For sediment particle sizes between 0.50 mm and 16 mm, the Helley-Smith bedload sampler has a near perfect sediment-trapping efficiency. For particles sizes smaller than 0.50 mm, the Helley-Smith sampler has a high bedload sediment-trapping efficiency because part of the sediement retained by the sampler has been transported in suspension and it cannot be quantified separately from the bedload. For particle sizes larger than about 16 mm, the Helley-Smith sampler has low sediment-trapping efficiency but this may be related to the paucity of coarse particles in transport in the calibration tests rather than reflecting an actual low trap efficiency for large-size particles. (USGS)...