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...

Redwood Creek, north coastal California, USA, has experienced dramatic changes in channel configuration since the 1950s. A series of large floods (in 1955,1964,1972 and 1975) combined with the advent of widespread commercial timber harvest and road building resulted in extensive erosion in the basin and contributed high sediment loads to Redwood Creek. Since 1975, no peak flows have exceeded a 5 year recurrence interval.Twenty years of cross-sectional survey data document the downstream movement of a 'sediment wave' in the lower 26 km of this gravel-bedded river at a rate of 800 to 1600 m a ' during this period of moderately low flows. Higher transit rates are associated with reaches of higher unit stream power. The wave was initially deposited at a site with an abrupt decrease in channel gradient and increase in channel width. The amplitude of the wave has attenuated more than 1 m as it moved downstream, and the duration of the wave increased from eight years upstream to more than 20 years downstream. Channel aggradation and subsequent degradation have been accommodated across the entire channel bed. Channel width has not decreased significantly after initial channel widening from large ( > 25 year recurrence interval) floods. Three sets of longitudinal surveys of the streambed showed the highest increase in pool depths and frequency in a degrading reach, but even the aggrading reach exhibited some pool development through time. The aggraded channel bed switched from functioning as a sediment sink to a significant sediment source as the channel adjusted to high sediment loads. From 1980 to 1990, sediment eroded from temporary channel storage represented about 25 per cent of the total sediment load and 95 per cent of the bedload exported from the basin.

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Abstract. Large, episodic inputs of coarse sediment (sediment pulses) in forested, mountain streams may result in changes in the size and arrangement of bed forms and in channel roughness. A conceptual model of channel organization delineates trajectories of response to sediment pulses for many types of gravel bed channels. Channels exhibited self-organizing behavior to various degrees based on channel gradient, presence of large in-channel wood or other forcing elements, the size of the sediment pulse, and the number of bed-mobilizing flows since disturbance. Typical channel changes following a sediment pulse were initial decreases in water depth, in variability of bed elevations, and in the regularity of bed form spacing. Trajectories of change subsequently showed increased average water depth, more variable and complex bed topography, and increased uniformity of bed form spacing. Bed form spacing in streams with abundant forcing elements developed at a shorter spatial scale (two to five channel widths) than in streams without such forcing mechanisms (five to 10 channel widths). Channel roughness increased as bed forms developed.