A methodology is presented to quantify the mean flow field in a natural river with a boat-mounted acoustic Doppler current profiler (ADCP). Moving-vessel (MV) and fixed-vessel (FV) survey procedures are used in a complementary fashion to provide an improved representation of mean three-dimensional velocity profiles along a cross section. Mean velocity profiles determined with FV measurements are rotated to a stream-fitted orthogonal coordinate system. The orientation of the coordinate system is established using MV measurements. The methodology is demonstrated using measurements obtained at two study sites on the lower Roanoke River for the mean annual flow (228 m3 s−1) and a flow that produces bankfull conditions at the sites (565 m3 s−1). Results at a meander bend identify well-known flow features, including a main circulation cell, outer-bank circulation, and separation at the inner bank. This methodology also provides a framework for comparing time-averaged velocity profiles from FV measurements with spatially averaged profiles from MV measurements. Results indicate that MV measurements can provide a reasonable estimate of the streamwise velocity at many locations. The MV measurements obtained here, however, were not sufficient to resolve the spanwise velocity component.