Using data from four field investigations between 2003 and 2009 along the Yellow River mainstream, we examined the transport features and seasonal variations of organic carbon, with a focus on contrasting the impacts of human activities with those of natural processes. Particulate organic carbon (POC) in the Yellow River originated mainly from the Loess Plateau, and thus the POC content in suspended sediments was much lower than in the world's other large rivers. Owing to both natural and human influences, dissolved organic carbon (DOC) has only a weak correlation with discharge. DOC varied as a result of human activities such as agricultural irrigation and pollution in the whole basin except for the upstream Qinghai–Tibetan Plateau. Our study also suggested that while reservoirs are a POC sink over short periods, a long-term POC storage flux cannot be easily estimated as discharge and sediment regulations have completely changed the relationship between the fluxes of water, sediments, and rainfall. However, this carbon sink can be obtained reliably through high-frequency sampling over long time periods. In addition, the annual water and sediment regulation (WSR) scheme has imposed an extremely severe human disturbance on the transport pattern of river organic carbon. Our study demonstrated for the first time that in a WSR event of less than 20 days, large proportions of the annual DOC (35%) and POC (56%) fluxes of the Yellow River were transported to the estuarine and coastal zone, potentially influencing estuarine and coastal geochemistry and ecosystems profoundly.

The Changjiang River supplies huge amounts of fresh water and dissolved and particulate substances to the East China Sea, thereby exerting a great influence on the coastal ecosystem. Meanwhile, the construction of the Three Gorges Reservoir (TGR) has reallocated the annual discharge, likely affecting the transportation of carbon in its various forms. The transport and transformation of carbon in Changjiang River and the effect of the TGR were discussed based on three field campaigns, a 1 year time series investigation, and historical data. Our results indicated the following: (1) Dissolved inorganic carbon (DIC) was derived from the upper stream and was significantly diluted downstream by the low‐DIC waters from two large lakes. Dissolved organic carbon (DOC) was a product of anthropogenic input and showed no clear relationship with discharge. particulate organic carbon (POC) within total suspended matter (POC%) was below the global average. (2) The TGR has not measurably affected the transport of DOC downstream of the reservoir dam. However, downstream grain size has decreased and autochthonous processes have increased, resulting in a sharp increase in POC% since reservoir construction. (3) For the period 1997–2010, estimated annual DIC flux was 16.9 Tg yr⁻¹. The regulation of river flow by the TGR has decreased the river DIC flux to the East China Sea in the autumn and increased it in the spring. Furthermore, the South‐North Water Diversion will reduce the high‐DIC water from the upper reach, thus affecting the biogeochemistry of the Changjiang estuary and the ecosystem of the nearby coastal ocean.