Jökull


Jökull - 01.06.2000, Qupperneq 28

Jökull - 01.06.2000, Qupperneq 28
Fiona S. Tweed Plate 2. Ice-dammed lake shorelines and sediments in Jökulsárgil, March 1991. - Strandlínur og set úr jök- ulstífluðu vatni við Jökulsárgil. Myndin er tekin í mars 1991. It is at present unclear whether small ice-dammed lakes in Jökulsárgil form and drain once a year or whether their formation and drainage is more frequ- ent. Likewise, establishing a recognized drainage mechanism for the ice-dammed lakes is problematic, given the predominantly shallow lake depths defined by the shorelines. Frequent formation and drainage of shallow ice-dammed lakes may be possible, the drainage of such lakes being periodically halted, per- haps by falling ice blocks temporarily blocking the ex- it portal. This type of “stop/start” drainage would give rise to the series of shorelines observed. CONCLUSIONS 1) Geomorphological evidence from Jökulsárgil indicates an ice-dammed lake occasionally forms in the valley. The frequency of formation and drainage of this ice-dammed lake are unknown at present, but it is suggested that it could form over winter and drain in early spring. 2) Modelling based on circular or semi-circular tunn- el cross-sections predicts that the tunnel at Sólheima- jökull does not close, even at the lower end of the assumed discharge range. This is not in agreement with observations. However, existing models of tunn- el dynamics define tunnel cross-sections that do not reflect the true form of many subglacial conduits. The tunnel at Sólheimajökull is broad and low rather th- an circular or semi-circular in cross-sectional form. An understanding of ice conduit dynamics indicates that the ice tunnel at Sólheimajökull can close per- iodically when the discharge of water through the tunnel is sufficiently low to permit overburden closure to exceed melting of ice from the tunnel walls. The periodic formation of an ice-dammed lake in Jökulsár- gil supports the conclusion that a broad and low tunnel collapses more rapidly than a circular or semi-circular tunnel. 3) The present configuration of the snout of Sólheima- jökull means that the river exiting Jökulsárgil is likely to continue to drain through the glacier by means of 26 JÖKULL No. 48
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