Evolution and controlling factors of natural levees during the past 4500 years derived from lowland archaeological ruins in central Kanto Plain, Japan

Abstract

Lowland archaeological ruins are frequently used in conjunction with sediment cores to reconstruct geomorphological evolution on alluvial plains. Lowland ruins are, as a rule, heterogeneously distributed across alluvial plains and retain short chronological records when compared to sediment cores. Along the historic banks of the Tone River, in the central Kanto Plain, thorough surveying prior to major construction projects in the Tokyo metropolitan area uncovered a dense swath of lowland archaeological ruins. This study demonstrates the evolution of natural levees during the past 4.5 kyr by compiling data from uniformly distributed lowland archaeological ruins in the central Kanto Plain. The location and age of 774 lowland archaeological ruins and 101 radiocarbon dates from surficial fluvial sediments in 42 sediment cores were compiled and arranged chronologically. As a result, levees and river terraces aged < 4.5 ka were found to be contiguous with modern sedimentology in the Arakawa Lowland, while relatively young natural levees aged < 2.8 ka are contiguous with the present Nakagwa Lowland. The Tone River migrated from the Arakawa Lowland to the Nakagawa Lowland at ∼5 ka. Subsequently, sea levels lowered between 4 ka and 3 ka. Due to sea level lowering, a minor tributary of the Tone River partially eroded the alluvial plain, and relatively old geomorphology persisted in the Arakawa Lowland. In contrast, in the Nakagawa Lowland, the major tributary of the Tone River eroded the entire alluvial plain. Following sea-level rises up to 2 ka, natural levees aggraded and formed new surface sediments in the Nakagawa Lowland due to large sediment discharge from the major tributary of the Tone River. Natural levee evolution in the central Kanto Plain is influenced by both river migration and sea-level fluctuation.