Measurements and analysis of the three-dimensional turbulence structure in a straight lowland river reach are presented. Accurate measurements of velocity profiles were taken with an acoustic Doppler velocimeter (ADV) and a multichannel micropropeller-based system. It is shown that analytical expressions derived from investigations of laboratory open-channel flows agree well with the measured data only in the central part of the river where flow can be considered weakly three-dimensional. At the same time, clear differences of empirical parameters between river and laboratory flows are detected. It was established that river turbulence is isotropic for spatial scales smaller than the river depth. The data also allowed the detection and analysis of the turbulence anisotropy impact on the formation of secondary currents. Special attention is paid to the investigation of coherent structures. Their spatial scales were evaluated applying the conditional-average procedure based on uw quadrant analysis. On average the scales of ejection and sweep events are as large as 1.5 times the flow depth. ¿ 1998 American Geophysical Union