The behavior of stable isotope ratios of attached matter, which consisted of algae and detritus in a reconstructed drainage canal, was analyzed for one year. Stable carbon isotope ratios in most investigation stations increased during the summer. This was attributed to the growth and photosynthetic activity of attached algae. Only one investigation station showed stable carbon isotope ratios that did not peak in the summer. This station was located in an ecologically conserved section with abundant riparian woods remaining. There was no summer peak due to shading by woods that prevented algae from sufficient photosynthesis. We observed that gray water raised stable nitrogen isotope ratios in water and attached matter of the drainage canal. A topographical peak of nitric acid concentration lagged behind the stable nitrogen isotope ratio. Leaching of chemical fertilizers from paddy fields increased the nitric acid concentration, while decreasing the stable nitrogen isotope ratios at the same time.