Thomas E. Lisle

Variability of bed mobility in natural, gravel-bed channels and adjustments to sediment load at local and reach scales

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 (тj( and measured surface particle size (D50) at bank-full discharge in six gravel-bed, alternate-bar channels with widely differing annual sediment yields. Values of тj and D50 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 reach-mean 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.

Thermally Stratified Pools and Their Use by Steelhead in Northern California Streams

Abstract Thermal stratification occurred in pools of three rivers in northern California when inflow of cold water was sufficiently great or currents were sufficiently weak to prevent thorough mixing of water of contrasting temperatures. Surface water temperatures in such pools were commonly 3–9°C higher than those at the bottom. Cold water entered pools from tributaries, intergravel flow through river bars, and streamside subsurface sources. In Redwood and Rancheria Creeks, cold water was protected where gravel bars encroached into pools that were scoured along bedrock banks, creating isolated backwaters. Sixty-five percent of the juvenile steelhead Oncorhynchus mykiss found in the Rancheria Creek study reaches moved into adjacent stratified pools during periods of high ambient stream temperatures (23–28°C). Fish showed a decline in forage behavior and increased agonistic activity just before movement into stratified pools. In the Middle Fork Eel River, pools deeper than 3 m stratified when surface flow ...

Particle Size Variations Between Bed Load and Bed Material in Natural Gravel Bed Channels

Particle sizes of bed load and bed material that represent materials transported and stored over a period of years are used to investigate selective transport in 13 previously sampled, natural gravel bed channels. The ratio (D*) of median particle size of bed material to the transport- and frequency-weighted mean of median bed load size decreases to unity with increasing drainage area, bank-full discharge, dimensionless stream power, and bed material sorting. In channels with high values of D*, significant volumes of fine bed load are transported during discharges that are less than bank-full, which is commonly associated with general entrainment of the coarse pavement in many gravel bed channels. This indicates transport of fine bed load over a more stable substrate of coarser bed material. The apparent breakdown in equal mobility of the bed as a whole may be caused by areal segregation of poorly sorted bed material into superiorly sorted patches of varying mean size. Likely sources of selectively transported fine bed load include fine patches that have low entrainment thresholds and high virtual particle velocities. A simple sediment budget applied to measurements from three channels indicates that velocities of material from fine patches in pools relative to velocities of average bed material are high in low-order channels and decrease distally as more bed material that represents the bed as a whole is accessed for bed load by deeper annual scour.

Response of a Channel with alternate bars to a decrease in supply of mixed‐size bed load: A Flume Experiment

The response of a channel with a topography and modeled bed material size typical of gravel bed rivers to reductions in sediment supply was investigated in a laboratory flume filled and fed with a sand-gravel mixture. After a series of quasi-stationary alternate bars were formed under equilibrium sediment transport, feed rate was reduced in two steps to one third and one tenth the initial rate as discharge was held constant. The primary response following both reductions was an increase in bed surface particle size as a corridor of intensive bed load transport contracted and local transport rates decreased. After the first feed rate reduction, the channel incised by twice the mean water depth, on average, and caused distal bar surfaces to emerge as terracelike features. Bar roughness decreased, and mean boundary shear stress exerted on bed surface particles increased. Little incision occurred after the second feed rate reduction.

Evolution of a sediment wave in an experimental channel

The routing of bed material through channels is poorly understood. We approach the problem by observing and modeling the fate of a low-amplitude sediment wave of poorly sorted sand that we introduced into an experimental channel transporting sediment identical to that of the introduced wave. The wave essentially dispersed upstream and downstream without translation, although there was inconclusive evidence of translation late in the experiment when the wave was only 10–20 grain diameters high. Alternate bars migrated through zones of differing bed load transport rate without varying systematically in volume, celerity, or transport rate. Sediment that overpassed migrating bars was apparently responsible for dispersion of the wave. The evolution of the wave was well predicted by a one-dimensional model that contains no adjusted empirical constants. Numerical experiments demonstrate, however, that the theory does not predict sediment waves that migrate long distances downstream. Such waves can only be explained by the following processes not represented by the theory: selective bed load transport, spatial variations in bar and other form roughness, the mechanics of mobile armor, and perhaps other mechanisms.

Evolution of a landslide-induced sediment wave in the Navarro River, California

A streamside landslide delivered 60 000 m3 of mixed-size sediment to the Navarro River, a sinuous gravel-bed channel (drainage area 535 km2), 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 1 m higher *E-mail: dsutherl@fs.fed.us. than the preslide elevation. As the wave profile flattened, its detectable leading edge extended downstream from 620 m in 1995 to 1600 m in 1997. Downstream advance of the wave was associated with coarsening of bed material. The sediment wave created a longitudinal disturbance in sediment transport. By using the dam as a reference datum of zero bed-load transport, we computed longitudinal variations in annual bed-load and suspended-sediment transport rates in 100 m increments downstream of the dam. These longitudinal variations were controlled by scour and fill of the bed and by abrasion of bed-load particles. Bed-load transport rates in the first and second years after the landslide increased in the landslide vicinity and then decreased downstream as sediment deposited behind the advancing leading edge of the wave. The location of peak bed-load transport rate advanced from the first year (400 m) to the second (800 m). We used a physically based, one-dimensional model (Cui et al., 2002b) to hindcast annual changes in transversely averaged bed elevation over the study reach. Agreement between measured and predicted bed elevations was very good. This result supports our conclusion that, once emplaced, sediment waves in gravel-bed rivers tend to disperse, with little or no translation.

