28 mg P/n^2/yr from additional surface run- 

 off. The swamp effectively removed 30% to 

 57% of these additions. 



The floodplain's capacity for improv- 

 ing water quality operates on two scales. 

 The first is the small, unleveed creek 

 swamps along tributary branches of the 

 major rivers. For example, Lowrance (1981) 

 found comparable swamps along the Little 

 River of Georgia reduced nitrate, sulfate, 

 calcium, and magnesium concentrations in 

 passage to the river. Reductions were 

 dramatic: runoff from a hogpen within 50 m 

 (164 ft) of the creek was not detectable 

 downstream. Lowrance (1981) concluded that 

 conversion of even a small part of the 

 floodplain riparian ecosystem to fields 

 would increase stream loadings of most 

 nutrients except phosphorus, with the 

 largest effect on nitrate movements. Ni- 

 trogen and carbon compounds, taken up by 

 anaerobic bacteria, are converted to gas- 

 eous forms by processes such as denitrifi- 

 cation and respiration during their pas- 

 sage through the riparian zone (Henderick- 

 son 1981). On a second and larger scale, 

 a complex network of distributaries in 

 some floodplains slowly partitions the 

 flow out over large areas of the flood- 

 plain, accomplishing nutrient reduction 

 of the same magnitude as tributaries 

 (Kitchens et al. 1975). 



The magnitude of the problem of point 

 source pollution is in many cases severe. 

 Industrial releases of lignins and wood 

 sugars affect the color and odor of the 

 Altamaha River for at least 40 km (25 mi) 

 downriver, contributing to the growth of 

 the white filamentous bacterium Sphaero- 

 tilus , which clogs fishermen's nets in the 

 tidal zone. Concentrations of RGB's ex- 

 ceeding FDA maximum levels (5 ppm) have 

 been found in many fish species in south- 

 eastern rivers (Veith et al. 1979). 



The filtering capacity of the swamp 

 could be useful in treating manmade efflu- 

 ents. Tributaries from the Savannah River 

 Plant (SO) have trapped and held radioac- 

 tive ^3^Ce, preventing major contamination 

 of commercial and sport fish species in 

 the Savannah River (Wharton 1977). A 

 cypress swamp above a peat substrate has 

 effectively treated the sewage of V.'i Id- 

 wood, FL, for 20 years. Bacterial levels 

 measured in the effluent were actually 



lower than those in the lake into which 

 they eventually emptied (Brown et al. 

 1974). For further reviews of swamp 

 filtering action, see Wharton (197C, 1977, 

 1980), Wharton and Brinson (1979a), and 

 Kibby (1979). 



The movement and immobilization of 

 pesticides and metals by humic substances 

 (humic and fulvic acids) is a particularly 

 important aspect of the swamp's filtering 

 process. Humic substances react with met- 

 als by exchange, adsorption and chelation. 

 This is particularly important in black- 

 water rivers (D0M> 10 mg/1 ) since signif- 

 icant quantities are complexed (Reuter and 

 Perdue 1977). Excess humic acids tend to 

 immobilize mercury (Miller et al. 1975) 

 and other heavy metals (Giesy and Briese 

 1978). The fate of these substances after 

 complexation is important. Humic acids, 

 being insoluble, tend to be immobilized as 

 bottom sediments. Soluble lighter fulvic 

 acids, constituting the bulk of DOC in 

 southeastern rivers, sometimes complex 

 with water contaminant metals; largely 

 unavailable as microbial foods, they do 

 not enter the downstream food chain 

 (Reuter and Perdue 1977). Some of these 

 flocculate with electrolytes at the inter- 

 face of fresh- and saltwater. 



Another important consideration is 

 the residence time required for processes 

 involved in water quality improvements. 

 Residence time is the time that water 

 remains over the floodplain floor and in 

 the sloughs and depressions. Examples of 

 processes are conversion of DOM to POM by 

 microbes (Slater 1954; Brinson et al. 

 1980) or by freezing (Giesy and Briese 

 1978), the complexing of metals with 

 organic matter, the adsorption of ions by 

 clay particles, and the establishment of 

 reducing conditions essential to operate 

 the metabolic pathways of sulfate reduc- 

 tion, denitrification and methanogenesis. 

 Obviously, any human activities which 

 decrease residence time, such as channeli- 

 zation, interfere with or eliminate these 

 vital floodplain functions. 



MODIFICATIONS OF RIVER AND FLOODPLAIN 



Construction or other modifications 

 on floodplains may cause profound changes 

 in their ecology. Man's direct or indirect 



105 



