Jökull - 01.01.2001, Blaðsíða 22

Knudsen et al.
less than

0.7 m for elevation (z). To be able to pro-
duce a map with this accuracy, model points must be
measured by ground survey and the location of these
points marked on the ground prior to taking the aerial
photographs. The model points can then be identified
on the aerial photographs. For this map 39 points were
measured with a GPS Trimble 4000 SSi (error in plan
coordinates less than 0.02 m and error in elevation less
than 0.05 m) and their location marked on the ground.
Since the main geomorphic impact of the Novem-
ber 1996 jökulhlaup on Skeiðarársandur was con-
centrated within the Gígjukvísl channel, the Public
Roads Administration commissioned Ísgraf to pro-
duce a similar map of this particular area based on
aerial photographs from August, 1992 (height of flight
5500 m, National Land Survey of Iceland). Common
points on these two sets of aerial photographs were
identified. The coordinates of these points were mea-
sured by ground survey. For the 1992 map 16 points
were measured with a GPS Trimble 4000 SSi. It was
possible to produce this map with about the same ac-
curacy as the 1997 map by using information from the
1997 map in the production of the 1992 map.
Due to the data being digital, maps of any scale
can be produced. At present, maps of scale 1:10,000
with 2 m contour lines have been produced. Maps of
the channel of Gígjukvísl in 1992 and 1997 with a
10 m contour interval are shown in Figures 2 and 3.
GEOMORPHIC CHANGES WITHIN THE
GÍGJUKVÍSL RIVER CHANNEL
The geomorphic impact of the November 1996 jökul-
hlaup within the Gígjukvísl channel is revealed by
comparing the 1992 and 1997 maps. This comparison
is complemented by our sedimentological studies in
the area before, during and after the jökulhlaup. The
location and surface elevation of selected transects are
shown in Figures 4 and 5. The first three transects
reach the glacier, which retreated by about 300 m be-
tween 1992 and 1997 and thinned by 50–60 m at the
snout.
The Skeiðarárjökull glacier advanced up to 1 km
during a surge and a small jökulhlaup in 1991 (Páls-
son et al., 1992; Björnsson, 1998). This glacier ad-
vance closed the former northwest outlet of the Há-
öldulón lake resulting in the reactivation of an older
and higher level outlet as can be seen by comparing
Figures 2 and 6. The advance of the glacier caused ge-
omorphic changes within the Gígjukvísl channel that
influenced the patterns and processes of erosion and
sedimentation during the November 1996 jökulhlaup.
An overview of these changes is summarised below.
Transect 1
This transect crosses the Gígjukvísl channel from the
Háöldukvísl channel mouth (left bank) to the glacier
(right bank) (Figures 4 and 5). It is characterised by
a depression between the ice margin and older pit-
ted sandur surface of higher elevation, Harðaskriða,
(Figure 1) a jökulhlaup fan, possibly dating back to
1922. Prior to the November 1996 jökulhlaup, the
Háöldukvísl channel was last flooded during the 1945
jökulhlaup (Ragnar Stefánsson, former warden of the
Skaftafell National Park, personal communication,
1990).
The erosion of the left bank amounted to 300 m
and the pre-existing channel aggraded by 6–12 m. Net
sediment deposition in the trench between the 1992
glacier margin and the pre-jökulhlaup 1996 margin
exceeds 12 m (Figure 5). It is possible that much of
the extreme northern part of this cross–section is still
underlain by sediment–covered ice from the glacier
retreat since 1991. The channel profile deepens to-
wards the south (Figure 5) where flows in the Novem-
ber 1996 jökulhlaup were concentrated along the left
bank, enhancing the erosional process.
The region between Gígjukvísl and transect 1
In 1992 this area was characterised by two alluvial
fans, each with an area of ca. 0.6 km

(B and C on
Figure 2), which formed during the 1991 surge. Simi-
lar fans have been reported from the 1890 and 1964
surges of Brúarjökull (Knudsen, 1995). The fans
were located at a poorly defined inter–lobate recess
in the snout of Skeiðarárjökull, in the same area as the
largest November 1996 jökulhlaup outlet.
The main Gígjukvísl outlet was described by Rus-
sell et al. (2001) who differentiated between the rising
(R on Figure 3) and waning–stage fans (W on Figure
3). Between 8 and 9 m of sediments were deposited
downstream of the surge–related fans (B and C on
22 JÖKULL No. 50