Jökull - 01.12.1983, Side 81

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Fig. 2. The discharge peak for the 1972 jökulhlaup
(redrawn from Rist 1973). The river Skeidará
transports the bulk of the flood water, while Gígja,
the second largest river draining Skeidarárjökull,
only floods when the hlaup is near its maximum.
The water types of the hlaup are superimposed on
the hydrographs: type 1 (No. 2 in Table 3) domin-
ates until March 18, when water of type 2 (No. 3,
Table 3), representing the bulk of the hlaup, takes
over. The hlaup in R. Gígja sets in at that time,
reflecting the increased flood rate. The cross hatch-
ed region on the Figure indicates mixing of the two
water masses. In early April the water is chemically
a mixture of type-2 water and normal Skeidará,
with fading influence of the former as time goes on.
Mynd 2. Jökulhlaupið 1972 (Rist 1973). Á myndina er
merkt þrenns konar vatn, sem aðgreina má í hlaupinu. Frá
upphafi hlaupsins til 18. mars var vatnið ríkt í Na og
kolsýru, en mjög snautt í magníum (2. dálkur í Töflu 3).
Frá 18. til 22. mars er vatnið blanda af fyrmefndri gerð og
hinu eiginlega og ómengaða hlaupvatni úr Grímsvötnum,
sem verður alls ráðandi eftir22. mars og til loka hlaupsins.
(3. dálkur í Töflu 3). Pað vatn var um 95% hlaupsins að
rúmmáli til.
particularly the low Mg (e.g. Lawrence et al. 1979) of
low-temperature exchange equilibria with clay
minerals suggesting that the initial partofthejökul-
hlaup water was modified by contact with geo-
thermally-altered bedrock at low temperatures.
This might be correlated with the bulging-up of the
glacier prior to jökulhlaups (Thorarinsson 1974)
which has been attributed to the introduction of
relatively small volumes of Grímsvötn water into
the ice/rock interface at the outlet of Grímsvötn.
This hypothesis accords with the high amount of
fine grained suspended load (clay) observed by
Tómasson et al. (1974, 1980) in the early 1972 dis-
charge. They state (op. cit. 1974, p. 14) that in the
initial phase of the flood the fmest-grained
suspended load consisted of clay minerals in
addition to sideromelane glass, with the clay
disappearing as the hlaup progressed. This was
Fig. 3. The alkali ratios, Na vs. K, for the Skeidará
hlaup of 1972. Type-1 water (circles, Fig. 1) domin-
ates over normal Skeidará water for a short time and
remains at a high, almost constant Na/K ratio until
mixing with type-2 water brings the Na/K ratio
down again. The compositions representing 90% of
the ílood volume areenclosed within a dashed curve,
and the discharge peak making up 60% of the flood
is enclosed by a solid line. The waning stage of the
hlaup is shown by triangles connected by a dashed
line. The discharge peak of the 1982-hlaup appears
in the lower right, showing remarkably consistent
values.
Mynd 3. Alkalimálmamir Na og K í Skeiðarárvatni.
Hringir tákna fyrsta hluta hlaupsins (Nr. 2 í Töflu 3) en
þríhyrningar meginhlaupið (Nr. 3 í Töflu 3). Próun efna-
samsetningarinnar er synd með brotinni línu og örvum.
JÖKULL 33. ÁR 79