The dominant species at the control site were Achnanthes 

 winutissiwa (August) and Hannaea arcus (June) . A. winutissima is 

 an attached species that pioneers recently scoured substrates and 

 resists detachment by high current velocities (Peterson and 

 Stevenson 1992) . Because of its attached habit of growth and 

 rapid rate of division, it often dominates diatom associations of 

 steep gradient mountain streams in western Montana . Hannaea 

 arcus is a clean-water species that prefers cold temperatures and 

 fast current velocities (Lowe 1974). 



The Flemming Bridge site also provided full support of its 

 beneficial uses, but with minor impairment due to siltation. 

 This site also had a borderline pollution index value that 

 indicated some nutrient enrichment. Nitzschia paleacea was the 

 co-dominant diatom here along with Achnanthes minutissima . 

 Nitzschia paleacea is a motile, free-living diatom that is 

 somewhat tolerant of organic pollution (Lange-Bertalot 1979) . 



The June collection from Two Mile Bridge indicated excellent 

 biological integrity with no impairment of aquatic life uses. 

 The August collection was dominated by Gomphonema olivaceoides. 

 This species prefers "cool fresh waters" (Patrick and Reimer 

 1975) and is "locally abundant in springs and small streams" 

 (Krammer and Lange-Bertalot 1986) . It is closely related to, and 

 considered by some taxonomists as a variety of, G. olivaceum, 

 which is a common diatom in calcareous, eutrophic rivers 

 worldwide (Krammer and Lange-Bertalot 1986) . 



Like other species in the genus Gowphonewa, G. olivaceoides 

 is a stalked diatom that may occur in large colonies (Cox 1996) . 

 This species has been recorded as the dominant diatom in the West 

 Fork of the Stillwater River near Nye and in the North Fork of 

 the Flathead River at the Canadian border (L. Bahls, unpublished 

 data) . Both are relatively unimpaired streams. 



