SUMMARY 



On August 21, 2001, periphyton samples were collected at 

 three stations on Cottonwood 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. 



Cottonwood Creek heads in the Big Snowy Mountains but flows 

 for most of its length through a grassland ecoregion. For this 

 reason, Cottonwood Creek metrics were compared to biocriteria for 

 both mountain streams and prairie streams. 



Cottonwood Creek supported a diverse algal flora indicating 

 nutrient-rich waters and a stable community with very little 

 disturbance. The presence/absence and relative abundance of 

 algal genera and species indicate an increase in sediment, 

 nutrients, organics, dissolved solids, and water temperatures as 

 one proceeds downstream. 



When compared to biocriteria for mountain streams, diatom 

 metrics at site 03 near the mouth of Cottonwood Creek indicated 

 partial support of aquatic life uses and moderate impairment. 

 The cause of this impairment was excessive siltation. However, 

 when the diatom metrics for Cottonwood Creek were compared to 

 criteria for plains streams, only minor impairment from organic 

 loading was indicated at the downstream site. Other sites fully 

 supported their aquatic life uses. 



The presence of the cyanobacterium Nostoc and a very large 

 number of diatoms in the family Epithemiaceae indicate that 

 nitrogen was likely the limiting nutrient in Cottonwood Creek. 



