To address the problems of low accuracy and weak anti-interference ability of traditional river flow monitoring methods during the high fl ood period,high-defi nition cameras were used to non-contact collect river surface images. The Sobel algorithm was applied to extract water level lines,and the optical flow method was employed to analyze the spatiotemporal motion characteristics of water surface pixels. Optical fl ow vectors were converted into actual fl ow velocities to achieve local velocity estimation. A state-space model was constructed with fl ow rate and water level as state variables,and regional fl ow velocity and water level as observations,incorporating a dynamic noise covariance online update mechanism. River flow was estimated by integrating multi-source observation data with iterative Kalman fi ltering operations. Experimental results showed that when the proposed method was applied to river fl ow monitoring during high fl ood period,the drift accumulation rate remained stable below 1.5%,enabling full-process optimization from water level extraction and local velocity estimation to global flow inversion. The application of the dynamic variance Kalman filtering algorithm effectively enhances the accuracy and stability of fl ow monitoring,providing a new and eff ective approach for river fl ow monitoring during high fl ood period.