Jökull - 01.12.1999, Page 48

Figure 1. Location of the field area within Iceland (inset) and the Gígjukvísl in relation to the margin of Skeið-
arárjökull. See box for area enlarged in Figure 2. — Staðsetning rannsóknarsvœðisins á Islandi (innsett),
Gígjukvísl og afstaða hennar tiljaðars Skeiðarárjökuls. Rammi sýnir stœkkað svœði á 2. mynd.
1997). Flows began in Gígjukvísl at around 1015h
on November 5th and are reported to have reached a
maximum discharge of c. 34,000 m3s-1 by c. 0300h
on November 6th (Snorrason etal., 1997). Thejökul-
hlaup had its longest duration in Gígjukvísl with flows
persisting well into November lth (Snorrason et al.,
1997). Waning stage flow concentrated on the Gígju-
kvísl outlet as Skeiðará and Súla outlets dried up. Vast
numbers of ice blocks, many over 10 m in diame-
ter, were transported by floating, rolling, and sliding
through the Gígjukvísl channel (Figure 1).
Jökulhlaups have been reported draining across
the Skeiðarársandur outwash plain since the 12</' cen-
tury (Thorarinsson, 1939; 1974; Björnsson, 1988;
1992) making the Skeiðarársandur outwash plain a
jökulhlaup “type-site” (Thorarinsson, 1974; Maizels,
1991; 1993a,b; 1995; Maizels and Russell, 1992).
However few studies have detailed the impact of a
high magnitude jökulhlaup on the geomorphology
of a single jökulhlaup channel (Kaldal, 1997). The
present Gígjukvísl channel has progressively incised
its course since its formation in the 1940's (Kaldal,
1997).
METHODS
The pre-jökulhlaup survey of 53 points was completed
between October 14//l and 23rt/ for the 2 km dis-
tance between the moraine and road, 1996 using an
electronic distance measurer accurate to ±5cm. Post-
jökulhlaup surveys of the Gígjukvísl river channel in
November and May 1997 added a further 100 points.
Maximum flood stage was easily measured by survey-
ing well-defined wash-limits on snow pack. Maxi-
mum water surfaces were identified along the west-
ern bank of the Gígjukvísl river upstream from the
road to the backwater lake level on the northern flank
of the moraine ridges. Within-channel sedimentary
46 JÖKULL, No. 47