INTRODUCTION 



This report evaluates the biological integrity, support of 

 aquatic life uses, and probable causes of impairment to those 

 uses, in the upper Blackfoot River and in four tributaries of the 

 upper Blackfoot River in western Montana. The purpose of this 

 report is to provide information that will help the State of 

 Montana determine whether these streams are 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 12 sites that were sampled from 

 mid to late June in 2001. 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; 



