Jökull - 01.12.1962, Síða 31

the discharge in this zone decreases rapidly as
the winter sets in and usually remains loio
throughout the winter season, loith the excep-
iion of individual high flows of 1 or 2 days
duration, caused hy thaws.
Zone 2 is composed of the middle reaches of
the rivers, from elevation 600 to elevation 300
metres approximately, or from the places men-
tionecl above down to the Thjórsá—Tungnaá
confluence, where the three headwater rivers
lmve converged to a single stream.
This zone has several important features (1)
liigh gradient and therefore high flow velocities
and relatively large heat gains from conversion
of kinetic energy, (2) rather deep channels,
effectively shielded against the chilling effect
of xoinds, (3) large heat exchange between
water and air due to high turbulence of the
flow and, (4) in the case of the Kaldakvísl and
Tungnaá rivers, an appreciable infloic from
ground water, the temperature of which is uni-
form throughout the year, a few degrees above
zero Centigrade (see map).
Zone 3 extends from el. 300 metres doion
to sea level. Its uppermost part, from the
Thjórsá—Tungnaá confluence down to the bend
near Búrfell, consists of a wide regular, shallow
channel cut into a lava flow, with a uniform
slope and relatively even rocky bottom. The
remainder of this zone is made of very xvide,
low-sloping and braided reaches, seperated by
low falls and rapids. Infloxv from ground-xvater
is rather unimportant in tliis zone, and inflow
from lakes is negligible.
ICE REGIMES
In countries with a predominantly contin-
ental climate, the winter ice regime of rivers
and other natural inland waters may, by and
large, be divided into three rnore or less sliarply
distinct phases or periods: (I) Freezing-over
period in early xvinter, (2) ice cover period from
early io late winter and. (3) breaking-up period
in early spring. This latter period is then fol-
lowed by the ice-free summer season which
completes the annual cycle. In some countries
this cycle is so stable that the beginning and
end of each of its phases or periods may be
predicted xvith an error of a few days only.
It is a common feature of the ice regime of
most Icelandic rivers especially in the loxver
coastal areas, thal this regularity is more or less
absent. Generally, the single annual cycle is in
these rivers refxlaced by a series of more or less
incomplete ones each winter. For instance,
freezing over may start in a cold period in
early xvinter, but before the rivers are frozen
over a warm spell sets in, the ice is broken
up xvhithout any intervening ice cover period,
and an ice-free period may then follow. This
may be repeated several times in each winter.
The three zones of the Thjórsá river system
mentioned above differ appreciably in respect
to ice conditions. In the uppermost zone, above
elevation 600 metres, the ice regime in many
years approaches tlrat of continental climates,
with a single, xvell-developed annual cycle.
There are, however, many exceplions, xvith txvo
or more cycles each xvinter, even seperated by
ice free periods, usually of a short duration.
When winter sets in, ice crystals are easily
formed in the shallow rivers of this zone. Large
amounts of frazil ice flow doxvn the rivers.
Anchor ice accumulates in reaches where the
river is floxving on a rock bottom. The anchor
ice obstructs the floxv and causes a rise in waler
level, usually 1—3 melres, xvith a resulting de-
crease in floxv velocity. The buoyancy of the
ice xvill thereby brmg some of it to the surface
where it will freeze and form a cover on the
river. This process may in some cases be inter-
rupted by a warrn spell before a complete
cover is formed. Nevertheless, in this upper-
most zone, a complete ice cover has usually
been formed on the rivers by the end of Nov-
ember. During the freezing-over period in this
zone, large amounts of sludge may be carried
inlo the next doxvnstream zone, but this ice
flow will come to an end xvhen the freezing-
over is completecl.
A characteristic feature of the middle zone
is large areas of open xvater, especially in the
Tungnaá and Kaldakvisl rivers. This is due to
(1) heat gains from the ground-water infloxv
and from the kinetic energy of the flow, men-
tioned above, but mainly to (2) ihe swift cur-
rent, xvhich prevents the formation of an ice
cover. During frost periods, especially when ac-
companied by high, dry winds (from N or NE),
the turbulent floxv in this zone gives rise to
great heat losses by convection, and large
amounts of ice are produced. Some of the ice
accumulates in inlets and other bank areas of
low velocity, where it freezes into a solid cover,
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