Late Pleistocene braided rivers of the Atlantic Coastal Plain, USA

ARTICLE IN PRESS

Quaternary Science Reviews 23 (2004) 65–84

Late Pleistocene braided rivers of the Atlantic Coastal Plain, USA David S. Leigh*, Pradeep Srivastava, George A. Brook Department of Geography, The University of Georgia, Athens, GA 30602-2502, USA Received 16 January 2003; accepted 3 July 2003

Abstract Infrared Landsat imagery (band 4) clearly reveals braided river patterns on late Pleistocene terraces of unglaciated rivers in the Atlantic Coastal Plain of the southeastern United States, a region that presently exhibits meandering patterns that have existed throughout the Holocene. These Pleistocene braided patterns provide a unique global example of river responses to late Quaternary climate changes in an unglaciated humid subtropical region at 30–35
north latitude. Detailed morphological and chronological results are given for the Oconee-Altamaha River valley in Georgia and for the Pee Dee River valley in South Carolina, including 15 optically stimulated luminescence (OSL) dates and four radiocarbon dates. Correlative examples are drawn from additional small to large rivers in South- and North Carolina. OSL and radiocarbon (14C) dates indicate distinct braiding at 17–30 ka, within oxygen isotope stage 2 (OIS 2), and braiding probably existed at least during parts of OIS 3 and possibly OIS 4 back to ca 70 ka. The chronology suggests that braiding is the more common pattern for the late Quaternary in the southeastern United States. Braided terraces appear to have been graded to lower sea-levels and are onlapped by Holocene floodplain deposits up to 10–60 km from the coast. The braiding probably reflects the response of discharge and sediment yield to generally cooler and drier paleoclimates, which may have had a pronounced runoff season. Sedimentation of eolian dunes on the braid plains is coeval with braiding and supports the conclusion of dry soils and thin vegetation cover during the late Pleistocene. Our chronological data contribute to a body of literature indicating that reliable OSL age estimates can be obtained from quartz-rich bed load sand from braided rivers, based on good correlations with both radiocarbon dates from braided fluvial sediment and OSL dates from stratigraphically correlative eolian sand. r 2003 Elsevier Ltd. All rights reserved.

1. Introduction Changes in river channel patterns from braided to meandering provide important evidence about late Quaternary environmental conditions and their geomorphic effects. Worldwide shifts from braided to meandering patterns were responses to deglaciation and pronounced changes in meltwater discharge and sediment yield during the terminal Pleistocene (Schumm and Brakenridge, 1987; Starkel et al., 1991; Knox, 1995; . Starkel, 1995; Blum and Tornqvist, 2000). In the eastern United States, braiding of glacial meltwater systems is exemplified by the Mississippi River valley (Saucier, 1994; Blum et al., 2000), but braiding from unglaciated watersheds is not well known. In contrast, European examples of terminal Pleistocene shifts from braided to meandering patterns are documented for several un*Corresponding author. Tel.: +1-706-542-2346; fax: +1-706-5422388. E-mail address: [email protected] (D.S. Leigh). 0277-3791/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0277-3791(03)00221-X

glaciated watersheds in upper midlatitude (45–50
north), marine west coast, and humid continental climatic regions of Europe (e.g. Loire R., France, Straffin et al., 2000; Meuse R., northwest Europe, Kasse et al., 1995; Warta R., Poland, Kozarski, 1991). However, globally there are no comparable records of braided to meandering transitions for relatively lowlatitude (30–35
north), unglaciated, humid subtropical regions like the southeastern United States. Here we present Landsat images that reveal distinct braided patterns on late Pleistocene terraces along rivers of the Coastal Plain in Georgia and the Carolinas (Fig. 1). We demonstrate a late Pleistocene age for these braid plains based on optically stimulated luminescence (OSL) and radiocarbon (14C) dates. These data suggest a regional pattern for late Pleistocene environmental conditions that caused braiding. The southeastern United States has received relatively little attention with respect to the character of late Quaternary fluvial systems and only meager evidence of braided patterns for late Pleistocene rivers in the

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Fig. 1. Southeastern United States showing rivers, sample locations, and USGS gaging stations.

Southeast has been reported (Thom, 1967; Ivester and Leigh, 2003). Braided river patterns in the unglaciated southeastern Atlantic Coastal Plain are very significant, because they imply pronounced differences in sediment and water discharge compared to modern conditions. Unlike the Mississippi River valley where glacial meltwater is thought to be the main cause of braiding (Saucier, 1994), the rivers of Georgia and the Carolinas have no direct linkage to glaciers and braiding implies climatically driven changes related to runoff, sediment yield, and bank stability. There is a need to clearly demonstrate braided patterns and their ages in the unglaciated landscapes of the Southeast. Although some previous studies have alluded to braided conditions, none has presented strong evidence to support their interpretations, and no direct numerical ages for braided patterns are known. In addition, most references to braided channel morphology are found in ‘‘grey’’ literature that is not widely accessible. Some of the best evidence is from Thom (1967) who recognized and mapped braided patterns on the first prominent terrace of the Little Pee Dee River and Lumber River. He tentatively inferred middle to late

Wisconsinan ages (OIS 2 and OIS 3) for the terrace. However, Thom did not provide a cause for braiding or any relation to environmental change. Segovia (1981) suggested that the Savannah River had a braided pattern during the late Pleistocene, but no direct evidence is provided. Soller (1988) conducted a very detailed analysis of the Cape Fear River valley and the nearby Pee Dee valley, but did not report braided patterns; instead he referred to meander scars and ‘‘scrollwork’’ on the late Pleistocene terraces. Schumm and Brakenridge (1987) did not mention braided streams in the Southeast in their overview of river responses to the last deglaciation. Walker and Coleman (1987) indicate that braided patterns existed during periods of low and rising sea level, but they provide no examples or dates and report that most braid patterns were obscured by subsequent sedimentation. Colquhoun et al. (1991) presented overviews of late Pleistocene geomorphic systems in the Southeast, but made no mention of braided river patterns. Schuldenrein (1996) suggested that braided patterns existed during the late Pleistocene but lacked supportive morphological evidence and dates. Ivester and Leigh (2003) indicated that

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Table 1 Morphological and hydrological characteristics of rivers in this study, based on data from the USGS gage River name and USGS gaging station #

Drainage area at gage (km2)

Valley gradient near gage

Channel widtha (m)

Mean annual floodb (m3 s1)

2-yr floodc (m3 s1)

Altamaha @Baxley: USGS gage# 02225000 Cape fear @Fayetteville: USGS gage # 02104000 Little Pee Dee @Galivants Ferry: USGS gage #02135000 Lynches river @Effingham: USGS gage #02132000 Pee Dee R. @Pee Dee: USGS gage # 02131000 Savannah R. near Clyo: USGS gage #02198500

30,044 11,383 7226

0.00022 0.00036 0.00021

180 70 60

2856 1419 375

3058 1291 343

2667 22,870 25,512

0.00017 0.00019 0.00030

35 108 100

185 1416 2107

152 1121 1824

a

Average obtained from USGS 7.5-min quadrangles within 200 m upstream of gage. Based on period of record prior to emplacement of large reservoirs in the basin. c Calculated by the Weibull method based on the pre-reservoir record. b

braided patterns were a probable source for late Pleistocene eolian sand dunes, but they lacked data about the regional extent or direct ages of the braids.

