INTRODUCTION 



This report evaluates the biological integrity, support of 

 aquatic life uses, and probable causes of impairment to those 

 uses at 11 stations on 0' Fallon Creek and three tributaries of 

 0' Fallon Creek in southeastern 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. 



The evaluations in this report are based on the structure 

 and species composition of the periphyton or phytobenthos 

 community. The periphyton community is a basic biological 

 component of all aquatic ecosystems. Periphyton accounts for 

 much of the primary production and biological diversity of 

 Montana streams (Bahls et al . 1992). 



Periphyton is a diverse assortment of simple photosynthetic 

 organisms called algae, and other microorganisms that live 

 attached to or in close proximity of the stream bottom. Many 

 algae, such as the diatoms, are microscopic. Diatoms are 

 distinguished by having a cell wall composed of opaline glass- - 

 hydrated amorphous silica. Diatoms often carpet a stream bottom 

 with a slippery brown film. 



