The operation and maintenance of the nine-foot channel also af- 

 fects benthic organisms. Dorris and Copeland (1962: 246, 247) reported 

 that winter drawdowns in the section of the Mississippi bordering Iowa, 

 Illinois, and Missouri significantly reduced the mean numbers of mayfly 

 naiads (Hexagenia rigida ) . Dredging operations destroy the benthos in 

 the channel and spoil placement may smother benthic populations. We do 

 not know how rapidly different types of benthic organisms can recolonize 

 areas which have been dredged or have received dredge spoil. 



A major study of the benthos in Pools 24, 25, and 26 of the Mis- 

 sissippi River and the lower 80 miles of the Illinois River was conducted 

 by Colbert et al. (1975) in 1974 for the St. Louis District, U.S. Army 

 Corps of Engineers. They took a series of benthic samples under high 

 flow conditions in July and another series during average flow condi- 

 tions in September. They generally sampled the following four habitat 

 types at each of thirteen locations in the Mississippi River: main 

 channel, side channel, river border area, and area downstream of dikes. 

 Definitions of habitat types are given *in Colbert et al. (1975: 16-17). 



Colbert et al. (1975: 94-95) found that side channels and river 

 border areas generally afforded the best habitat for benthic organisms, 

 while the main channel was the poorest habitat. Total density of 

 benthic organisms, species diversity, number of taxa, number of oligo- 

 chaetes, oligochaete biomass, and total biomass (exclusive of clams) 

 were significantly lower in the main channel. Colbert et al. (1975: 95) 

 felt that relatively high current velocity and the coarse, shifting 

 substrate in the main channel were the main factors limiting the ben- 

 thic organisms. Colbert et al. (1975: 84) also found that the density 

 of aquatic insects, and the average number of taxa and diversity of 

 all benthic organisms in the Mississippi River were significantly greater 

 in September during average flow conditions than in July when the flow 

 was high. On the basis of these results, they reasoned that the effects 

 of natural environmental conditions (current velocity, shifting substrate) 

 in the main channel are more critical than the effects of river traffic, 

 since the benthic organisms increased during low flow when effects from 



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