Late Quaternary Geomorphic Development of Alluvial Fan-Source Basin Systems: The Yamagata Region, Japan

Abstract

Although alluvial-fan development reflects the fluctuation in the amount of sediments supplied from its source basin, previous alluvial-fan studies tended to pay insufficient attention to the erosional history of the hillslopes in the source basin. This paper discusses the Late Quaternary geomorphic development of three alluvial fan/source basin systems in the Yamagata region of northeastern Japan, with special reference to changes in hillslope processes and their influence on the sediment supply and fan development. All fluvial surfaces and hillslopes in the fan/basin systems were classified based on chronological and morphological criteria. Around the Last Glacial Maximum, periglacial processes prevailed on mountain slopes to form smooth and rounded slopes. After the onset of a warm and humid climate during the Pleni Glacial-Late Glacial transition, the magnitude of periglacial processes declined and the smooth slopes were incised by shallow slope failure and gullying. Although this change in slope processes was common to all the three basins, fan aggradation occurred in two basins but degradation in one basin. Such different responses of the river basins to the Late Quaternary climatic change are concordant with those reported from other two regions in Japan. Morphometric measurements performed on the slope classification maps have revealed that the areal ratio of the incised parts to all mountain slopes corresponds to the mode of fan development since the Late Glacial. This finding provides a three-stage model of the development of the fan/basin systems, showing that the progress of the slope incision led to changes in the amounts of the sediment supply and the size of the active aggradational fan. © 1994, The Association of Japanese Geographers. All rights reserved.