Previous studies on sorption processes in running waters lack a quantification of phosphorus sorption in relation to phosphorus mass balances of entire river sections. The present study was designed to investigate the influence of sorption processes on the phosphorus mass balance in a medium-sized lowland river. Riverbed surface sediments were sampled in the lower River Spree in February and in July 2002, and used for sorption shaking experiments. Phosphorus analyses results were evaluated using Langmuir-isotherms. Organic sediments revealed sorption properties that were affected by incubation temperature in a manner opposite to those of clastic sediments. Particulate iron is assumed to be the dominant sorption site of phosphorus in the lower River Spree. Clastic sediments act as phosphorus sinks, especially in summer, when soluble reactive phosphorus (SRP) concentrations in the water body are relatively high. Organic river substrates were found to be a source rather than a sink of phosphorus for the water body, but even in the worst case of a storm event, including complete resuspension of an organic surface layer, the contribution of desorption to the riverine phosphorus load of a eutrophic river was negligible (at maximum 2.5% of SRP load during vegetation period).