SUMMARY 



In May and August 2001, periphyton samples were collected at 

 six stations on Big Spring Creek near Lewistown, Montana for the 

 purpose of assessing whether the creek is water-quality limited 

 and in need of TMDLs . The samples were collected following DEQ 

 standard operating procedures, processed and analyzed using 

 standard methods for periphyton, and evaluated following modified 

 USEPA rapid bioassessment protocols for wadeable streams. 



Results from the samples collected in May could not be 

 compared to biological criteria Montana streams because these 

 samples were collected outside of the summer sampling "window" . 

 The sample collected below Lewistown in May contained an 

 unusually large number of motile, mesotrophic to eutrophic 

 diatoms that may indicate siltation and nutrient problems. 



In August, the site just below Big Springs supported a cold- 

 water flora that is typical of a spring creek. The moderate 

 impairment noted here is caused by natural thermal stress. 



Diatom metrics for site 02 above Lewistown indicated minor 

 impairment from disturbance and possible low levels of toxic 

 chemicals. Aquatic life uses were fully supported. Site 03 

 below Lewistown also provided full support of aquatic life uses, 

 with minor impairment indicated from siltation and toxics. 



Diatom metrics for site 05 near the mouth of Big Spring 

 Creek indicated moderate impairment and partial support of 

 aquatic life uses . The probable cause of this impairment is 

 nutrient enrichment . The bioassessment ratings for sites 03 and 

 05 would be the same if diatom metrics for these sites were 

 compared to criteria for prairie streams instead of mountain 

 streams . 



