Jökull - 01.01.2001, Blaðsíða 29

Changes in the Gígjukvísl river channel
increased to about 100 m (Figure 5) during the surge
and jökulhlaup in 1991.
The large surge-related fan downstream of tran-
sect 2 (A in Figure 2) has a similar morphology when
the 1992 and 1997 maps are compared. However, the
80 m contour line is less fan-like on the 1997 map and
has a more glacier–parallel orientation (Figures 2 and
3). Several November 1996 jökulhlaup outlets extend
in a line westward from transect 2 (Figure 3). Flows
from these outlets modified the clearly defined shape
of the 1992 fan and infilled an area immediately to the
west of transect 2. Although some kettles formed fol-
lowing the 1996 jökulhlaup on the lower portions of
the 1991 surge fan, the bulk of the fan has survived
intact.
The island on transect 3 (at a distance of 400-
900 m, Figure 4) underwent major erosion during the
November 1996 jökulhlaup (Figure 3), truncating a
distinctive anatomising esker system. The erosion of
the right bank of the main channel cuts through a por-
tion of the 1991 surge–fan (fan A in Figure 2) as
well as older sediments. The jökulhlaup flows also
reworked the northwestern side of the island.
Transect 4
This transect is located where the Gígjukvísl river
runs through a large moraine constriction. The snout
of Skeiðarárjökull was situated at these moraines in
1904 (map from Danish Geodetic Institute, 1:50.000,
based on measurements from 1904, published in
1941). The moraine is presently ice–cored (Kaldal,
1997; Russell et al., 1999). Despite erosion of the
right bank by 170 m and the left bank by 50 m, the
riverbed elevation remained unchanged.
Transect 4 controlled the level of the backwater
lake which developed upstream. At its peak this back-
water lake temporarily stored 60–120 10


m

of wa-
ter (Russell and Knudsen, 1999a). As the ice–cored
moraine ridges on either side of the river channel
eroded, the capacity of the cross-section increased,
lowering the level of the backwater lake even when
the discharge was increasing.
The region between transect 3 and 4
Prior to 1991 this area was the site of a lake (Fig-
ure 6) which filled with sediments during the surge.
In 1992 the Gígjukvísl river had a braided course in
this region becoming channelised where it turns south.
From there the river flows through moraines situated
at the ice margin in 1904.
A heavily kettled “expansion bar” downstream of
the south island at transect 3 (at a distance of 1200–
2000 m, Figure 5) had a surface elevation between 66
and 68 m in 1997, whilst only 60 m in 1992. Some of
the largest kettle–holes on this bar resulting from the
November 1996 jökulhlaup extend through the entire
bar thickness.
In the centre of the Gígjukvísl channel, north of
the moraine gap (transect 4), the jökulhlaup bar sur-
faces have an elevation between 62 and 64 m in 1997,
compared to 60 m in 1992. The 1997 channels at this
location are, however, at an elevation of 60 m. This
may suggest waning–stage scour to pre–jökulhlaup el-
evations at this location.
The western part of the Gígjukvísl catchment,
west of the moraine gap, contains a southeastward–
draining confluent river system, which is flowing
around a series of older fluted surfaces. Comparison
of the 1992 and 1997 photos reveals very little change
in this area, with the same patterns of fluted surfaces
being clearly visible on both photographs. The 70 m
contour across this system has not changed its posi-
tion between 1992 and 1997. The greatest change dur-
ing this period is concentrated along the ice–marginal
zone where a series of new kettled–outwash fans pro-
vide evidence of ice–proximal aggradation, confined
to a distance of less than 500 m from the glacier mar-
gin. The extent of ice-proximal aggradation coincides
with an elevation of approximately 70 m. The limited
extent of jökulhlaup fans in this region could be due to
a combination of both limited flow duration and lim-
ited sediment availability for the western Gígjukvísl
outlets. The backwater lake may have also limited the
transport of jökulhlaup sediment from this section of
the glacier margin.
The region south of transect 4
At transect 5 the erosion of the right bank was 50 m
and the erosion of the left bank was 600 m during
the November 1996 jökulhlaup. The riverbed is 4 m
higher in 1997 than in 1992 (Figure 5). At transect
6 the erosion of the right bank was 500 m and the
JÖKULL No. 50 29