Bed material waves are temporary zones of sediment accumulation created by large sediment inputs. Recent theoretical, experimental and field studies examine factors influencing dispersion and translation of bed material waves in quasiuniform, gravel-bed channels. Exchanges of sediment between a channel and its floodplain are neglected. Within these constraints, two factors influence relative rates of dispersion and translation: (1) interactions between wave topography, flow and bed load transport; and (2) particle-size differences between wave material and original bed material. Our results indicate that dispersion dominates the evolution of bed material waves in gravel-bed channels. Significant translation requires a low Froude number, which is uncharacteristic of gravel-bed channels, and low wave amplitude which, for a large wave, can be achieved only after substantial dispersion. Wave material of small particle size can promote translation, but it primarily increases bed load transport rate and thereby accelerates wave evolution.

Incubating salmonid eggs in streambeds are often threatened by deposition of fine sediment within the gravel. To relate sedimentation of spawning gravel beds to sediment transport, infiltration of fine sediment (<2 mm in diameter) into clean gravel beds, bed material size distributions, scour-fill depths, and sediment transport during 10 storm flow events were measured in three streams of north coastal California. Although suspended sediment comprised most (75-94%) of the clastic load during storm flows, bed load material (0.25-2 mm) accounted for most (70-78%) of the fine sediment accumulated in experimental gravel implanted in the streambeds. Sand trapped in the interstices of the top several centimeters formed a seal that impeded deeper deposition of very fine sand and finer material. The seal was responsible at least in part for a decrease in the rate of fine-sediment accumulation with increasing cumulative bed load transport. Areas of the streambeds commonly scoured or filled 0.1 m or more during storm flows, and thus scour and fill commonly created a sandy layer at least as thick as the seal formed by sediment infiltration. Scour could erode eggs laid in the bed and expose deeper levels of the bed to infiltration by fine sediment, but at the same time could allow fine sediment to be winnowed away. Great temporal and spatial variation in sedimentation in these streams suggests that individual storms of moderate size pose a threat to eggs in many but not all areas selected by fish for spawning.

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...

During waning flood flows in gravel‐bed streams, finegrained bedload sediment (sand and fine gravel) is commonly winnowed from zones of high shear stress, such as riffles, and deposited in pools, where it mantles an underlying coarse layer. As sediment load increases, more fine sediment becomes available to fill pools. The volume of fine sediment in pools can be measured by probing with a metal rod, and, when expressed as the fraction (V*) of scoured residual pooi volume (residual pool volume with fine sediment removed), can be used as an index of the supply of mobile sediment in a stream channel. Mean values of V* were as high as 0.5 and correlated with qualitative evaluations of sediment supply in eight tributaries of the Trinity River, northwestern California. Fine‐sediment volume correlated strongly with scoured pool volume in individual channels, but plots of V* versus pool volume and water surface slope revealed secondary variations in fines volume. In sediment‐rich channels, V* correlated positively with scoured pool volume; in sediment‐poor channels, V* correlated negatively with water‐surface slope. Measuring fine sediment in pools can be a practical method to evaluate and monitor the supply of mobile sediment in gravel‐bed streams and to detect and evaluate sediment inputs along a channel network.

Controls on pool formation are examined in gravel-and cobble-bed rivers in forest mountain drainage basins of northern California, southern Oregon, and southeastern Alaska. We demonstrate that the majority of pools at our study sites are formed by flow obstructions and that pool geometry and frequency largely depend on obstruction characteristics (size, type, and frequency). However, the effectiveness of obstructions to induce scour also depends on channel characteristics, such as channel gradient, width:depth ratio, relative submergence (ratio of flow depth to grain size), and the calibre and rate of bed material supply. Moreover, different reach-scale channel types impose different characteristic physical processes and boundary conditions that further control the occurrence of pools within a catchment. Our findings indicate that effective management of pools and associated aquatic habitat requires consideration of a variety of factors, each of which may be more or less important depending on channel type and location within a catchment. Consequently, strategies for managing pools that are based solely on single-factor, regional target values (e.g. a certain number of wood pieces or pools per stream length) are likely to be ineffective because they do not account for the variety of local and catchment controls on pool scour and, therefore, may be of limited value for proactive management of complex ecosystems.