Jökull - 01.01.2001, Page 28

Knudsen et al.
Figure 2) between 1992 and 1997. The bed eleva-
tions of the eastern margin of the waning–stage jökul-
hlaup fan (W) change from 80 m to 92–94 m during
the same period. The greatest aggradation (16 m) oc-
curred in the vicinity of the rising–stage fan (R), cre-
ating the greatest potential of complete burial of ice
blocks, as demonstrated by numerous kettle–holes in
this region (Fay, in press). With this high aggradation,
the two 1991 surge fans (B and C on Figure 2) were
probably completely buried during the 1996 jökul-
hlaup, although parts of these fans may have been
eroded during the rising–stage of the flood. The apex
of these fans in 1992 was at an elevation of 88 m,
whilst the jökulhlaup fan elevations at the same loca-
tions were 98–100 m in 1997. High sediment flux dur-
ing the waning jökulhlaup flow stage may have buried
the 1991 western surge fan (B). Localised backwa-
ter conditions early in the rising flow stage, combined
with high sediment flux within the flow, may have re-
sulted in the burial of the eastern surge fan (C). The
small sandur to the east of the rising–stage fan ag-
graded from an elevation of 90 m in 1992 to 100 m in
1997.
Transect 2
This transect shows cross-profile changes within one
of the narrowest November 1996 proglacial channels.
Major erosion of the island at the central part of
this transect (distance 300-700 m, Figure 5) was ob-
served during the rising flow stage on November 5th,
1996. This erosion resulted in marked lowering of
the level of a large backwater lake which accumulated
upstream during the early rising flow stage. The ero-
sion concentrated mainly on the right bank where it
amounted to 220 m. In contrast, erosion of the left
bank was only 50 m (Figure 5). Such high erosion
rates on the right bank could be due to the presence of
relatively easily eroded fluvial and glacial sediments,
as opposed to the buried glacier ice within the left
bank. The area at the glacier margin (distance 0–
300 m, Figure 5) was over-washed during the jökul-
hlaup. Therefore, some of the distinctive troughs may
represent channels occupied by 1996 jökulhlaup wa-
ter flowing obliquely around the island. The elevation
of the main channel was 80 m in 1992 and 78 m in
1997.
The region between transects 1 and 2
The proglacial lake Háöldulón (Figure 2) dominated
this area in 1992. The lake was noticeably altered by
the 1992 surge when the glacier advanced 200 m into
it during a total advance of about 700 m in this region
(Pálsson et al., 1992). The surge closed the former
northwest outlet of Háöldulón resulting in the reacti-
vation of an older and higher level outlet and about
2 m increase of the lake level (Figures 2 and 6). Tran-
sect 2 crosses the outlet that was reactivated during
the 1991 surge.
The former area of Háöldulón has been com-
pletely filled with sediments and is now occupied by
the westernmost part of the waning–stage Gígjukvísl
jökulhlaup fan (fan W in Figure 3). The former lake
basin is now 80 m a.s.l. just upstream of transect 2 and
85–90 m a.s.l. at transect 2, whereas the lake level was
79 m a.s.l. in 1992.
Transect 3
This transect shows a combination of erosion and de-
position. Erosion of the right bank of the main chan-
nel was 220 m, whilst erosion of the left bank was
only 50 m. The pre-existing channel area aggraded
by about 6 m. The northern bank of the island (dis-
tance 500 m, Figure 5) was also eroded by jökulhlaup
flows exiting the glacier from the north. These flows
also resulted in the infill of a well–defined channel cut
by 1991 surge–related flows exiting the ice margin.
The irregular topography on the northern 300 m of this
profile represents jökulhlaup deposition on top of the
ice–marginal landscape created by meltout from the
1991 surge.
The region between transects 2 and 3
This area is located downstream of lake Háöldulón
and contains the largest fan formed during the 1991
surge (fan A). The three fans (A, B and C in Figure 2)
indicate where bedload–rich flows escaped the glacier
at the end of the surge. In 1992 the river flowed along
the southern fringe of fan A and eroded the north side
of Harðaskriða. The 1992 Gígjukvísl channel was
braided from the fan to transect 4, but before the surge
the channel was incised and sinuous (Figure 6). The
pre–surge channel width at transect 2 was 50 m, but
28 JÖKULL No. 50