Benthic algal communities developed on riverbeds lose their biomass gradually under normal stream conditions or suddenly in floods by the shear force of the current (Allan 1995, Biggs Close 1989). Although this tearing-off process is important from the aspect of organic load to the down-reach of the river, few studies have been conducted to assess the factors affecting the process (Biggs Thomsen 1995, Horner et al. 1990). External factors, such as the current velocity and texture of the substratum, and innate factors, such as the algal species composition and degree of biofilm development, should contribute to the tearing-off processes. The objective of this study was to clarify the factors concerning the tearing-off processes of algal communities under light run-off conditions without the movement of a riverbed substratum. Observations were performed at the middle reach of the Tama River (length ca. 100 km; drainage area ca. 1200 km~2), which runs through southwest Tokyo, Japan. The water at the study site was transparent, and the concentration of nutrients was low. Field experiments were performed to examine the influence of the current velocity, the texture of the substrate surface, and the degree of algal development on the tearing-off process in which the cobbles with algal communities on the riverbed were transferred to sites with faster current.
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