6 R. M. Anderson, J. B. Layzer, and M. E. Gordon 



Corbicula densities may be a result of our greater sampling efficiency, 

 especially in detecting small individuals. Although high densities of 

 Corbicula may negatively affect mussels (Clarke 1988), the adverse 

 effects reported usually have occurred at substantially higher Corbicula 

 densities than those observed in our study (Gardner et al. 1976). 

 Corbicula is generally more tolerant than unionids of environmental 

 stress (Horn and Mcintosh 1979). Tolerance to, and even preference for, 

 silted areas by C. fluminea has been reported (Belanger et al. 1985). The 

 increase in Corbicula densities could be a result of an increasingly 

 favorable substrate. 



The source of silt in the LSF has not been documented. However, 

 strip-mining activities have increased greatly since 1981, and heavy 

 siltation has been observed at a few sites within the affected area (S. A. 

 Ahlstedt, personal communication; G. A. Schuster, personal communi- 

 cation). On several occasions we also observed a very fine sediment at 

 Ritner Ford (station 15). These silt deposits varied in depth, with the 

 greatest accumulation occurring along the sides of riffles. Silt deposits 

 appeared to be transient and were readily flushed during freshets. 



If the observed silt was a result of strip-mining, it may have 

 contained potentially toxic concentrations of metals such as aluminum 

 or zinc (Dick et al. 1986). "Yellow boy," a ferric precipitate often 

 associated with mining activity, was observed in Lick Creek, Jones 

 Hollow, Baker Branch, and several other unnamed tributaries of LSF. 

 Most of these tributaries are intermittent. Although we never saw 

 yellow boy in the river itself for more than a few meters below a source 

 during low flow, we observed these precipitates to be readily transported 

 during high flows. 



The upstream extent of the mussel die-off in LSF appeared to be 

 between sites 6 and 8. Because site 6 is a commonly used ford, the 

 absence of live unionids in 1981 and in 1987 is probably a result of 

 physical disturbance. Moreover, a cursory examination of a riffle just 

 upstream from site 6 indicated the presence of live unionids, including 

 P. fabula. All of the tributaries that drain known strip mines in this 

 watershed enter the LSF downstream from site 6. Moreover, the pattern 

 of mining in the LSF appears to be correlated with the sequential die- 

 off of mussels. Intensive mining began in the lower portion of the 

 watershed and has been progressing upstream. In 1981 a rich mussel 

 fauna existed throughout the LSF, and there was relatively little mining. 

 By April 1985, P. fabula had been extirpated from sites 13 and 14 but 

 persisted at site 8 (Ahlstedt 1986). In November 1984, a permit was 

 issued to strip-mine 100 ha in the Kidd's Branch sub-basin of the LSF. 

 Kidd's Branch enters LSF between sites 6 and 8. By 1987, P. fabula and 

 most other unionids had disappeared from site 8. This pattern of mining 



