The flow in natural river channels is typically turbulent with complex velocity patterns. The spatial and temporal variations in velocity may have a significant impact on channel morphology and ecology. For example, the presence of secondary currents has been shown to be an important control on bank erosion in meander bends. The strength of secondary currents depends in part on both the channel morphology and the flow rate. Detailed measurements of velocity profiles can, therefore, provide useful information for a variety of river-related studies. Fixed-vessel measurements using an Acoustic Doppler Current Profiler (ADCP) acquire time series velocity data that allows the determination of mean and fluctuating profiles. Field techniques to obtain fixed-vessel ADCP measurements are discussed and the effect of boat motion and bed sediment movement on the measured profiles is assessed. The findings indicate that if care is taken to secure the boat within the channel, the effect of boat motion is small.