Motion pictures taken at Duck Creek, a clear stream 6.5 m wide and 35 cm deep near Pinedale, Wyoming, provide detailed, quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them. Bed shear stress was approximately 6 Pa (60 dynes cm−2), which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material; and transport was almost entirely as bedload. The displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones, and rarely by brief sliding of large, angular ones. Entrainment was principally by rollover of the larger particles and liftoff of the smaller ones, and infrequently by ejection caused by impacts, whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones. The displacement times averaged about 0.2−0.4 s and generally were much shorter than the intervening repose times. The collective motions of the particles were characterized by frequent, brief, localized, random sweep-transport events of very high rates of entrainment and transport, which in the aggregate transported approximately 70% of the total load moved. These events occurred 9% of the time at any particular point of the bed, lasted 1–2 s, affected areas typically 20–50 cm long by 10–20 cm wide, and involved bedload concentrations approximately 10 times greater than background. The distances travelled during displacements averaged about 15 times the particle diameter. Despite the differences in their dominant modes of movement, the 8–16 mm particles typically travelled only about 30% slower during displacement than the 2–4 mm ones, whose speeds averaged 21 cm s−1. Particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely, with diffusivities of 4.6 and 0.26 cm2 s−1, respectively. The bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler. The 2–4 mm particles were entrained 6 times faster on finer areas of the bed, where 8–16 mm particles covered 6% of the surface area, than on coarser ones, where they covered 12%, even though 2–4 and 4–8 mm particles covered practically the same percentage areas in both cases. The 4–8 and 8–16 mm particles, in contrast, were entrained at the same rates in both cases. To within the statistical uncertainty, the rates of distrainment balanced the rates of entrainment for all three sizes, and were approximately proportional to the corresponding concentrations of bedload.

Field measurements in a sand-bedded river and in two gravel-bedded ones are compared to examine controls on boundary shear stress fields, sediment transport processes, and sorting in meanders. Analysis of detailed flow field measurements in the sand-bedded river meander and over a gravel-bedded alternate bar reveals a well-defined spatial structure to the magnitude and sign of forces controlling boundary shear stress that arise from topographicallyinduced spatial accelerations. The relationship between bedload transport and boundary shear stress fields in river meanders varies with size and heterogeneity of bed material. In bends of moderately to well sorted sand in flows generating boundary shear stresses well above critical (such as in large sandy rivers), downstream varying boundary shear stress is matched by topographically-induced cross-stream transport of sediment. In meanders with high excess shear stress but poorly sorted coarse sand and fine gravel, boundary shear stress variation downstream is partially matched by surface grain size adjustments and by net cross-stream sediment flux.Maxima of bedload transport rate and boundary shear stress do not correspond in some areas. In gravel-bedded meanders with low excess boundary shear stress and low sediment supply, bedload may be much finer than the bed surface, and significant areas of bar surface are covered with grain sizes that constitute a very small portion of the bedload. Substantial bedload transport may only occur over a narrow portion of the bed width where boundary shear stress relative to critical stress of the surface is highest and where the sediment flux from upstream is locally concentrated. In· this case, grain size adjustments dominate over topographically-induced cross-stream sediment transport in controlling the relationship between boundary shear stress and bedload transport fields.

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Alluvial mountain streams exhibit a range of channel forms: pool±riffle, plane bed, step±pool and cascades. Previous work suggested that these forms exist within discrete, and progressively steeper slope classes. Measurements conducted at over 100 sites in west-central and central Idaho confirm that slope steepens progressively as one moves from pool±riffle, to plane bed, to step±pool, and finally to cascades. Median slope for pool±riffle topography is 0Á0060, for plane beds 0Á013, for step± pools 0Á044, and for cascades 0Á068. There is substantial overlap in the slopes associated with these channel forms. Pool± riffle topography was found at slopes between 0Á0010 and 0Á015, plane beds between 0Á0010 and 0Á035, step±pools between 0Á015 and 0Á134, and cascades between 0Á050 and 0Á12.Step±pools are particularly striking features in headwater streams. They are characterized by alternating steep and gentle channel segments. The steep segments (step risers) are transverse accumulations of boulder and cobbles, while the gentle segments (pools) contain finer material.Step wavelength is best correlated to step height which is in turn best correlated to the median particle size found on step risers. This result differs from past studies that have reported channel slope to be the dominant control on step wavelength. The presumed geometry and Froude number associated with the features under formative conditions are consistent with the existence field for antidunes and by extension with the hypothesis that step±pools are formed by antidunes.

Pb xs ratio in suspended sediments to determine the time since the particles were tagged by precipitation-derived radionuclides (i.e. the age of the suspended sediment). In addition, an alternative model is presented to determine the fraction of the sediment that is 'newly tagged'. These two models are applied to three catchments -Old Woman Creek, Ohio; Weeks Bay, Alabama; and South Slough, Oregon -and yield similar findings at all three sites. Sediment ages increase from 0 in newly tagged material, to 50-80 days in rivers, to about 80-100 days in the estuaries, to about 200 days in the sediment traps, to about 300 days on surface bottom sediments. Alternatively, the percentage new sediment decreases from 100 per cent in newly tagged material, to about 35-50 per cent in rivers, to 25-35 per cent in the estuary, to less than 10 per cent in the sediment traps, to 1-4 per cent on the surface of the bottom sediments.