Scour was responsible for most of the US bridges that collapsed during the past 40 years. Due to erosion and refilling of riverbed deposits under strong current, the maximum scour depth is difficult to measure with existing technologies during a flood event. In this study, a new methodology is proposed to embed permanent magnets in a natural rock and integrate the so-called smart rock into the process of bridge scour for real-time monitoring. Once properly designed, the smart rock can continually fall into the bottom of a gradually growing scour hole during a flood event and thus register the maximum scour depth. Emphasis in this study is placed on the working principle of smart rocks, the maximum measurement distance with a magnetometer, and various calibration and validation tests. It was demonstrated that the magnetic field intensity is a function of measurement distance and magnet orientation and, once calibrated, can be strongly correlated with the increase in scour depth over time.