Nagara rivermouth barrage is located at a 5.4km upstream site from the rivermouth and its upstream reach is changed to a reservoir area. An analysis was carried out with the monitoring data to investigate the production process of phytoplankton during retention time in the just upper region of the barrage. There were two kinds in the production process of the phytoplankton. Pattern A was large self-multiplication of the phytoplankton and Pattern B was flowing down of phytoplankton masses with high concentration. It was made clear that Pattern A occurred with enough dissolved inorganic nutrients (DIN, PO_4-P) and a medium flow rate when water temperature and solar radiation were more than 22 degrees and 20 MJ/m², respectively.