Abstract. Local variations in boundary shear stress acting on bed-surface particles control patterns of bed load transport and channel evolution during varying stream discharges. At the reach scale a channel adjusts to imposed water and sediment supply through mutual interactions among channel form, local grain size, and local flow dynamics that govern bed mobility. In order to explore these adjustments, we used a numerical flow model to examine relations between model-predicted local boundary shear stress (ri) and measured surface particle size (Ds0) at bank-full discharge in six gravel-bed, alternate-bar channels with widely differing annual sediment yields. Values of ri and Ds0 were poorly correlated such that small areas conveyed large proportions of the total bed load, especially in sediment-poor channels with low mobility. Sediment-rich channels had greater areas of full mobility; sediment-poor channels had greater areas of partial mobility; and both types had significant areas that were essentially immobile. Two reachmean mobility parameters (Shields stress and Q*) correlated reasonably well with sediment supply. Values which can be practicably obtained from carefully measured mean hydraulic variables and particle size would provide first-order assessments of bed mobility that would broadly distinguish the channels in this study according to their sediment yield and bed mobility.
IntroductionChannel evolution is a response to runoff and sediment supply involving mutual interactions among channel form, bed material size, and hydraulic forces. In the short term these interactions are driven by spatial variations in boundary shear stress acting on bed material of varying mobility. In most gravel-bed channels, mean boundary shear stress only slightly exceeds the threshold for particle entrainment at channelforming (bank-full) flows [Parker, 1979;Andrews, 1983]. Such channels are commonly referred to as "threshold channels."A question that we address is, How well are boundary shear stress and bed-surface particle size adjusted within a reach of a threshold channel? Four degrees of adjustment could govern channel evolution: (1) Variations in bed-surface particle size are balanced by variations in boundary shear stress so that threshold conditions are met uniformly over the channel. Con- Flume experiments have indicated that the heterogeneity of particle sizes in gravel-bed channels provides a capacity for adjusting to changes in sediment load through changes in the mobility of the bed surface. Dietrich et al. [1989] fed mixed-size sediment at a high rate into a narrow flume containing bed material with the same size mixture as the feed and then reduced the feed rate in two steps after achieving equilibrium in sediment transport during each step as boundary shear stress was held approximately constant. At the initial, highest feed rate a coarse surface layer was not evident. After each subsequent reduction in feed rate the surface coarsened over most of the bed. In total, a 90% reduction in feed rate resulted i...