PART IV 



RIVERS 



Compared to coastal dredging, little research has been conducted on im- 

 pacts of river dredging. There are many data gaps in ecological impacts of 

 river dredging. Most research was conducted in the upper Mississippi River 

 and may only be partially applicable to other river systems. 



ASSESSMENT OF IMPACTS AT THE DREDGING SITE 



Water Column Impacts 



Impacts to the water column of both "new work" and maintenance dredging 

 are generally slight. Most severe dredging impacts are to the river bottom 

 substrate. Turbidity from both "new work" and maintenance dredging is tem- 

 porary and is usually less than turbidity associated with natural flooding. 

 Most rivers that are used for navigation are naturally turbid and usually 

 turbidity from dredcina exeeds background levels for only a short distance 

 downstream. Both Claflin (1973) and Held (1978) noted that runoff from the 

 deposition area created more disturbance than was created by the cutter head. 



Turbidity impacts Clearwater streams, particularly those used by sal- 

 monids. There are numerous references on the adverse impacts of suspended 

 particles (Stern and Stickle 1978). Impacts include interference with respira- 

 tion, abrasion to the gills, pathological changes to the gill structures, 

 changes in blood chemistry, and disruption of migration. However, there is 

 little evidence that the excavation phase of dredging operations actually 

 causes any of the problems listed. Fortunately, navigational dredging is 

 rarely conducted in Clearwater streams. 



A minor concern is the entrainment of slow moving nekton. Dutta (1976) 

 reported entrainment of as many as 26,GCC salmon fry per day by a hydraulic 

 dredge. It should be noted that this loss occurred when up to 20 million or 

 more fry per day v/ere passing through the area. Conducting dredging operations 

 at slack periods of fish migration can minimize losses of juveniles and dis- 

 ruption of adult movement. 



Bottom Impacts 



Routine maintenance dredging causes some short-term disruption of bottom 

 faunas, but there is little evidence that the disruption is long-term. How- 

 ever, the alteration of rivers through new channel construction or deepening 

 projects has severe direct and indirect impacts on the entire river and flood- 

 plain ecosystem. Short-term impacts include direct destruction of organisms 

 such as mussels, changes in bottom substrate, and downstream sedimentation. 



The literature indicates that dredging removes 75-100% of the benthic 

 organisms from the dredge cut (U.S. Army Corps of Engineers, San Francisco 

 District 1975). With "new work" dredging, the replacement fauna may take 2 yr 

 or more to recover and will be different from the original. The transitional 

 fauna will consist of an abundance of opportunistic species; however, species 

 diversity will be limited. 



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