In Sagae reservoir, the water level change drastically more than 50m throughout the year. Because of lower water level from summer to fall, temporary ecotone appears in upper reaches of reservoir and provides habitats of a variety of plants and terrestrial invertebrates every year. We hypothesized that these organisms generated in ecotone may contribute to aquatic ecosystem of reservoir as sediment organic matter after they are submerged and decomposed. To verify this hypothesis, we analyzed food web in the upper reaches of Sagae reservoir using carbon and nitrogen stable isotope analysis. In the ecosystem of this reservoir, producers as energy source were sediment, particulate organic matter (POM) and autochthonous biofilm, primary consumers were benthic animals and fish larvae, and secondary and tertiary consumers were adult fishes throughout the year. The mixing model using isotope ratios showed that sediment were originated from Scirpus asiaticus grew in the ecotone and flow-downed soil from upper stream. Our study suggested that the aquatic ecosystem in Sagae reservoir was supported by not only autochthonous production but also allochthonous matter made from terrestrial plants in ecotone.