particle is compared to the total size composition of the riffle to determine the percent of riffle material 

 that is moved during the annual runoff. The resulting percentage of movable particles becomes an 

 index of riffle equilibrium called the riffle stability index (RSI). 



It has been suggested that an RSI value of 70 or higher is a warning sign for Idaho's belt geology 

 streams, similar to those found in the Flathead Basin; an RSI value greater than 90 indicates that a 

 watershed is out of equilibrium with respect to the balance between sediment loads and water yields 

 (Kappesser, 1993). 



During the summer of 1993, riffle stability index measurements were made at nine sites in upper Big 

 Creek from below the Lakalaho Creek junction upstream to within one-half mile of Road #1696 

 crossing. The RSI values ranged from 65 to 95, with eight sites having RSIs greater than 70, and three 

 sites having RSIs greater than 90. The three sites with RSIs greater than 90 have a relatively high 

 percentage of small particles, suggesting that sediment has accumulated in those areas. Also, the 

 mean size of the largest moving particle for all sites was about 5.5 inches, a further indication that 

 stream energy is high enough to move even large cobbles during annual peak flows. These results 

 suggest that portions of Big Creek's channel is unstable and has a limited capacity to absorb additional 

 water yield increases from hillslope development in the headwater basin. 



McNeil Core Sediment IVIeasurements 



The size range of the streambed materials is indicative of fish spawning and incubation habitat. 

 Increased fine sediments reduce pool depth; interstitial spaces needed for invertebrate production, and 

 reduce embryonic survival of fry (Everest et al. 1987, Weaver and Fraley 1991). A McNeil corer 

 (McNeil and Ahnell, 1964) is used to collect streambed samples which are dried and sieve analyzed to 

 detennine the particle size distribution, for materials less than 6.5mm in diameter (fines). As part of the 

 Flathead Basin Forest Practices - Water Quality, and Fisheries Cooperative Research Program, Fraley 

 and Weaver established a correlation between the streambed fines and the bull trout survival in the 

 Flathead River Basin. A statistically significant correlation was identified, that streambed fines greater 

 than 35% resulted in decreased survival of bull trout (Weaver and Fraley 1991). Base on this research, 

 the Flathead National Forest uses the criteria that streams with greater than 35% fines are considered 

 threatened, while a streams with greater than 40% fines are generally considered impaired. These 

 threatened and impaired determinations do not necessarily correlate with MDEQ threatened and 

 impaired designations associated with beneficial use support since the MDEQ may use values that can 

 vary when reference conditions imply other numbers would better represent MDEQ guidance for 

 making beneficial use support determinations (Water Quality Assessment Process and Methods, 

 Appendix A of the 2000 303(d) List). 



Since 1982 McNeil core samples have been taken in a sampling reach of Big Creek, near the 

 Skookoleel bridge crossing (road # 31 6E). Table - 5 reports the results of the McNeil core monitoring 

 program. The increasing trend of fine streambed sediments starting in 1989 is thought to be the 

 movement of the earlier upland erosion sediments through the streambed monitoring reach in lower Big 

 Creek. After the flushing flows in 1992 there has been a decline in the streambed fines in this 

 monitoring reach. 



Table - 5: McNeil Core samples (%fine sediment <6.4mm) in Big Creek. 



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