INTRODUCTION 



This report evaluates the biological integrity, support of 

 aquatic life uses, and probable causes of impairment to those 

 uses in the East Boulder River, a tributary of the Boulder River 

 in the Yellowstone River Basin of southcentral Montana. The 

 purpose of this report is to provide information that will help 

 the State of Montana determine whether the East Boulder River is 

 water-quality limited and in need of TMDLs . 



The federal Clean Water Act directs states to develop water 

 pollution control plans (Total Maximum Daily Loads or TMDLs) that 

 set limits on pollution loading to water-quality limited waters. 

 Water-quality limited waters are lakes and stream segments that 

 do not meet water-quality standards, that is, that do not fully 

 support their beneficial uses. The Clean Water Act and USEPA 

 regulations require each state to (1) identify waters that are 

 water-quality limited, (2) prioritize and target waters for 

 TMDLs, and (3) develop TMDL plans to attain and maintain water- 

 quality standards for all water-quality limited waters. 



Evaluation of use support in this report is based on the 

 species composition and structure of the periphyton (benthic 

 algae, phytobenthos) community at five sites that were sampled in 

 1998 and 1999 . The periphyton community is a basic biological 

 component of all aquatic ecosystems. Periphyton accounts for 

 much of the primary production and biological diversity in 

 Montana streams (Bahls et al . 1992). 



Plafkin et al . (1989) and Stevenson and Bahls (1999) list 



several advantages of using periphyton in biological assessments: 



• Algae are universally present in large numbers in all 



streams and unimpaired periphyton assemblages typically 

 support a large number (>30) of species,- 



