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 Jokulls serve as huge water-reservoirs, 

 from which the 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 immediately, 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 glacier-mud, becomes filtered 

 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, wiiile 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 "jokulvatn" (milky- white glacier- water) 

 which may have a muddy, yellowish colour or a chocolate-brown 



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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 than later in the day. Differences 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 

 dry and warm summers the clear rivers are but small, while the 



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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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