are subject to drying out during the summer, and to winterkill when there 

 is ice and snow cover coupled with an oxygen demand exerted by sediment. 

 These areas would have no fish until they were reflooded by rising water 

 levels and fish were recruited from the river. 



While the effect of summerkill or winterkill on fish populations in 

 the total river system is probably not detectable, the local effect on 

 a favorite fishing area can be disastrous, though temporary. Management 

 of water levels in the pools, or construction of low levees to retain 

 water in fishing areas during low-flow periods might alleviate the problem 

 temporarily, although sedimentation continues to make the lakes and 

 backwaters shallower and more subject to summerkill and winterkill. 



Effects of Other Operation and Maintenance Activities . In addi- 

 tion to pool regulation, other methods of operation and maintenance 

 of the nine-foot navigation system include: maintenance dredging of 

 the channel, disposal of dredged material, the construction and main- 

 tenance of regulatory works such as dikes and bank revetments, and 

 the maintenance of the lock and dam structures (Colbert et al . , 1975: 117). 

 The potential environmental impacts of these practices were described 

 by Colbert et al. (1975: 120-127). However, an extensive literature 

 search yielded very little information on actual impacts that have 

 occurred within Pools 24, 25, and 26 as a result of these procedures. 



Kelley (1949, in Keenlyne, 1974: 145) reported that occasional 

 complaints were received by him from people living along the river 

 regarding the disposal of dredge spoil. He stated that the greatest 

 concern was the placement of spoil on fish spawning beds in slough* 

 mouths. Reportedly, this material washed back into the river and 

 disturbed either fish or fishing. 



In 1969, a dredge spoil survey was conducted by several state 

 conservation agencies, the U.S. Fish and Wildlife Service, and the U.S. 

 Army Corps of Engineers (Robinson, 1970). The study recommended spoil 

 and no-spoil areas for dredge disposal from Hastings, Minnesota to Cairo, 

 Illinois to prevent further destruction of aquatic habitat. The Corps of 

 Engineers now coordinates annually with the U.S. Fish and Wildlife Service 

 and the state conservation agencies regarding dredge spoil disposal. 



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