In Japan, various reservoirs have been built since ancient times to relieve water shortages in agriculture and for irrigation. While many of these reservoirs continue to serve local communities, changing land use patterns and population decline have left many unmanaged, posing significant risks, especially during the rainy season. Highland reservoirs, in particular, can cause rapid downstream flooding and mudslides in the event of a collapse. Due to their age, relevant authorities often lack comprehensive hydrological data for these reservoirs. This study uses GIS to analyze elevation data of a highland reservoir to assess its hydrological conditions, contributing to disaster management and flood prevention efforts by providing critical insights into potential hazards. The results highlight the importance of such reservoirs in both flood risk mitigation and sustainable regional development.