Growth and detachment of the attached-algae in river bed is one of the key factors in ecological system in the river. The detachment process of the algae can be categorized into three types, namely 1 Natural detachment due to aging, 2 Detachment due to flow and bed material load and 3 Detachment caused by rolling of the bed material. To evaluate the detachment processes during flood, we conducted a series of field survey in a site in the Johge River. In the period this survey had been conducted, a flushing discharge and a flood due to typhoon were observed. The chlorophyll-a densities on the bed material before and after floods were measured. Based on the field survey results, the detachment processes of algae are evaluated, and a new model to estimate an amount of detachment during the flood is proposed.
