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

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1. Rivers are subject to thresholds of several types that define significant changes in processes and morphology and delimit distinctive riverine landscapes and habitats. Thresholds are set by the conditions that govern river channel process and form, amongst which the most important are the flow regime, the quantity and calibre of sediment delivered to the channel, and the topographic setting (which determines the gradient of the channel). These factors determine the sediment transport regime and the character of alluvial deposits along the channel. 2. Changes occur systematically along the drainage system as flow, gradient and sediment character change, so a characteristic sequence of morphological and habitat types – hence of riverine landscapes – can be described from uplands to distal channels. The sequence is closely associated with stream competence to move sediment and with bank stability. 3. The paper proposes a first order classification of river channel and landscape types based on these factors. The riverine landscape is affected seasonally by flow thresholds, and further seasonal thresholds in northern rivers are conditioned by the ice regime. 4. It is important to understand geomorphic thresholds in rivers not only for the way they determine morphology and habitat, but because human activity can precipitate threshold crossings which change these features significantly, through either planned or inadvertent actions. Hence, human actions frequently dictate the character of the riverine landscape.

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A new set of field data facilitates a detailed analysis of variations in bed material grain size within two confluent gravel-bed rivers in northeastern British Columbia, Canada. A preliminary assessment of grain-size variability establishes a basis for examination of the spatial pattern of grain-size change. Standard ANOVA techniques are inappropriate because individual samples have unequal variances and are not normally distributed. Alternative tests for homoscedasticity and comparison of means are therefore utilized. Within-site, between-sample variability is not significant. The grain-size distributions that were obtained at individual sites are therefore representative of the depositional environments that were sampled. In both rivers mean grain size does vary significantly between sites and there is therefore a basis for examining the data for spatial patterns such as downstream fining.Textural variations along the two rivers studied here are complex and show negligible overall fining (in over 100 km). This is the consequence of a large number of tributary inputs and non-alluvial sediment sources which are the legacy of Late Pleistocene glaciation. The identification of lateral sources like these is fundamental for understanding textural changes within rivers. The sedimentary link (a channel reach between significant lateral sediment inputs) provides a means of isolating fluvial maturation processes (abrasion and sorting) from contingent lateral inputs. Strong fining trends are apparent in most links and classification of grain-size measurements according to their location within particular links greatly improves the statistical explanation of textural variation. Identification of sedimentary links provides a means of applying models of fluvial fining processes, so isolation of link networks will aid the development of basin-scale models of textural variation.