Jökull - 01.12.1977, Síða 75

Fig. 2. Tlie eastern ice caul-
hron after the jökulhlaup in
February 1977. Scale: about 3
km across the cauldron between
the outermost crevasses.
Mynd 2. Eystri sigketillinn eft-
ir hlaup í febrúar 1977. Um 3
km eru þvert yfir ketilinn milli
ystu sprungna.
Photo Oddur Sigurdsson,
February 14, 1977.
activity. Björnsson (1975) advanced the explana-
tion tliat the jökulhlaups in Skaftá were re-
leased from a water cupola which is formed at
a geothermal area as meltwater is trapped be-
neath a depression in the glacier surface. The
evidence in favour of this explanation is publish-
ed in the present paper.
According to Gísli Sigurdsson (pers. comm.),
the farmer at Búland in Skaftártunga, jökul-
hlaups have occurred in Skaftá almost every
year as far back as he remembers, say to about
1910. Most of the jökulhlaups before 1955 were
small in volume and the river could be crossed
by horses within one day from the start of the
jökulhlaups. A sulphurous smell was felt from
the water. According to Sigurjón Pálsson (1968
and pers. comm.), who was a farmer at Sandar
in Medalland from 1919 to 1947, jökulhlaups
were frequently observed during the first half
of the present century in thc river Kúdafljót,
which is fed by Skaftá. (The farm Sandar was
situated on an island in Kúdafljót). The inter-
vals between the jökulhlaups wliich could be
observed in Kúdafljót were variable, but never
ntore tlian 3 to 4 years. A faint sulphurous
smell was felt from the water. Drinking water
was never collected from the river cluring the
jökulhlaups. The largest jökulhlaup before 1955
was observed in early September 1938.
ín 1783 several jökulhlaups occurred in
Skaftá. Thorarinsson (1974) considered likely
that these jökulhlaups were causecl by volcanic
activity at the site fronr which the recent jiikul-
hlaups originate.
HYDROLOGICAL OBSERVATIONS
Sigurjón Rist has estimated the discharge of
all jökulhlaups in Skaftá since 1955. His limni-
graph is placed at Skaftárdalur (Fig. 1). The
liydrographs for the first ten jökulhlaups are
sliown in Fig. 3, together witli the flow summa-
tion curves. The hydrographs for the jökul-
lilaups in December 1974 and February 1977
were published in Jökull by Rist (1976).
The time interval between jökulhlaups has
varied from 1 to 3(0 year. The duration of
each jökulhlaup has been from one to two
weeks. The volume of the jökulhlaups has
varied fronr 50 • 10° m3 to 250 • 10® m3. The
total volume of water whicli drainecl in the
jökulhlaups during tlie last 22 years (1955—
1977) was 2.2 km3. The flow summation curve
in Fig. 4 shows that the runoff rate from the
cauldrons lias increased continuously since the
jökulhlaups started in 1955.
THE SUBGLACIAL HEAT SOURCE
Analysis of hydrological data makes possible
an estimate of the strength of the subglacial
lieat source.
JÖKULL27. ÁR 73