Vegetat i ve Cover . Vegetation with deep root structures provides a resis- 

 tance to bank erosion. Dense ground cover on the buffer provides an increase 

 in the roughness of the buffer, causing a decrease in the velocity of flow 

 over the buffer. This, in turn, reduces the potential for erosion of the 

 buffer surface and the development of a channel through the buffer. This is a 

 primary reason why a buffer should not be disturbed. 



When extending historical erosion rates, the vegetative pattern should 

 be considered. No compensation for vegetation is required if the vegetation is 

 comparable between the buffer and the area that eroded during the period of 

 historical erosion. If the vegetation type or density changes within the 

 buffer, or between the buffer and the area of historical erosion, then the 

 historical rate of erosion should be modified according to the type of change 

 and the discussion in the preceeding paragraph. 



Permafrost Banks. The erosion of permafrost banks is a more complicated 

 process than unfrozen bank erosion. Various investigators have studied the 

 process; some have concluded that permafrost increases bank erosion, others 

 have decided that permafrost decreases bank erosion. Scott (1978) reviewed 

 previous investigations and added his own investigation of five rivers in 

 arctic Alaska. He concluded that the net effect of permafrost is to create 

 greater channel stability than is found in rivers of similar size in nonperma- 

 frost environments. However, banks which are ice-rich will likely have less 

 stability and higher erosion rates than other permafrost or nonpermaf rost 

 banks. 



When using past records to predict future conditions, the thermal con- 

 dition of the banks should be considered. Past thermal conditions of the 

 banks are generally not known, consequently, it must be assumed that they were 

 similar to the current condition. If the banks are ice-rich, the safety factor 

 applied to the buffer width should be larger. 



Channel Aufeis. Aufeis development in the active channel of a river can 

 cause a larger percentage of the snowme I t runoff to flow across the buffer 

 than otherwise would be expected. Doyle and Childers (1976) show a photograph 



98 



