After the identification of soft algae, raw periphyton 

 samples were cleaned of organic matter using sulfuric acid and 

 potassium dichromate, and permanent diatom slides were prepared 

 using Naphrax, a high refractive index mounting medium, following 

 Standard Methods for the Examination of Water and Wastewater 

 (APHA 1998) . For each slide, between 296 and 464 diatom cells 

 (592 to 928 valves) were counted at random and identified to 

 species. The following were used as the main taxonomic and 

 autecological references for the diatoms: Krammer and Lange- 

 Bertalot 1986, 1988, 1991a, 1991b; Patrick and Reimer 1966, 1975. 



Lowe (1974) was also used as an ecological reference for the 

 diatoms. Bahls et al . (1984) provide autecological information 

 on important diatom species that live in the Southern Fort Union 

 Coal Region of Montana, including many of the diatom species 

 found in the Tongue River. 



The diatom proportional counts were used to generate an 

 array of diatom association metrics (Table 2) . A metric is a 

 characteristic of the biota that changes in some predictable way 

 with increased human influence (Barbour et al . 1999) . 



Metric values from the Tongue River were compared to numeric 

 biocriteria developed for streams in the Great Plains Ecoregions 

 of Montana (Table 3) . These criteria are based on metric values 

 measured in least-impaired reference streams (Bahls et al . 1992) 

 and on metric values measured in streams that are known to be 

 impaired by various sources and causes of pollution (Bahls 1993) . 



The criteria in Table 3 distinguish among four levels of 

 impairment and three levels of aquatic life use support: no 

 impairment or only minor impairment (full support) ; moderate 

 impairment (partial support) ; and severe impairment (nonsupport) . 

 These impairment levels correspond to excellent, good, fair, and 

 poor biological integrity, respectively. 



