GIS analysis of depositional slope change at alluvial-fan toes in Japan and the American Southwest

Slope change at alluvial-fan toes provides a clue to understanding differences between alluvial fans and other depositional landforms. This paper analyses topographic data for 430 alluvial fans and their adjacent lowlands in Japan and the American Southwest (ASW), typical humid and arid regions with abundant alluvial fans. Arcs showing fan toes were identified and representative areas above and below each arc were then delineated based on a GIS buffer analysis. The mean slope of the area above (SA) and below the fan toe (SB), and the ratio of SA to SB (RS) were computed. Overall fan slope (SO), total fan length (L) and angle of the fan apex (AF) were also measured. Relationships among these morphometric parameters reflect differences in climate and sedimentary processes. Fans in the ASW tend to have straight profiles with similar values for SA and SO, indicating that fan gradients from deposition resulted from low-frequency/high-magnitude storms have been preserved well under an arid climate. Higher RS values result from good preservation of both steep fan surfaces and gentle adjacent lowlands, which is also ascribable to infrequent sediment redistribution. In Japan, SA tends to be smaller than SO, and RS is often close to unity reflecting frequent sediment redistribution on fans and across fan toes under a humid climate. RS in both the regions tends to converge into unity with increasing L. Longer alluvial fans are more strongly dominated by fluvial processes than by mass-flows, especially at their toes. Longer fans also tend to have small AF, leading to fluvial processes similar to those on floodplains. Sediment re-mobilisation at the toes of such fans is more frequent, resulting in smaller differences between SA and SB.