LATEST HOLOCENE CHANNEL AND FLOODPLAIN EVOLUTION FOR SMALL STREAMS IN THE SOUTHERN BLUE RIDGE MOUNTAINS

More than 50 small stream channels (drainage areas <400 km²) and floodplains were surveyed in the upper Little Tennessee River valley near Franklin, North Carolina. Optically stimulated luminescence, radiocarbon, and ¹³⁷Cs dating, along with allostratigraphy, establish a chronology for historical terrace formation and initiation of the modern floodplain in the smallest tributaries (<25 km²) at circa A.D. 1920. Terrace/floodplain initiation immediately followed a period of hillslope erosion and valley-bottom sedimentation related to widespread timber harvest. Historical terrace sedimentation occurred primarily by vertical accretion of overbank sediments, whereas bed material of prehistoric and historic channels remained at about the same elevation. Measurement of ¹³⁷Cs in historical terrace soil profiles indicates that aggradation of the historical terrace had essentially ceased by the 1950s, and probably earlier, because the ¹³⁷Cs is concentrated in the surface A horizon. Lateral channel migration and formation of the modern floodplain for <25 km² streams has occurred at a minimum average-long-term rate of about 0.3 to 5.0 cm/year, based on measurements of floodplain widths and an A.D. 1920 date for initiation of the floodplain. This rate of lateral erosion exhibits a strong positive correlation to drainage area and can be modeled by linear regression. Allostratigraphy and chronologies for the main stem of the upper Little Tennessee River (>100 km²) indicate a different pattern for this larger trunk stream. An historical terrace has not evolved along the main stem, where active accretion of overbank sediment still is occurring on the prevailing valley flat, similarly to the past 140 years. Both small tributaries and the main stem have had rapid historical vertical accretion rates on valley-flats of about 1-2 cm/yr, whereas prehistoric vertical accretion rates were about an order of magnitude lower. Such rapid rates continue along the main stem, whereas sedimentation on the historical terrace along small tributaries essentially has ceased since the modern floodplain formed. These observations indicate a spatially distributed pattern of historical channel evolution and overbank sedimentation, which probably has little relation to historical mill dam placement and removal.