Increase in stream temperature related to anthropogenic heat input from urban wastewater

To better understand long-term temperature changes in urban streams, we investigated stream temperatures in the central Tokyo area and its suburbs from 1978 through 1998. Stream temperature data were analyzed together with data on thermal effluents of urban wastewater and air temperature for the same period. Statistical analyses indicated that the stream temperature in winter and early spring increased at a rate of 0.11–0.21 °C/year in segments that had a considerable increase in wastewater heat input over the same period. These segments showed an appreciable change in the relationship between air temperature and stream temperature, which suggests that the increase in anthropogenic heat input from wastewater was the main cause of the long-term increase in stream temperature. Other possible factors such as increasing air temperature and heat exchange with seawater were found to have comparatively minor influences.