On 5–6 July 2017 (J17), an unusual series of rainfalls induced a concentration precipitations in Northern Kyushu, Japan, reaching 516 mm 24 h Asakura City, first its history since the beginning observation 1976. This triggered unprecedented hydro-meteorological hazards (landslides, debris flows, and floods) forested mountainous areas, such as Akatani watershed, where estimated discharge reached 520 m3/s at outflow. It numerous deaths, structural damages, destruction river channels, deposition sediment flood plains. If smaller-scale have usually driven authorities to build protection systems J17 crisis called for full remodeling it, interrupting waterways reshaping slope shapes structures. Considering means deployed by central government this reconstruction, event “re-construction” rivers, modifying hydrosystem’s functioning watershed scale. Thus, our objective is show that after relative stability watershed’s hydrosystem over 75 y, exceptionality forced us rethink organization, using reference post-disaster reconstruction. The research relies on field surveys 2019 2022, interviews officials, GIS analysis based historical aerial photograph interpretation geospatial data. revealed (a) geometry channels was completely redesigned match new (b) control structures were multiplied restructure slopes, breaking angles into subwatersheds. case then illustrates range measures developed Japanese engineering reduce hydrological risks.