Restoration of the Heights of the Riverbed and Banks of Kusatsu River in Southeastern Shiga Prefecture: 100 Years of Changes Based on Village Maps in the Early Meiji Era

Abstract

In this study, we restored the heights of the riverbed and banks of the Kusatsu River in the southeastern part of Shiga Prefecture, well-known as a typical raised-bed river, in the early Meiji period according to numerical information of the river in the “Kurita-Gun Kaku-son Ezu (Handmade local map of each village in Kurita County)” prepared in 1874.　Next, by comparing the heights from information of the maps with those from a leveling survey conducted in the late 1970s, we examined the changes of the heights during ～100 years since the early Meiji era.　As a result, the following points were clarified. In the early Meiji period, the river was a raised-bed river with the surrounding lowlands higher than the riverbed in the section 2–12 km from the river mouth.　However, downstream from the confluence with the Minogo River, its tributary (8.2 km point from the river mouth), the difference in height between the lowlands and the riverbed reached 4–6 m.　Upstream from the confluence, the difference in height between the lowlands and the riverbed was only 2–3 m. In the Meiji period (1868–1912), the reconstruction of banks upstream caused the riverbed to rise because of sediment accumulation, resulting in the creation of the raised-bed river with a marked difference in elevation between the low land and the riverbed in upstream from the confluence.　On the other hand, downstream from the confluence, the riverbed was slightly lowered in 2–4.5 km section from the river mouth, but no major changes were observed.　It was difficult to dig the riverbed of downstream to mitigate the disaster because of three under-the-riverbed tunnels made after the early Meiji era.　In addition, the left bank of the river, which had been lower than the right in the early Meiji era, was raised to prevent the bank collapsing. This study revealed that the timing of the development of a raised-bed river (riverbed rise) varies from place to place even in a single river, and that a riverbed rise occurred even in the modern era.