2. Study area Sites chosen for study include braided terraces along medium and large rivers of the southeastern Atlantic Coastal Plain in Georgia, South Carolina, and North Carolina (Fig. 1, Table 1). These rivers originate on the clay-rich saprolite weathered from the Paleozoic crystalline rocks of the Piedmont and Blue Ridge, and then they flow across the sandy terrain of the Cretaceous and younger sedimentary rocks of the Coastal Plain province. Detailed morphologic, stratigraphic, and chronologic data are provided for the Oconee-Altamaha and Pee Dee River valleys, and recognition of braided patterns is provided for several other river valleys. The present morphology of all these rivers is characterized as meandering channels with bed loads of sand and fine to medium gravel. Abundant suspended load is carried by Piedmont-sourced rivers, which have most of their drainage on clayey saprolite. Meade et al. (1990) suggests that the largest part of the modern sediment load for the Piedmont-sourced rivers probably is carried as suspended load, which has been demonstrated for the Chattahoochee River in northern Georgia (Faye et al., 1980). The channel beds are sandy with occasional gravelly riffles. The channel banks typically are cohesive, composed of sand, silt, and clay, and are covered with dense vegetation. The present floodplains of these rivers would be considered lowenergy cohesive floodplains (Order C1) by the classification of Nanson and Croke (1992), having clay-rich soils, common meander cut-offs, backswamps, natural levees, and crevasse splays. The braided terrace remnants typically exhibit an interwoven pattern of sand ridges and intervening silty and clayey swales. Alluvial surfaces immediately

predating those of the braided terraces also exhibit meandering patterns, indicating that the rivers changed from meandering to braided and back to meandering during the late Quaternary. The climate in the southeastern Atlantic Coastal Plain is humid subtropical. Summers are long and hot, and winters are short and mild. Mean annual precipitation in the region is 1100–1200 mm yr1. Maximum precipitation occurs during the months of January–March (100–130 mm mean monthy totals) and again during July–August (120–200 mm mean monthly totals). The mean annual temperature is about 15–20
C. Mean annual precipitation and evapotranspiration are about equal, but evapotranspiration exceeds precipitation about 6 months out of the year (Markewich and Markewich, 1994). Native vegetation is deciduous, mixed, and southern pine forests, with pine forests most common on the lower Coastal Plain. Riparian vegetation is typically a very dense mixture of shrubs, vines, and trees.

3. Methods 3.1. Imagery Landsat images that captured unusually wet soils and inundated floodplains were preferentially selected for study by analyzing flood records of the US Geological Survey (USGS) and matching floods with cloud-free images. Landsat scene LT5017038009805210, recorded on February 21, 1998, was used for rivers in Georgia, and Landsat scene LT5016036009506910, recorded on March 10, 1995, was used for rivers in the Carolinas. Both images captured the landscape shortly after a long period of persistent heavy rains and overbank flooding. These images made it easy to discern floodplains from terraces and to recognize the ridge and swale topography of the braid plains using the infrared band (band 4), which accentuates low lying wet soils as dark tones and

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slightly higher, well drained soils as light tones. Crosssections and longitudinal profiles in the Oconee and Altamaha River valleys were derived from digital raster graphics (DRGs) of USGS 7.5 min topographic maps having 5 or 10 ft contour intervals. 3.2. OSL and

14

230
C in the later step has been experimentally tested on riverine dune sediments from southern Georgia by Ivester et al. (2001), and it was found to be appropriate with the ratio of irradiated OSL to natural OSL remaining stable. The complete measurement cycle was repeated three times, each preceded by regeneration

C sampling, processing, and analysis

Samples were collected for OSL and radiocarbon (14C) dating during field visits in 2002. We typically obtained OSL samples by boring an 8 cm diameter hole with a hand auger and then pounding a 5 cm diameter sample coring tube (30 cm long) into the sediment at the desired sampling depth. The sample tube is equipped with an internal stainless steel sleeve that was removed and immediately sealed at both ends to shelter the sample from exposure to light. In some cases outcrops were sampled by pounding the 5 cm diameter corer laterally into a portion of the exposure that had been scraped back into moist sediment. Radiocarbon samples were obtained from river cutbanks, backhoe pits, or bucket auger holes. Care was taken to select uncarbonized pieces of wood that showed no evidence of abrasion from fluvial transport. Radiocarbon dating was done at the Center for Applied Isotope Studies, University of Georgia by either scintillation counting or accelerator mass spectrometry using standard methods. Several centimeters of sample were removed and discarded from each end of the stainless steel sleeves in a dark room. Then, samples for OSL dating were taken from the center of the remaining sediment. These samples were sequentially pretreated with 10% HCl and 30% H2O2 to remove carbonates and organic matter, respectively, and then sieved to obtain the 150–200 mm size fraction. Density separation using Napolytungstate (2.58 g/cm3) was carried out to separate quartz from most feldspar minerals. The sample was further etched with 40% HF for 80 min to remove the alpha skin and more feldspars. A bath in 12 N HCl for 30 min followed to remove the fluorides. The separated grains were mounted as monolayers on stainless steel discs using Silkosprayt. All measurements were carried out using a standard Riso TL/OSL system. Blue LEDs focused at 470 nm were used for optical stimulation of quartz extracts. The detection optics utilized Hoya 2U340 and Schott BG-39 filters coupled to an EMI 9635 QA photomultiplier tube. Beta irradiation was applied using a built-in 25 mCi 90Sr/90Y beta source. Paleodose estimates were made by the single aliquot regeneration (SAR) protocol (Murray et al., 1997; Murray and Wintle, 2000). The aliquots of separated quartz were preheated at 220
C for 60 s and the natural OSL was measured. A test dose (usually 10% of the expected paleodose) was administered and the sample was again heated to 220
C for 60 s and OSL measured to check sensitivity. A heating temperature of 220–

Fig. 2. OSL data from sample BH 5.2, indicating the quality of data used in this study, including: (a) OSL depletion in quartz with increasing stimulation time; (b) standard OSL growth in response to regenerated dose showing the paleodose corresponding to the natural signal. Lx/Tx is test dose corrected ratio of OSL signal; and (c) frequency distribution of paleodoses measured on 10 aliquots.

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doses selected to bracket the natural dose. The recycling ratio between the first and the fifth point was ranged within 0.95–1.05 for aliquots. Data were analyzed using Duller’s (1999) ANALYST program. Representative examples of our OSL data are presented in Fig. 2, and all ages are given with two standard deviation error estimates (72 sigma). The dose rate estimates were obtained using the elemental composition of the sample matrix. We used ZnS(Ag) thick source alpha counting to estimate uranium (U) and thorium (Th). Potassium (K) was measured by atomic absorption (AA) and inductively coupled plasma spectrometry (ICP) from liquid solutions obtained by dissolving the samples in a cocktail of hydrofluoric (HF), nitric (HNO3), and hydrochloric (HCl) acids. Luminescence dating of fluvial sediments may suffer due to problems related to poor and/or inhomogeneous bleaching. However, recent studies demonstrate the validity of dating quartz-rich fluvial sands from riverine settings that are similar to the ones that we sampled (Colls et al., 2001; Srivastava et al., 2001; Stokes et al., 2001; Wallinga et al. 2001; Rittenour et al., 2003). We validated our fluvial OSL ages by dating eolian sand by OSL that directly overlies braided terrace sediments, and also by comparing our fluvial OSL ages to finite radiocarbon dates in stratigraphically similar positions at three separate sites. The OSL samples dated in our study area demonstrate low disc to disc paleodose variation, suggesting a uniform bleaching of the sands. In addition, plots of the paleodose versus natural signal for individual discs of all the samples (as in Colls et al., 2001; Rittenour et al., 2003) exhibited low correlations (R2 values of 0.0–0.3), implying that the discs with higher natural signals have relatively lower paleodose. This indicates that higher natural intensities were not due to poor bleaching. Ivester et al. (2001) demonstrate the excellent suitability of eolian sediments for OSL dating in this region. They analyzed two samples of eolian dune sediments collected at the surface that produced 0.0170.02 ka age estimates, indicating that full bleaching is achieved by sands typical of the study region. In addition, Ivester et al. (2001) indicate excellent correspondence between 14C and OSL dates, which suggests adequate and consistent bleaching of the sediments.

