What would be the best way to estimate velocity from a video?
You can probably address that by tracking debris across frames, assuming that they move at the speed of water and aren't too far apart in successive frames.

Maybe start with the old stuff: Shi, J.; Tomasi, C., "Good features to track," in Computer Vision and Pattern Recognition, 1994. Proceedings CVPR '94., 1994 IEEE Computer Society Conference on , vol., no., pp.593-600, 21-23 Jun 1994.