Particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field. This has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows. Lift and drag forces are determined on clustered hemispherical particles of varying size. In the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion. On the stoss side of the cluster, drag is uniform regardless of the separation of the component particles, but lift is shown to increase when particle separation is small, so affecting stability. This mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape. On average, 46% of clustered particles are entrained by flood flow compared to 87% of particles in open plane‐beds. The influence of clusters is a major determinant of sedimentary sorting.

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A new sensor for the continuous and unmanned detection and recording of bedload motion is described. The sensor acts in the same fashion as a conventional metal detector but is elongate and installed permanently in the bed of an alluvial channel. Artificial clasts are labelled with short lengths of ferrite rod and are seeded upstream from the sensor, replacing bed particles. The entrainment of seeded clasts takes them over the sensor where they distort the magnetic field and produce a change in inductance that is detected and recorded. A field installation demonstrates the value of the sensor by revealing for the first time in coarse‐grained alluvium the spasmodic nature of particle motion reminiscent of kinematic waves. It also illustrates the importance of pebble clusters in delaying particle entrainment.

Flow separation is a common feature within alluvial channels that occurs at abrupt changes in the bed geometry. At a wide range of scales, it is a process that can exert considerable control over the segregation and deposition of heavy mineral grains. Fluid separation generates a region of high bed shear stress that can entrain heavy minerals, and a region of low velocity that is a preferred site for the deposition of denser particles. Significant concentrations of heavy minerals, sometimes up to eight times the background value, may be formed by flow separation processes. A review of the occurrence of flow separation illustrates that it may be responsible for the accumulation of heavy mineral grains at a range of geomorphological scales.

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