Forbes and Richardson found the usual rate of flow at normal stages to 

 be l%-2% miles per hour (1920: xli) . Starrett found the current speed 

 in 1966 to be about 0.6 mph at normal stages. This reduction in cur- 

 rent velocity compounded the siltation problem that already existed in 

 the river (Starrett, 1971: 272; Bellrose et al. , 1977: C-12). The reduced 

 current velocity resulting from the dams and reduced Lake Michigan diversion 

 may also be preventing the reestablishment of several mussel species: 



The navigation dams possibly have reduced the flow of the 

 current enough to make the environment in the river unsuit- 

 able for the reestablishment of several current-inhabiting 

 species of mussels present before 1900. (Starrett, 1971: 346) 



Experiments by Ellis showed that most of the common fresh-water 

 mussels were unable to maintain themselves in sand or gravel bottoms 

 when a layer of silt from h," to 1" deep was allowed to accumulate on 

 the bottom (1936: 39, 40). He found the yellow sand-shell to be one 

 of the least resistant mussels he tested and while this mussel was not 

 found alive in Alton Pool by Starrett in 1966 (1971: 356), it was col- 

 lected in the Pool at Meredosia in pre-dam surveys (Table 6 ) . Ellis 

 also found that silt may reduce survival of young clams by destroying 

 mucus threads used by the animal for anchoring (1931: 6, 7). 



Starrett felt that sedimentation was of major importance in ex- 

 plaining the decline of the mussel fauna in the river: 



The increase in the sluggishness of the river, as men- 

 tioned above and the increased planting of row crops on the 

 watershed have, in the author's opinion, made siltation in 

 the past 30 years /mid-1930 's to mid-1960's/ an important 

 factor adversely affecting the survival of mussels and 

 other organisms in the Illinois River and its bottomland 

 lakes . . . (1971: 272) 



The resuspension of bottom materials by barge traffic adds to 

 the turbidity of river water. Starrett in 1964 found that in Alton Pool 

 (river mile 65.1) the passage of two towboats increased the turbidity 

 from 108 to 320 Jackson Turbidimeter Units (1971: 273). Increased 



43 



