and may be measurable; however, the long-term response may be so gradual 

 that the changes will not be noticeable for decades. 



In addition to these general statements pertaining to all rivers, 

 a few characteristics of arctic and subarctic rivers are introduced below. 

 Flow stops in many rivers for much of the winter. Those rivers that continue 

 to flow in the channel beneath the ice or in the gravel beneath the channel 

 have the potential to develop auf ei s , which is ice that forms upon itself by 

 a series of overflows. The remaining flow is considered vital to fish over- 

 wintering areas. 



At breakup, the water levels of large snowmelt floods are often in- 

 creased by ice jamming or aufeis in the channel. After the snowmelt flood, 

 flow may decrease significantly for the rest of the summer except for a few 

 short duration events in response to summer storms. Very low summer flow is 

 especially common on the North Slope, which is semiarid, receiving only 

 150 mm of precipitation annual ly. 



In subarctic Alaska, glaciers feed many rivers, resulting in generally 

 more uniform flows through the summer. Diurnal fluctuations are evident in 

 these rivers near their headwaters. Associated with glaciers are glacier 

 dammed lakes that can empty rapidly causing extensive flooding downstream. 



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