benthic organisms (about 20) in unpolluted areas than in polluted areas 

 (about 6) of the Mississippi River (1927: 166). Wiebe concluded that 

 changes in the bottom fauna were primarily due to the lower dissolved 

 oxygen content of polluted water (1927: 166). 



Species Composition Changes . The lack of reliable pre-dam data 

 concerning benthos in the study area prevents a comparison of pre- and 

 post-dam species compositions. 



Effects of the Navigation System on Benthos . Again, lack of 

 historical data prohibits an analysis of the effects that construc- 

 tion of the navigation dams had on benthos in the study reach. Al- 

 though historical data are available for Keokuk Pool, upstream from 

 the study area, we do not feel that the effects of the high-head 

 power dam at Keokuk are comparable to effects of the relatively low- 

 head navigation dams in the study reach. The gates at the Keokuk 

 dam are on the top of the dam, and the drop and the force of the water 

 coming over the dam present an effective barrier to the movements of 

 several species of fish. In addition, sediment accumulates readily 

 above the Keokuk dam. In contrast, dams 24, 25, and 26 have gates 

 which open from the bottom of the dam, and were purposely designed 

 to pass sediment which would otherwise accumulate in the main channel 

 immediately above the dam. During high flows in the spring, most of 

 the gates are open from the bottom and are out of the water, and most 

 fish which make upstream spawning migrations in the spring can probably 

 negotiate these dams. 



29 