Fine bed material in pools of natural gravel bed channels

Natural gravel bed channels commonly contain a fine mode of sand and fine gravel that fills voids of the bed framework of coarser gravel. If the supply of fine bed material exceeds the storage capacity of framework voids, excess fine material forms surficial patches, which can be voluminous in pools during low flow. Data collected in 34 natural channels in northern California and southern Oregon indicate the following. (1) Fine material on the bed surface can be readily winnowed and transported at high particle velocities, much of it in intermittent suspension. Fine material can dominate the bed material load in gravel bed channels, but its abundance on the bed surface is limited by its increasing mobility as hiding places among prominent particles are filled. (2) Fine material in pools is typically replaced many times per year. (3) The proportion V* of residual pool volume filled with fine bed material correlates with annual sediment yield in channels whose parent material produces abundant sandy sediment. (4) Temporal and spatial changes in V* appear to correspond to variations in the balance between sediment inputs and water discharge. These results suggest that V* can be used to monitor and evaluate the supply of excess fine material in gravel bed channels and that samples of fine material in pools can characterize the fine, mobile mode of bed material load. The sediment load of a stream channel (the amount supplied and transported over a period of time) can be difficult to evaluate. Contributions from hillslopes can be quantified with erosion surveys and sediment budgeting, but uncertainty in routing makes it difficult to assess the load at any point in a channel network, and direct measurement of transport rates is notoriously difficult. Bed material typically furnishes the bulk of the annual bed load, especially in more distal channels, but the active volume that contributes to the load is highly variable and difficult to evaluate. It may be easier to detect variations in load by examining the mobility of the bed surface of gravel bed channels. Bed material in gravel bed channels characteristically includes a wide range of particle size, and the bed can become

Effects of Coarse Woody Debris and its Removal on a Channel Affected by the 1980 Eruption of Mount St. Helens, Washington

During the May 18, 1980, eruption of Mount St. Helens, Washington, a pyroclastic surge introduced large volumes of coarse woody debris (CWD) and fine-grained sediment to Clearwater Creek, approximately 15 km northeast of the summit. Effects of controlled CWD removal on sediment storage, substrate, and pool frequency and volume were measured in four reaches, each with three 200-m segments, from 1982 to 1990. In each reach the upstream segment served as a control with no debris removal, and CWD was totally and selectively removed from the other two segments. Unique among similar experiments are the large size and volume of CWD and the large inputs of fine-grained sediment. Except for segments of two reaches that received debris torrents, the Clearwater channel thalweg scoured until 1985. In three reaches, total debris removal caused additional scour and coarsening of the bed surface compared to segments with no or partial debris removal. Pools contracted from 1982 to 1985 and expanded afterward, especially in control segments. Total debris removal apparently caused pools to become shallower and, in segments of low sinuosity, decreased the frequency of major pools. Habitat complexity decreased after total debris removal, as indicated by a decrease in the standard deviation of residual depth and an increase in the size of substrate patches.

A Sorting Mechanism for a Riffle-Pool Sequence

Transport of coarse, heterogeneous debris in a natural stream under a wide range of flows usually results in a remarkably stable, undulatory bed profile, which manifests an in transit sorting process of the bed material. In general, finer material representative of the bulk of the normal bed load resides in the deep sections, or pools, below flood stages. At high flows, pools may scour to immovable boulders or bedrock. Coarser material transported at more infrequent flows forms the shallow sections, or riffles. Above a flow threshold, the pool fill material is often scoured and deposited in part over the riffles (Lane and Borland, 1954; Andrews, 1977). Interactions among coarse particles in motion in turbulent flow tend to concentrate them in groups with the coarsest particles at the surface (Langbein and Leopold, 1968). Incipient accumulations of coarse particles may be perpetuated by altering the flow conditions which influence bed load transport. As flow increases, submergence of the riffle-pool bed topography modifies its effect on the local hydraulic conditions. At low flow, mean volocity and water surface slope are greater, and mean depth is less at a riffle than at a pool. Competence is greater at the riffle. As the stage increases, the water-surface profile tends to even out as the hydraulic gradient over a riffle decreases and that over a pool increases (Leopold and others, 1964). Corresponding values of velocity and depth tend to converge, although depth often less so (Richards, 1976; Lisle, 1976; Andrews, 1977). The convergence of respective values of water-surface slope over a riffle and pool and the greater depth of the pool cause mean shear stress, τ = γRSE (where γ is the specific gravity of water, R is the hydraulic radius or approximately the mean depth, and SE is the energy gradient), to increase more rapidly at the pool (Leopold and others, 1964). As a result, competence as measured by velocity or bottom shear stress should become more evenly distributed over a riffle and pool (Richards, 1976) or even reversed in. hierarchy at high flow (Gilbert, 1914; Keller, 1971).

Effects of Woody Debris on Anadromous Salmonid Habitat, Prince of Wales Island, Southeast Alaska

Abstract The effects of woody debris on anadromous salmonid habitat in eight streams on Prince of Wales Island, southeast Alaska, were investigated by comparing low-gradient (1-9%) first- or second-order streams flowing through either spruce-hemlock forests or 6-10-year-old clear-cuts, and by observing changes after debris was selectively removed from clear-cut reaches. Woody debris decreased the rate of shallowing as discharge decreased, thus helping to preserve living space for fish during critical low-flow periods. Debris dams were more frequent in clear-cut streams (14.9/100 m), which contained more debris, than in forested streams (4.2/100 m). As a result, total residual pool length (length when pools are filled with water but there is no flow) and length of channel with residual depth greater than 14 cm—the depth range occupied by 84% of coho salmon (Oncorhynchus kisutch)—were greater in clear-cut streams than in forested streams. Greater volumes of woody debris in clear-cut streams produced greater...