In both blackwater and alluvial sys- 

 tems organic iriatter represents the link 

 between the river and its floodplain. Most 

 of this organic matter is in the dissolved 

 form termed dissolved organic matter (DOK) 

 or dissolved organic carbon (DOC), com- 

 posed principally of humic substances 

 leached from soil, peat, and leaf litter. 

 For example, up to 95% of the total 

 organic matter in the Altamaha River was 

 DOK (Reuter and Perdue 1977). Total 

 organic matter averages around 15 mg/1 

 (Windom et al. 1975), ranging up to ICO 

 mg/1 in waters leaching peat deposits 

 (Malcolm and Durum 1976). These materials 

 are often chemically and biologically 

 inert (i.e., refractory) with concentra- 

 tions changing principally in response to 

 discharge additions or dilutions. A small 

 proportion of humic substances flocculate 

 in fresh water and can be seen as "silts" 

 on white sand bars, or are rolled as bed 

 load particles (J.H. Reuter, Department of 

 Geophysical Science, Georgia Institute of 

 Technology, Atlanta; personal communica- 

 tion). 



PHYSICOCHEMICAL CHARACTERISTICS OF FLOOD- 

 PLAIN SOILS 



The alternation of inundation of 

 floodplains during extended high flow per- 

 iods of the river with drydown periods 

 during low flow conditions produces a spec- 

 trum of soil types across the floodplain. 

 These soil types are associated with 

 elevational gradients which in turn dic- 

 tate flooding frequency and duration: the 

 hydroperiod. Differences in elevation and 

 hydroperiods are the basis of a system of 

 classifying the environmental and biotic 

 zonation that result from this continuum 

 of fluctuating water levels and soil mois- 

 ture. A system of six zones, developed by 

 the National Wetlands Technical Council 

 (NWTC) (Larson et al. 1901), provides a 

 convenient framework for portraying the 

 relationship between the bottomland hard- 

 wood community and environmental factors 

 necessary for effective management consid- 

 erations. Throughout the remainder of 

 this report these zones will be referred 

 to as either ecological or bottomland 

 hardwood zones. 



Briefly, the classification generally 

 corresponds to the following broad geomor- 

 phologic floodplain features: 



Zone I: river channels, oxbow lakes, 

 and permanently inundated backsloughs 



Zones II-V: the active floodplain 

 including swales (II and III), flats 

 and backswamps (IV), levees, and 

 relict levees and terraces (V) 



Zone VI: the floodplain-upland tran- 

 sition to terrestrial ecosystems 



Examples of floodplain zonation are 

 depicted in Figure 15. An idealized 

 floodplain proceeds sequentially from the 

 river channel to the surrounding uplands 

 (Zone I-VI) along a gradually increasing 

 elevational gradient (Figure 15A). The 

 presence of natural levees interrupts this 

 sequence (Figure 15B); depending on eleva- 

 tion, the levee may be characteristic of 

 Zones II, III, IV or V. Accordingly, 

 levees are generally excluded from the 

 NWTC zonal concept. Other geomorphic fea- 

 tures (Figure 15C) contribute further to 

 the complexity of zonation patterns on 

 most southeastern floodplains (see Chap- 

 ter 4). 



Flooding produces and regulates the 

 chemical properties of floodplain soils by 

 (1) continually depositing and replenish- 

 ing minerals, including essential nutri- 

 ents on the floodplain (the mineral sub- 

 sidy); (2) producing anaerobic conditions 

 in the soils; (3) importing particulate 

 and dissolved organic matter (POM, DOM); 

 and (4) removing or exporting accumula- 

 tions of organic detritus (principally 

 degraded leaf litter). The degree to 

 which these processes operate in the six 

 zones is determined by the hydroperiod 

 (Table 6). 



An example of the relationship of 

 floodplain soil types to bottomland hard- 

 wood zones is illustrated in Figure 16 for 

 an alluvial river floodplain, the Congaree 

 River (SC). The bulk of the floodplain 

 floor is Tsw Caw silty clay loam, support- 

 ing principally Zone IV forest. However, 

 variations in microrelief or subsurface 

 water table height can make differences 

 in surface soils even in small quadrats. 

 Reynolds and Parrott (1980), found spe- 

 cific soil differences on a 1-ha plot 

 coincidental with different patterns of 

 tree distribution and postulated (from 28 

 wells) that water table differences ac- 

 counted for the numerous soil differences 



23 



