Jökull - 01.12.1962, Síða 32

but middle of the river remains open. The
greatest part of tlie ice is, however, carried
doxvnstream. At certain places, where the chan-
nel is deep and the velocity drops below 0.5—
0.6 m/s, an ice cover will extend across the
river and form a bridge. The mass of the sludge
will be passed under such a bridge and will
continue its downstream journey.
Anchor ice may also be formed in some places
in this zone. In the lower parl of the Tungnaá
River, ice jams may form, usually 1—3 times
each winter.
Due to the high heat gains referred to above,
a moderate rise in air temperature may be
sufficient to rencler the heat balance of the
rivers in this zone positive, and this may occur
although the air temperature is still well beloio
zero. With a positive heat balance, the turbul-
ence of the floxu causes a rapid deterioration
of the ice. The open water areas xuill increase,
and thereby the heat losses and the positive
heat balance is reduced. It may or may not
shift over to a negative balance again, depend-
ing upon the xueather. Given the variable
xueather the result is usually that the heat bal-
ance fluctuates around zero for most of the
xuinter. The ice regime therefore consists of
alternate periods of freezing-over and thaxuing,
or a number of incomplete, little developecl ice
cycles.
This applies in particular to the Tungnaá
and Kaldakvísl rivers, xuith their high heat gains
from ground-water infloxu, ancl more uniform
gradient within this zone, than in the case of
Thjórsá River, where the infuence of ground-
waler is negligible and xuhere the high gradient
is confined to a relatively short reach.
The most salient feature of the ice regime
of the lowest zone are the so-called ice jams,
i. e. large accumulations of sludge ice, that occur
predominantly at three places in this zone, viz.
at. Urriðafoss, Búði and SW of Búrfell, and
cause very considerable rises in water level.
As previously stated, this zone consists of
wide, braided sections of a low gradient sepe-
rated by low falls and rapids. In the first days
of frost periocls, large quantities of ice flows
doxun the river frorn the middle zone and much
ice is produced in the loxuest zone as xuell. The
braided sections, wilh low velocity and the
xuater divided into a number of small, irregular
channel, between sand bars greatly impede the
floxu of ice xuith t.he result that the ice starts
to accumulate in these sections. An ice cover
is formed and it progresses rapidly upxuard un-
til it reaches higher gradients at the next up-
stream rapids. If the floxu of ice is maintained,
it xuill be carried under the ice cover as long as
the velocity is above the crilical value 0.5—0.6
m/s and accumulate beneath it. The buoyance
of the waler will lift the cover and cause further
rise in water level behind it, until the upstream
velocity is reduced beloxu critical velocity. When
this stage is reached, the sludge will no longer
floxu under the cover but xuill freeze at its up-
stream edge thereby extending the cover further
upstream. In this manner the jam formation
proceeds up the rapids. The max rise in water
level occurs immediately beloxu the section with
ihe greatest slope.
The characteristics of each of the three main
ice jams of the loxuest zone are shoxun in Table
3. When the frroduction of ice in the open
xuater sections comes to an end either due to
xuarmer weather or because an ice cover has
formed on most of the river, the building up of
the jam also stops, since that process is entirely
dependent upon an adequate supply of ice
from upstream. Immediately, the heat of the
water and the current begin to deteriorate the
jam from beneath starting at its upstream end,
and proceecling doxunward. The part of the jam
lying over the cleepest channel is first xueakened
and falls clown, xuhile the ice on both sides
remains in place and often forms precipitous
xualls, resting on the bottom.
The formation of an ice jam is usually broken
up into several phases, seperated by periods of
partial deterioration of the part already formed.
The ice cover on the low-gradient reaches of
ihis zone is normally much less stable than
that of the uppermost zone. A narrow channel
usually remains open throughout the xuinter.
This is due to the closeness of the lowest zone
io the ocean, witli resulting higher air tempera-
ture, more frequent rains and, in general,
xuarmer, but at the same time more variable
weather. There is thus no single annual ice
cycle to speak of in the lowest zone either,
but a series of more or less incomplete ones.
On the coloured map accompanying the paper
the ice conditions generally prevailing by mid-
xuinter in the Thjórsá river system are in-
dicated.
30 JÖKULL