Denitrification, plant uptake and sedimentation are important mechanisms of nutrient retention in floodplains. Due to nutrient retention, nitrogen (N) and phosphorus (P) loads of the river decrease, which is a primary aim in European water quality management. However, the relative contribution of the various retention mechanisms to nutrient retention is largely unknown, as is the total retention that may be achieved. The aim of this study was the estimation of nutrient retention by the mechanisms of plant uptake, silt sedimentation and denitrification for the Dutch Rhine distributaries over one hydrological year (April 2000–March 2001). N and P retention in these floodplain stretches were estimated by means of measured process rates in the floodplains and one-dimensional hydrodynamic and sedimentation models.Estimated total retention of N (all mechanisms together) is less than 3% of the annual load, which can be considered as insignificant. The total retention of P was 5 to 18% of the annual P load, which is significant. Floodplain sedimentation was the most important retention mechanism for both N and P. As excavation of the floodplain is expected to increase annual sedimentation rates, floodplain lowering as carried out in river restoration projects, may lead to a considerable increase in retention of P, but will hardly affect retention of N, because N was mainly present as dissolved nitrate in the river water. Copyright © 2004 John Wiley & Sons, Ltd.