■Long-term impacts are more subtle but potentially much more severe. These 

 include changes in hydrology and stream gradient that impact the river, 

 swamps, backwaters, and the entire floodplain (Simons et al. 1975). 



The literature about ecological impacts of channelization of large 

 streams is limited. Numerous references to channelization of smaller streams 

 for flood control document many detrimental impacts to fish and other aquatic 

 and terrestrial wildlife. Generally, channelization eliminates wetlands and 

 backv/aters, destroys fish cover, causes the water temperature to rise, in- 

 creases sediment load, increases turbidity, and makes other physical-chemical 

 changes to the stream and its floodplain. Darnell et al. (1976) provided a 

 thorough discussion of channelization impacts. These changes are gene-ally 

 detrimental to game and forage fish and wildlife populations but increase 

 rough (nongame) fish populations. In the absence of definitive research on 

 the impacts of channelization on larger streams, we can assume that similar 

 adverse impacts will occur. New channel construction may also be expected to 

 result in accelerated industrial development which decreases aquatic habitat 

 (U.S. Army Corps of Engineers, Office of the Chief of Engineers 1972). 



ASSESSMENT OF IMPACTS OF DISPOSAL ALTERNATIVES 



Riparian Disposal 



Dredged material is often hydraulically placed above the normal water 

 level in bottomland forests, old fields, or other floodplain areas near the 

 dredging site. Impacts can range from slight to severe, depending on many fac- 

 tors. Trees vary in their resistance to siltation (Teskey and Hinckley 1977). 

 Depending on the depth of fill and characteristics of the fill material, the 

 plant community may be slightly to drastically affected. Siltation increases 

 dieback and reduces stem height and diam.eter growth. Thick deposits of dredged 

 material may result in the eventual death of most species of trees (Larson 

 1974). Willows ( Salix spp.) are well adapted to survive covering by sand. They 

 quickly develop adventitious roots. Cottonwood ( Popul us deltcides ) and river 

 birch (Betula nigra ) also survive fairly well (Larson T^JTy. Willow and Cot- 

 tonwood are early colonizers of the wetter portions of the new fill material. 

 In general, the new con: .ities are less diverse, less productive, and less 

 valuable to wildlife than the original community (KcMahon and Eckblad 1975, 

 Vanderford 1979). The soil is porous, subject to large fluctuations in temper- 

 ature, and nutrient poor. Colonization by plants is slow. Ziegler and Sohmer 

 (1977) reported that early colonizers of Mississippi River dredged material 

 islands consisted of only two grasses, a sedge, and tumbleweed ( Amaranthus 

 sp,). Later a few vines and shrubs such as poison ivy (Rhus sp.), riverbank 

 grape (Vitia riparia ), and black raspberry (Rubus occidental is ) encroached 

 from the fringes of surrounding forests. High exposed areas in Pool 9 of the 

 Mississippi River were found to be virtually unvegetated after 35 yr (McMahon 

 and Eckblad 1975). However, along the shore where moisture is available, dense 

 stands of willows occur and provide shade for a variety of smaller plants 

 (Larson 1974). 



In most river floodplains, the long-term succession pattern proceeds from 

 willow-cottonwood to mixed hardwoods, i.e., silver maple ( Acer s accharinum ), 

 pin oak ( Quercus palustris ). and hickories ( Carya spp.) (Klein et al. 1975). 

 Similar succession will occur on dredged material deposits unless the eleva- 

 tion is high, in which instance succession will be retarded due to xeric 

 conditions. 



60 



