Distinctive Lateral Erosion along Rivers as Part of Strath-Terrace Formation during an Unprecedented Rainfall Event in Catchments Rich in Deposits Accumulated under the Past Cold Climate in Hokkaido, Japan

Detecting impacts of contemporary climate change on geomorphic processes and landforms may require some strategies that take account of the infrequent event-based nature of many geomorphic processes, and that consider varying magnitude of the impacts of single events. Practical strategies to overcome these difficulties could include selection of study areas where landform sensitivity to climate change is high, and incorporation of analysis of the time-dependent evolution of landforms that enables the characteristics of contemporary processes to be placed in historical context. Typhoon1610 brought heavy rainfall to Hokkaido, the most northerly main island of Japan, in late August 2016 after three typhoon-induced rainfall events in the same month. Debris flows and river floods occurred as a result of the unprecedented rainfall, causing distinctive lateral erosion along rivers in the northern Hidaka Range and the western Tokachi Plain. Hillslopes of the northern Hidaka Range are extensively mantled by poorly sorted soils/sediments with angular gravels, that are thought to have accumulated in the last glacial period and the early stage of the post-glacial period. Alluvial fans and fluvial fill terraces also formed in the cold periods in the western Tokachi Plain. These hillslope and river deposits were substantially eroded during the passage of the typhoon and supplied debris flows and meandering flood flows. Previous studies using laboratory experiments and model calculations have shown that lateral erosion of a river takes place in a flood when sediment supply is large, and that the lateral erosion is the essential process for strath-terrace formation. The lateral erosion of the rivers during the typhoon event is therefore the process for strath-terrace formation in the studied catchments where there is a large supply of deposits accumulated in the past cold periods. Our chronological data indicate that the laterally-eroded river terraces developed in the Little Ice Age, and the fluvial process induced by the rainfall of Typhoon1610 should have had the most significant impact along the rivers in the study area since then. We conclude that the event and the consequent landform change is a fluvial geomorphic response due to contemporary climate change in an area where landform sensitivity to climate change is high.