4. No specific information was available on the effects of Dams 

 24, 25, and 26 on fish migration. 



5. In the period 1903 to 1944, several fishes which prefer a 

 clear-water environment with sand or gravel bottoms and appreciable 

 current declined or disappeared from the upper Mississippi River: the 

 Ozark minnow, blackfin shiner, redfin shiner, steelcolor shiner, 

 Southern redbelly dace, freckled madtom, and crystal darter. In addi- 

 tion to these forage fishes, three commercial fishes (pallid sturgeon, 

 river redhorse, and brown bullhead) and a predatory fish (alligator gar) 

 virtually disappeared from the upper Mississippi. The pallid sturgeon 

 was considered rare in the upper Mississippi River in the period 1876- 

 1903, and was thought to prefer a swift-water habitat. The river red- 

 horse is intolerant of turbidity and siltation, and the brown bullhead 

 is also considered sensitive to turbid waters. These changes in the 

 fish fauna of the upper Mississippi River are probably related to reduc- 

 tions in current velocity caused by the navigation dams and to increased 

 sedimentation and turbidity brought about by both navigation dams and 

 increased sediment input from tributaries. 



6. Winter drawdowns in Pools 24, 25, and 26 in the 1940*s often 

 resulted in large fish kills. Winter drawdown has not been practiced 

 by the St. Louis District since 1970. 



7. There is very little information on the actual impact on 

 fishes of maintenance dredging of the channel, disposal of dredged 

 material, and the construction and maintenance of regulatory works such 

 as dikes and bank revetments. Dredging and deposition of dredge 

 spoil may affect fish food organisms. Fish are known to congregate 



in the tailwaters below the dams, and fishermen likewise congregate 

 there. The fish probably gather below the dams for several reasons: 

 (a) the water is swift and well-oxygenated below the dams, (b) the dams 

 may impede the normal upstream movements of certain fishes, and (c) 

 there is an abundance of food organisms, such as insects, which are 

 continually swept off the dams and locks. In a similar fashion, 

 wing dikes and riprap may offer good habitat and a source of insect 

 food for certain species of fish. 



189 



