We investigated natural disasters such as floods and sediment movements in the Nakatsugawa watershed over a period of 406 years, using literatures including historical documents. We found that natural disasters have occurred irregularly, and classified the 406�]years period into three categories, High Frequency (HF), Middle Frequency (MF), and Low Frequency (LF), based on disaster frequency. The HF category is divided into three periods, (A. C. 1687�]1705, A. C. 1789�]1794, and A. C. 1842�]1870), MF into two periods (A. C. 1795�]1841 and A. C. 1928�]2005), and LF into three periods (A. C. 1600�]1686, A. C. 1706�]1788, and A. C. 1871�]1927). Historical activities using the riverfront or using its water resource were recorded mainly during LF, indicating that LF periods would have been considered safe because few disasters occurred. We compared time series of the categories in the watershed with other areas and found that the time series were not dominated by external events(affecting areas outside the watershed)such as climate changes, or large earthquakes. Watershed�]specific events leading to classification of periods as HF., MF,. or LF depended on the existence of unstable remanent sediment in the watershed. We verified these findings with historical documents and photographs.

Key words�FLong term dynamics, Sediment disaster, Historical manuscripts;Kiso River, Mt. Ena