deposition which flattens downstrean 

 reaches. Deposition by tributaries often 

 increases river slope in the region imine- 

 diately downstream of their junction with 

 the main river. This increase in slope 

 results frotn elevation of the river chan- 

 nel bed due to sediment accretion at the 

 juncture of the two streams. 



Basic features of swamp rivers and 

 streams are their sinuous meanders (Figure 

 3). Meandering is one way the river accom- 

 modates slope. Meanders lengthen the path 

 of the water, adjusting the energy of the 

 flow to a uniform rate of energy loss per 

 unit of stream length (Leopold and Lang- 

 bein 1966). In other words, the flow rate 

 is stabilized as the slope of the stream 

 is made more uniform and less steep by 

 increasing the distance that the water 

 traverses in its vertical descent from the 

 Piedmont to the coast. The process can be 

 compared to a skier whose rate of descent 

 is slowed by meandering: the consistency 

 and resistance of the snow determine the 

 meander path which he follows for his 

 particular weight. The path of the water 

 bends as uniformly as possible (conforming 

 to a sine-generated curve), minimizing 

 bank erosion and the expenditure of energy 

 (Leopold and Langbein 1S66). After the 

 water rounds the meander curve, it flows 

 straight until centrifugal and inertial 

 forces are diminished to the point where 

 gravity can turn the water back downs lope 

 and the process is repeated. The greater 

 the volume, velocity, or density (water 

 containing sediment weighs more than water 

 without sediment), the greater the force 

 and the further the river travels before 

 it is turned back to form the next meander 

 loop. It follows that meander length and 

 radius are closely related to the width of 

 the river. The width of meanders is a 

 function of water volume, velocity, and 

 density. The development of meanders 

 occurs at bankful (flood) stages (Thorne 

 and Lewin 1979). Meanders occur at fairly 

 consistent intervals of 7 to 15 times the 

 width of the channel (Dury 1977). 



River slope adjusts naturally to the 

 velocity required to transport the load of 

 water and sediment supplied by the drain- 

 age basin. As slope increases, a stream 

 must cut down (degrade) or develop a sinu- 

 ous course. A meandering reach is stabler 

 than a straight reach (Yang and Song 1979) 

 because it more closely approaches uni- 



formity in the rate of work over the 

 various riverbed irregularities than does 

 the straight channel (Leopold and Langbein 

 1966). In addition, the meandering river 

 and its floodplain apparently present the 

 most efficient geometry to accommodate the 

 mean and extremes of flow variability that 

 have occurred throughout its history. 



The types of sediment through which 

 meanders pass are important. In a river 

 system, meanders will occur where bank 

 material is comparatively uniform. Mean- 

 ders move slowly if banks are cohesive 

 (silt or clay) and rapidly if banks are 

 easily erodible (sands, silty sands). 

 Depending on the type of sediment load, 

 meander wavelength can vary ten-fold at a 

 given discharge (Schumm 1969; Thorne and 

 Lewin 1979). 



FEATURES OF MODERN FLOODPLAINS 



Although floodplains appear flat and 

 featureless from the air, in reality they 

 are characterized by diverse topographic 

 features as a result of the continual, 

 dynamic reworking of their sediments by 

 rivers. The low topographic relief of the 

 floodplain landscape is deceptive; a mat- 

 ter of inches in elevation may produce 

 quite distinct ecological zones (see Chap- 

 ter 4). The following sections detail the 

 origin and the dynamic nature of the major 

 geomorphic features of the floodplain 

 (Figure 4). 



Channels 



The river channel processes, as dis- 

 cussed earlier, create and maintain the 

 floodplain. As a precursor to developing 

 meanders, basically unstable flows in the 

 stream create a series of pools (scoured 

 areas) and riffles (areas of redeposition) 

 through erosion of the stream bank. Ero- 

 sive forces continue to act in meanders. 

 Scouring occurs on the concave bank of a 

 meander; conversely, scoured material is 

 deposited on the opposite convex bank 

 (Figure 3). Good basic references on 

 channel geomiorphology are Leopold et al. 

 (1964), Allen (1965) and Schumm (1971) as 

 well as overviews by Thorne (1977) and 

 Winger (1981). 



The stream channel morphology (width, 

 depth, slope, and meander characteristics) 



8 



