This paper focuses on reservoirs in Sanuki (Kagawa Prefecture), Japan. Sanuki, which faces the Inland Sea and has low annual rainfall, has some 14,000 reservoirs in a small area of 1,877 square kilometres. Today, countless small reservoirs are neglected and are disappearing along with the rural areas in disrepair. Disused for agricultural purposes, reservoirs are also at risk from disasters caused by climate change. The traditional reservoir culture is disappearing, except to be preserved and maintained as cultural heritage. Huge man-made structures that could not have been achieved by ordinary human endeavour, such as Mannoike (Manno-Pond), which is said to have been created by Kukai, one of Japan's foremost Buddhist monks, have changed the landscape of Sanuki. It can be said that the motivation for building huge reservoirs was drought risk and flood risk, especially drought risk. However, it is also important to note that there were many more small reservoirs managed by individuals than could be imagined. The findings of palaeoclimatology, while rewriting the history of Japan, remind us of the roots of contemporary problems beyond the relationship between climate change and the historical event of the construction of giant reservoirs. In particular, more sophisticated reservoir development research based on a softclustering approach, focusing on the difference between labour politics and local economies as peasant economies and the construction of giant reservoirs as national and regional projects, could become an important subject for a viable commons in the future.