Casino Creek would have been rated as suffering from minor 

 silation, and both Casino Creek and Big Spring Creek below Casino 

 Creek would have been moderately impaired by silation. Below 

 Lewistown, the siltation index would have indicated severe 

 impairment and nonsupport of aquatic life uses. 



The spilled oil may have served as a substrate for motile 

 diatoms and may have had the same effect on the siltation index 

 as do fine particles of inorganic sediment. An assessment of Big 

 Spring Creek conducted in August 1998 indicated no siltation 

 problems in Big Spring Creek above Lewistown and only minor 

 siltation below Lewistown (Bahls 1999) . Certain cold season 

 species of Navicula and Nitzschia were also more abundant in the 

 winter samples. This may explain all or part of the elevated 

 siltation index values in March as compared to August. 



The disturbance index (% abundance of A. minutissiwa) 

 indicated no significant physical, chemical, or biological 

 disturbance had occurred in the days and weeks prior to sampling. 

 The percent dominant species values were marginally large at two 

 sites because of an abundance of cool season diatoms. Abnormal 

 cells, which may result from toxins in the water, were not found 

 at any of the sites. 



Casino Creek had slightly more than a third of its diatom 

 flora in common with sites in Big Spring Creek above and below 

 Casino Creek (Table 5) . It is not unusual for a tributary stream 

 to have a diatom flora that is this different from that of the 

 mainstem, especially if the streams have sources that are 

 different geologically or hydrologically . 



The two sites in Big Spring Creek above and below Casino 

 Creek had very similar diatom floras, sharing nearly 80% of their 

 diatoms. This would indicate that Casino Creek and its load of 

 waste oil did not have a significant effect on Big Spring Creek. 



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