and rivers have made drastic changes in 

 their courses, the floodplain has become 

 a succession of relict surfaces, each 

 bounded by terraces older than those 

 closer to the river. Their importance 

 lies in the hydrologic control they still 

 exert over the modern floodplain. 



At least three terraces can usually 

 be found in southeastern floodplains. The 

 Holocene terrace is usually the most re- 

 cent; such terraces are known as "first 

 bottoms." The next lowest terrace is known 

 as the Terrace I, Deweyville, or in South 

 Carolina "second bottom," and is distin- 

 guishable en many southern floodplains, 

 including the Altamaha (GA), Pee Dee (SC), 

 Cape Fear (NC), the Pearl and Pascagoula 

 (MS), and the Sabine, Trinity, and Brazos 

 (TX) (Gagliano and Thom 1967). Terrace I 

 sediments were deposited during a fluvial 

 period 17,000 to 36,000 years ago with 

 flows that were five to seven times great- 

 er than at present forming giant meander 

 scars or a braided topography of sandy 

 bars and fossil dunes (Pee Dee River, SC) 

 (Thom 1967). In South Carolina, Terrace I 

 lies 1.5 to 3.0 m (5 to 10 ft) higher 

 than the modern floodplain and 1.5 to 

 6.0 m (5 to 20 ft) below a still higher 

 Pleistocene fluvial or river terrace known 

 as Terrace II (Gagliano and Thom 1967). 

 Another floodplain terrace classification 

 scheme for the Ouachita River of Arkansas 



and Louisiana combines three terraces into 

 a "Deweyville sequence" lying between the 

 original pre-Wisconsin glacial sediments 

 deposited in the Sangamon interglacial 

 period (the Prairie Terrace) and the 

 modern Holocene floodplain (Saucier and 

 Fleetwood 1970). 



Prehistoric floodplain surfaces still 

 function in the modern hydrologic regime. 

 Some are inundated by present high water, 

 and relict channels, ridges, and swales 

 bear vegetation associations indistin- 

 guishable from those on their recent ana- 

 logues. The Pleistocene has left its 

 imprint in many other ways. The mouths of 

 numerous rivers at the coast (Roanoke, 

 Chowan, NC; Escambia, Choctawhatchee, FL) 

 are narrow, drowned floodplains entrenched 

 during the Woodfordian phase of Wisconsin 

 glaciation. The sediments of Terrace I 

 may Tiave provided much of the sands for 

 the barrier islands of the gulf and Atlan- 

 tic coasts (Thom 1967). 



Other ancient floodplains have been 

 variously used by man. Along the Waccamaw 

 (SC) the relict ridges support roads and 

 pine plantations while the swales bear bog 

 vegetation. Along the Roanoke (NC) row- 

 crop agriculture occupies most of the 

 ridges of the higher Pleistocene terraces 

 (3 m or 10 ft above MSL). 



15 