4. Results 4.1. Oconee-Altamaha river valley The CV, BC, and BH sample sites on terraces in the Oconee and Altamaha River valleys are unequivocal examples of braided channel patterns (Fig. 3). They closely resemble late Pleistocene braided terraces

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described in the lower Mississippi River valley (Saucier, 1994) and modern braided rivers that have bar-braided morphology. The BH site shows that meandering patterns existed on the older fluvial surface north and east of the braided surface (Fig. 3, BH), indicating a meandering pattern that changed to braiding. This older meandering surface is at approximately the same elevation as the braided terrace. Widths of the braided terraces at the three localities studied slightly exceed floodplain widths, but at most other localities in the Altamaha River valley the floodplain width exceeds that of braided terrace remnants, because of lateral channel migration during the Holocene. However, in the lower Altamaha valley the braided terrace is onlapped and buried by the floodplain, which obscures width estimates for the braided terrace. The remnant braid bars at all three sites stand about 0.5–1.5 m above the intervening channels, which are typically one-third to one-half as wide as the modern channel, which ranges from about 50–60 m wide at CV to 200–250 m wide at BH. The bars primarily are composed of medium and coarse sand, whereas the paleochannels contain 1.0–1.5 m thick clayey soils unconformably overlying sands and fine gravelly sand. The clayey textured soils in the channels probably were derived from post-braiding flood drapes that slowly accreted in the channels throughout the Holocene. In fact, soils mapped in the swales include ones that are ‘‘flooded periodically’’ (Coxville series) and ‘‘subject to overflow’’ (Johnston series), and one soil (Rains series) is noted to have been covered with light grey sandy ‘‘overwash’’ (Rigdon, 1975) indicating frequent flooding of the braided terraces in some localities. The sedimentary texture of the braided terraces is quite similar to that of the modern channel bed, predominantly sand and fine gravelly sand. Sandy soils on bars typically exhibit cambic B horizons (Bw) to incipient argillic horizons (Bt), and clayey soils in swales typically exhibit gleyed B horizons (Bg) or incipient argillic horizons that are gleyed (Btg). Both pedogenic settings tend to have A and E horizons as epipedons. Typical solum thickness is 1.0–1.5 m. The cutbank exposed on the east side of the Altamaha River at the BC site allowed examination of a 4–5 m thick section of sedimentary structures within the fill of the braided terrace that overlies sandstone bedrock. It showed that the terrace consists of a sequence of sand bar architectural elements composed almost entirely of tabular cross-beds, horizontally laminated sand beds, trough cross-beds, and thin horizontal beds of gravel and silt (Fig. 4). Vertical accretion facies were absent across 750 m of the outcrop examined, except at the top of the section in swale/channel fills where 0.5–1.5 m of the vertical accretion facies abruptly overlie the 3–4 m thick section sand bar facies. The unconformity between the vertical accretion facies and the sand bar facies does

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Fig. 3. Landsat images (infrared band 4) and cross-sections of three sample localities in the Oconee-Altamaha valley, including: (a) lower Oconee River valley near Charlottsville, GA (CV); (b) Bullard Creek Wildlife Management area near Baxley, GA (BC); and (c) big Hammock Wildlife Management area near Glennville, GA (BH). The UTM (NAD83) coordinates for sample sites are provided in Tables 2 and 3.

not exhibit a buried soil profile in the top of the sandy facies, indicating that little time elapsed before the bar facies were draped with clayey sediment. The longitudinal valley gradient of the braided terrace in the Oconee and Altamaha River valley (0.00025) has a slightly steeper slope than that of the

modern floodplain (0.00020) and water surface (Fig. 5). The sandy braided terrace surface passes beneath the clayey floodplain about 55 km upvalley from mean sea level where the floodplain deposits onlap the braided surface. The upper end of the onlapped floodplain (near Ludowici, Georgia at US Highway 301/84) is

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Fig. 4. Sedimentary structures typical of the braided terrace outcrop at the Bullard Creek site at OSL sample site BC1.

Altamaha River Valley Longitudinal Profile 35 CV Site

30

BC Site

Elevation (m)

25 BH Site

20

a ah am Alt

ee on Oc

14-52 ka Dune Bands of Ivester et al. (2001)

15 10

Water Surface Floodplain/Backswamp Braided Terrace Bar Tops Projected Braided Terrace

5 0 0

10

20

30

40

50

60

70

80

90

100 110 120

130 140 150

Distance up the Altamaha/Oconee Valley from Tidal Zone (km) (zero distance at I-95) Fig. 5. Longitudinal profile of the Altamaha River valley showing the elevations of the straight line of water surface intersections, the modern floodbasin/backswamp surface, and the ridge crests of prominent braided terrace surfaces. Note that the braided terrace passes beneath the modern floodplain about 50 km upvalley from the tidal zone. Distances are measured upstream from Interstate Highway 95, which is 0 masl on the Altamaha River channel.

characterized by many bands of dunes dated by Ivester et al. (2001) to 14–52 ka (Fig. 5), including separate bands of dunes at 14–18, 20–22, 33–43, and 38–52 ka ages (including 1-sigma errors). These dunes emerge above the floodplain and are successively older with distance from the river toward the northeast. The upstream end of this dune field (about 65 km upvalley) exhibits braid patterns between the dunes, but the braid

patterns are completely buried by clayey floodbasin sediments several kilometers downstream. Sand quarries in the midst of this dune field expose a conformable boundary between the eolian sand and the underlying fluvial sand associated with the braided terrace. Ivester et al. (2001) report a radiocarbon age of 43,79076270 14C yr BP (UGA-7293) from a cypress knee recovered from the fluvial sand beneath 20 ka

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Table 2 Optically stimulated luminescence (OSL) dates and supporting data Sample

Age (ka)72-sigma

Unit

Depth (cm)

UTM zone 17 (NAD83) coord.

U (ppm)

Th (ppm)

K (%)

Paleodose (Gy)

Dose rate (Gy/ka)

BH-2 BH-3 BH5.2 BH-6 BH-7 BC-1 BC-2 CV-1 CV-2 PD-2 DL-2 DL-3 DL-4 DL-5 DL-6

18.671.9 28.174.9 17.672.6 19.172.9 30.675.4 12.571.9 23.475.7 27.974.4 22.474.2 68.6710.5 15.071.4 16.671.8 20.272.4 19.173.2 13.872.2

Dune Braid bar Dune Braid bar Braid bar Braid bar Braid bar Braid bar Braid bar Braid bar Dune Dune Dune Scroll bar Scroll bar

180–210 150–180 180–210 150–180 90–120 440 235 150–180 150–180 300 180–210 180–210 160–190 200–230 120–150

400806E; 397667E; 397577E; 397474E; 397520E; 360068E; 360207E; 349816E; 349488E; 639550E; 613733E; 608944E; 608536E; 608508E; 612602E;

0.770.1 5.4471.1 0.8170.1 0.8270.3 0.7770.04 0.7570.2 1.770.4 0.6770.09 2.0570.5 0.9470.15 0.6370.2 0.9370.4 0.9370.1 6.371.1 1.070.3

1.670.4 3.670.3 0.970.4 5.471.2 0.0270 2.670.8 4.471.5 1.070.3 5.071.7 1.970.5 2.370.6 1.070.1 1.270.5 1.470.1 2.270.9

0.24 0.72 0.18 0.17 0.15 0.40 0.41 0.38 0.47 0.49 0.89 0.82 0.77 0.89 1.19

10.370.5 57.077.0 9.071.0 14.271.0 13.072.0 9.371.0 24.075.0 17.472.3 26.373.4 56.077.0 18.170.8 18.670.8 22.072.0 42.075.0 19.972.7

0.570.05 2.070.2 0.570.05 0.770.1 0.470.04 0.770.08 1.070.1 0.670.05 1.270.2 0.870.07 1.270.1 1.170.1 1.170.1 2.270.2 1.470.1

3525897N 3526958N 3526407N 3526361N 3526487N 3536238N 3536493N 3544039N 3544914N 3773402N 3808822N 3822132N 3821846N 3821366N 3807119N

Notes: Dune sample water contents were assumed to be 1075% and braid and scroll bar samples were assumed have 1575% (by weight). Cosmic ray contribution is assumed to be 150730 mGy per annum.

