The Botany of Iceland - 01.12.1914, Síða 25

PHYSICAL GEOGRAPHY
209
insula, and Skogafoss and Seljalandsfoss below Eyjaíjallajökull. Tliese
cascades have a height oi' about 100 metres and more. The largest
rivers, only, have been mentioned above, but in addition to these,
hundreds of streams of greater or lesser ATolume occur, often with
beautiful waterfalls and cascades in connection with picturesque
clefts and rocks.
As mentioned above, the torrential and changeable glacier-rivers
have a destructive influence upon the cultivation and vegetation of
the plains. The greensward is torn off and large areas are covered
by gravel, therefore the level country south of Vatnajökull is in
several places turned into a desert almost destitute of vegetation
except where special natural conditions afford a shelter from the
destructive effect of the rivers. Where the action of the glacier-rivers
is suddenly arrested hy any natural phenomenon the level country
again becomes quickly covered with plants. As an example may be
mentioned the fact that Hverfisfljót, in the year 1783, was forced
out of ils hed by a great lava-stream, and a considerable stretch
of land — Brunasandur — which had previously been irrigated by
cold and torrential river-branches Avas freed from these, only a few
clear streams of filtered glacier-water with a slight current issuing
from the edge of the lava-streams and flowing down the level
country; so that where in 1783 there Avas a gravelly and sandy
flat withóut plant-life and without means of sustenance for huinan
beings there is now a parish with seven farmsteads and abundance
of meadows and pasture-lands for the sheep and cattle of the in-
habitants. In itself the glacier-water is not inimical to vegetation;
it is only tlie torrential current, the changeableness of the water-
courses, and the low temperature of the water which have a de-
structive effect upon plant-growth; where tliese factors are not active,
the glacier-water, with its contents of fine clay, is on the contrary
a fertilizer; therefore in the neighbourhood of the mouths of the
largest glacier-rivers, where there is only a slight current and the
water has become warm on the way, fertile tracts of meadows are
often found where the glacier-water is profitably utilized for irriga-
tion. Water from rivers such as Thjórsá and Hvítá has, by analysis,
been proved to contain an unusually large quantit\r of alkali and
phosphoric acid.
Lakes. There are many lakes in Iceland, but the majority of
them are of sinall size. The largest lakes — Thingvallavatn and