The Botany of Iceland - 01.12.1914, Blaðsíða 22

206
THORODDSEN
melting of the snow their size varies from day to day and from
year to year; many glacier-streams which disappear entirely during
winter, carry in warm summers an immense volume of water. Al-
most all Icelandic glaciers rest on soft rocks (tutl' and breccia) upon
which erosive action is very active; therefore, the Icelandic glacier-
rivers carry down an immense amount of rock in the form of mud,
sand, gravel and blocks of stone; for this reason they are hardly
ever found entering fjords or deep bays, these having quickly be-
come íilled up in cases where they formerly so entered, while the
Jökulls (glacier-bearing mountains) are surrounded by large sandy
and gravelly tracts which for the most part owe their origin to
the rivers.
Taking the whole of the island into account, rivers containing
glacier-water are decidedly in the majority. South of Vatnajökull
clear water is almost unknown, as all rivers and brooks originate
in the glaciers. There all the rivers llow down to the coast by
short courses in torrential current, and during summer some of
them are so broad that it lakes hours to cross them — but then
it must be remembered that it is necessary, in the middle of the
river, to go a long way round, on account of the current and depth.
On the flat, sandy tracts the rivers are constantly changing their
course, and greater and smaller changes take place daily. AIl glacier-
rivers branch abundantly.
In accordance with the slope of the land, the longest and
largest rivers flow in South Iceland towards the south-west and
south, and in North Iceland towards the north; the majority of
them rise on the plateau at a height of 600—900 metres above
sea-level, a fact which should be correlated with tlie limits of the
glaciers in the interior. Although the Icelandic rivers carry a com-
paratively great volume of water yet they are not navigable, because
of their usually steep fall, their torrential current, and their tendency
to spread out and subdivide into numerous branches in the low
land. The largest rivers of Iceland are as follows: — From the
south edge of Vatnajökull rise Jökulsá í Lóni, HornaQarðarfljót,
Jökulsá á Breiðamerkursandi, Skeiðará and Núpsvötn; the last three
are considered to be the most dangerous glacier-rivers of Iceland.
From the west edge of Vatnajökull rise Hverfisfljót, Skaftá and the
well-supplied Kúðafljót; this last also receives a large supply of
water from Mvrdalsjökull. From the latter another river flows down
— the short, but torrential Jökulsá á Sólheimasandi, also called