Shifting of the Arakawa River in the Kanto Plain, Central Japan, during the Late Holocene: A Geomorphological Approach

抄録

A geomorphological survey map of the Arakawa Lowland (the lower Arakawa River basin in the Kanto Plain, central Japan) documents several groups of paleomeanders suggestive of channel shifts of the Arakawa River during historical times (last ca. 2,000 years). However, the sequence and timing of these changes of the river are unclear. I examined geomorphological features of paleomeanders and lithological analyses of natural levee deposits, along with historical records on channel modifications, to summarize the current state of knowledge. I differentiate four groups of paleomeanders with different meander wavelengths along the present Arakawa River. The set with the smallest wavelengths is on the Moto-Arakawa River, considered to be a distributary prior to river modifications starting in the 17th century. The second smallest is of late 19th century derived from earlier topographic maps. The set with middle-sized wavelengths can be correlated with the main trunk in the Middle Ages, and the set with the largest wavelengths is suggested to be from an ancient large river that combined the flows from the Tone and Arakawa Rivers. Lithological analyses of deposits along the large paleomeander indicate sediment input from the Tone River watershed. More historical and geomorphological evidence and dating results are required to confirm these arguments.