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Abstract. Over 1400 tracer pebbles 16-256 mm in diameter were tracked for 2 years in six reaches of Allt Dubhaig, Scotland, a small gravel-bed river along which shear stress and bed surface grain size decrease toward a local base level. Pebble movement was sizeselective both within and between reaches. Within reaches the decrease in mean travel distance with increasing grain size is strongest in the coarse tail of the size distribution. Particle shape has a minor secondary effect. A nondimensional grain velocity, averaged over the duration of competent flow, is used to compare different size classes and reaches. Over 90% of its variance is explained by relative grain size and reach Shields stress. The pattern of size selectivity is consistent with single-event tracer results elsewhere, bedload trap data from our distal reach, and the concept of partial mobility. It provides a mechanism for strong downstream fining by selective transport and deposition along rivers in which stress declines toward base level. The nondimensional prediction equation for grain ,velocity may be of use in other rivers but requires testing. We show in this paper that tracer movement was sizeselective both within and between reaches, discuss the link with the observed pattern of downstream fining along the river, and seek general nondimensional relationships applicable across reaches and potentially to other rivers. Attention is focused on the long-term and large-scale mobility of different size classes of bed material, not the possible influence of bed microtopography or event magnitude and duration on individual particle displacements. The approach is one-dimensional: only the longitudinal dispersion of tracers is considered, and it is related to reach properties that are averaged across the channel width and along the channel over several widths. Field Site and MethodsAllt Dubhaig is a small stream in the Scottish Highlands which has an alluvial channel extending for 3.5 km from an area of hummocky moraine and rock outcrops (the last of which defines our zero datum for distance downstream) to a local base level imposed by an alluvial fan (Figure 1). The long profile of the alluvial channel is strongly concave, with a reduction in slope from •0.02 to 0.002 and then to 0.0002 after an abrupt change from gravel to sand bed material at 2.8 km.

Bed load was trapped during flood events over a 20-month period at the lower end of the Allt Dubhaig, a small river in Scotland with rapid downstream fining of gravel bed material on a slowly aggrading concave long profile. The channel bed near the trap is predominantly gravel with a secondary sand mode. Total transport in each event depended mainly on peak shear stress, rather than duration over a threshold. Bed load was mainly sand in smaller events, bimodal in intermediate events, and mainly gravel in the biggest floods. Mean and maximum grain diameter both increased with peak shear stress, but in different ways. Analysis of fractional transport rates and maximum grain size in relation to peak shear stress suggests that gravel transport is slightly size selective but sand transport is close to equal mobility. The slight selectivity in gravel transport is consistent with previous field studies of near-equilibrium unimodal beds and supports assumptions made in the numerical model of Hoey and Ferguson (1994), which successfully simulates the observed amount of downstream fining over the 2.5-km upstream of the bed load trap.

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