turbidity. Impoundment of the upper Mississippi River as part of the 

 nine-foot navigation system probably reduced current velocities and increased 

 siltation rates. The combined effects of turbidity and siltation altered 

 specific riverine habitats and probably contributed to the decline of these 

 species. Dams and impoundments ranked sixth in a list of factors re- 

 sponsible for the decimation of certain native Illinois fish (Smith, 

 1971: 14). Excessive siltation ranked first (Smith, 1971: 8). 



Effect of Dams on Fish Migration . Dams can block the natural 

 migration and dispersal of fish (Smith, 1971: 13). Hub ley (1961: 8) 

 reported that the locks and dams were not barriers to channel catfish 

 movements in the upper Mississippi River from Bay City, Wisconsin to 

 Lansing, Iowa based on recovery of tagged fish. 



No specific information was available on the effects of locks and 

 dams 24, 25, and 26 on fish migration. 



Effects of Winter Drawdowns . An early operational procedure of the 

 nine-foot navigation system that had a measurable effect on fish was 

 winter drawdown. To provide adequate depths of water for winter navi- 

 gation in the lower Mississippi River, it was sometimes necessary to 

 release water from the upper navigation pools. Carlander (1954: 23) 

 stated that these sudden and drastic lowerings of water in the upper 

 pools often left thousands of fish stranded in pools isolated from the 

 main channel. On many occasions, thousands of fish were killed. Green- 

 bank (1946, in Keenlyne, 1974: 24) found that winter drawdowns led to 

 oxygen depletion and fish destruction and that they seemed to have had 

 a greater deleterious effect on game fishes such as bass, bluegill, and 

 crappie than on yellow perch, pike perch, catfish, and the rough or 

 commercial fishes. 



Winter drawdowns also prompted fish movements from the backwater 

 areas. Christenson and Smith (1965: 46) reported that falling water 

 levels during the winter were accompanied by a definite movement out 

 of backwater areas in Pools 8 and 9 by carp, Northern pike, crappies, 

 spotted sucker, and bowfin. They also found that movement of these fish 

 was intensified by a sharp drop in the water stage. 



In interviews with commercial fishermen, Smith (1946: 6) reported 



119 



