Geomorphic variation of a transitional river: Blue Ridge to Piedmont, South Carolina.

摘要

Field data was collected systematically to characterize the geomorphic variations in a river transition from the southern Blue Ridge to the Piedmont physiographic regions in South Carolina. Ten study reaches were surveyed for cross-sections and longitudinal profiles. Surface grid samples of bed material collected. Downstream hydraulic geometry and downstream fining of bed material were analyzed using traditional power functions and exponential decay relationships. Reach-scale channel bed morphology (bedforms) was analyzed under the assumption that the transition in bedforms is related to changes in hydraulic geometry and sediment characteristics. Well-developed downstream trends of hydraulic geometry variables (width, depth and velocity) and bed material fining were observed. However, variations within the general trends reflect the influence of a key transition zone characterized by substantial tributary inputs, drastic decreases in slope, and the presence of erosion-resistant bedrock knickpoints. Bedforms were distinguished using a regime diagram, an approach that utilizes hydraulic and sediment data that is independent of drainage area. Plots of relative grain submergence (R/D84), relative form submergence (R/H), Darcy-Weisbach friction factor (f) and slope vs. area reveal trends in the data that are not discernible with downstream models. Hydraulic, sediment, and bedform data suggest that structurally controlled breaks in slope are influential to bedform characteristics, resulting in forced morphologies that may not be distinguished using simple downstream models. This corroborates the utility of scale-independent methods, especially in mountain or transitional environments where fluvial controls may be longitudinally forced and sporadic. These results have implications for river management and restoration approaches in such environments, as scale-independent models and landscape-scale perspectives can be beneficial to management objectives.