Jökull - 01.12.1969, Qupperneq 137

mS /sek.
Fig. 11. Jökulsá á Fjöllum, Dettifoss,
1939/40-1965/66.
Distribution of weekly flow.
flows are equally frequent above and below
this line.
Consider for example the river Vatnsdalsá.
There is no doubt that flooding in the river
occurs during spring thaws. The thaw in the
low highlands drained by the river takes usu-
ally place in May. If the climate would be-
come milder in the drainage area of Vatns-
dalsá, for example as it is presently on the
low highlands of southern Iceland, river Vatns-
dalsá would change its outlook.
Indeed the flow patterns of river Laxá in
Nesjum in Hornafjördur, river Skógá, and river
Olfusá, all locatecl in southern Iceland, is dif-
lerent from Vatnsdalsá as they are characteris-
ed by heavy floods in late winter but hardly
any spring floods. On the other hand if the
climate turned colder, these rivers in southern
Iceland would trend toward heavy spring
floods.
Consider next the ílow pattern of Thjórsá.
This river drains the interior highlands and
possesses distinct peaks in the flow rate during
spring thaws and later during melting of
glaciers. For the present weather conditions
winter thaws cause sometimes increased l'low
as the fluctuations of flow rates in February
and March show. The flow pattern of the
glacial river Jökulsá á Fjöllum reflects colder
weather conditions since the flow is not effect-
ed by winter thaws.
This discussion makes it therefore clear that
climatic changes will shift peaks nf maximum
flow rates even from one season to another. It
should be pointed out that the diagrams give
only an indication of the parts of the year
when there is greatest probability for major
floods.
It was stated previously that climatic change
to cooler conditions would be accompanied by
heavier floods because of increased availability
of sudden appearance of warm and moist air
masses. If the climate becomes colder it can-
not be too much emphasized that there is an
increased danger of flooding in areas of the
Móberg-formation, which results from increas-
ed period of time of frozen ground and ac-
cumulation of ice in depressions causing very
rapid surface run off.
REFEREN CES
Jónatansson, S. 1968: Flood in river Ellidaár.
February ’68. Report Ahnenna bygginga-
félagið, Reykjavík. (Icelandic).
Jónsson, S. 1960: Supplement report of S.Í.R.
p. 95. Reykjavík. (Icelandic).
Kjartansson, G. 1969: This symposium.
Linsley, R. K. 1960: The common ground of
meteorology and hydrology. Bull. Amer.
Met. Soc. 41, 423-428.
Rist, S. and Sigvaldason, H. 1968: Long term
recorcls of Icelandic rivers. Orkumál, 18.
National Energy Authority, Reykjavík.
Thorarinsson, S. 1969: This symposium.
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