exceeded 521,000/1 at this station, located just below the Illinois 

 River confluence, but T_. minimum occurred at a density of only 123/1 

 at a side channel station Immediately upstream at Mississippi River 

 mile 220.7, and no T_. minimum occurred in samples below river mile 

 215.1 on the Mississippi, or at Illinois River mile 23.1, just upstream 

 from the confluence (Colbert et al. , 1975: 51). 



Colbert et al. (1975: 54) concluded that the side channels played 

 a major role in the productivity of the Mississippi River: 



119. It is apparent that side channels are a very important 

 habitat in the Upper Mississippi River, particularly during 

 high-flow periods. They provide refuge in the form of slower 

 moving waters, which leads to increased plankton abundance, 

 which in turn provides a food source for phytoplankton feeders 

 such as zooplankton, certain benthic insects, and particularly 

 planktivorus fishes such as paddlefish, shad, and larval fishes. 



120. During reduced river stages, the impounded river 

 channel becomes increasingly important as abundance of phyto- 

 plankton increases due to larger areas of slack water and 

 associated reduced turbidity. Some side channels (river mile 

 201.3) have less input to the river proper since they actually 

 become cut off from the river during low stages. 



Colbert et_ al. (1975: 66) reached the following conclusions re- 

 garding zooplankton: 



Based on statistical analyses . . . , it was determined that 

 total zooplankton density, zooplankton species diversity, 

 and concentrations of Cladocera, Copepods, and Rotifera 

 increased significantly between July (high flow) and Sep- 

 tember (low flow) . Evenness index showed the reverse trend 

 in that it decreased significantly between July and Septem- 

 ber. Statistical treatment of data combined for both 

 sampling periods indicated that only the rotifers were sig- 

 nificantly greater in side channel habitats. 



Colbert et al. (1975: 61) indicated that the difference in flow 

 regimes prior to the July and September sampling was the main reason 

 for the marked difference in zooplankton populations in the main 

 channel: 



10 



