Aspect‐induced differences in hillslope processes

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Abstract
The type and relative rate of geomorphic process activity on residual hillslopes in the White River Badlands of South Dakota were found to vary as a function of aspect‐induced differences in topoclimate. Drainage densities on north‐facing slopes several orders of magnitude greater than those on slopes facing south indicate that north‐facing slopes experience greater fluvial erosion. Comparative analysis of mass movement features suggests that north‐facing slopes are prone to relatively high‐magnitude, low‐frequency failures, whereas south‐facing slopes experience far more frequent but much smaller movements.Variations in process activity can be explained in large part by aspect‐induced differences in moisture regime. Field monitoring of moisture conditions indicates that north‐facing slopes maintain higher and less variable moisture levels. South‐facing slopes are not only drier in‐ general but are subject to more frequent and intense episodes of wetting and drying. Differences in moisture conditions promote differences in weathering activity as evidenced by thicknesses of slope regoliths. Higher moisture levels on north‐facing slopes lead to more rapid dissolution of binding cements and consequently, thicker regolith covers. On south‐facing slopes, however, regolith covers are virtually non‐existent due to both drier conditions and episodic desiccation, which triggers small rockfalls. Greater fluvial erosion on north‐facing slopes is a consequence of higher antecedent moisture levels, which retard infiltration. Further, the unconsolidated regolith cover is more erodible than the relatively unweathered materials exposed on the surfaces of south‐facing slopes. Resistance to slumps and mudflows is lower on north‐facing slopes because the weathered regolith is less cohesive than the unweathered parent material and also because the regoliths are periodically saturated.