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.

A streamside landslide delivered 60 000 m 3 of mixed-size sediment to the Navarro River, a sinuous gravel-bed channel (drainage area ‫؍‬ 535 km 2), at the end of the annual high-runoff period in spring 1995. The deposit formed a 9-m-high dam that partially breached within several hours, but recessional flows entrained little material until the following high-runoff season. The landslide afforded the opportunity to measure the evolution of a sediment wave from its inception to near-obliteration and, particularly, to test relative tendencies for translation and dispersion of a sediment wave in a natural gravel-bed channel. This study represents a simple case: The wave originated from a single input, the preexisting channel was relatively uniform, and resistant banks prevented adjustments in width. We surveyed channel topography over a 1.5-4.5 km reach centered on the landslide dam each year from 1995 to 1999, and we sampled bed material downstream of the dam in 1995 and 1997. Landslide material was coarser than ambient bed material, but all sizes were mobilized by subsequent peak flows. Abrasion of weathered and fractured graywacke sandstone landslide material was roughly an order of magnitude greater than the ambient river gravel. The sediment wave dispersed and mostly disappeared within a few years with no measurable translation. Sediment filled the reservoir created by the eroding landslide dam until throughput of bed load was restored in 1998. The stationary wave crest eroded until in 1999 it was Ͻ1 m higher

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The dynamics of sediment transport capacity in gravel-bed rivers is critical to understanding the formation and preservation of fluvial landforms and formulating sediment-routing models in drainage systems. We examine transport-storage relations during cycles of aggradation and degradation by augmenting observations of three events of channel aggradation and degradation in Cuneo Creek, a steep (3%) gravel-bed channel in northern California, with measurements from a series of flume runs modeling those events. An armored, single-thread channel was formed before feed rates were increased in each aggradation run. Output rates increased as the channel became finer and later widened, steepened, and braided. After feed rates were cut, output rates remained high or increased in early stages of degradation as the incising channel remained fine-grained, and later decreased as armoring intensified. If equilibrium was not reached before sediment feed rate was cut, then a rapid transition from a braided channel to a single-thread channel caused output rates for a given storage volume to be higher during degradation than during aggradation. Variations in channel morphology, and surface bed texture during runs that modeled the three cycles of aggradation and degradation were similar to those observed in Cuneo Creek and provide confidence in interpretations of the history of change: Cuneo Creek aggraded rapidly as it widened, shallowed, and braided, then degraded rapidly before armoring stabilized the channel. Such morphology-driven changes in transport capacity may explain the formation of flood terraces in proximal channels. Transport-storage relations can be expected to vary between aggradation and degradation and be influenced by channel conditions at the onset of changes in sediment supply. Published in 2011. This article is a US Government work and is in the public domain in the USA.