H41D-0756
Controls on Branching in Valley Networks
Branching valley networks are a widespread planetary feature, yet significant questions remain about one of their most visually striking properties: how does the interaction between hillslopes and channels determine whether or not a valley branches, and what is the nature of the transition from the unbranched to the branched state? I address these questions by examining a simple case in which erosion is dominated by detachment-limited stream incision and slope-dependent creep, such that the long-term evolution of the topography can be modeled with a nonlinear advection-diffusion equation. Basic topographic dimensions of the equilibrium model solutions, such as valley spacing and relief, are functions of the ratio of the characteristic timescales for diffusion and advection, which can be expressed as a quantity analogous to a Péclet number, Pe. In a landscape consisting of first-order valleys, valley spacing narrows linearly as the rate of stream incision quickens relative to the rate of hillslope transport (Pe increases). This scaling regime is bounded by two critical values of Pe. The lower of these is the critical value for the formation of valleys, below which the solutions are unchanneled hillslopes. The upper value marks the onset of branching: valleys develop tributaries, and the spacing among the second-order valleys begins to widen. Two mechanisms contribute to this abrupt change in behavior. First, as Pe increases, there is a transition from a regime in which the equilibrium spacing of first-order valleys is stable with respect to perturbations in valley width or depth, to a regime in which the predicted spacing is unstable. The range of Pe over which this transition occurs corresponds to the critical Pe for branching. Second, tributaries form when Pe for the valley side slopes reaches the critical value for valley formation. The formation of tributaries accelerates the evolution of the topography away from the unstable equilibrium and toward a state consisting entirely of branched valleys. In nature, irregularities in the initial topography of an evolving landscape will drive the drainage network toward the branched state rather than the unstable state of narrowly-spaced, unbranched valleys. I compare these results with observations of experimental and natural valley networks, and explore the implications for valley network morphology at scales much larger than a hillslope length.
H41D-0757
Climatic, tectonic and lithologic controls on the size distribution of sediments supplied to channels: implications for transient evolution of bedrock river profiles
Recent theoretical, laboratory and field investigations strongly suggest that the bedload grain size distribution is a first-order control on river channel slope, bedrock incision rate, and the landscape response time to changes in tectonic and climatic forcing. As the size distribution of sediment supplied from local hillslopes and tributaries can dominate the bedload grain size distribution in a river, the processes generating hillslope sediments may exert a fundamental control on channel dynamics. Although few data are available to constrain estimates of the sediment distribution supplied to channels, we propose several broad hypotheses to guide exploratory modeling: Coarse size distributions will be favored by stronger rocks, colder and dryer climates, and by more rapid rates of rock uplift and landscape denudation; finer size distributions are favored by the opposite conditions. Here we combine models for bedrock incision by saltating bedload and the topographic dependence of precipitation and temperature with heuristic relations for the grain size distribution supplied by hillslopes to channels, to explore the sensitivity of river longitudinal profiles to variations in climate, rock strength and rates of relative baselevel lowering. We use two metrics to represent sediment size distributions, the fraction of total supply greater than a threshold size (2mm) and the median size of this coarse, bedload-size fraction. Preliminary model predictions suggest the potential for both positive and negative feedbacks in transient profile evolution. For example, accelerated rock uplift creates elevated topography favorable to cooler conditions and more efficient mechanical weathering and coarse sediment production. Increased supply of coarse sediment causes local channel steepening and greater overall profile relief, reinforcing the elevation-driven cooling effect. Conversely, enhanced orographic precipitation favors more efficient chemical weathering and fine sediment production. However, wetter conditions, combined with more rapid relative base level lowering, may also lead to increased landsliding and enhanced supply of bedrock-derived rock fragments directly to channels. Many intriguing questions emerge from this work, including the response time of hillslopes to changes in weathering regime, the relative importance of soil production and transport versus landsliding in delivering coarse sediment to the channel, and climatically- sensitive biological influences on rates and styles of coarse sediment production and weathering on hillslopes.
