the lake has grown as
the glacier terminus has retreated. The area of lakes
by Fláajökull was 1.7 km2 in 2018 and by that time
terminus lakes had started to merge north of Miðfell
and by Merkifell. Further retreat of Fláajökull will
eventually lead to the formation of three or four lakes
in bedrock depressions. The largest lake will become
> 4.5 km long, > 200 m deep and ∼6 km2 in area if
the glacier retreats out of the subglacial depressions
(Björnsson, 2009a).
Viðborðsjökull
Viðborðsjökull, a small outlet glacier between Fláa-
jökull and Hoffellsjökull, retreated by 4 km in the pe-
riod 1904–2018 (Figure 22). A small lake was formed
in dead ice in front of the glacier in the early 1980s
26 JÖKULL No. 69, 2019