Jökull - 01.01.2001, Side 30

Knudsen et al.
erosion of the left bank was 200 m. At the eastern
end of the former bridge, as well as further east, the
bed elevation was 4–5 m higher in 1997 than in 1992.
The bed elevation west of the former bridge was un-
changed (Figure 5).
Whereas this part of the Gígjukvísl river channel
was virtually unaffected by the 1991 surge and jökul-
hlaup, it underwent an overall widening and shallow-
ing during the 1996 jökulhlaup. Downstream of the
moraine constriction the erosion was concentrated on
the left bank. At transect 6 the erosion has shifted
to the right bank. Net deposition dominates the old
channel course with 3–4 m of aggradation.
CONCLUSIONS
This paper presents maps that show the channel of
Gígjukvísl after the 1991 surge of Skeiðarárjökull
and following the November 1996 jökulhlaup. These
maps have been used to measure changes in the
Gígjukvísl channel resulting from the jökulhlaup. The
main geomorphic impact consists of channel change
brought about by bank erosion of up to 300 m at
the main Gígjukvísl outlet (transect 1) and within–
channel deposition between 6 and 12 m. Aggradation
rates decrease markedly downstream, with bank ero-
sion of 600 m downstream of the moraine constriction
(transect 5) accompanied by localised within–channel
aggradation of only 4 m. Comparison of 1992 and
1997 aerial photographs also provides a clear picture
of 300 m of glacier snout retreat and thinning of 50-
60 m during the study period.
The Gígjukvísl channel system underwent spec-
tacular transformation from a complex system of low
capacity channels and proglacial lakes to a large, high
capacity channel scaled to November 1996 jökulhlaup
flows. The overall size of the Gígjukvísl channel in-
creased, reducing flood–flow resistance and decreas-
ing future potential for the formation of backwater
lakes. The drastic change within the Gígjukvísl chan-
nel was brought about by the fact that the proglacial
trench in which the river system is located had only
recently (post–1954) been created. As such, the prox-
imal Gígjukvísl channel had never experienced a high
magnitude jökulhlaup, in contrast to the Skeiðará
channel system, which had adjusted to successive
jökulhlaups over the previous decades, and where the
overall geomorphic impact of the November 1996
jökulhlaup was much less spectacular. Due to the
extensive bank erosion during the jökulhlaup, the
present Gígjukvísl channel is now well–adjusted to
high magnitude flood flows reducing the geomorpho-
logical impact of any subsequent jökulhlaups.
ACKNOWLEDGEMENTS
This work was funded by the Icelandic Public Roads
Administration. Ground survey was carried out by
Jón S. Erlingsson and Halldór S. Hauksson from the
Iclandic Public Roads Administration.
ÁGRIP
BREYTINGAR Á FARVEGI GÍGJUKVÍSLAR Í
JÖKULHLAUPINU Í NÓVEMBER 1996
Ný kort af farvegi Gígjukvíslar eftir framhlaup Skeið-
arárjökuls árið 1991 og eftir hlaupið í nóvember 1996
eru notuð til að leggja mat á breytingar á farvegi
Gígjukvíslar sem áttu sér stað í hlaupinu í nóvem-
ber 1996. Helstu breytingar á farveginum eru allt
að 300 m rof á árbakka á móts við stærsta útfall
hlaupsins (snið 1) og hækkun á árbotni þar á bilinu
6 til 12 m. Botninn á farveginum hækkar minna eftir
því sem neðar dregur og í sniði 5 var rof á árbakka
600 m en botnhækkunin aðeins 4 m. Við samanburð
á kortunum má sjá að jaðar jökulsins hefur hopað
um 300 m og lækkað um 50-60 m á árabilinu 1992
og 1997. Þær miklu breytingar sem urðu á farvegi
Gígjukvíslar stafa af því að lægðin sem áin rennur
í meðfram jökuljaðrinum er nýleg og hefur myndast
við hop jökulsins eftir 1954. Farvegur Gígjukvíslar
hafði því aldrei áður tekið við vatnsmagni úr stóru
jökulhlaupi, ólíkt farvegi Skeiðarár sem hefur mót-
ast af mörgum stórum jökulhlaupum á undanförnum
áratugum enda urðu breytingar á farvegi Skeiðarár í
hlaupinu í nóvember 1996 mun minni en breyting-
arnar á farvegi Gígjukvíslar. Vegna hins mikla rofs
á árbökkum Gígjukvíslar í hlaupinu í nóvember 1996
þá getur núverandi farvegur Gígjukvíslar flutt meira
vatnsmagn en áður og því má búast við minni breyt-
ingum í næstu jökulhlaupum.
30 JÖKULL No. 50