Abstract. This paper reported a dramatic channel incision (>10 m in the deepest cut) during the past 10 y or so in the lower Pearl River, the second largest river in terms of water discharge in China. The channel incision had caused changes both in the channel geometry as well as in the river hydraulics. Also, the water exchange between the two major tributaries of the Pearl River, the Xijiang and Beijiang, had been significantly changed due to the channel incision. The rapid channel incision was principally the result of extensive sand mining in the lower Pearl River and the delta region due to the booming economy in the Pearl Delta region. Slight increase of water discharge and significant decrease of sediment load since the early 1990s in both the Xijiang and Beijiang also likely contributed to the observed dramatic river bed downcutting to some extent. This has important implications for river management, as the large Chinese rivers have seen a dramatic depletion of sediment fluxes due to the combined effects of declining rainfall, dam constructions, water diversion, reforestation and afforestation, and sediment mining over the recent decades.

The sources and dynamics of riverine carbon have been discussed extensively, but knowledge about the transformation from DIC into organic carbon (OC) is still poorly understood. In this study, we conducted a comprehensive investigation on the riverine carbon, stable carbon isotopic components and C/N ratios for different seasons, including an extreme flood event, in the lower Xijiang and its three tributaries. Detailed analyses are also performed for soil samples across the study region. Downstream increase in δ¹³C_DIC and downstream decrease in both δ¹³C_POC and C/N have been observed for all the tributaries. Meanwhile, positive shift of δ¹³C_DIC and negative shift of δ¹³C_POC are also observed from summer to winter. These observations likely indicate that the isotopic compositions of both DIC and POC are significantly affected by in-river primary production that converts DIC into organic matter through photosynthesis. It is estimated that the percentage contribution of the riverine aquatic primary production to the riverine POC in the Xijiang and three tributaries of Guijiang, Hejiang and Luoding is respectively 7.1%, 43.2%, 36.4% and 9.9% in rainy season, and 35.6%, 47.3%, 50.3% and 40.1% in dry season. Based on the stoichiometry involved in chemical weathering of the bedrocks, the transformation of the carbonate-sourced DIC to POC is further quantified to be 3.4–20.5% in rainy season, and 12.3–22.1% in dry season. This may suggest an important sink of atmospheric CO₂ in river systems that was largely ignored previously