cause-and-effect relationships. Man's activities in the river basin 

 resulted in increased industrial, municipal, and agricultural pollution. 

 The construction of levee districts and drainage of bottomlands for 

 agricultural purposes removed fish spawning and feeding areas. Car- 

 lander (1954: 25) described this complexity: 



Man has changed the Upper Mississippi River both delib- 

 erately and indirectly. These changes have had their ef- 

 fect both on fish and on fishing methods. It is almost 

 impossible to separate the effects of the various changes, 

 or even to say whether the individual changes were favorable 

 or unfavorable to the fishery resources of the river. 



Effects of Increased Water Area and Reduced Discharge . Two in- 

 itial effects of impoundment were an increase in the permanent water 

 area and a decrease in river discharge. New aquatic habitat was 

 created by inundation of terrestrial areas. Within the study area 

 of Pools 24, 25, and 26, there are approximately 73 square miles of 

 aquatic habitat at normal pool elevations (Colbert et al. , 1975: 32). 

 Although this certainly represents a gain in aquatic habitat due to 

 impoundment, comparable pre-construction acreage was unavailable at 

 this writing. 



Carlander (1954: 21) felt that the decrease in river current af- 

 fected the fish and fishing more than the increase and stabilization 

 of the water area. Although fish spawning and feeding areas were in- 

 creased following impoundment, Carlander (1954: 21) noted that as 

 the current slowed, silt settled out and covered these important areas, 

 Bellrose et al. (1977: C-112) found that this also occurred in the 

 Illinois River valley following impoundment. 



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