Concentration and divergence of sediment in an erosional landscape

Abstract

In an eroding landscape, the erosional source area (Ae) may be larger or smaller than the depositional or storage area (Ad). This corresponds to areal concentration (Ae/Ad > 1) or divergence (Ae/Ad < 1) of sediment. We investigated this in an area of the Ouachita Mountains for three different time periods: before the early 1800s (pre-European settlement), early 1800s to 1990s, and post 1990s. Pre-1800, the forest was mainly undisturbed and soil loss was dominated by slow erosion and mass wasting from ridge tops. In the latter period establishment of all-terrain vehicle (ATV) trails created a small area of rapid, concentrated, persistent erosion. In the middle period, logging operations resulted in short-lived erosion hotspots scattered throughout the landscape. For the pre-1800s period, we estimated Ae/Ad based on the spatial distribution of alluvial, colluvial, and upland potential source area soils. Ae/Ad = 0.81, and <1 even when alluvial soils are not included. For the most recent era, field studies documented the eroding surface area of trails, as well as the area of near-trail sediment deposits and of deposition in smaller stream channels. Ae/Ad = 6.60, indicating concentration. In the intermediate era, conditions were more similar to the pre-European condition, as harvested areas and temporary unpaved roads recover quickly to pre-disturbance conditions and use of permanent roads was far lower before the ATV trails. The strongly dissected, steep topography, humid subtropical climate, limited potential for agriculture, and the nature of the ATV trail erosion all play roles in creating the sediment divergence in earlier eras and sediment concentration more recently. This suggests a need for more case studies to develop more general principles or guidelines to predict sediment concentration and divergence.