listed stream are at least partially impaired as a result of sediment (Blackfoot Challenge 

 2005). It is elevated anthropogenic sediment from chronic sources that commonly limits 

 a stream's reproductive capacity. High levels of fine sediment in the Blackfoot River 

 system have significant implications for native fish recovery as well as sport fisheries. 

 Generally, the basin's natural high levels of sediment reduce flexibility of land managers 

 to contribute sediment to the system. In the case of bull trout spawning streams, fine 

 sediment levels (<6.35mm) are near or above threshold limits (30%) above which 

 survival decreases significantly (Shepard et al 1984). Land managers must understand 

 that naturally high levels of sediment in many streams (and spawning sites) provide little 

 room for additional human caused sedimentation. 



This baseline provides a potential tool for land managers to interpret the value of 

 existing spawning sites for multiple salmonids of the Blackfoot River and for monitoring 

 fiature instream sediment changes. However, these data were collected in low flow years 

 and may not fiiUy representative. Interpretation should also be considered within the 



context of channel slope 

 and channel -type. 



Certain streams such as 

 spring creeks and E-type 

 channels also naturally 

 support higher levels of 

 fine sediment. 



From the 



perspective of general 

 trout spawning success, 

 the core sample data 

 provides two accepted 

 methods of assessing the 

 value of spawning sites. 

 These are measures of 

 percent fines (<0.84 and 

 <6.35mm) and central 

 tendency (Fredle index 

 and geometric mean 

 diameter) of the sample. 



Fine sediments 

 (<0.84mm in diameter) 

 are considered among 

 the most detrimental to 

 incubating eggs (Reiser 

 and White 1988; Hall 

 1984). It is this material 

 that most often enters 

 important spawning 



streams as a result of 

 human activities (Reiser 

 and White 1988; 



Index mean+SD 



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Figure 56. Geometric mean (top in mm) and fredle index mean 

 (bottom) for 29 spawning streams in the Blackfoot Watershed, 2004-05. 



80 



