TOPOGRAPHY AND DRAINAGE 



Most of this subarctic region was scraped and eroded by the 

 vast continental glaciers of the Pleistocene. The relief is thus 

 subdued, and vast stretches of plains characterize the central por- 

 tions of the region, while the remainder, except for the valleys of 

 the Mackenzie and Yukon Rivers, is mostly rolling plateaus and 

 dissected uplands, with rugged mountains in southern Alaska. 

 The soil mantel is very thin and the entire area is underlain by 

 permafrost. 



The valley of the Yukon River has many areas of flatlands, some 

 of which are thousands of square miles in extent. The river, with 

 its many tributaries, takes a circuitous route through the rolling 

 mountains of the Yukon plateau and the low hills in the depression 

 between the Brooks Range and the Alaskan Range, to empty into 

 Norton Sound, where it is building a swampy delta. The Bering 

 Sea coasts of Alaska are generally adjoined by lowlands. Back of 

 the shallow Bristol Bay lie a large number of lakes at low eleva- 

 tion. Norton Sound is likewise surrounded by low land. Kotze- 

 bee Sound is surrounded by lagoons, barrier beaches, and flat 

 moors. Harbor conditions throughout are unfavorable because 

 of shallow water and of insufficient protection. 



Separated from the Yukon River by the northern Rocky Moun- 

 tains and the Yukon plateau, the Mackenzie River drains the east- 

 ern slope of these uplands, and flows along their foothills, receiv- 

 ing water from the Peace and Liard Rivers and the two vast lakes 

 of the north. Great Slave Lake and Great Bear Lake. Because 

 the Mackenzie has such a great latitudinal extent, there is a dif- 

 ference of about 3 weeks in the length of the navigation period 

 between the northern and southern sections. Delay in the navi- 

 gable season is also due to the fact that although the river ice 

 usually breaks up in middle or late May, the ice along the shores 

 of Great Slave Lake does not break away until early June and 

 the lake is not ice-free until mid-June. Ice begins to form again 

 in October in the delta. Freeze-up on the upper Mackenzie comes 

 in mid-November and on its southern tributaries by late November. 



Other than these two major systems, the rest of this subarctic 

 region is drained by ill-defined streams which wander aimlessly 

 over the glaciated surface, where irregularities cause numberless 

 lakes to form. In the summer, these areas are alive with hungry 

 mosquitoes, and the soft, water-soaked ground makes transporta- 

 tion overland extremely difl^cult or impossible. 



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