Although increasingly acknowledged as an important factor in bedload transport, few data on the effects of bed conditions on entrainment in gravel-bed rivers exist. This reflects to some degree the substantial difficulties involved in obtaining accurate threshold measurements in such channels. Using a sensor which allows initial movement of labeled particles to be detected during flood flow, the entrainment thresholds of clasts incorporated into the bed microtopography have been compared with those in exposed, open plane-bed positions. The threshold velocity of clasts from cluster bedforms, the most prevalent type of bed microtopography, is found to be above that of more exposed particles of like size and shape. Measurements indicate that the initial movement of ∼70% of particles in gravel-bed rivers will be directly influenced by the presence of neighboring grains. Bed microtopography plays an important role in delaying incipient motion. The influence of microtopography may be invoked to account for phenomena such as discontinuous particle movement and variations in the composition of bedload during discharge events.

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