Table 8 compares the metal content of sediment samples collected in the Blackfoot River before 

 (1973) and after (1975) dam failure. Except for Cd, higher concentrations of metals occurred in 

 the 1975 sediment sample than occurred in the 1973 sample. The 1975 sample was not collected 

 at the same site as the 1973 sample and diis may account for some of the variation. For purposes 

 of comparing the two samples, only the Tyler Sieve (mesh) U.S. Standard No. (-100 ) fraction is 

 shown in Table 8. 



Table 8. 



Concentration of metals in sediment collected at Pop's Place in 1973 and below 

 Shoue Gulch in 1975 (-100 fraction). 



Analyses of the two tailings samples collected by the Anaconda Company gave the results shown 

 in Appendix G. 



DISCUSSION 



Water quality and biological sampling showed that the Mike Horse Dam failure adversely 

 effected fish and bottom fauna populations in the upper Blackfoot River, although it was not 

 specifically determined how these effects occurred. 



A possible cause for mortality of smaller cutthroat trout is sediment deposition during the 

 cutthroat spawning and rearing period which prevented egg and fiy development and, 

 consequently, resulted in reduced numbers of young-of-the-year trout. Some other mechanism, 

 such as high concentrations of suspended sediment, may have been responsible for reduction in 

 larger ti-out of both species as well as for young-of-the-year brook trout from the 1974 fall 

 spawning season. Suspended sediment can cause physical damage to the fish, but some literattire 

 indicates that large quantities of sediment are needed to cause direct fish mortalities (Cordone 

 and Kelly 1961). Suspended sediment samples were not collected in 1975 but turbidity readings 

 were made. The highest ttirbidity recorded during the period was 5200 JTU on June 25 in 



See Spence (1975), Appendix D, p. A75, Field No. 8 and Lab No. 8. 

 See Appendix E, Table IV (-100 fraction) in this report. 



17 



