Size and distance of travel of unconstrained clasts on a streambed

We have examined the relation between the mean distance of movement during individual flood events and the mean particle size of unconstrained clasts in gravel bed rivers. Distance of movement was scaled using the mean distance of movement of the median size group of the surface material; size was scaled using the median b axis diameter of the subsurface material. The scaled data are fitted with a nonlinear function. Clasts larger than the median of surface material exhibit a steep decline of mobility, with an upper limit for mobility near D i / D 50sub = 10. Smaller materials exhibit a less sensitive relation. We suppose that trapping of small clasts affects their travel distance. These results indicate that the movement of free surface stones is size preferred. A subordinate analysis of free, constrained and buried particles reveals that bed surface structure acts to reduce differential movement.