Table 3 Radiocarbon dates Lab #

UTM zone 17 (NAD83) coord.

Sample depth (cm)

Material dated

13 C corrected date (14C yr BP)

71 standard deviation

Calendar age (cal yr BP)

2-Sigma calendar age (cal yr BP)

UGA-10297 UGA-10667 UGA-10670 UGA-10843

612431E; 609208E; 609208E; 360207E;

260–275 320 300–350 550

Uncarbonized acorn Uncarbonized twigs Uncarbonized hardwood Exterior rings of uncarbonized log

11,470 12,990 12,940 30,940

50 60 210 1800

13,444 15,662 15,565 n.a.

13,188–13,768 16,182–14,573 16,356-14,347 n.a.

3806724N 3822241N 3822241N 3536493N

dunes. Dunes occur sporadically on the braided terrace remnants along the entire length of the Altamaha drainage within the coastal plain, and in some places it is apparent that eolian reworking of the braid bars has obscured their distinct braided patterns. The 14C and OSL dates (including 2-sigma range) that we obtained in the Altamaha-Oconee valleys from sites CV, BC, and BH (Figs. 1 and 3) generally indicate a Last Glacial Maximum (LGM) age for the braided terrace sediments within marine oxygen isotope stage 2 (OIS 2), ranging from 11 to 36 ka with an average age of 23.4 ka (76.2 ka) from the seven OSL samples (Table 2). This age range closely matches the most recent phase of widespread eolian sedimentation of riverine dunes in Georgia at 15–30 ka (Ivester et al., 2001), and corroborates the contention of Ivester and Leigh (2003) that dune sedimentation was genetically associated with braided rivers. Results from individual sample sites are presented below and in Tables 2 and 3. The Charlottsville site (CV) yielded OSL ages of 22.474.2 and 27.974.4 ka (Fig. 3) from separate sand bars (sand ridges) that were sampled at a depth of 150– 180 cm below ground surface (Table 2). Considering the

2-sigma error, these give essentially the same age. This is not surprising, because the samples are from adjacent sand ridges separated by a swale (paleochannel). The Bullock Creek site (BC) yielded OSL ages of 23.475.7 and 12.571.9 ka from outcrop localities spaced about 300 m apart (Fig. 3). The 23.475.7 ka age comes from a depth of 2.4 m and directly overlies a log at 5.5 m depth dated to 30,94071800 14C yr BP (UGA-10843). The log was found within the basal braided stream deposits just above sandstone bedrock, and it represents the lower boundary of the braided river strata. We view the 23.475.7 ka OSL age to be in good agreement with the underlying radiocarbon age. The 12.571.9 ka OSL date at BC is for a sample collected at 4.4 m depth in a position within a morphologically separate sand ridge compared to the site with the 23.475.7 ka age. We believe that the sample depth has little interpretive value, relative to the other OSL date, because of the laterally shifting nature of braided channels. Thus, we view the 12.571.9 ka age to represent one of the final channel positions on the terrace. Soil development in the sand ridge from which the 12.571.9 ka sample was taken is consistent with that

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interpretation, because a weakly developed soil (Bw horizon) was apparent above that sample, whereas a better expressed soil (with a weak Bt horizon) was found in the sand ridge dated to 23.475.7 ka. The Big Hammock site (BH) yielded two OSL dates from an infilled parabolic dune that overlies the braided terrace (Fig. 3). We consider these eolian samples as ‘‘controls’’ that should be less than or about equal to the age of the adjacent and underlying braided fluvial surface. Indeed both dune samples yielded the youngest ages of the five samples at the BH site, with dates of 18.671.9 and 17.672.6 ka. The 18.671.9 ka date comes from the distal or leading edge of the parabolic complex, and the 17.672.6 ka date comes from eolian sand that probably ‘‘filled’’ the parabolic outline subsequent to stabilization of the leading edge. A sample collected from fluvial sediment directly beneath the dune (from beneath a drainage ditch that was cut through the dune) yielded an age of 30.675.4 ka, and a sample from the fluvial surface downwind from the dune yielded an age of 28.174.9 ka. As expected, both samples are older than the overlying dune. One more fluvial sample (BH-6) was collected on the braided surface immediately windward of the dune, and this locality should have been the source of eolian sand from the braided terrace. This sample yielded an age of 19.172.9 ka, which is in very close agreement with the age of the dune. In summary, the BH samples indicate that the braided surface was active within a 15–36 ka age range and that the fluvial samples are in good agreement with the stratigraphic relationship between the dune and underlying braided terrace. Also, our OSL dates are in agreement with a radiocarbon date (14,6907250 14C yr BP, W-5790) reported by Markewich and Markewich (1994) from a correlative terrace remnant immediately to the south across the river from the BH site, which calibrates to about 16.8 ka in calendar years. 4.2. Pee Dee River valley Distinct braiding is apparent at many localities along the first prominent terrace of the Pee Dee River (Fig. 6). This first terrace is named the Wando surface and has been assigned a late Pleistocene age ranging from 90 to14 ka (Thom, 1967; Owens, 1989; this paper). Based on two radiocarbon dates from samples beneath dunes, Thom (1967) indicated that the Wando terrace in the Pee Dee valley was active just prior to 17,000 and 36,000 14C yr BP. Owens (1989) assigned an age of 90 ka to the Wando terrace, based on correlation with marine corals in the Charleston area. We located an exemplary site about 25 km south of Interstate Highway 95 on the eastern side of the valley (Fig. 6). Here the braid bars are very closely associated with large coalescing infilled parabolic dunes, indicating

