The Botany of Iceland - 01.12.1914, Blaðsíða 21

PHYSICAL GEOGRAPHY
205
Rivers. Iceland possesses numerous rivers of considerable size;
their volume is due to the damp climate of the land and to the
great number of glaciers. The Jökulls serve as huge water-reservoirs,
from which tlie majority of the larger rivers are supplied, and the
glacier-plateaus also attract rain and other atmospheric precipitation,
so that the supply is always uniform. The volume of water is
greatest in the lower-lying districts because the plateau above 500
—600 metres, to a great extent, is covered with lava, gravel and
loose sand which absorb all the moisture; but it reappears further
down, and below the 500 metre line^there are extensive bogs both
in the lowest part of the plateau, in the lowlands and in the val-
leys. In the lava-deserts both on the plateau and in the lowlands,
the rain and melted snow disappear iinmediateljr, and large tracts
are quite destitute of water; but at the edge of the lava numerous
springs bubble forth from the earth. The water which at first is
usually turbid and intermingled with g'lacier-mud, becomes fdtered
by passing through the lava, and is therefore very pure and clear
in the springs. When there is no outlet, water is sometimes found
in the lava-field itself, at the bottom of deep clefts; as for instance,
the renowned, cold, crystal-clear water in the lava-clefts near Thing-
vellir. In early summer, during sudden thaw, while the frost-layer
still persists in the ground, extensive gravelly and clayey flats on the
plateau and in the lowlands are turned into morasses, but when the
ice of the subsoil melts later in the summer outlets are again opened
for the surface-water so that the gravel-fields are drained. In the val-
leys numerous springs make their way through the basalt along the
mountain-sides, often in long rows between the layers, and can be
detected by the green mosses which grow luxuriantly around them.
Icelanders draw a distinction between “bergvatn” (mountain-
water) which is clear, and “jökulvatn” (milky-while glacier-water)
which may have a muddy, yellowish colour or a chocolate-brown
colour according to the amount and kind of glacier-clay carried in
it. The amount of clay in the glacier-water is larger in summer
than in winter and, again, the glacier-rivers are clearer and of less
volume in the morning tlian later in the day. DiíTerences in the
weather — cold or warm or wet or dry years -— have the greatest
influence as regards the volume of water in the glacier-rivers. In
drv and warm summers the clear rivers are but sinall, while the
water carried by the glacier-rivers increases to double or three times
the usual volume. As the glacier-rivers are so dependent upon the
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