steeper slope. Fewer animals were found in channeled areas and their 

 number decreased with increasing sediment movement and accumulation. 

 Accumulation of both algae and sediment accompained by high decomposition 

 rates decreased available oxygen and increased hydrogen sulfide concentra- 

 tion, thereby limiting many benthic animals. 



The canyon and control transects had many species in common but the 

 general composition of assemblages differed (Tables 3 and 4) . The number 

 and kinds of animals at the two shallow-water canyon stations were vari- 

 able with seasonal lows occurring in winter when topographic changes were 

 greatest. Most species were present only intermittently; the few con- 

 stant ones were also found at the same depths on the control transect. 



The most convincing evidence of sediment movement as the main factor 

 controlling benthic faunal distribution in the study area came from the 

 comparison of ridge stations at the same depth arranged by substrate 

 stability along the Kaiser Company trestle (Table 1). The total numbers 

 of worms increased with increased stability. Crustaceans were most 

 abundant at low and intermediate substrate stability; mollusks were more 

 abundant with increasing stability. 



The distribution and relative abundance of many species were similar. 

 The worms, Amaeana ooaidentalis , Nothria elegans , Lumbvinevis nr. luti , 

 and the crustaceans, Paraphoxus dahoius , Euphilomedes ohlonga, E. 

 aaraharodonta , were characteristic of deeper stations and found in areas 

 of increasing substrate stability on the ridge. The worms, Dispio 

 unoinata, Sooloplos armigev , Onuphus evimita, Paraphyoxus luaubrans , 

 cunuaeans , and the mollusk, Olivella , were characteristic of shallow 

 stations and found in areas of decreasing substrate stability along the 

 ridge. The crustaceans, Euphilomedes longiseta, the mollusk, Tellina 

 modesta, and the common sand dollar, Dendraster exoentriaus , were charac- 

 teristic of stations of intermediate depths and found in areas of inter- 

 mediate substrate stability along the ridge gradient. 



The overwhelming trend was for species composition and abundance to 

 change in a similar manner with stability changes rather than with depth. 

 Sediment movement was the most important environmental parameter affect- 

 ing the distribution of macroinvertebrates at the control area and in 

 the canyon. 



Many small crustaceans (amphipods and ostracods) are well adapted to 

 shifting or gradual accumulation of sediment. They are characteristic 

 of coarse, unconsolidated sediments (Barnard, 1963; Masse, 1972), but 

 intolerant of the mass accumulation of sediments in the burial experiment. 

 The crustaceans' presence at the shoreward ridge stations indicated that 

 sediment probably did not move in mass slumps; instead, the sediment crept 

 gradually down adjacent terrace walls, as suggested by Shepard and Dill 

 (1966), creating an environment to which crustaceans can readily adjust. 



No active or passive migration of species characteristic of shallow- 

 water stations along the control or canyon transects was evident which 



