PARTICLE-SIZE VARIATIONS BETWEEN BED-LOAD AND BED MATERIAL IN NATURAL GRAVEL-BED CHANNELS

Particle sizes of bed load and bed material that represent materials transported and stored over a period of years are used to investigate selective transport in 13 previously sampled, natural gravel bed channels. The ratio (D*) of median particle size of bed material to the transport- and frequency-weighted mean of median bed load size decreases to unity with increasing drainage area, bank-full discharge, dimensionless stream power, and bed material sorting. In channels with high values of D*, significant volumes of fine bed load are transported during discharges that are less than bank-full, which is commonly associated with general entrainment of the coarse pavement in many gravel bed channels. This indicates transport of fine bed load over a more stable substrate of coarser bed material. The apparent breakdown in equal mobility of the bed as a whole may be caused by areal segregation of poorly sorted bed material into superiorly sorted patches of varying mean size. Likely sources of selectively transported fine bed load include fine patches that have low entrainment thresholds and high virtual particle velocities. A simple sediment budget applied to measurements from three channels indicates that velocities of material from fine patches in pools relative to velocities of average bed material are high in low-order channels and decrease distally as more bed material that represents the bed as a whole is accessed for bed load by deeper annual scour.