Some results of an experimental study of the three-dimensional structure of the turbulent flow within a lowland river are presented. The feasibility of the description of the streamflow turbulence structure with the use of modern models of free-stream turbulence was assessed. The application of turbulence models to the description of streamflow was demonstrated to require an updata of the model's parameters. A simple empirical relationship that allows for the effect of roughness on the river-depth distribution of turbulence intensity was proposed. Secondary currents of turbulent origin were studied and the feasibility of their simulation was assessed. The local isotropy of streamflow turbulence was numerically evaluated based on the analysis of spectral characteristics.