Due to the influences of the two disposal periods and other factors, a 

 mixture of successional types was present at the site. Birds, small mammals, 

 and deer were abundant. The elevated area probably helped certain species of 

 mammals, such as rabbits, survive the periodic flooding of the area. 



On the negative side, the changed elevation and vegetation probably ad- 

 versely impacted fish, aquatic mammals, and waterfowl (conclusions are partial- 

 ly our own subjectively derived from data presented). 



Out-of-Channel Disposal 



Dredged material from channel maintenance operations is often placed in 

 areas adjacent to the navigation channel in medium to shallow depths within 

 the river. Potential adverse impacts include turbidity, sedimentation, burial 

 of organisms, changes in substrate composition and bottom topography, blockage 

 or filling of side channels, and releases of noxious materials and nutrients. 



Turbidity from disposal operations temporarily reduces light penetration 

 (which impacts primary productivity) and flocculates plankton. Generally, 

 these impacts appear to cause little impact. Increased stream turbidity is 

 usually of short duration and confined to a small area (Great River Environ- 

 mental Action Team I, Water Quality Work Group 1978). In clearwater streams, 

 turbidity may act as a barrier to migrating salmon (Darnell et al. 1976) but 

 dredging can be timed to avoid periods of migrations. 



Other water column impacts include increased biological oxygen demand and 

 release of noxious materials, such as sulfides, methane, ammonia, and heavy 

 metals. Impacts should be minimal unless the disposal is in an area where dil- 

 ution is poor. For reviews of turbidity impacts see Darnell et al. (1976) and 

 Stern and Stickle (1978). 



Great River Environmental Action Team I, Water Quality Work Group (1978) 

 conducted a water quality study of downstream impacts of dredging and disposal 

 at Mississippi River mile 827, immediately downstream from Minneapolis-St.Paul . 

 They found that physical and bacteriological parameters returned to background 

 concentrations within 1.3 km (0.8 mi) downstream of the disposal discharges. 

 Chemical parameters returned to background within a much shorter distance. 

 Impacts were generally localized due to dilution and the sorptive capacity of 

 rapidly settling resuspended particles. 



In our opinion, sedimentation is a much more serious concern than turbid- 

 ity, but sedimentation impacts can be minimized through careful disposal. Sed- 

 imentation dramatically decreases hatch^bility and survival of fish eggs and 

 fry (Hassler 1970); organic sediments reduce the oyxgen level (Phelps 1944); 

 the abundance and diversity of benthic organisms are reduced, particularly 

 mussels (Ellis 1936); and aquatic plants are adversely impacted (Langloise 

 1941). 



Apparently, severe sedimentation impacts are rare from the disposal of 

 dredged material into the river channel. Dredged rr.aterial from navigational 

 projects appears to pose the greatest sedimentation threat when it is placed 

 in, or adjacent to, backwaters (Great River Environmental Action Team I 1979). 



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