An evaluation system for the water environment of a river basin (Web-EGIS) has been developed based on a geographic information system. The environmental data were constructed on the Web-EGIS server, enabling us to show the spatial and seasonal variations of the data from client computers via the internet web system. The system also can simultaneously indicate land-utilization information map around the river basin together with the environmental data. This Web-EGIS was applied to the Ongagawa River basin located on the northern part of Kyushu Island, Japan. Spatial and seasonal variations of the environmental data originated from nature and from human activities could be clearly distinguished. The exact pollution point into river water could be specified by using the Web-EGIS system.