Percent 



0.31 0.83 126 2.S2 5.03 



Size class (Inches') 



Figure 44. Pebble count particle distribution at bankfiill riffle 

 cross-sections in three reaches of the upper Blackfoot River. 



Mean wetted widths (pools and riffles) and pool length all increased in the 

 downstream direction. Mean residual pool depth decreased from 4.2' in the upper reach 

 to 3.3' (middle reach) to 2.0' in the lower reach. 



We completed pebble counts for both the bankfiill (Figure 44) and wetted-width 

 (riffle-crest) cross-sections. For the wetted-width surveys, we recorded a large decrease 

 in suitable spawning-size (0.3-2.5') gravels in the downstream direction. In the upper 

 reach cross-section, we found well-sorted substrate ranging from a 0.04" to 5.0" diameter 

 with 89% of the surface substrate falling into the 0.3-2.5" diameter size range. In the 

 middle reach cross-section, we 

 found alluvial gravel with a 

 smaller range of particle sizes 

 (compared v^th the upper 

 reach) ranging from very fine 

 sand to small cobble, a bimodal 

 distribution with 19% of the 

 sample comprised of sand 

 (modal class), 28% <0.3" 

 diameter and 70 % of the 

 sample within the medium to 

 coarse gravel (0.3-2.5") 

 substrate range. For the lower 

 reach cross-section, we found a 

 poorly sorted, more 



heterogeneous mixture of 

 substrate in riffles, with a much 

 wider range of substrate sizes 

 ranging from clay/silt to 

 boulder (<0.02" to 20"). A 

 veneer of silt and clay covered 

 the surface and interstitial 

 spaces of the substrate and only 

 28% of the sample gravels fell 

 within the suitable spawning- 

 size range. For bankfull 

 measurements, we found a 

 bimodal distribution at all three 

 samples, with very fine sand 

 and smaller particles being a 

 dominant size-class. 



Water temperatures show a wide range of variability within and between reaches 

 (Figure 45, Appendix I). Winter water temperatures were significantly lower in the lower 

 reach compared with the upper reach (P < 0.001) during core winter months (January and 

 February) with mean water temperatures ~4 " F above freezing in the upper reach, but at 

 or near freezing in the middle and lower reaches. Summer water temperatures were 

 significantly warmer in the lower reach compared with the upper reach (P < 0.001) during 



>> 25 



Figure 45. Monthly ma,\imum water temperatures at three locations 

 of the upper Blackfoot River January-October 2003. 



60 



