northward into the polar basin. In Eurasia, the Ob, Yenisei and 

 Lena are the major north-flowing rivers, the Pechora, Indigirka 

 and Kolyma being of lesser importance. The Mackenzie river 

 system is the largest of the North American rivers draining into 

 the Arctic Sea. The Colville, Coppermine, Back, Thelon, Du- 

 bawnt. Kazan, Churchill, Nelson, Fort George, Great Whale, 

 Koksoak and George rivers are some of the smaller north-flowing 

 rivers of Arctic America. 



The rivers that flow from the south present problems in the 

 spring breakup. While the lower portions of these rivers are still 

 ice-locked, the upper reaches are being exposed to the longer days 

 and warmer temperatures of spring. Ice in the upper sections 

 thaws, breaks loose, and starts moving down stream. Its progress 

 is blocked by the still frozen lower sections, and flooding and ice- 

 damming, are common spring occurrences. Another effect of the 

 alinement and direction of these rivers is that, although the upper 

 portions may be ice-free early in the summer, the rivers cannot be 

 entered until the ice at the mouths has broken up. Local river 

 traffic can operate in the ice-free areas, but all outside traffic must 

 wait until the summer season has progressed northward to free 

 the river outlets. 



VEGETATION 



The abundance of lakes and rivers in the northern regions might 

 give the impression that it is an area of heavy precipitation. This 

 is not the case. The Arctic has scant precipitation, receiving only 

 10 to 15 inches a year. Aridity is one of the main controlling 

 factors in the region, together with low evaporation and low tem- 

 peratures. Under such circumstances soil development is at a 

 minimum and decay is prevented. The combination of drought, 

 inadequate soil, frozen subsoil and short growing season produces 

 conditions inimical to tree growth. Hence, the Arctic is essen- 

 tially a treeless tundra. Clumps of trees are found in areas where 

 soil is sufficiently deep, as on terraces, raised beaches and deltas, 

 but they are limited in size and number. 



Despite the lack of forests, the tundra is luxuriant in vegetation. 

 There are innumerable flowering plants, mostly perennials, which 

 burst into bloom and complete a life cycle in the short season from 

 June to August. These plants also show adaptation to the envir- 

 onment in other ways; for example, they have abnormally devel- 

 oped subterranean stems and roots spreading laterally in the 

 shallow layer above the permafrost; they have leathery, waxy or 



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