Summary 



In August 2003, periphyton samples were collected from 3 sites on the Yaak River and 1 

 site on the West Fork of the Yaak River in northwestern Montana for the purpose of assessing 

 whether these streams are water-quality limited and in need of TMDLs. The samples were 

 collected following MDEQ standard operating procedures, processed and analyzed following 

 standard methods for periphyton, and evaluated following modified USEPA rapid bioassessment 

 protocols for wadeable streams. 



An unusually large percentage of the pollution-tolerant species Synedra rwnpens and a 

 large percentage of abnormal diatom cells suggest moderate impairment by heavy metals in 

 the West Fork of the Yaak River. There are no known sources of heavy metals in the West Fork 

 drainage. The West Fork site also had the lowest diatom species richness, diatom species 

 diversity, and pollution index values of all four sites that were sampled in the Yaak. Values for 

 these metrics indicated minor impairment. 



Diatom metrics suggest that the three sites on the main Yaak River were subject to minor 

 stress from unknown causes. All three sites supported a small number of abnormal diatom cells 

 and the Whitetail Camp Ground and Sylvanite sites had elevated values for the disturbance 

 index. Paralleling a downstream increase in abundance of pollution-tolerant algae, the pollution 

 index declined in a downstream direction but remained above the criterion for minor impairment 

 at all three sites. The siltation index peaked at Whitetail Camp Ground but remained just below 

 the threshold for minor impainnent. Values for the diatom species diversity index also declined 

 in a downstream direction. 



The sample from Sylvanite was dominated by cells and polysaccharide stalks of the 

 diatom Didymosphenia geminata. Colonies of this diatom look like dirty cotton balls. This is a 

 widespread boreal species that is often reported to produce nuisance standing crops in northern 

 streams of Europe and North America. 



