depressions or swales. Vegetation quickly 

 invades and stabilizes each point bar 

 ridge, encouraging further deposition. 

 Ridges are composed priiriarily of sands. 

 Silts and clays are deposited mainly in 

 swales, forming a sticky clay subsoil 

 (gley), sometimes called "blue mud." Mate- 

 rial eroded from the concave side of one 

 meander loop is deposited on the convex 

 side of the next downstream meander. The 

 floodplain is thus "reworked" to the depth 

 of the deepest part of the channel. Sedi- 

 ments are resuspended by powerful bottom- 

 flowing crosscurrents (Figure 5). Meanders 

 migrate because of the constant erosion 

 (undercutting) and/or slumping of the con- 

 cave bank laterally and downslope. Meander 

 migration is slow (<3 m or <10 ft/yr) in 

 small southeastern rivers, particularly 

 those with forested banks. On the other 

 hand, meanders in India's huge Kosi River 

 moved 750 m (2,460 ft) in 1 year (Wolman 

 and Leopold 1957). 



Dune Deposits 



Aeolian dunes form when strong winds 

 blow exposed sand from point bars or other 

 sources onto the floodplain. Dunes 13.7 m 

 (45 ft) high sometimes are formed by the 

 deflation (wind removal) of point bar 

 sands and other bare areas of the flood- 

 plain (Allen 1965). Several linear series 

 of large dunes occurring on the east side 

 of the Altamaha River (GA) floodplain are 

 of probable aeolian origin (Bozeman 1964). 

 So extensive are these dunes that the pro- 

 posed Big Morter-Snuffbcx Project (Soil 

 Conservation Service) recommended that 

 they be artifically joined to create a 

 huge levee to block off part of the flood- 

 plain and divert water from eventually 

 flowing into certain tidal river distribu- 

 taries. Aeolian dunes and those associated 

 with the relict braided stream channels 

 (e.g.. Little Pee Dee floodplain, SC; Thorn 

 1967) probably were formed by gale-force 

 Pleistocene winds blowing across the 

 unvegetated part of the floodplain from 

 the southwest. Dune chains are more likely 

 to be formed where discharge varies widely 

 and the floodplain is not heavily vege- 

 tated (Allen 1965). Discharge is thought 

 to have varied miuch more during the Pleis- 

 tocene when strong seasonality developed. 



Scour Channels, HuniP'Ocks and " Mini-Basins" 



Scour channels, hummocks, and mini- 



basins are microtopographic features 

 producing only slight elevational and 

 drainage changes; however, their effect 

 on plant species distribution is often 

 marked. 



Scour channels are small v/aterways 

 within the floodplain generally formed 

 during high water as flows seek shortcuts: 

 for example, cuts or chutes across bends, 

 or tributary connections to the main 

 channel. A high percentage of sand is 

 present in the scour channels (and on the 

 adjacent floodplain as well) because scour 

 channels are areas where sheet flow may 

 carry a substantial bed load of sand 

 across the floodplain flats. 



Hummocks are small "islands" left 

 after years of erosion by scour channel 

 currents. Usually the curved channels in 

 hummock terrains are weblike, weaving 

 around the bases of trees which rriay be 

 "stooled," often bearing ferns and shrubs 

 on swollen bases. The top of hummocks may 

 bear trees characteristically found in 

 areas of higher elevation, although in 

 som.e cases trees such as tupelo gums and 

 cypress form hummocks themselves. 



Minibasins are shallow depressions 

 that sometimes occur between tree bases. 

 Some may be created by swirling water; 

 others are of ambiguous origin. They are 

 frequently filled with rainwater. Any 

 detritus trapped in them is rapidly decom- 

 posed by frequent fluctuations between dry 

 and moist conditions. This is in contrast 

 to areas around the drier, raised tree 

 bases where detrital accumulations tend to 

 increase floodplain floor elevations. In 

 addition, rriuch of the aerobic-anaerobic 

 nutrient cycling is accomplished in rain- 

 filled minibasins (Wharton and Brinson 

 1979b) (see Chapter 3). 



PALEO-GEOMORPHOLOGY 



It is now recognized that Pleistocene 

 ice age climates and hydrology strongly 

 influenced both terrestrial and aquatic 

 landforms. Glacial and interglacial peri- 

 ods during the Pleistocene produced dra- 

 matic changes in climate, precipitation, 

 and sea level. Increased precipitation 

 and more intense, frost action during 

 glacial advances caused considerable down- 

 slope movement and subsequent transloca- 



12 



