Abstract: As a crucial terrestrial source, the source, transport, and flux of riverine particulate organic carbon (POC) to the ocean are currently of significant interest. However, human activities, such as the construction of reservoirs, are changing the composition of river POC, potentially impacting the source-sink processes between land and sea, as well as biogeochemical cycles. To address this issue, the Liaohe River System was selected for this study, in which 14 samples were collected in July 2023, and the trends in POC content and sources within the drainage basin were analyzed using biogeochemical methods and gene detection technology. The potential mechanisms by which riverine primary production (Rpp) has become a dominant POC source in the Liaohe River System was explored and those in other typical Chinese rivers were compared. Results indicate that the Rpp was the predominant source of POC in the Liaohe River System, and Trebouxiophyceae and Cyanobacteria were the primary contributors. Additionally, animals may also play a significant role as POC sources in rivers, warranting further attention in future analyses. The retention effect in reservoirs could alter the composition of river plankton and thus significantly affect the POC sources. Moreover, the proportions of plankton-derived POC in the Changjiang River, Huanghe River, and Zhujiang River in the continent, and rivers in Taiwan and Hainan islands have also seen notable increases. These changing trends could lead to substantial shifts in the patterns of POC sources and sinks across watersheds, estuaries, and continental shelves, meriting considerable attention.