The bed sediment and particulate organic matter (POM) samples collected from Ishikari river and its tributaries were analyzed for organic C, total N and their isotopic composition. The δ¹³C of POM in Ishikari river ranged from −34.6 to −24.6‰, suggesting that contributions from the C₄-based vascular plant organic matter was negligible and detritus from C₃ plants and C₃ plant dominated soil and fresh water phytoplankton contributes the POM load in Ishikari river system. The δ¹⁵N isotopic ratio of POM ranged from −4.8 to +15.8‰, with an average of 4.5±4.3‰ in the Ishikari river system. The δ¹⁵N ratio of sediment samples range from −0.92 to +8.71‰, with an average value of 3.8±2.5‰ in the Ishikari river system. The δ¹⁵N isotopic ratio of POM samples taken from Barato lake – an Oxbow lake in Ishikari river – shows strong inverse relation with the dissolved nitrate concentration in the lake water indicating that significant portion of the POM being derived from phytoplankton that were utilizing nitrate that had been subjected to denitrification. The molar C/N ratio of POM sample ranged from 5.3 to 17.8, with an average of 9.8±2.8 in the Ishikari river system. The correlation plots between molar C/N ratio and N isotopic ratio indicates that most of the POM samples fall between mixing zone of aquatic source (phytoplankton) and terrestrial source (soil organic matter and plant detritus) in Ishikari river system.