Jökull - 01.12.1976, Blaðsíða 45

the area was eroded by an ice cap which flowed
towards the south or east, perpendicular to the
coast. In this area one finds two of the best
known lateral glacier lakes in Iceland, Græna-
lón and Vatnsdalslón. A few glacier-margin
lakes are found in the active volcanic zone.
Most of these lakes are proglacial (or frontal)
and located in depressions between the NE-SW
running hyaloclastic ridges. No transverse val-
leys have been eroded in this relatively young
landscape. Therefore, lateral lakes are rare in
the active volcanic zone. A number of proglacial
lakes have turned up in this century in front
of the rapidly retreating glaciers. Most of them
have disappeared again and only the main pro-
glacial lakes are shown in Fig. 2.
Most lateral lakes are at present sources of
jökulhlaups, however small. Some of the pro-
glacial lakes are potential sources of jökulhlaups.
Hamarslón and Hvítalón can become damrned
when the neighbouring glaciers surge (Frey-
steinsson, 1972). Further, the lakes south of
Langjökull were dammed by the glacier up to
the late 1920:s. To the south the lakes are bound
by the lava sheet Lambahraun and when the
glacier advances the natural outlets of the north-
western edge of the lake can be dammed up
(see Wright (1955), Kjartansson (1938), Thorar-
insson (1939, 1966), Green (1952) and Sigbjarn-
arson (1967)).
Glacier advance would result in the forma-
tion af small lakes in ravines at the glacier
margin. Frequent but small jökulhlaups may
occur from these lakes. The advance of the out-
lets of Mýrdalsjökull, like Höfdabrekkujökull
and Sólheimajökull in the last few years, has
already caused small jökulhlaups (Einarsson
1973, see also Björnsson and Einarsson, this
issue p. 58). One of the lakes which would
form again is Ker which up to the 1950:s was
dammed between the mountain Kerfjall and
Múlajökull, SE-Hofsjökull (Gestsson, 1956). But
thickening of the ice barrier of already existing
lateral lakes woulcl decrease the frequency of
jökulhlaups and increase the water volume.
JÖKULHLAUPS FROM GRÆNALÓN AND
VATNSDALSLÓN - TRIGGERING
Data are available on the topography and
the jökulhlaup history for the two main ice-
dammed marginal lakes at Vatnajökull.
Grænalón is the largest marginal lake in Ice-
land (64°11'N, 17°24'W), Áskelsson (1936), Kjart-
ansson (1938), Thorarinsson (1939). The lake is
located in an ice-free tributary valley which is
dammed up by the western margin of Skeidar-
árjökull, which is an outlet at the southern
edge of Vatnajökull. During the last century the
lake drained over a col at 635 m to the river
Núpsvötn. But in 1898 the lake was emptied
in a jökullilaup which ran subglacially the
whole waterway down to the rivers Súla and
Blautakvísl at Skeidarársandur. From 1901 up
to 1935 the lake drained again over the col. In
1935 a jökulhlaup occurred again. The total
volume of the jökulhlaup was about 1500- 106
m3. The lake area before the jökulhlaup was
19 km2. The maximum depth of the lake was
180 m. Fig. 3. shows a cross-section of the lake
and the glacier at the start of the jökullilaup
in 1935.
The next jökulhlaups occurred in 1939, 1943,
1946, 1949 and 1951. Since 1951 the jökulhlaups
have become more frequent and less volumin-
ous, occurring once or even twice a year.
Further, the jökulhlaups do not run subglacial-
ly beneath the whole glacier but lift a shallow
ice barrier at the glacier margin and run along-
side tlie glacier down to Skeidarársandur. Typi-
cal values for the latest jökulhlaups are: total
volume of 200 • 108 m3 and maximum discharge
of 2000 m3/sec The maximum area of the lake
is about 13 km2 and the lake level drops only
about 20 m in each jökulhlaup (see Rist 1970,
1973).
Vatnsdalslón is dammed up in the tributary
valley Vatnsdalur by the eastern margin of
Heinabergsjökull (Thorarinsson 1939). The first
jökulhlaup in recent times occurred in 1898. Up
to that date the lake drained over a col at 464
m into the valley Heinabergsdalur. The total
volume of the jökulhlaup was 120- 106 m3 and
the estimated maximum discharge 3000 m3/sec.
Fig. 4 shows the cross-section of the glacier and
the lake at the start of the first jökulhlaups at
the end of the last century; lake area 1.9 km2.
In the present century the jökulhlaups have be-
come more and more frequent and less volum-
inous, occurring even twice a year (Rist 1973).
However, the drainage is still subglacial be-
neath the whole length of the glacier.
Consider the triggering of the jökulhlaups. A
floating model predicts a jökulhlaup when the
JÖKULL 26. ÁR 43