IntroductionMost alluvial stream channels change remarkably little over periods that can be as long as years or decades, even though they may be regularly subjected to sediment-transporting flows. Stream channels are stability-seeking entities. What we see is the product of a succession of changes; the morphology that persists is that which is most nearly stable in face of the usually imposed flows. Stability is gained when stream energy can be dissipated without the accomplishment of significant channel-deforming work, even though sediment transfer, including the exchange of sediment at the channel boundaries, may still occur. Energy-dissipating structures develop at all morphological scales within the channel, including the scales of channel pattern, pool and riffle, and sedimentary bed forms. However, the most basic level at which stability develops in gravel-bed channels is that of the granular boundary materials, the potentially mobile sediments themselves.Grains are entrained in stream channels when the force of water acting on the alluvial bed material overcomes particle inertia. Shields [1936] expressed the force balance at entrainment as a "mobility number," a function of the ratio of fluid shear stress exerted on the bed to (submerged) particle weight. [Gomez, 1993]. None of these factors eliminate the fundamental effect of particle weight. However, when particles become interlocked, the relative effect of particle weight becomes comparatively less dominant.• In gravel-bed streams with low rates of bed material transport, we have observed much more complex grain structures, which we call stone cells [cf. Gustavson, 1974]. The purpose of this paper is to describe these features, to obtain some phenomenological understanding of the conditions under which they develop and persist, and to explore their influence on the promotion of streambed stability.
2.Field Observations