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a sedimentary linkage between the braided fluvial and eolian transport systems. Like the BH site in Georgia, distinct meandering patterns are visible on a pre-braided fluvial surface east of the braids (PMT on Fig. 6), indicating that the river pattern was meandering at some time prior to becoming braided. We obtained one sample at this locality from a braid bar that produced an OSL date of 68.6710.5 ka (Fig. 6, sample PD-2; Table 2). This age predates those reported above for fluvial sediments in Georgia, and it appears to be a reliable estimate. The cross-cutting relationships of different braid surfaces and dunes (Fig. 6) indicate that the PD-2 site is on one of the oldest braided surfaces (braided terrace 3 of 4). In addition, recall that Owens (1989) indicated that the Wando surface was up to 90 ka in age. Our OSL age of 68.6710.5 ka at PD-2 suggests that braided patterns were present during the early Wisconsinan of OIS 4, but more chronological data probably are needed to fully confirm this result. Further north in the middle Pee Dee valley (Fig. 7), immediately south of the Fall Line boundary between the Piedmont and Coastal Plain near US Highway 401, braid patterns are juxtaposed and transitional to sandy scroll bar patterns of a low-sinuosity meandering pattern. In addition, cross-cutting relationships of different aged braided patterns are apparent in the vicinity of the Highway 401 locality. We dated fluvial and eolian sediments along US Highway 401 near the transition from the braided to the scrolled surface by both 14C and OSL (Fig. 7, samples UGA-10667 & 70, DL-3, 4, 5). Two separate radiocarbon samples from immediately above the contact with channel-lag sands returned a calendar-year 2-sigma age range of 14.5– 16.4 ka (UGA-10667, 10670; Table 3), indicating that the braided paleochannel, immediately to the east of the dated site, was active ca 17 ka. A sample from a sandy scroll bar 1.1 km to the southwest of this radiocarbon site is dated by OSL to 19.173.2 ka (DL-5), and eolian samples from parabolic dunes that were blown from the scrolled fluvial surface yielded ages of 16.671.8 and 20.272.4 ka (DL-3 & 4). These OSL dates correlate well with the radiocarbon dates and together they indicate that the braided to scrolled transition in this part of the Pee Dee valley occurred ca 17 ka. At another locality in the Pee Dee River valley, 14 km south of US Highway 401, known as the Damon paleomeander (Fig. 7), we dated one of three exceptionally large cutoff meanders with distinct scroll bar patterns by both 14C and OSL. A 14C date from an acorn, situated atop gravelly sand and at the base of 2.7 m of peaty and clayey fill in the paleomeander, yielded an age of 11,470750 14C yr BP (UGA-10297), which calibrates to a 2-sigma calendar range of 13.2– 13.8 ka, indicating that the scrolled paleomeander was active at ca 14 ka. In addition, the scroll bar nearest to the radiocarbon site (inner channel bank) yielded a very

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Fig. 6. Landsat image (band 4) of the Pee Dee River valley just south of US Highways 76 and 301 showing OSL site PD-2 (left frame) where a 69710 ka date was obtained. The geomorphic map (right) illustrates the separate units in the valley with ‘‘BT’’ referring to progressively older braided terraces. BT1 is the youngest and BT4 is oldest. Terrace elevations are given in meters above mean sea level in the central portion of the map. PMT refers to a terrace with large paleomeanders that predates the braided terraces. The Socastee Terrace is thought to be about 200 ka and the Waccamaw is early Pleistocene (Owens, 1989). The UTM Zone 17 coordinates for the center of the Landsat scene are 639,934E, 3,770,934N, and UTM coordinates for sample PD-2 is provided on Table 2.

correlative OSL date of 13.872.2 ka (DL-6), which also was corroborated by an OSL date of 15.071.4 ka (DL-2) from a parabolic dune overlying older scroll bars in the central portion of the scrolled floodplain (Fig. 7). Taken together, the 14C and OSL dates at the two localities shown in Fig. 7 indicate that the braiding changed to a low sinuosity scroll bar pattern ca 17 ka, and that a very high sinuosity scrolled meander pattern was established by 14 ka. In addition, the geometry of the exceptionally large paleomeanders and scroll bars indicates that bankfull discharge was significantly greater than present during the latest Pleistocene. Sand and gravel borrow pits on scroll bars, and the depth to gravel in the paleomeanders, at the Highway 401 and Damon sites indicate that the thickness of scroll

bar sands is about 3–4 m. Thom (1967) notes that the thickness of Wando terrace sediment rarely exceeds 9.1 m. The maps of Thom (1967) and Owens (1989) indicate that the Wando surface is approximately 5 km wide in the region where we present age estimates, whereas the Holocene floodplain is only 0.5–1.5 km wide (Figs. 6 and 7). Thom (1967) plotted the longitudinal gradient of the Wando surface in the Pee Dee valley as being essentially equivalent to the floodplain down to about 40 km from the coast, but he did not map the braided Wando terrace downstream from that point. However, it is apparent on maps and on the Landsat image that the braided terrace is onlapped by the modern floodplain near the confluence with the Little Pee Dee River valley approximately 20 km from the coast.

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Fig. 7. Landsat image (band 4) of the Great Pee Dee River valley near US Highway 401 showing both braided and scrolled patterns. Dates associated with labels are listed in Tables 2 and 3. The Holocene floodplain is delineated with a black line. The UTM Zone 17 coordinates for the center of this scene are 611692E, 3,815,944N, and UTM coordinates for samples are provided on Tables 2 and 3.

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4.3. Tributaries to the Pee Dee Braided patterns are readily apparent along tributaries to the Pee Dee River, including the Little Pee Dee (Fig. 8) and Lynches River (Fig. 9). The locality along the Lynches River, a small western tributary to the Pee Dee (Fig. 1), clearly illustrates that braiding occurred in streams with catchments smaller than 2000 km2. Thom (1967) mapped the gradient of the braided Wando surface in the Little Pee Dee valley as passing beneath the floodplain near the confluence with the Pee Dee at about 20 km from the coast. Cross-sections of the Little Pee Dee valley provided by Owens (1989) indicate that the braided deposits of the Wando terrace are about 2 m thick. Like the Pee Dee valley, the width of the Wando terrace in the Little Pee Dee is about two to four times that of the Holocene floodplain. 4.4. Savannah River valley

Fig. 8. Landsat image of the Little Pee Dee valley immediately south of US Highway 501. The drainage area above this site is about 6500 km2. The UTM Zone 17 coordinates for the center of this scene are 656,399E, 3,757,071N.

Fig. 9. Landsat image of the Lynches River valley about 7 km downvalley from Interstate Highway 95 and 120 km upvalley from the coast. The drainage area above this site is about 2500 km2. The UTM zone 17 coordinates (NAD83) for the center if this scene are 600,923E, 3,761,406N.

The lower Savannah River exhibits a braided terrace that is reworked and overlain by parabolic dunes about 40 km upstream from the tidal zone (Fig. 10). Many of the braid bars appear to have been reworked by wind, forming bands of small coalescing parabolic dunes, but braid bars are still apparent beneath and between the

Fig. 10. Landsat image of the eastern side of the Savannah River valley about 17 km upvalley from Interstate Highway 95 and 40 km upvalley from the coast. The floodplain is the homogenous dark area on the left half. Some of the braid bars (right half) have been reworked by the wind to produce parabolic dunes. The drainage area above this site is about 31,200 km2. The UTM zone 17 coordinates (NAD83) for the center if this scene are 486,106E, 3,584,766N.

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dunes. Topographic maps show that the elevation of the braided terrace is about equal to the floodplain at this locality, and it appears that this is the ‘‘onlap’’ segment where floodplain deposits begin to cover the braided terrace surface. The width of the braided terrace at the locality depicted in Fig. 10 is about equal to that of the floodplain. A late Pleistocene age is indicated for the braided terrace shown in Fig. 10, based on OSL and 14C dates that have been obtained on the first prominent terrace in the Savannah River valley about 45–50 km upstream at the pre-Clovis Topper Site (Goodyear, 2002), where 14C and OSL dates indicate a late Pleistocene age for the terrace ranging from about 15 to 40 ka (Forman, 2002; Goodyear, 2002; Karabanov et al., 2002). 4.5. Cape Fear River valley Braided patterns also are visible in the Landsat image of the first prominent terrace to the Cape Fear River near Tar Heel, North Carolina (Fig. 11). Soller (1988) mapped this locality as the Wando terrace, which he described as the first prominent terrace having ‘‘wellpreserved meander scars and scrollwork features’’. Distinct scroll bars are noted further north along the big bend in the Cape Fear River immediately southeast of Fayetteville, North Carolina, but scroll bar patterns

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appear to be less common than braid patterns along most of the valley, based on the Landsat image. It is possible that the late Pleistocene Wando terrace was very near the threshold separating braiding from meandering, and braiding may have been spatially and temporally variable. However, many parts of the Wando terrace in the Cape Fear valley are clearly braided. Soller (1988) maps the gradient of the Wando terrace as slightly steeper than the modern floodplain in the upper Coastal Plain, but he maps the Wando terrace as flattening-out at about 30 km from the coastline to form an unusually concave profile. He attributed concavity to neotectonic activity, but it is more probable that the braided surface passes beneath the floodplain at about 30 km upstream from the coast, given current understanding of eustatic effects on river systems (Blum and . Tornqvist, 2000) and results of this study. Soller notes the thickness of the alluvium in the Wando terrace is 1–3 m. The maps of Soller (1988) and Owens (1989) indicate the Wando surface to be approximately 4 km wide in the valley in the vicinity of Fig. 11, whereas the Holocene floodplain is o0.5 km wide and incised beneath the Wando terrace.

