The bed of an alluvial river is highly susceptible to changes during the course of its existence. Besides variations of the large scale topography and plan form of the river, smaller scale dune forms can be observed. These recurring dune forms migrate on top of the large scale topography and can yield local yet important variations in the flow field. In order to study the effect of migrating dune forms on the flow characteristics and consequently the erosive capacity of an alluvial river, an experiment with mobile bed has been carried out in a laboratory flume representing a sharp meander bend. In this experiment, changes to an initially flat, slightly sloped river bed under a steady flow and sediment discharge were observed until a recurring pattern of migrating dune forms could be seen on top of the characteristic pool-bar topography of meander bends. Once the dune forms were established, an Acoustic Doppler Velocity Profiler (ADVP) was placed in several positions alongside the river bend and used to measure the flow depth and flow characteristics under the influence of the passing dunes. Several times during the experiment, the topography was mapped using laser altimetry on a grid of large spatial resolution in order to isolate the dune forms from the large scale topography and determine the dune characteristics and the dune celerity. In this paper the large scale topography and dune characteristics will be shown and the effect of the migrating dune forms on the flow field and the erosive capacity will be discussed in detail.