This paper presents a case study of a prototype B-type hydraulic jump produced downstream an irrigation-regulating gate in a natural river. The inflow Froude number is ${\mathrm{Fr}}_1=5.1$ and the Reynolds number is $\mathrm{Re}=9\times {10}^5$ . Detailed air–water flow properties and free-surface fluctuating features are investigated on site in the central flow region using intrusive phase-detection probes and nonintrusive acoustic displacement meters. Challenges in employing the phase-detection probe in high-momentum natural water are addressed, including the necessary postprocessing for uncertainty mitigation. While good agreement between the present data and previous laboratory measurements is achieved for the air concentration and interfacial velocity distributions, scale effects are observed for bubble frequency, challenging the validity of previous empirical prediction deriving from laboratory experiments. A greater percentage of large bubbles are detected in the full-scale jump. The free-surface fluctuating characteristics are investigated together with the surface wave propagation in the tailwater. The jump length and roller length in the B-jump are discussed with comparison to existing model equations. Although the investigation is limited to one case, it provides the first physical data obtained in full-scale natural water and an important benchmark data set for future study of scale effects in self-aerated open channel flows.