Summary of Effects of the Nine-Foot Navigation System on Fish in the 

 Upper Mississippi River 



The dams constructed in the 1930 's as part of the nine-foot navigation 

 system impound water during low river stages to provide a depth of at least 

 nine feet in the channel. The dams initially increased the amount of slack- 

 water habitat available during the normal low-flow periods, with the 

 greatest increases occurring in the reaches closest to the dams. During 

 high river stages, the gates in the navigation dams are opened, and water 

 levels are free to fluctuate as they did in pre-construction times. 



The environmental impacts resulting from the operation and main- 

 tenance of this navigation system have concerned biologists since its 

 inception. Methods of operation and maintenance with potential impacts 

 on the river biota include pool regulation, maintenance dredging of the 

 channel, disposal of dredged material, the construction and maintenance 

 of regulatory works such as dikes and bank revetments, and the main- 

 tenance of locks and dam structures (Colbert et al. , 1975: 17). 



The difficulty encountered in determining the specific effects 

 various alterations have had on the fish and wildlife of the upper 

 Mississippi River can be attributed to the paucity of pre-construction 

 data mentioned elsewhere in this report. Gunter (1957: 13) in ad- 

 dressing this problem at the 22nd North American Wildlife Conference 

 stated: 



It has been suggested to me that some statement about 

 the amount of the wildlife and aquatic life decrease along 

 the river should be made. It would be a matter of comparing 

 what the valley was in its pristine glory as a wildlife 

 habitat and what it is today. But this is impossible. 

 Wildlife biologists did not exist in the days when the life 

 along the river began to decline. All we can say today is 

 that wildlife and aquatic life has declined because its 

 habitat has largely been destroyed. Great changes have 

 taken place and their general outlines are obvious. The 

 specific and exact changes are unknown. 



Other changes were taking place in the Mississippi River valley 

 during this period that compounded the problem of identifying particular 



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