Disclosed is a composite hydrological monitoring system, in which a counterweight component and a test component are respectively connected to both opposite ends of a strip and a plurality of sensors are disposed at different vertical positions. Accordingly, the scour depth can be measured by sensing the location of the counterweight component, whereas the water level and/or flow velocity can be determined by signals from the sensors. When the counterweight component moves downward with sinking of the riverbed, the strip would be pulled down and thus causes the test component to present a change in mechanical energy. Accordingly, the sinking depth can be measured by sensing the change of the mechanical energy. Additionally, since the water level variation would cause signal changes of the sensors arranged in a row along a vertical direction, the change of water level can be determined accordingly.