This paper evaluated the long-term variations of the input loads of BOD and COD from Akashi river (128.4 km2) and sewerage to the sea using the public water quality observation data set and GIS population and land use analyses. We obtained the following results: 1) BOD and COD input loads from rivers from the late 1970s to 2000s have been reduced by 84% and 69%, respectively, 2) BOD input load from river basin including sewage-treated water has been reduced by 40% from the late 1970s to 2000s, while COD input loads are not reduced (6% increase), 2) BOD and COD input loads were provided 70% by the sewerage and did 30% by the river. These results show the sewage improvement makes the restriction of organic pollution in rivers though the population increased about three times since 1980s.
