The recent research on the widespread presence of microplastics in the Ottawa River has underscored the potential environmental implications for this vital aquatic ecosystem. In this study, we conducted a numerical analysis of microplastic transport in a 400-km stretch of the Ottawa River, spanning from Chats Falls to Carillon generating station. We developed a three-dimensional hydrodynamic model of the domain using the TELEMAC modeling system. The model was validated through a comparison of the measured and modeled water level data. The results of the hydrodynamic model were then employed in a three-dimensional Particle Tracking Model (CaMPSim-3D), developed by the National Research Council Canada, to predict the fate and transport of microplastics in aquatic environments. This model was utilized to identify accumulation zones for two different groups of microplastics with densities greater and lesser than that of water, aiming to explore the influence of particle density on their distribution within the simulated domain. To evaluate the performance of the model, the results of the numerical simulation were qualitatively compared with previously collected field data on microplastics in the domain. This study provides valuable insights into the accumulation zones of various types of microplastics in the Ottawa River, offering guidance for decision-making processes and identifying hotspots for further investigation, cleanup initiatives, and future remediation activities.