Hachirou-gata is the secondary largest lake in Japan following Lake Biwa. In 1957, the "National Hachirou-Gata Reclamation Project" was started, and the entire project was completed in 1967. Out of 22, 024 ha in Hachirou-gata, reclamation land totalled 17,239 ha. The drainage inside the central area is pumped out by a central canal and north and south pumping stations constructed at both ends of the canal. The phosphate concentration of the drainage of the south pumping station was 0.47mg L^<-1>, and was more than 3 times higher than that of the north pumping station. We investgated the cause of this difference. The results obtained were as follows. We measured the phosphate concentrations of the drainage of 40 first class branch drainage canals and the highest concentration was 1.62 mg L^<-1> at Right 3 Station (LD-G1). This branch drainage canal flowed into the main drainage canal beside the south bridge; for that reason the phosphate concentration of the drainage of the south pumping station was higher than that of the north pumping station. Then, we investigated the water in the small canals which were around branch drainage canal LD-G1, and many of them showed higher concentrations of phosphate. The water which flowed in these small canals originated from the bottom of these canals.