5. Discussion 5.1. Time of braiding

Fig. 11. Landsat images of the Cape Fear River valley about 25 km downvalley from Interstate Highway 95 near Tar Heel, South Carolina, and about 120 km upvalley from the coast. Soller (1988) indicated a late Pleistocene age for this terrace. The drainage area above this site is about 15,500 km2. The UTM zone 17 coordinates (NAD83) for the center if this scene are 702,305E, 3,849,151N.

Our results indicate that braiding was well expressed in the Oconee-Altamaha valley during OIS 2 and late OIS 3 (11–36 ka), and that the Pee Dee valley exhibited braiding prior to 17 ka and possibly back to 70 ka (Fig. 12). Meandering appears to have been well established by 12–14 ka and has persisted until the present time. The time of braiding correlates very closely with the latest period of eolian sand dune sedimentation associated with rivers on the Coastal Plain in Georgia at 15–30 ka (Ivester et al., 2001; Ivester and Leigh, 2003), and we argue for a genetic linkage between the braided floodplain and eolian dune sedimentary systems. That is, satellite imagery, stratigraphic observations, and dates (both 14C and OSL) indicate that the braid plains were an important source of eolian sand during the late Pleistocene. Thus, braiding in the Oconee and Altamaha rivers may have been initiated much earlier than 36 ka, given that Ivester et al. (2001) dated riverine dunes in the Altamaha River to 45.0277.37 ka by OSL, and wood in alluvium beneath dunes on the braided was terrace dated to 43,79076270 14C yr BP (UGA-7293). Elsewhere in southern Georgia riverine dunes were active at 65–85 ka (Ivester et al., 2001), suggesting braiding during OIS 4. Our 69 ka date for braided sediments along the Pee Dee also suggests braiding during OIS 4. Thus, the available evidence indicates that

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Sea Surface Temperature (Celcius)

20

18

PD braids co PD dunes o o Oc-Alt. braids o o o o oo o Alt. dunes o ooo

Alt. braids c Alt. dunes o o

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OIS 5

8 0

10000

20000

30000

40000

50000

60000

70000

80000

90000 100000 110000

Calendar Years Ago


0




0

Fig. 12. North Atlantic (37 45 N, 10 10 W) sea surface temperatures over the last 107 ka (Bard, 2002, 2003) and associated chronology for braiding and eolian dunes in the Oconee-Altamaha (Alt.) and Pee Dee River (PD) valleys. The letter ‘‘c’’ represents 14C dates, the letter ‘‘o’’ represents OSL dates, and the lines represent the 2-sigma error ranges associated with the dates. The arrow associated with the ‘‘PD braids’’ indicates that the braided patterns pre-date 17 ka. The 14C date from the Altamaha River and most of the OSL dates from Altamaha dunes are from Ivester et al. (2001).

environmental conditions during OIS 3 and possibly OIS 4 were sufficient to favor braiding as global ice volume steadily expanded and as the global temperatures fell (Fig. 12). We have insufficient evidence to evaluate whether or not braiding reverted to meandering during short-lived interstadials, but the limited data of Leigh and Feeney (1995) indicate that unusually large meandering patterns existed in the Ogeechee River valley of southeast Georgia ca 28–31 ka. It is apparent that sandy braided and low sinuosity scrolled patterns lasted until about 14 ka, based on our youngest OSL dates for braids in the Oconee-Altamaha valley and based on 14C and OSL dates from large paleomeanders in the Pee Dee valley. In addition, Paleoindian sites occur within the meandering floodplain deposits of Coastal Plain rivers in Georgia (Anderson et al., 1990; Georgia Site Files, unpublished), also indicating that meandering was well established by 12–14 ka. We believe meandering patterns prevailed from 12 ka to present, as only meandering patterns are visible on post-braided geomorphic surfaces. The transition from braided to meandering that we observe from 14 to 17 ka is in close agreement with the time of a major shift in regional flora and progressive warming.

Landscapes of the Southeast were dominated by sparse covers of northern pine (Pinus), spruce (Picea), and herbs during the LGM, whereas after 14 ka regional dominance of oak (Quercus) and other deciduous trees prevailed (Watts, 1980; Kneller, 1996; Kneller and Peteet, 1999; Lamoreaux, 1999;). Such shifts in climate and vegetation patterns probably were associated with changes in discharge and sediment yield that in turn influenced channel patterns. 5.2. Regional extent of braiding Braided river patterns appear to be widespread in the southeastern Atlantic Coastal Plain of Georgia and the Carolinas during the late Pleistocene. This is indicated by examples from temporally correlative sites that are widely distributed. Our data also indicate that much of the drainage system on the Coastal Plain experienced braiding, including large, medium, and small rivers. We lack data about the Piedmont, but some previous studies have suggested late Pleistocene braiding in Piedmont drainages (Schuldenrein, 1996; Segovia et al., 1981; Leigh, 1996). However, unambiguous data are not available and clear evidence of braiding on Piedmont streams remains to be documented.

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Maps and Landsat imagery generally show the braided terraces to be wider than the Holocene floodplain. This may be the result of more time available for braiding than meandering and it suggests that braided river facies dominate the late Quaternary sedimentary record in river valleys of the Southeast. Longitudinal profiles of the braided terraces indicate that the rivers were graded to lower sea level or to the Continental Shelf during eustatic lowstands. The slightly steeper gradient of the LGM surface is onlapped by Holocene floodplain sediments in the near-coastal reaches of riverine systems (10–60 km from present coast). These observations on the horizontal and vertical extent of the braided terraces are in close agreement with . generalizations made by Blum and Tornqvist (2000) regarding worldwide patterns of river responses to eustatic sea level and climate changes during the late Quaternary. Our evidence of braided patterns in the Atlantic Coastal Plain clearly supports the hypothesis that unglaciated catchments in humid subtropical regions have experienced environmental changes in the late Pleistocene sufficient to produce braided channel patterns. Many examples of braided to meandering transitions are noted for the Pleistocene–Holocene transition in northern and northwestern Europe (Kozarski, 1991; Kasse et al., 1995; Straffin et al., 2000), but they represent late Pleistocene periglacial regions where the land surface and hydrological conditions were very different from those in the southeastern USA. Thus, our examples are significant because they indicate pronounced river responses to glacial–interglacial cycles in relatively low latitude regions that are not known to have had periglacial conditions. We are not aware of any other examples of braided-meandering fluctuations for unglaciated humid subtropical regions of the world. 5.3. Causes of braiding The cause of braiding has stimulated intensive research in fluvial geomorphology for about 50 years (e.g., Lane, 1957; Leopold and Wolman, 1957; Osterkamp, 1978; Chang, 1985; Schumm, 1985; Ferguson, 1987; Begin, 1991; Van den Berg, 1995; Lewin and Brewer, 2001; Simpson and Smith, 2001; Van den Berg and Bledsoe, 2003). Although no single equation or variable has proven to be entirely satisfactory for discriminating braided versus meandering patterns, four drivers of channel change are commonly cited as influential, including: (1) discharge variability, (2) bed load size and abundance, (3) valley gradient, and (4) bank erodibility (Knighton, 1998; Simpson and Smith, 2001; Bridge, 2003). Knighton (1998) states that none of the four drivers is independently sufficient to cause braiding, but that ‘‘abundant bed load, erodible banks,

