Figure 4. Diagram of an idealized allu- 

 vial floodplain with various depositional 

 environments. RC = river channel; K = 

 direction of meander movement; L = natural 

 levee; P = point bar deposits (alternating 

 ridge and swale topography); B = back- 

 swamps; C = channel fill deposits (former 

 ox-bow lake); R = ridge (former natural 

 levee around adandoned channel); OC = 

 overflow channel; S = swale deposits. 



depends on long-term patterns of flow 

 (Blench 1972). High flow regimes are 

 principally responsible for the formation 

 of channels, while low flows are responsi- 

 ble for only minor adjustments in channel 

 morphology (Keller 1977). The process of 

 channel formation is greatest at bankful 

 stage. Therefore, the annual to biennial 

 flood intervals are more important than 

 periodic catastrophic flooding (Wolman and 

 Miller 1968). When flows exceed the 

 capacity of the river channel, the entire 

 floodplain becomes the channel, and addi- 

 tional physical factors come into play. 



Since current velocities are a func- 

 tion of the slope of the water surface, it 

 is not surprising that water velocities 

 over the floodplain during overbank flows 

 are comparable to the mean velocities of 



natural channels and transport sands and 

 silts (Wolman and Leopold 1957). In fact, 

 during overbank flows when the waters 

 leave the meandering channel, the flood- 

 plain surface becomes the high water 

 channel with the current directed straight 

 downslope (valley-axial direction) short- 

 ening the path of flow and increasing the 

 slope of the water surface (Carlston 

 1965). Only the structure of the intact 

 forest impedes the ravaging flov/s and pre- 

 vents catastrophic scouring of the flood- 

 plain surface and valley walls. 



Under certain conditions, specialized 

 channels are formed. Termed braided and 

 anastomosing channels, they are character- 

 ized by the main river channel dividing 

 into numerous interconnected channels. 

 Braiding results from a change in grade or 

 slope so abrupt that coarse sediment, 

 usually sand, is precipitously deposited. 

 Braiding, however, can occur at any point 

 in a stream where large deposits of coarse 

 sediments occur. For example, large 

 amounts of sand brought down by a channel- 

 ized reach of a tributary (Flat Branch) of 

 the Alcovy River (GA) have been deposited 

 on the main stream floodplain, causing the 

 main flow to braid. Braiding may also 

 occur at river confluences (e.g.. Little 

 Pee Dee and Lumber River, SC). Braiding 

 is often a temporary phenomenon. When a 

 river divides and rejoins on a vegetated 

 floodplain and the channel configurations 

 are relatively unchanging, it is better 

 termed an anastomosing stream. Both Four 

 Hole Swamp (SC) and parts of the Chipola 

 River (FL) are examples of anastomosing 

 channels. 



natural Levees 



During periods of overbank flow, as 

 waters spread out over the floodplain, 

 water currents abruptly slacken, and sus- 

 pended sands and silt are deposited as a 

 levee Immediately adjacent and parallel to 

 the channel . 



Natural levees (Figures 4-6) are best 



developed on concave stream banks. They 



occur along straight reaches, 



they are usually higher on one 



on the other. Some large rivers 



mile-wide levees; however, the 



river in the Southeast has levees 



30 m (98 ft) and ICO m (328 ft) 



also may 

 although 

 side than 

 may have 

 average 

 between 



wide. Natural levees slope gently to 

 flood basins and backswamps. The height 



