tion and deposition of surface materials 

 (Whitehead and Barghoorn 1S62). The dimin- 

 ished relief and aggraded (filled in) 

 valleys of today's Piedmont are evidence 

 of the tremendous impact of the lateral 

 migration of soils during that era (Eargle 

 1940). Many common Piedmont soils are 

 underlain by organic deposits as much as 

 3.7 m (12 ft) thick representing downslope 

 transport and deposition; in one study in 

 South Carolina more than 50% of the sur- 

 face was underlain by these soils (Eargle 

 1940). Pollen dating of the soils indi- 

 cated they were deposited more than 35,000 

 years ago, possibly during the waning of 

 the first of the two periods of Wisconsin 

 glaciation, the so-called Altonian sub- 

 stage. 



Some floodplains in the Southeast 

 apparently were affected by the climatic 

 changes associated with continental gla- 

 ciation. One striking feature reflecting 

 these past climatic regimes is the dramat- 

 ic discrepancy between the size of the 

 floodplain and the size of the present- 

 day river. Today many streams are too 

 small (in terms of discharge volume and 

 meander dimensions) to have produced such 

 wide floodplains. Such streams are de- 

 scribed as "underfitted" (Dury 1977). This 

 phenomenon is common in alluvial rivers 

 and may occur in coastal blackwater 

 streams (Wharton 1977). A growing body of 

 evidence indicates that the geomorphology 

 of underfitted stream floodplains can be 

 explained by the sequence of different 

 hydrologic regimes resulting from prehis- 

 toric climates (Fisk 1947, 1951; Schumm 

 1971; Dury 1977; Froehlich et al. 1977; 

 Mycielska-Dowgiallo 1977). 



Floodplain width is a function of 

 sediment deposition and redistribution by 

 meandering during periods of greatest 

 stream discharge, coupled with periods of 

 relatively high sea level. Increased dis- 

 charge over that of the present was prob- 

 ably due to increased precipitation. An- 

 cient flow regimes can be determined 

 through studies of ancient paleochannels 

 in present floodplains (Schumm 1971; Dury 

 1977; and others). Dury calculated, from 

 ratios of former to present channel bed- 

 widths and meander wavelengths, that dis- 

 charge 12,000 years ago was 18 times 

 greater than that at present. Sediment 

 delivery rates were three times those of 

 today. This increased- discharge was at 



least in part due to a pluvial (rainy) 

 period of much greater rainfall occurring 

 18,000 to 10,000 years ago (Thorn 1967; 

 W.G. Kclntire, Louisiana State University 

 Center for Wetland Resources, Baton Rouge; 

 personal communications). Discharge for 

 many streams subsided about 10,000 years 

 ago. Runoff decreased to one-seventh of 

 its magnitude 2,000 years earlier (Dury 

 1977). Streams and rivers began to assume 

 their underfit characteristics at this 

 time. 



Climatic changes, coupled with the 

 more subtle influences of change in grad- 

 ient brought about by lowered sea levels 

 (Figure 7) or tectonic rebound of the 

 land, formed another characteristic geo- 

 morphic feature of southeastern flood- 

 plains — the floodplain terrace. Increased 

 flow volume or, in some cases, an in- 

 creased gradient, changed the hydrologic 

 regime and created a new floodplain sur- 

 face, often lower than the old one. De- 

 creased flow volume or increased sediment 

 sometimes reversed the sequence, filling 

 the floodplain back up with new sedi- 

 ments. In any event, steplike terraces 

 resulted, many of which are remnants of 

 prehistoric surfaces. This sequence of 

 alternating high (degrading) flows and 

 lower (aggrading) flows is diagrammed in 

 Figures 7 and S. Because precipitation 

 generally has declined into modern times, 



THOUSANDS OF YEARS BEFORE PRESENT 



60 40 20 5 



100 



g 



o 



500 



Figure 7. Sea level changes between the 

 Sangamon interglacial period (S) and 

 modern times (M) covering two periods of 

 Wisconsin glaciation, the Altonian (AS) 

 and the Woodfordian substages (WS), and a 

 warmer interglacial period, the Farmdalian 

 (F). Periods of entrenchment (E) occurred 

 during glacial buildup. (A) represents 

 periods of alluviation when alluvial river 

 valleys were filled with sediments. (Mod- 

 ified from Saucier and Fleetwood 1970.) 



13 



