Streams of the Piedmont
Northwest of the Brevard Zone, trellis stream drainage dominates, with master streams trending parallel to the underlying northeast-southwest structural grain of the basement rock. Southeast of the Zone, drainage systems are dendritic, trending perpendicular to bedrock structure. White (1953) observed that in the foothill zone of the Piedmont of North Carolina and Virginia, northeast-flowing tributaries to the southeast-flowing master streams are long and subparallel, whereas opposing tributaries are short and irregular in direction. He hypothesized the former existence of a drainage system flowing northeast toward the Potomac, which has since been dismembered by the headward erosion of the southeast-flowing streams. Each of the northeast-flowing streams has been captured and diverted in turn, beginning at the southeast and ending near the Blue Ridge on the northwest. Evidence supporting this hypothesis includes the relatively sharp elbows of capture to the northwest, where capture would have been later, and the presence of what White interpreted as strath terraces across the divides between present major streams such as the Yadkin and Catawba.
First-order streams in the Piedmont generally flow on saprolite. Second and higher-order streams generally have cut down through the saprolite into weathered rock and bedrock, with depth of incision into bedrock increasing with stream order (Costa and Cleaves, 1984). Valley alluvial fills are thin, rarely greater than 10 m. Wolman and Leopold (1957) found depth of valley fills in North Carolina and South Carolina to be a function of stream length. The regression equation T = 1.35 L 0.45 (where T is thickness in meters and L is length in kilometers) predicts fill thickness fairly accurately.
Along many streams,the levels of terraces on the two sides of the stream do not match, and most are strath terraces (cut on bedrock). Terrace chronologies are therefore more difficult to work out than in areas with thicker valley fills and paired terraces. (See discussion of terraces in Ridge and Valley section). Segovia (1981) , however, worked out a tentative terrace chronology for the Savannah River in Elbert and Hart Counties, GA. Radiocarbon and prehistoric cultural remains to date the floodplain and low terraces, and soil-weathering correlations to date the higher terraces. He mapped five major stratigraphic units: terrace remnants of deeply weathered alluvium (unit IVb, presumed early Pleistocene to early Wisconsinan in age), late-Pleistocene terraces (unit Iva, with alluvium up to 4 m thick mantling bedrock islands, estimated to date between 15,000 and 30,000 yearsbased on soil characteristics), and three younger units representing intervals of floodplain-aggradation during the Holocene. The latter consist of early- to mid-Holocene terraces (unit III, up to 4.5 m thick, radiocarbon-dated between 10.4 and 8.7 ka), mid- to late-Holocene terraces (unit II, up to 3.4 m thick, dated between 7.5 and 3.8 ka), and a cap of modern alluvium dating from the last 350 yr and representing anthropogenic influence (unit I, up to 1 m thick). Segovia (1981) also hypothesized two important episodes of Holocene landscape stabilization associated with soil development and minimal fluvial sedimentation along the Savannah River floodplain, dated between 8.7 and 7.5 ka and between 3.8 and about 350 yr, respectively.
The human impact on Piedmont streams has been profound. Trimble (1974) noted that early travelers in the southern Piedmont reported clear streams flowing over rocky beds. With the beginning of widespread agriculture, streams became muddy and began to aggrade. By the latter half of the 19th century, this aggradation became extreme in small streams, with stream beds rising as much as 3-6 m, actually burying bridges in some cases. The streams that once flowed freely over rocky beds became sluggish and swampy, sometimes without definite channels. As a consequence of the decreased upland erosion and decreased sediment load since the 1930s, these streams are now degrading. Detrimental effects continue, however, because sediment from these lower-order streams is now moving into and swamping some areas along larger streams.
Encyclopedia ID: p1539
