We estimated the contribution of reservoir-derived plankton released from an upstream dam to particulate organic matter (POM) relative to terrestrial allochthonous (fallen leaves) and instream autochthonous (downstream epilithic algae) sources in the Uji River, Japan, to investigate the influence of the reservoir plankton on downstream POM composition. Four types of POM such as suspended fine, suspended coarse, benthic fine and benthic coarse POM were collected and then analyzed using two types of mixing models combining δ13C–δ15N: a standard linear model (SLM) and a concentration-weighted model (CWM), which are compared with the microscopic examination. Results demonstrate that the three trophic sources were isotopically distinct, and all POM samples are plotted inside the mixing triangle defined by the three end members in the δ13C–δ15N biplot. SLM underestimated the terrestrial source contribution and overestimated that of the reservoir plankton, suggesting that despite popular application of the SLM method, CWM was more appropriate for the source partitioning of riverine POM particularly in the case where large differences in source concentrations of C and N are present. Reservoir plankton contribution was highest in suspended fine POM (S-FPOM), accounting for 47%. Nitrogen in S-FPOM was found to be supplied mostly by the reservoir plankton (68%). These results collectively suggest that reservoir plankton from dams can greatly influence downstream S-FPOM composition and play a role in supplying nitrogen to tailwater ecosystems. Our findings on the riverine POM source partitioning in a tailwater channel should be useful in assessing downstream heterotrophic food webs and nutrient transport in response to the plankton-derived POM originating from upstream reservoirs. Copyright © 2013 John Wiley & Sons, Ltd.