A TEST OF EQUAL MOBILITY IN FLUVIAL SEDIMENT TRANSPORT - BEHAVIOR OF THE SAND FRACTION

Mobile fine sediment was caught in six screened pit traps installed in the bed of a cobble gravel channel. Sample periods lasted from several hours to several days, so the trapped material represents a good integration of the range of sizes in traction up to a limit near 10 mm imposed by the screens. All the traps, distributed along a bar, caught similar amounts of material, of similar texture. The peak transport rate for the < 2-mm fraction, corresponding with the lower portion of the bimodal bed material, was about 3 x 10(-4) kg s-1 m-1. Observations continued for 6 weeks, through the nival flood on the stream. There was a sensible threshold for motion of material of all sizes, but minor transport occurred below it. Material up to 210-mu-m in size, including conventionally classified "wash" material, moved with near equal mobility over the season, but there were departures in individual sample periods. "Equal mobility" of fine material appears at best to be a statistical phenomenon which holds for a limited range of grain sizes over a seasonal period.