Human and natural activities regulate organic matter transport in Chinese rivers

•Riverine COD content showed high values in the cropland-dominated areas.•Three seasonal variation types were identified for station-based riverine COD.•Variations in riverine COD were co-determined by human and natural factors. Rivers connect terrestrial and aquatic ecosystems and export approximately 55.47 % of the net terrestrial carbon fixation. However, due to unavailable high-frequency monitoring data, litter is known about diurnal variation in riverine carbon transport on a national scale. Based on daily measurements between March 2021 and February 2022 at 1491 stations across China, this study clarified the spatiotemporal variations in riverine organic matter indicated by chemical oxygen demand (COD). Spatially, COD content showed a spatial pattern with high values in the northwest (p