Radioactive tracer techniques may be useful for assessing water transport and the overall effects of concurrent biogeochemical processes in river?reservoir systems. In this study, we show that radium isotopes can assess the hydrodynamics and sediment/nutrient retention in the Xiaolangdi Reservoir, the largest impoundment along the Yellow River, China. Activity ratios of 224Ra/226Ra and 223Ra/226Ra were used for water mass age calculations in the riverine, transition, and lentic reaches of the reservoir. Water ages were combined with the length scale of three river?reservoir zones to determine water transport rates of 3.6 ± 1.2, 1.3 ± 0.3, and 0.16 ± 0.14 km/day, respectively. Radium ages were also used to quantify the net retention of sediment and nutrients in different parts of the river?reservoir system. Suspended sediment was removed at a rate of 1.4 ± 0.6 g/m3/day, mainly in the riverine zone. Nutrient dynamics were more complicated, with addition or removal at different rates within the three zones.