In the absence of specific information on releases and impacts of con- 

 taminated material in freshwater, the reader is referred to the discussion on 

 contaminants in Part III - aquatic disposal in estuaries. Remember, however, 

 the influence of salinity. Generally, toxicity increases as water becomes 

 softer. Sodium, potassium, calcium, and magnesium have all been found in cer- 

 tain instances to be capable of antagonizing the ions of several heavy metals 

 thereby reducing their toxicity (Tarzwell 1957). For additional discussions 

 (of a general nature and not specific to rivers) see the section on "biocon- 

 centration" in Morton (1977). The reader may also wish to consult the Appen- 

 dix of this review. 



Thalweg Disposal 



Environmentally acceptable disposal areas are limited. The current com- 

 mon practice of shoreline disposal creates many environmental problems as 

 discussed in previous sections. LaGasse et al . (1976) suggest that disposal 

 in the thalweg or main river channel may be an environmentally acceptable 

 alternative. Miller (1973) further notes that the thalweg is generally rela- 

 tively barren of invertebrates and the U.S. Army Corps of Engineers, Portland 

 District (1973) notes reduced turbidity and suspended sediment problems with 

 thalweg disposal. However, caution is urged as Hawkinson and Grunwald (1979) 

 have shown that catfish overwinter in deep water of the main Mississippi River 

 channel. Commercial fishermen have also reported that the main channel is a 

 valuable wintering area for fish (letter of 17 January 1980 from John P. 

 Wolfin, U.S. Fish and Wildlife Service, St. Paul, Minn.). 



Thalweg disposal consists of dredging a shoal area and depositing the 

 material in the adjacent pool downstream or scraping a shoal (agitation dredg- 

 ing) and letting the current take the sediments downstream to the next pool. 

 LaGasse et al . (1976) indicates this technique could be employed at certain 

 sites during maintenance dredging and might have wide application for emer- 

 gency dredging. A discussion of the practicality of this technique from the 

 geomorphic standpoint is beyond the scope of this review. For detailed dis- 

 cussions the reader is referred to LaGasse (1975), Simons et al . (1975), and 

 LaGasse et al . (1976). 



Habitat Development 



Terrestrial development . Dredged material is often deposited into the 

 river margins or other shallow waters so that the disposal area becomes ter- 

 restrial. This destroys an existing habitat and the newly created habitat is 

 often of marginal value to wildlife (McMahon and Eckblad 1975, Vanderford 

 1979). However, valuable wildlife habitat can be developed through the appli- 

 cation of well-established agricultural and wildlife management techniques 

 (Larson 1974, River Studies Center 1975, Smith 1978). Terrestrial habitat 

 development can be used as an enhancement or mitigative measure at new or 

 existing disposal sites. Smith (1978) further stated that regardless of the 

 condition or location of a disposal area, considerable potential exists to 

 convert it into productive habitat. Small sites in densely populated areas 

 may be managed for small animals adapted to urban life. Larger tracts may be 

 managed for a variety of wildlife including waterfowl, game, or endangered 

 species. 



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