reduced current and favorable conditions for sedimentation. Gale 

 (1971) reported that fingernail clams select mud substrates in preference 

 to sandy mud and sand. Abundant populations of fingernail clams over 

 soft mud bottoms in navigation pool number 19 on the Mississippi River 

 were reported by Gale (1969) and have been observed by the author. 



If the high navigation dams constructed in the 1930's did reduce 

 the current and increase sedimentation in parts of the Illinois River, 

 then the habitat suitable for fingernail clams may have increased, with 

 a benefit to the mollusc-eating fish. It is puzzling that conditions 

 have been so dramatically different since 1955, when a die-off of finger- 

 nail clams and snails occurred in the middle section of the Illinois 

 River (Mills, et al., 1966). As late as 1973, the fingernail clams had 

 not returned to areas of the river where empty shells indicated they 

 had formerly been abundant. 



The high navigation dams have a significant influence on dissolved 

 oxygen (DO) levels and waste assimilative capacity in the Illinois River, 

 according to Butts et al. (1975): 



"The dams are significant reaeration sources for 

 waters overflowing them. The extent of the aeration 

 establishes the bases for the configurations of the DO 

 sag curves. However, the dams should not be considered 

 wholly beneficial. On the contrary, their existence 

 lessens the capability of the waterway to assimilate 

 organic waste by: 1) Increasing the time-of-travel 

 and thus lengthening incubation periods in each pool, 

 2) Increasing the depth of flow and decreasing stream 

 velocities thus lowering the reaeration capability of 

 the pooled water, 3) Encouraging deposition and ac- 

 cumulation of solids on the pool bottom thereby 

 creating benthic biochemical oxygen demands." 



Changing agricultural practices beginning c. 1940 



Starrett felt that the increased sluggishness of the river and the 

 increased planting of row crops in the Illinois basin in the last 

 30 years have made siltation an important factor adversely affecting 

 the survival of mussels and other organisms in the Illinois River and 

 its bottomland lakes (Starrett, 1971). 



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