Real-time topographic data are useful for river bed evolution analysis and engineering assessment. In this article, a real-time online monitoring system for riverbed terrain is developed based on ultrasonic technology, 4G
wireless transmission and network database. An ultrasonic sound speed calibration device is presented to eliminate the influence of temperature and salinity on measurement results. The sound speed is obtained by inversion and is calibrated in real time according to the time intervals of ultrasonic transmission and reflection signal acquired by timer. The system is applied to the terrain monitoring in the downstream of Yaojiang Gate. In monitoring period, the river bed silted by 0.041 m, while the GPS-RTK observation result is 0.050 m, consistent with each other in general. Practice has shown that the presented system is reliable and feasible in real-time monitoring of river terrain.