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and a relatively high stream power are probably necessary.’’ The present-day meandering river channels cited in our study fall below the classic threshold of braiding determined by Leopold and Wolman (1957), but the largest rivers are within the field of sand-bed braided streams of Kellerhals (1982) (Fig. 13) when their valley slope (instead of channel slope) is plotted against the 2yr discharge. This suggests that the Coastal Plain rivers are situated so that a small change in any one of the four drivers could potentially cause a distinct change in the channel pattern. Also, the close association of scroll bar and braided patterns observed in the Pee Dee valley indicates that the late Pleistocene channels were near the threshold between braiding and meandering. 5.3.1. Paleoclimate The paleochannels of the Atlantic Coastal Plain are discussed below in the context of the four drivers. However, an overarching influential factor is paleoclimate. Pollen records and global circulation models (GCMs) indicate that late Pleistocene paleoclimatic in the Southeast was significantly cooler and drier than during the Holocene (Watts, 1980; Whitehead, 1981; Delcourt and Delcourt, 1985; Prentice et al., 1991; Kneller, 1996; Bartlein et al., 1998; Kutzbach et al., 1998; Lamoreaux, 1999). Much drier and windier conditions than present during the LGM also are indicated by widespread eolian sediment transport (Ivester et al., 2001). Thus, generally cooler and drier mean annual conditions probably drove changes in the fluvial conditions. Pollen reconstructions for the LGM in the Carolinas indicate mean July temperatures 8–10
C cooler than present and January temperatures 8–12
C cooler than present, while annual precipitation was estimated at 700 mm or 40 percent less than present (Prentice et al., 1991). The GCM output for 21 ka indicates July temperatures 2–3
C cooler than present and January temperatures 16-18
C cooler than present (Bartlein et al., 1998, pp 570–571; Kutzbach et al., 1998). The GCM annual precipitation was about 12 percent less than present, with all months of the year drier than present except for April and May, which are modeled with 0.5–1.0 mm/day more precipitation. Modeled snow depth was 4–7 cm/month more than present for all winter months (December–March). Modeled runoff was significantly greater than present (by 1–2 mm/day) only during the months of March and April, while the remainder of the year had less or equal runoff. While these GCM data should be viewed with caution, they provide support for widely variable hydrological conditions that may have influenced fluvial conditions in the region and they introduce the idea of a snowmelt component, which is not part of the present-day hydrological regime.

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80 10-2

Sand Bed Braided Channels (Kellerhals, 1982) 0.44 Slope = 0.0125Qbkf (Leopold and Wolman, 1957) Q2.0 vs Valley Slope of USGS Gages in this Study

Slope

"B ra "M ea ided " nd er ing " Le op old an d

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olm

an 's

C

P

LP

(1 95 7) Bo un S da ry Lin A e

L 10-4 101

102

103

104

105

Discharge (m3s-1)

Fig. 13. Sand bed braided streams (J) presented by Kellerhals (1982) compared to meandering rivers at USGS gage sites of the Atlantic Coastal Plain cited in this study (m). The solid line represents the classic boundary between braided and meandering streams of Leopold and Wolman (1957). Discharges for Kellerhals’ (1982) sites include 2yr floods, mean floods, and bankfull floods in that order of preference, whereas data for this study (triangles) only include 2-yr floods calculated from pre-reservoir time periods. Valley slope is used for the Atlantic Coastal Plain sites (labeled), whereas slope type (valley or channel slope) is not specified for Kellerhals’ (1982) data. However, sand bed braided streams tend to have very low sinuosity, so valley slope and channel slope probably are similar in the Kellerhals (1982) sand bed braided channels. Labels are: A represents the Altamaha River at Baxley, C represents the Cape Fear River at Fayetteville, L represents the Lynches River at Effingham, LP represents the Little Pee Dee at Galivants Ferry, P represents the Pee Dee River at Pee Dee, S represents the Savannah River at Clyo. Characteristics of these gage sites are given on Table 1.

5.3.2. Daily and seasonal discharge regime Highly variable daily discharge has been cited as a cause for braiding, but recent studies have shown that it is not a key causative factor for braided channels (Knighton, 1998), and some have referred to fluctuations in discharge as ‘‘myth’’ with respect to a cause for braiding (Bridge, 2003, p. 156). In any case, there is no reason to think that late Pleistocene rivers in the study area had a widely fluctuating daily discharge, particularly because they were not fed by diurnal variations in glacial meltwater. Relatively little is known about seasonal variation in paleodischarge of Atlantic Coastal Plain Rivers during the late Pleistocene. However, Leigh and Feeney (1995) suggested that bankfull discharge of the Ogeechee River in southeast Georgia was significantly larger than present ca 28–31 ka, based on the large dimensions of two radiocarbon-dated paleomeanders. In addition, while pollen data and paleoclimate simulations suggest that annualized LGM conditions were significantly drier than present, model results also indicate that the months of March and April had significantly greater runoff and that snow depths were significantly greater than present.

Thus, it is possible that rivers of the Atlantic Coastal Plain experienced a pronounced spring flood season that was punctuated by snowmelt runoff and larger-thanmodern spring floods. Indeed, a dry season is supported by eolian dunes on the LGM floodplain, which would require dry soils throughout much of the year to prevent vegetation and moisture from stabilizing the sandy soils. However, large annual floods must have occurred to cause lateral erosion and sandy bed form development. Although increases in bankfull discharge could induce greater unit stream power, which might favor braiding, such changes are not sufficient in themselves to fully explain braiding (Lewin and Brewer, 2001). In fact, changes in discharge could simply be accommodated by enlarged meander patterns as implied by the data of Leigh and Feeney (1995) and as illustrated by unusually large paleomeanders in the Pee Dee valley (Fig. 7). In summary, change in the magnitude of seasonal discharge and bankfull floods does not seem to be a compelling cause of braiding for late Pleistocene rivers of the Atlantic Coastal Plain. However, seasonal (spring) bankfull discharges may have been greater than present. 5.3.3. Bed load Increased bed load size and abundance have been extensively cited as key agents of braiding (see Knighton, 1998; Bridge, 2003). Ferguson (1987) concluded that changes in sediment supply were very important with respect to channel patterns and could ‘‘mimic, exaggerate, or conceal the effects of altered discharge regime on channel pattern.’’ Schumm (1985) states that the ratio of bed load to suspended load is very important in determining channel pattern and that pattern change can occur without a change in the size of bed load sediment. Smith and Smith (1984) found that sandy bed load additions caused by the entry of William River into dune fields induced channel widening and braiding. There is no evidence at our study sites to indicate that the size of bed sediment was larger during the late Pleistocene than at present. In fact, exposures of late Pleistocene bed sediment reveal very similar sizes compared to the modern channels (sand with some gravel). Instead, an increase in the abundance of bed load appears to be a much more plausible explanation for braiding in the Coastal Plain. It is likely that the drier climate of the late Pleistocene favored greater sediment yield in a manner consistent with a shift from humid to drier conditions illustrated on the classic sediment yield curve of Langbein and Schumm (1958). Greater erosion of sandy soils characteristic of the Coastal Plain would certainly introduce more sand to streams. In fact, historical agriculturally induced erosion of sandy soils caused braiding in Coastal Plain streams of southwestern Georgia (Magilligan and Stamp, 1997). Increased erosion of soils on the Piedmont would also

