Size-selective bedload transport is well established in channels dominated by gravel. This study investigates the mobility of gravel-sized clasts in a channel dominated by sand to quantify the annual frequency of gravel movement. A Helley-Smith bedload sampler was deployed at three locations in the low-gradient, sand-dominated San Antonio River. The smallest flow in which a given gravel fraction was captured defines the mobilizing discharge from which an annual frequency of occurrence was derived using flow duration analysis. Results document that smaller gravel fractions (2–4 mm) move between 54 and 96% of the year. Larger gravels are less mobile at all sites, but grain diameters between 16 and 32 mm mobilize every year on average, depending on river location. Comparison to mobilizing flows established for 10 rivers with different bed material textures documents the higher annual gravel mobility in sand-dominated channels. These results shed light on the extent of gravel mobility in the distal portion of watersheds, and help explain patterns in the gravel-sand transition in riverbed material.