Jökull - 01.12.1970, Blaðsíða 38

Fig. 9.
cases this drainage was controlled by fluted
ground moraine. Once the ice front started
retreating down a reverse slope, or if a sequ-
ence of substantial moraines blocked the sea-
ward route of meltwaters, drainage parallel or
sub-parallel to the ice front became dominant.
An examination of the maps accompanying this
paper clearly indicates that after 1904, when
the drainage was largely beyond the outermost
moraines, the drainage system gradually became
more complicated reaching its most complex
form during the decade 1935—45. After 1945
the development of the proglacial lakes led to
a less complicated drainage system so that by
1965 only three major channels carried all the
meltwaters to the coast.
It is interesting to speculate what will happen
with further retreat of the ice margin. If the
subglacial surface to the north of the area just
east of the Breidárlón becomes progressively
lower (in 1965 the altitude of this divide was
45 m), it is possible that the east flowing
drainage (i.e. towards the Jökulsárlón) could
cut back across the relatively short distance of
300 m, which in 1965, formed the drainage
divide between the Jökulsárlón and the Breid-
árlón drainage. Since in 1965 the altitude of
the surface of the Breidárlón was 27.5 m, the
altitude of the surface of the long narrow pro-
glacial lakes leading to the Jökulsárlón was 19.5
m and the altitude of the Jökulsárlón itself
was 5.5 m, it may well be that the Jökulsá will
eventually act as a major outlet of the Breidár-
lón, and the Fjallsá will only carry meltwaters
from Fjallsjökull.
The situation in the eastern half of the area
is a little different. The drainage divide separ-
ating the drainage flowing to the Jökulsárlón
and that flowing to the Stemmulón is at 40 m.
However the surface of the upper Stemmulón
is at approximately 9 m whereas the surface of
the proglacial lake to the west of the divide is
at approximately 20 m, so if the sub-glacial
surface does decrease in altitude it is likelv
that this lake would drain eastwards into the
Stemmulón. However, looking further ahead,
since the surface of the Stemmulón is at ap-
proximately 9 m and that of the Jökulsárlón
is at 5.5 m, if these water levels are maintained
during further retreat of the ice front and the
sub-glacial surface does decrease in altitude
there could be a reversal of drainage so that
tlie Stemmulón is drained into the Jökulsárlón.
The history of drainage evolution in this
area and some suggestions about future deve-
lopment are not only of academic interest. The
36 JÖKULL 20. ÁR