Table 3. Changes in levee height in upper, 

 middle, and lower reaches of typical 

 southeastern floodplain rivers. 



Figure 5. A meander bend and cross sec- 

 tion showing levee (L) and ridge (R) and 

 swale(S) topography so common on modern 

 and relict surfaces. Pioneer and suc- 

 cessional plant species anchor the newly 

 formed sandy ridges leading to even more 

 deposition at each high water episode. 

 Normal helicoidal currents (C) conduct 

 sediments up the bai- slope. 



of levees diminishes as rivers approach 

 the coast because the stream's energy to 

 move sand also decreases downstream (Table 

 3 and Figure 6). Some deeply incised 

 streams with headwaters arising from clay- 

 rich soils (Tallahala River, MS) have 

 barely discernible levees. Kany black- 

 water streams have unimpressive levees, 

 apparently due to lack of sufficient 

 gradient in the outer Coastal Plain. A 

 breach (or crevasse) in the levee may pro- 

 duce an alluvial fan-shaped feature termed 

 a crevasse-splay deposit which spreads out 

 over the floodplain (Allen 1965). 



Floodbasins, Flats and Backswamps 



The term floodbasin specifically 

 applies to vast underfitted floodplains 

 (floodplains developed under a signifi- 

 cantly higher flow regime than at present) 

 where channel meanders may occupy only a 

 portion, or belt, of the floodplain width. 

 Along southeastern rivers that are not 

 markedly underfitted, the floodplain 

 between the natural levees and high valley 

 wall is generally called ambiguously a 

 "backswamp" or more succinctly a "flat" 



where elevational relief is limited to 

 shallow depression basins and almost 

 imperceptible rises. The term backswamp 

 also may be applied specifically to peat- 

 forming environments occupying relict 

 channels along the outer rim of the flood- 

 plain. 



Floodbasin, flat, and backswamp sedi- 

 ments are composed of fine silt and clay 

 particles. Acid backswamps are environ- 

 ments where deposition is minimal and are 

 constantly wet, having water tables at or 

 near the surface. Most of the floodplain, 

 however, dries out annually. The fine 

 clays tend to dry and crack in polygonal 

 patterns, allowing oxygen to enter. Depos- 

 its in these areas vary with frequency of 

 flooding, proximity to the channel, sedi- 

 ment load, flow velocity, and substrate 

 texture. In many eastern floodplains 

 devoid of significant relief, overbank 

 deposits may be coarse and layered. Coarse 

 layers represent the rise to maximum stage 

 of an individual flood; the alternating 

 fine layers represent recession of flow 

 (Allen 1965). 



P oint Bars and Ridge and Swale Topography 



Host deposition occurs along the main 

 channel of the swamp stream. Materials 

 are eroded from concave sides of channel 

 meanders and redeposited on convex bends 

 to form point bars (Figure 3). Small 

 ridges formed on the point bar by deposi- 

 tion of bed load material during floods 

 form a temporary natural levee on the con- 

 vex side of meanders. The crests of these 

 ridges may stand higher than natural lev- 

 ees on the concave side. As the river bed 

 moves laterally and downstream (Figure 5), 

 d series of ridges' forms with intervening 



10 



