Abstract Water and soil erosion are serious issues in the Loess Plateau in China. The carbon and nitrogen cycles along river catchments during the soil erosion process have historically been poorly understood. In this study, we investigated the concentrations and isotopic signatures of carbon and nitrogen in river water, river-suspended materials and soil. Our results revealed that, in river catchments, from the water sources to their confluence with larger rivers, (1) the isotopic signatures of dissolved inorganic carbon (DIC) varied from −9.2 to −6.7‰ in May and from −12.9 to −9.3‰ in August. In addition, δ13CDIC values were more negative in August than in May. These differences may be related to changes in the source and soil erosion. (2) The NO3− nitrogen isotopic signature of the water ranged from 5.1 to 10.3‰ in May and from 3.5 to 6.4‰ in August. The δ15Norg of the suspended material also became more positive; similar trends can help us to understand the N cycle during soil erosion. Our investigation suggests that soil CO2/carbonates and soil NO3− are dissolved into the Loess Plateau rivers during rainfall and soil erosion. The concentration and the isotopic signatures of carbon and nitrogen in rivers can be used for the quantitative evaluation of carbon and nitrogen transfer from the soil to the Loess Plateau rivers.