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increase the volume of sandy bed load, but Piedmont soils are approximately 50 percent clay and also would yield a high proportion of clay-rich suspended sediment. However, gullying and mass wasting might deliver proportionally more sand from the decomposed rock underlying the soils. Late Pleistocene periglacial activity and accelerated mass wasting is documented for high elevation parts of the Blue Ridge Mountains (Mills and Delcourt, 1991), but late Pleistocene periglacial activity is not known for the Piedmont. However, Mills and Delcourt (1991) suggested the possibility of accelerated mass-wasting activity on the Piedmont, based on the data of Eargle (1977). If Piedmont-derived sediment yields were high during the late Pleistocene, then the net effect may have been for the streams to respond to more sandy bed load, rather than to a corresponding increase in suspended load. Our examination of outcrops revealed virtually no overbank facies in the substrata of the braided terraces, and it is possible that large overbank floods were rare during the late Pleistocene even though bankfull floods might have been larger than present. Thus, the suspended load may have been contained within the channel and transported through to the Atlantic Ocean. Alternatively, overbank deposits may have formed, but were not preserved due to rapid lateral reworking of floodplains. In summary, it is reasonable to conclude that sediment yield to Coastal Plain streams increased during the late Pleistocene in response to generally drier climatic conditions and altered vegetation cover. Sediment delivery to streams may have been maximized during a pronounced period of runoff during the spring, and mass wasting may have delivered sediment to streams at relatively higher rates than present. Increased sediment yield probably led to a higher proportion of bed load in stream channels, which favored a braided pattern. 5.3.4. Bank erodibility Bank erodibility is a necessary element of channel widening as well as an important source of sediment (Knighton, 1998). Bank erodibility is cited as the cause for braiding in case studies where rivers flow into landscapes that favor sandier channel banks. For example, Simpson and Smith (2001) found that the Milk River in Montana changed from meandering to braiding when the silt plus clay content of channel banks dropped from 68% to 18%, and Smith and Smith (1984) observed braiding when the William River entered a landscape of dune fields, which led to inherently unstable banks. In addition, vegetation cover and density has been linked to bank stability as a driver of channel form (Nevins, 1969; Gran and Paola, 2001). Again, in relation to drier climates of the late Pleistocene, it is reasonable to think that the rivers of the Coastal Plain had more erodible channel banks. In

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addition to the possibility of greater sediment yield from Coastal Plain soils adding more bed load to the system and causing more erodible banks, it is likely that reduction in the density of vegetation cover on banks (due to drier conditions) also favored braiding. Today the channel banks have a dense cover of vines, shrubs, and trees that depend on moist soils. Drier conditions during the late Pleistocene could have significantly reduced riparian vegetation cover and thus increased bank erodibility. In fact, dunes on the braided surfaces support the idea of a reduced vegetation cover in riparian areas, because eolian sedimentation clearly indicates that the braided floodplain was dry and had relatively little vegetation. In addition, the lack of vertical accretion facies associated with the braided terraces indicates that the late Pleistocene channels probably had sandy banks that were inherently unstable. 5.3.5. Valley gradient Empirical and theoretical data indicate that a threshold value of slope discriminates braiding from meandering (e.g. Fig. 13, Leopold and Wolman, 1957; Osterkamp, 1978; Ferguson, 1987), but the critical factor probably is a high level of unit stream power, rather than just a steep slope (Knighton, 1998). The gradients of braided channel patterns studied in the Altamaha, Pee Dee, and Cape Fear valleys are mapped as slightly steeper than the modern valley. The slight difference may be the result of the rivers being graded to the emergent Continental Shelf and lower sea-level during the LGM, or perhaps due to slight neotectonic tilting as suggested by Brown and Oliver (1976), as well as by Soller (1988). However, if neotectonic tilting were the cause, then the initial gradients of the braided terraces would have been equivalent or less than the gradients of modern valleys. Thus, it is doubtful that slope was the sole cause of braiding. In fact, the valley slopes and 2-yr flood discharges already place the largest of the studied rivers within the field of modern sand bed braided streams (Fig. 13). Therefore, it appears that steeper slopes were not a requirement for late Pleistocene braiding. Instead, late Pleistocene braiding appears to have been caused by erodible banks, abundant bed load, and perhaps greater stream power caused by large spring floods. 5.4. Reliability of OSL ages from alluvium An important aspect of this research is that our OSL age estimates for the braided fluvial sediments are very correlative with 14C dates from associated strata and with OSL ages from overlying eolian sediments. Also, our OSL age estimates are very compatible with OSL ages for eolian sands reported by Ivester et al. (2001) from a dune field in the Altamaha River valley that

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correlates with the LGM braided terrace discussed there. Thus, our results show that reliable OSL ages can be obtained from quartz-rich bed load sediments in braided river systems of the southeastern United States within the millennial resolution required for chronostratigraphic studies of this kind. However, it is uncertain how vertical accretions of suspended sediments would behave with respect to OSL dating. This conclusion supports recent studies (Colls et al., 2001; Srivastava et al., 2001; Stokes et al., 2001; Wallinga et al., 2001; Rittenour et al., 2003) contending that reliable OSL ages can be obtained from quartz-rich fluvial sediments, and has important implications for future studies of alluvial chronologies in the Coastal Plain of the southeastern United States.

6. Summary and conclusions This study illustrates that the large rivers of the southeastern Atlantic Coastal Plain and their tributaries had braided patterns during OIS 2, OIS 3, and possibly during OIS 4. Optimal expression of the regional braided patterns occurred at 17–30 ka. These braided patterns are clearly seen on Landsat images (particularly infrared band 4) taken during wet periods of the leaf-off season. The images, stratigraphic observations, and age estimates indicate that eolian dunes are coeval with braid plains and scroll bars. Our chronological data, along with inferential data from riverine dunes (Ivester et al., 2001), indicate that braiding may have occurred as early as 70 ka, with continued development at 40–45 ka, and excellent expression throughout the LGM at 17–30 ka. Meandering patterns appear to have been reestablished by 14–16 ka with large wavelength scroll bar patterns that changed to much more sinuous meanders by 14 ka. We argue that braiding was induced by high sediment loads and channel bank instability related to generally drier climatic conditions that may have had a punctuated runoff season in the spring causing larger bankfull discharges than present. The results suggest that braided conditions may have been present throughout much of OIS 4, OIS 3, and OIS 2, but more data are needed to determine if channel pattern oscillations from braided to meandering occurred prior to the LGM. The data suggest that braiding is the most common channel pattern during late Quaternary time for the study area. Braided terraces appear to have been graded to lower sea-levels and are onlapped by Holocene floodplain deposits up to 10–60 km from the coast. Braiding of southeastern Coastal Plain rivers is significant, because it illustrates that paleoenvironmental changes in unglaciated catchments of the humid subtropical Southeast during the last glacial period were of sufficient magnitude to cause braiding. Such low latitude examples of braided-meandering transitions associated with

the Pleistocene–Holocene boundary are absent from the literature. A tangential result of our research is that reliable age’s estimates were obtained by OSL analysis of fluvial bed load sediments deposited by braided rivers of the Coastal Plain.

Acknowledgements An earlier version of this manuscript benefited from thoughtful comments provided by Michael Blum and . . Torbjorn Tornqvist. We thank The University of Georgia Vice President for Research and Dean of the Franklin College of Arts and Sciences for help in funding the operation of the Optically Stimulated Luminescence Laboratory in the Geography Department. The South Carolina Department of Natural Resources and the South Carolina Department of Transportation provided funding for some of the dating of samples from the Pee Dee valley, which was facilitated in part by Brockington and Associates, Inc.

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