The measurements and primary analysis of turbulent flow across a compound channel (River Severn, England) are presented. The velocity was measured using a three-dimensional acoustic Doppler velocimeter in combination with a directional current meter. The statistical flow structure is examined against existing analytical formulations derived for single channel flows based on laboratory studies. The existence of a vertically two-layer structure around the interface between the main channel and the floodplain is demonstrated, indicating (1) a vertical shear-dominated flow zone near the bed; and (2) away from the bed a transverse shear-dominated flow zone with enhanced turbulent mixing. The temporal spectra clearly reveal the occurrence of anisotropic turbulence both in the main channel and over the floodplain. The present findings necessitate the resolution of both transverse and vertical structures for advanced modeling of compound channel flows. The measured data can be used to assess the performance of mathematical river models.