Historical erosion and sedimentation in two small watersheds of the southern Blue Ridge Mountains, North Carolina, USA

- Historical erosion rates were locally high in Blue Ridge drainage basins.
- Moderately disturbed mountain basins erode like heavily disturbed Piedmont basins.
- Sediment delivery is lower than published drainage area curves suggest.
- Historical floodplain alluvium contributes little to mountain sediment budgets.
- Sediment budget uncertainty is problematic for historical reconstructions.

Sediment bodies produced during historical periods of human land use, sometimes referred to as legacy sediment, may be found in various locations within drainage basins, and potentially remobilized by hydrogeomorphic processes accompanying land use change. The amounts and locations of stored legacy sediment can be significant factors in modern drainage basin function and should be accounted for when possible. In this study, late nineteenth-century erosion and sediment storage were investigated and used to construct approximate sediment budgets for two small Blue Ridge Mountain drainage basins in North Carolina (USA). Erosion was quantified using a distributed implementation of the Universal Soil Loss Equation (USLE), and calibrated on the separate bases of soil profile truncation data and recent published rates for long-term erosion in the region. Sediment yield information was reconstructed from pond sediments trapped behind a mill dam. Alluvial sediment storage was quantified using field studies of streambank and floodplain sediment profiles and digital elevation data. Colluvial storage was calculated as a residual in the sediment budget, and further evaluated using soil profile studies of footslope deposits, and soil survey maps. The proportions of erosion accounted for by the different budget terms in the most reliable budget are: 28% sediment yield (i.e., a sediment delivery ratio of 0.28), 69% colluvial storage, and 3% alluvial storage. Blue Ridge basins with low levels of ground disturbance erode like Piedmont basins at high levels of ground disturbance, primarily due to higher slope angles. Sediment delivery ratio is high relative to those given for much larger basins in the adjacent and more frequently studied Piedmont province, and generally in accordance with published sediment delivery curves that reflect higher hydrogeomorphic connectivities within smaller basins. Low values for alluvial storage in Blue Ridge basins may be explained by high sediment transport within steep channels. Colluvial storage values have high uncertainties because of the well known problem of error accumulation in residual budget terms. Field data on colluvial deposits reported here are not sufficient to correct for this problem, and in general, the acquisition of accurate field data on historical colluviation remains an important methodological issue in historical sediment budgeting.