Record of the Sedimentation and Environmental Change on Several River Drainages in the Holocene, the Central Part of North-east Japan

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The Sendai Coastal Lowland, which faces the Pacific Ocean, is 50 km long from north to south and 10-15 km wide from east to west. Several beach ridge ranges are distributed parallel to the coastline. Back marshes are distributed in the inner area of the beach ridge ranges and the level of these marshes is less than three meters above sea level. The Nanakita River, the Natori River and the Abukuma River flow through the lowland. There are natural levees along these rivers, as well as several Natural levee and Abandoned channel (NL-AC) systems," which run into the back marshes from the main river courses.The purpose of this study was to clarify the fluctuation of sediment load that were transported from the upper drainage area to the lower drainage area during the late Holocene. The developmental periods of NL-AC systems were determined and the volume of sediment load that constructing natural levees ware calculated. Fifteen NL-AC systems that run into the back marshes from the present channel were focused in this study.The result of stratigraphical survey and textural analysis, natural levee deposits, 2-3 meters in thickness and consisting of silt and sand, lie directly on the clayey back marsh deposits. The C-14 age of the top of the back marsh deposit indicates beginning of levee deposition. Abandoned channels, which contributed to the development of levees, can be clearly traced on the surface as narrow and long-shaped depressions. They are filled by humic clay deposit, 2-4 meters in thickmess. The C-14 age of the base of the humic clay deposit in abandoned channel indicates the age of the end of channel activity. The periods of channel activity of these NL-AC systems were confirmed to be two specific periods as follows : 2,500-2,400 yrBP and 1,600-1,500 yrBP. Moreover, the active periods correspond to the periods when the rise in sea-level fluctuation in the late Holocene. The volume of sediment that consists each NL-AC system was estimated 3-5×10^6m^3.