Jökull


Jökull - 01.06.2000, Side 27

Jökull - 01.06.2000, Side 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

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