Jökull - 01.12.1981, Side 100

Mórilla concentrations may be a reflection of
a short sample period in the middle of the melt
season, whilst peak concentrations are often
found to occur early in the melt season. This is
generally held to be a flushing out mechanism
whereby the first high flows remove the avail-
able sediment with subsequent flushing
occurring only with flows which exceed the
previous ones. The fact that the two very
similar discharge peaks produced markedly
different suspended sediment coíicentrations
is possible evidence of this mechanism in
operation in the River Mórilla.
In a predominantly basalt terrain, it is per-
haps no surprise to record very low solute
concentrations. The relatively good corre-
lation between total hardness (Ca + +, Mg + +)
and conductivity may be a good indicator that
Ca + + and Mg + + are probably the major ions
in solution. The overall pattern is an inverse
one in relation to discharge with higher con-
ductivities at low flow and this is best
accounted for by dilution effects. In terms of
total daiiy transport from the catchment dur-
ing the study period, solute load remains fairly
constant although becoming relatively more
important during periods of low flow where it
accounts for c.30% of the total daily transport.
This effect is likely to increase into autumn
and winter when discharge and therefore sus-
pended load is low (Vivian and Zumstein, 1973).
On the other hand, suspended load varies
markedly with the highest values coinciding
with high values of discharge.
CONCLUSIONS
In spite of the brevity of the study period,
some general conclusions can be drawn on the
basis of the above observations. Firstly, the
response time of meltwater discharge to
meteorological change in the basin appears to
be about 12 — 24 hours and this is superim-
posed upon diurnal fluctuations in discharge.
However, studies elsewhere indicate that the
time lag is likely to shorten as the season
proceeds and run-off becomes rapid from the
bare ice surfaces. Secondly, although the pat-
tern of suspended sediment concentration fol-
lows that noted elsewhere and a good corre-
lation was achieved between concentration
and discharge, it has no simple relationship
with discharge since, depending on recent
flow history, different concentrations can be
produced with similar volumes of discharge.
Finally, both suspended and solute concen-
trations are low in comparison with other
glacial streams. When converted to total daily
transport rates per unit area, Kaldalónsjökull
(40 km3) yields 7750 kg/km2/day (with the
low rates at the beginning and close of the
melt season not included). 0strem’s (1975) data
for Norwegian glaciers shows that Nigards-
breen (40 km2) yields 9058 kg/km2/day and
Engabreen (39 km2) yields 9645 kg/km2/day
whilst some smaller glaciers have much higher
transport rates. One reason for the relatively
low transport rates at Kaldalónsjökull might
be the recent rapid retreat of the snout to a
virtually stagnant state, starved of ice by the
rock bar above (Fig. 1). Fresh glacial erosion
will be limited under such conditions and will
be less than that effected by a more mobile
glacier. Debris delivery to the meltwater
stream may therefore be less in spite of a
greater proglacial area freshly exposed by
retreat.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the support of
the University of Aberdeen, Carnegie Trust and the
Royal Geographical Society towards the funding of
The Aberdeen University North-West Iceland Ex-
pedition, 1979.
REFERENCES
Church, M. and Gilbert, R. 1975: Proglacial
fluvial and lacustrine environments, in
Jopling, A. V. and McDonald, B. C., eds.,
Glaciofluvial and Glaciolacustrine Sedimentation.
Soc. Econ. Palaeont. and Min. Spec. Pub.
23.
Eythorsson, J. 1960: Jöklabreytingar 1958/59
og 1959/60, Jökull, 10: 30-32.
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