Abstract

Recent burns in the western United States attest to the significant geomorphic impact of fire in mountainous landscapes, yet we lack the ability to predict and interpret fire-related erosion over millennial time scales. A diverse set of geomorphic processes is often invoked following fire; the magnitude of postfire erosional processes coupled with temporal variations in fire frequency dictate the extent to which fires affect sediment production and landscape evolution. In the Oregon Coast Range, several models for long-term rates of soil production and transport have been tested and calibrated, although treatment of fire-related processes has been limited. Following recent fires in the Oregon Coast Range, we observed extensive colluvial transport via dry ravel, localized bedrock emergence due to excess transport, and talus-like accumulation in adjacent low-order valleys. Soils exhibited extreme but discontinuous hydrophobicity, and no evidence for rilling or gullying was observed. Using a field-based data set for fire-induced dry ravel transport, we calibrated a physically based transport model that indicates that soil flux varies nonlinearly with gradient. The postfire critical gradient (1.03), which governs the slope at which flux increases rapidly, is lower than the previously estimated long-term value (1.27), reflecting the reduction of slope roughness from incineration of vegetation. By using a high-resolution topographic data set generated via airborne-laser swath mapping, we modeled the spatial pattern of postfire and long-term erosion rates. Postfire erosion rates exceed long-term rates (which average 0.1 mm·yr−1) by a factor of six, and subtle topographic variations generated local patches of rapid postfire erosion, commonly >1 mm·yr−1. Our simulations indicate that fire-related processes may account for ∼50% of temporally averaged sediment production on steep hillslopes. Our analysis provides a mechanistic explanation for the coincident early Holocene timing of increased fire frequency and regional aggradation in Oregon Coast Range drainage basins. Given the sensitivity of steep hillslopes to fire-driven transport, changes in climate and fire frequency may affect soil resources by perturbing the balance between soil transport and production.

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