The distribution and biomass of Peridinium bipes f. occultatum were observed in autumn 1991, 1992 and 1994 at the head of the Shorenji Dam reservoir (Mie prefecture, Japan). Distributions show that Peridinium vertically migrated, and dispersed at the plunging point and horizontally moved to upper and lower stream in the study area. From these observations and informations in references, we supposed that the spatial biomass of Peridinium would be changed by “vertical migration” and “entrainment” ; horizontal “import” and “export” ; “outflow” ; and “growth” (including several biological processes). In the study area, biomass of Peridinium increased in rate similarly to growth day by day, and periodically varied about 2/3 of maximum biomass in a day. We estimated transport rates (cells·s^-1) by vertical profiles of the current velocity and the population density, and could explain this diurnal change of biomass. Thus, we concluded that the horizontal import and export will mainly change red tide's biomass at the head of the reservoir, and with the growth control, the flow control in the reservoir will be also important to suppress the fresh water red tide.