CHAPTER 1 



MODERN AND PALEO-GEOMORPHOLOGY OF FLOODPLAINS 



INTRODUCTION 



The complexities of hydroperiod, cli- 

 mate, soils, and watershed characteristics 

 have produced an often bewildering mosaic 

 of vegetative zones and associations in 

 the bottomland hardwood community. To 

 understand better the complex relationship 

 between hydroperiods and the bottomland 

 hardwood community, one must first con- 

 sider the geomorphology of the floodplain 

 itself. The biota of the floodplain cannot 

 be wisely interpreted or managed, nor can 

 the impact of man's modifications be cor- 

 rectly evaluated without understanding 

 watershed-dependent floodplain hydrology 

 and geomorphology. The biota alone pro- 

 vide too narrow a viewpoint. 



The energy of flowing water and the 

 sediment load of river flows are ulti- 

 mately responsible for the geomorphic 

 landforms on southeastern floodplains. 

 This chapter discusses processes of water 

 and sediment distribution on floodplains 

 and landforms characteristic of both 

 modern and ancient environments. 



ORIGIN AND DYNAMICS OF FLOODPLAINS 



The flows and sediments carried by a 

 river are responsible for the origin, 

 character, and maintenance of the flood- 

 plains and their forest cover. The gently 

 sloping coastal plains of the Southeastern 

 United States provide an ideal environ- 

 mental setting for floodplain formation. 

 Erosional and depositional processes cul- 

 minate in a sinuous river channel within a 

 broad flat plain bounded by valley walls 

 or bluffs--the floodplain (Figure 2). 



Striking examples (Figure 2) of 

 floodplain formation occur along rivers in 

 the study area at the fall line, the 

 abrupt transition between the Piedmont and 

 the Coastal Plain. The excess energy of 

 river flows in passage over the bedrocks 



and red clays of the Piedmont begins to 

 dissipate at the fall line, where the 

 river first encounters the easily erodible 

 sedimentary strata of the Coastal Plain. 

 This dissipation results in deposition of 

 alluvium (sands, silts, and clays) which 

 in turn is reworked by riverine processes 

 into meanders. The residual energy of the 

 flowing water is expended by this lateral 

 meandering which serves to widen the river 

 valley. Rivers adjust their slopes by 

 meandering until they reach a nearly 

 steady-state, with sediment load balanced 

 with water velocity and volume. As the 

 floodplains widen out, more sediments can 

 drop out from overbank deposition. Remark- 

 ably broad, flat floodplains are the 

 result of these processes. Mountain and 

 Piedmont rivers, on the other hand, al- 

 though they form floodplains when topogra- 

 phy and soils permit, still retain much 

 potential channel energy. 



First order variables that determine 

 the behavior of water and sediments in- 

 clude climate, geology, soils, land-use, 

 and vegetation (Morisawa and LaFlure 

 1979). Variables that describe the chan- 

 nel are velocity, slope, flow-depth, 

 plan-form (shape from an aerial view- 

 point), and width (Gregory 1977). Other 

 variables are meander length and meander 

 belt width (Blench 1972), discharge (vol- 

 ume of water/unit time), and roughness of 

 river bed (bottom presence of trees, 

 cobbles, dunes, etc.) (Leopold and Wolman 

 1957). These variables are extremely 

 interdependent. 



The dominant depositional/erosional 

 processes on floodplains are: (1) point 

 bar deposition, (2) overbank deposition, 

 and (3) sheet or gully erosion (scour) and 

 redeposition on floodplain surfaces in a 

 sequence of floods (Sigafoos 1964). 



The point bar is built on convex 

 banks of streams or river meanders by lat- 

 eral accretion (Figure 3). Since deposi- 



