[A-CG42] Water and sediment dynamics from land to coastal zones

[ACG42-P12] Historical diversion of the Tone River from the Tokyo Bay to the Pacific Ocean: A low flow analysis

*Josko Troselj¹, Naota Hanasaki¹ (1.National Institute for Evironmental Studies, Center for Climate Change Adaptation)

Keywords:paleohydrological modelling, H08 global hydrological model, Tone River diversion, Edo Period, low flows navigable paths, food and comodity supply

The Tone River was originally flowing towards the Tokyo Bay through nowadays the Edo River. However, during the Edo Period in 17^th century, the flow was diverted towards the Pacific Ocean. The main reason for conducting the river flow diversion was, according to the study of Koide (1975), to increase and stabilize the levels of low flows within the watershed. With the stable and sufficient low flows for navigation, transport of large amounts of goods from the eastern Kanto Plain towards inland areas and ultimately to the capital Edo became safer and economically more efficient. To our knowledge, the historical Tone River diversion have been not yet quantitatively assessed and discussed using a hydrological modelling framework. Here, we model the Tone River before and after the diversion, its lessons from past applications and implications for present and future climates. We applied the H08 global hydrological model with reconstructed historical flow direction maps at 60-arcseconds spatial and 1-day temporal resolutions forced by 92 AMeDAS stations from 2013 to 2022. We confirmed that flows were diverted towards the Pacific Ocean. Additionally, we found that navigability paths for low flows (twenty-Q₂₀ and thirty-Q₃₀ percentiles of annual river discharge) became smoother and conveniently connected between the Pacific Ocean and Tokyo Bay afterwards. Thus, the societal implication of the diversion had numerous advantages at the construction time. Three centuries after the diversion, inland transportation in the 20^th century has gradually lost its importance due to the development of sophisticated rail network in the region. Our simulation results support the claims of Koide (1975) that the diversion of Tone River expanded the Kanto Plain’s business and trade possibilities, which sustained historical food and commodity supply of the capital Edo.

Reference: Hiroshi Koide, 1975: Tone River and Yodo River - Historical Development in Eastern and Western Japan