Jökull - 01.12.1976, Side 42

Marginal and Supraglacial Lakes in Iceland
HELGI BJÖRNSSON,
SCIENCE INSTITUTE, UNIVERSITY OF ÍCELAND
ABSTRACT
About twenty glacier-margin lakes are known
at present in Iceland. The lateral lakes are found
mainly at the southern margin of Vatnajökull.
Most of them cause jökulhlaups, however small
for the present. Grœnalón and Vatnsdalslón are
the best known lateral lakes in Iceland. The
proglacial lakes vary in number and size from
year to year. Some of them are potential sources
of jökulhlaups.
Two types of glacier-surface lakes exist in
Iceland. First, ice sink-holes which can be found
in the ablation area of slowly moving glaciers.
Such lakes collect melt-water from the glacier
surface and drain even daily during the summ-
er. Sink-holes at Esjufjöll, SE-Vatnajökull, are
outstanding examples. Second, lakes in ice
cauldrons at geothermal areas. Such a lake was
observed after a jöliulhlaup from the Katla area
in 1955. A lake is also frequently observed in
the cauldron northwest of Grimsvötn which is
the source of jökulhlaups in the river Skaftá.
Data are available for the triggering of jökul-
hlaups from Grœiialón and Vatnsdalslón as well
as for Grímsvötn. The jökulhlaups are trigger-
ecl before the glacier is floated, that is before
the “pressure barrier” is reduced to zero in a
simple floating model. The barrier is pene-
trated by some tunnel effect which is a subject
to further study. The penetration may be caus-
ed by melting of convected water. Melting fo-
cussed in narrow doivncuttings may not be bal-
tinced by inflow of ice when the pressure
barrier has been reduced to a certain value.
Fluctuations in the subglacial ivater pressure
may play an important role in the penetration
of the pressure barrier.
SYMBOLS
g acceleration gravity
z vertical coordinate for glacier elevation
40 JÖKULL 26. AR
zb glacier bed-rock elevation
Hj glacier thickness
z0 datum level for glacier elevation
zs glacier surface elevation
zw elevation of the top surface of a subglacial
water layer
k shear stress
L transition length
Pj ice overburden pressure
t time
T height of a potential barrier
W width of a potential barrier
x horizontal coordinate for distance measured
down-glacier
cp potential for water pressure in the (x, z)-
plane
Pj density of ice
pw density of water
INTRODUCTION
Glacier lakes can be found in a number of
different locations. According to tlieir geo-
graphical setting one can distinguish between
marginal, supraglacial and subglacial water re-
servoirs. The present paper gives a summary
of glacier-margin and glacier-surface lakes . in
Iceland. The paper is an extension to the
author’s paper on subglacial water reservoirs in
Iceland (Björnsson, 1975).
Summary of Basic Theory
The conditions for the formation of a water
reservoir at the bed of a temperate glacier are
discussed by Nye (1976) and Björnsson (1974,
1975). The relation between the slope of the
water/glacier boundary and the glacier surface
slope is given by the gradient
Vzw =-------V*s (1)
Pw-Pi