SUMMARY 



On August 8, 2000, periphyton samples were collected from 

 two stations on Casino 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. 



Casino Creek heads in the foothills of the Big Snowy 

 Mountains and flows for most of its length through a grassland 

 ecoregion. For this reason. Casino Creek metrics were compared 

 to criteria for both mountain streams and prairie streams . 



When compared to biocriteria for mountain streams, the 

 siltation index indicated partial support of aquatic life uses at 



both sampling sites. Siltation was worse at the upper site. 

 When compared to criteria for plains streams, the siltation index 

 indicated only minor impairment and full support of aquatic life 

 uses at C 1, and no impairment at C 2 . 



A relatively large number of teratological diatom cells 

 indicated possibile chemical toxicity or environmental stress at 

 C 2, and only partial support of aquatic life uses. The source 

 of this toxicity or stress is unknown. 



The non-diatom algal assemblages and the diatom pollution 

 index both indicated small increases in nutrient concentrations 

 and organic loading between sites C 1 and C 2. 



The relatively low similarity index indicated that sites C 1 

 and C 2 had only somewhat similar diatom floras and that a 

 moderate amount of environmental change had occured between the 

 two sites. 



