Jökull - 01.06.2000, Blaðsíða 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