Jökull - 01.06.2000, Blaðsíða 27

An ice-dammed lake in Jökulsárgil
blockage of drainage routes by ice collapse (see Ball-
antyne and McCann, 1980), and it is possible that
such events at Sólheimajökull could be attributed to
ice disintegration around the exit portal. Whilst it is
generally believed that conditions could promote the
short-term blockage of the portal area, it is unclear at
present whether these conditions would be capable of
blocking the outlet on a more permanent basis.
GEOMORPHOLOGICAL EYIDENCE AND
IMPLICATIONS OF AN ICE-DAMMED LAKE
IN JÖKULSÁRGIL
Visits were made to Jökulsárgil to establish the
presence or absence of an ice-dammed lake in the
valley. During the summer of 1990, lake shorelines
and sediments were observed in Jökulsárgil that had
not been present during the previous summer. Sever-
al shorelines could be identified; the first approxima-
tely 21.5 m above the deepest part of the valley floor,
and a further three discontinuous shorelines somewhat
lower down on the valley sides at 8.5 and 3.5 m
respectively. The clarity of the lower shorelines was
obscured by scree falls on the valley flanks. Lake sedi-
ments were present, overlying scree on the western
slopes of Jökulsárgil, and could also be observed in
patches on the eastern valley side. It was conclu-
ded that an ice-dammed lake had formed and drained
between the previous visit to the site in July-August
1989 and the visit made in the summer of 1990. The
valley is seldom visited, as it is not easily accessible,
and no further information concerning the status of
the lake or the times of both formation and drainage
could be ascertained from other visitors to the valley.
In an attempt to witness the presence of an ice-
dammed lake in Jökulsárgil, a visit was made to the
site in early March 1991. Evidence for the temporary
existence of an ice-dammed lake over the winter was
present in the form of lake shorelines (the highest
at 8 m above the valley floor), sediments, and shatt-
ered, platy lake surface ice still present on the valley
floor (Plates 1 and 2). The freshness of the evidence,
particularly the lake surface ice which had not yet
been removed by the river, led to the conclusion that
the ice-dammed lake had occupied the valley until
shortly before the site visit.
There are several reports of small jökulhlaups into
the Jökulsá (e.g. Sigurðsson, 1989; Sigurðsson et al.
1992) and more must surely be expected, given both
the modelling predictions and the geomorphological
evidence for the existence of an ice-dammed lake
described above. Reynir Ragnarsson (personal comm-
unication, 1991), the local police officer and private
pilot, reported that the farmer at Sólheimar had alle-
gedly observed an ice-dammed lake in the valley dur-
ing the summer of 1988, prior to the visits made as
part of this study.
Plate 1. Shattered platy lake surface ice in Jökulsárgil,
March 1991. -Isskœni við Jökulsárgil, í mars 1991.
JÖKULL No. 48 25