Abstract: Image-based experiments have been conducted to measure low-speed streaks in the near-bed region of open channel flow. The measurements involve tracing the streaks with natural sand, recording the streak images by a high-resolution camera, and extracting geometrical features of the steaks through mathematical morphology procedure. Analysis of the measured data yields the following findings: ① The streak width is symmetric with respect to the midline of the bed, attaining its maximum value at the center and decreasing gradually toward the side wall; ② At friction Reynolds number from 276 to 465, the low-speed streaks vary from 20.8 to 29.8 in dimensionless width, from 99.3 to 126.7 in dimensionless interval, and from 23.4% to 15.9% in percentage area; and ③ An increase of friction Reynolds number leads to an increase in the width and interval of streaks together with a reduction in the percentage area. The experimental results indicate that image processing technique provides an efficient and accurate tool for investigating complex low-speed streaks in open channel flow.