Jökull - 01.12.1969, Side 130

challows, where Laxá leaves the lake, and
/líverts the surface water, i.e. the ice forming
water, to the bottom resnlting in the forma-
tion of anchor ice. For the physical aspects
of ice phenomena the interested reader is re-
ferred to Rist (1962).
There were three causes for the choking up
of Mývatn outlet:
Causes
1. Slush ice, anchor ice, snow and pack-ice
2. First lowering of water levels, then anclior
ice on outlet controls, as bottorn ice coat.
Closure of channels between ice stacks by
small lumps of ice hraking from the ice
sheet.
3. Break up of the ice cover of Mývatn, drift-
ing ice chokes up the outlet channels.
Weather
Strong north-blowing wind with snow drifting,
severe frost.
Westerly wind against the current, clear and
dry atmosphere, rapid evaporation frorn open
lake surface, frost (—6° C or more).
Strong easterly wind with thaw.
1 caused the most harassing jammings and
lasted longest.
2 occurred quite frequently, but did not last
as long as 1 and therefore did not inflict
as serious power produtcion disturbances.
On the other hand it repeatedly occurred
unexpectedly.
3 occurred very seldom.
As a solution to this ice trouble the Geira-
stadir channel was deepened and in 1959 the
Laxá Power Company had a gate-dam con-
structed there at the outlet. Instead of distri-
buting the outflow across a shallow area and
into three channels, now the entire outflow
runs in winter time only via Geirastadir
channel.
As of result of this deepening and operation
the water needs now 11 hours shorter time
than previously to flow out of Mývatn itself
and reach 2 m fall.
The outflow from Mývatn is now without
any ice problems witli the additonal asset for
the people of Mývatn district that Laxá river
no longer forms ice jams between the farnts
Arnarvatn and Helluvad which it tendecl to
do formerly, perhaps once during each 5 to 10
years.
The temperature of the outflowing water is
largely dependent on the extent of pools of
open water at the Álar shallows. For example
when it became colder after the warm weather
in November, last year (1968), the water temp-
126 JÖKULL 19. ÁR
erature at Geirastadir fell as low as 0.2° C,
while now during these days early in Febru-
ary 1969 with air temperature of —22° C and
north-blowing wind the water-temperature is
as high as 1.2° C. At present the openings are
as small as tlieir minimum extent shown in
Fig. 2 and the ice is 45 cm thick. In freezing
weather as now, the evaporation from the
water near the outlet is so intense that the
view across the river becomes blurred, but this
only applies to a short distance since at Plof-
stadir the heat has completely vanished away
and frazil ice formation has begun.
The ice problems in Laxá are best summariz-
ed by the following quotation (Rist 1961):
“The water from Mývatn will be available
for power production but the operators at
Laxá power stations for sure will not have a
constant easy day. ln severe frost periods Laxá
flows like a thick slush ice porridge and in the
struggle with the slush and pack ice nothing
is of effective use but a deep and voluminous
reservoir.”
REFERENCES
Sigfinnsson, Jóhannes. 1957: Bárugarðarnir við
Mývatn. Náttúrufræðingurinn 27, 67—72.
Reykjavík. (Icelandic.)
Rist, Sigurjón. 1961: Vötn, Náttúra íslands p.
183. Álmenna bókafélagið, Reykjavík.
— 1962: Winter Ice in Thjorsá River System.
Jökull 12, 1-30.