In larger rivers, most natural buffers that were maintained to protect 

 scraped sites in active floodplains failed within a couple years. At Middle 

 Fork Koyukuk R i ver-Upstr earn a 30-m wide, I- to I . 5-m high heavily vegetated 

 buffer protecting an inside meander site was breached in I year; at Sagavan- 

 irktok River, a 30-m wide, 0.5-m high gravel buffer protecting a mid-channel 

 site was breached in I year; and at Dietrich River-Downstream a 50-m wide 

 and 0.5- to I -m high gravel and sparsely vegetated buffer protecting a site 

 on the edge of the active floodplain of a braided river was breached in 2 

 years. These buffer failures have all created permanent channel changes 

 through the mined areas of these sites. 



At pit sites located in inactive floodplains and terraces, buffers 

 composed of incised banks and heavily vegetated riparian zones ranging from 

 50 to 90 m in width were sufficient in protecting the pits from active 

 channel diversion at sites up to 13 years old. However, most of these sites 

 (three of five) are located on sma I ler rivers with relatively stable chan- 

 nels and are on the inactive side of the floodplain. On the other hand, at 

 the oldest pit site (Tanana River-Downstream) a 50-m wide buffer separated 

 the pit from an erosional zone of a side-channel of this braided river. 

 During 1977 and 1978 this buffer was being actively eroded. It is not known 

 how wide the buffer was at the completion of the mining activity. 



One mining method (pits) and one site location (separated from the 

 active floodplain) frequently led to the creation of high quality habitat 

 that resulted in an increase of water birds (Biotic Group III). As previous- 

 ly discussed, this method created a habitat type that frequently was not 

 readily available in adjacent floodplain reaches. The quality of this habi- 

 tat was related to its size, shoreline diversity (configuration), water 

 depth diversity, shoreline cover, presence of islands, and food 

 ava i I ab i I i t y . 



Other characteristics occurred that were not directly related to the 

 location or operation of the material site but that reduced detrimental 

 impacts to the terrestrial biota. At those sites where access to the flood- 

 plain had to pass an incised bank, gravel fill ramps (Figure 82) reduced the 

 overal I impact. At sites where incised banks were cut for access severe 



265 



