prairie streams. 



An elevated siltation index indicated minor impairment at 

 Interstate 15 (Table 8) . Species of Navicula and Nitzschia 

 appeared as major species (>10% abundance) for the first time at 

 Interstate 15. Most species of Navicula and Nitzschia are motile 

 and adapted to living on silty, aggrading substrates. The total 

 relative abundance of species in these genera, together with the 

 relative abundance of Surirella species, is used to calculate the 

 siltation index. 



Other periphyton metrics indicated good water quality and 

 biological integrity at Interstate 15. Specific conductance was 

 considerably lower at 1-15 and at downstream sites than it was at 

 Highway 221 (Figure 2) . Interstate 15 had about 40% of its 

 diatom flora in common with the next upstream site at Highway 

 221, which is quite a large similarity index considering the 

 distance between these two sites (>20 miles) and the several 

 intervening tributaries, including Muddy Creek. 



Along with Achnanthes minutissima , the pollution- tolerant 

 species Cymbella microcephala (also a salinity indicator) and 

 Navicula cryptotenella dominated the diatom association at Kerr 

 Bridge (Table 8). The large percentage of A. minutissima (37%) 

 here resulted in borderline values for the diversity index, 

 disturbance index, and percent dominant species, all indicating 

 minor impairment (Table 8) . 



Chemical, physical, and biological disturbance (e.g., 

 toxins, abrasion, and invertebrate grazing) may all cause an 

 increase in the abundance of Achnanthes minutissima . The 

 specific cause or causes of disturbance at Kerr Bridge are 

 unknown, but channel downcutting and instability may be involved. 

 The Kerr Bridge site had over half of its diatom flora in common 

 with the 1-15 site (Table 8), which is about 10 miles upstream. 



16 