H41D-0758
Mining soil survey databases to explore lithologic, climatic and topographic controls on hillslope production of bedload-size rock fragments
The grain size distribution of sediments supplied by hillslopes to channel networks may strongly influence landscape dynamics at both the long time scale of landscape evolution and the short time scale of channel response to changes in land use. Little is known, however, about how lithology, climate, and the processes and rates of sediment production and transport on hillslopes, control the grain size distribution supplied to channels. A wealth of soil size distribution measurements have been collected and archived by state and local agencies, which to our knowledge have not been systematically examined to uncover patterns in the mass fraction and size distribution of rock fragments large enough to move as bedload in rivers. Here we report results obtained from data mining soil survey archives in the western United States. To focus on hillslopes with active sediment production, we exclude data from soil pits dug in valley bottoms, terraces and other topographic settings where long-term net accumulation is likely. Among the many questions that can be explored with these data, we are interested in how the mass fraction of rock fragments (>2mm) and their size distribution, averaged over the soil column, varies with factors such as depth-to-bedrock, hillslope gradient, lithology, and mean annual temperature and precipitation. We are also looking for variations with depth within the soil column, that might provide insight into soil disturbance, transport, and weathering regimes. Several general patterns emerge from our work to-date. A large fraction of hillslope soil pedons do not contain any rock fragments, with lithology possibly the dominant control. Where the rock fragment mass fraction is greater than about 0.3, there is commonly an increase in rock fragment concentration with depth within the soil column. Pedons with lower depth-averaged rock fragment concentrations more commonly show no gradient in concentration with depth, suggesting that the disturbance processes that produce well-mixed soils are effective at mechanically weathering rock fragments. Steeper hillslope gradients correlate with greater rock fragment concentrations, but surprisingly we see no significant correlation with depth-to-bedrock. In general, rock fragments constitute a mode distinct from the sand-sized and finer portion of the bulk soil size distribution, supporting the use of bi-modal distributions in modeling of sediment supply to channels.
H41D-0759
Numerical Study of Growth and Degradation of Fluvial Hanging Valleys due to Climate Variability
An increased rate of base level fall may cause a knickpoint to migrate up a trunk stream and tributaries within a drainage basin. Recent studies have focused attention on those tributaries that are unable to incise as quickly as the trunk stream. As a consequence, hanging fluvial valleys form at these tributary mouths. Such stationary knickpoints at tributary mouths are uncommon, and where found, their height and occurrence is limited. Given a theory for how such hanging valleys can form, this study addressed the question of why they are not more common. A numerical model of bedload-saltation erosion for a tributary junction experiencing base-level fall demonstrates how such hanging valleys may form and subsequently degrade through perturbations in climate- controlled parameters. Increased frequency of bedload mobilization and enhanced bedload supply can drive the degradation of hanging fluvial valleys. In particular, when channel aggradation overtops a knickpoint, the knickpoint tends to be removed during subsequent degradation of the alluvial surface. Channel narrowing, larger bedload clasts, and increased bedload supply are factors that could inhibit the formation or maintenance of hanging fluvial valleys. Although bedload-saltation models predict that hanging valleys should be quite common in regimes of rapid erosion, the frequency and magnitude of climatically induced change in sediment loads typically overwhelms those factors that, in the absence of such bedload variability, promote hanging valley formation.
H41D-0760
Channel Morphology Response to Differential Rock Uplift Rates in the San Gabriel Mountains, CA
The San Gabriel Mountains in southern California provide an excellent opportunity to study landscape response to both spatial and temporal rock uplift patterns. Both published low temperature thermochronometry data and our preliminary cosmogenic erosion rate data show a strong gradient in uplift rates from west to east, ranging from <0.1 mm/yr to >1.0 mm/yr respectively. Cosmogenic 10Be in stream sands from 13 basins have already been measured, and 50 more are currently in progress at Purdue University's PRIME Lab. These erosion rates will be a major focus of this study, revealing in detail the nature of the relationship between uplift rates and landscape morphology, including the channel steepness index. These erosion rates also allow for analysis of correlations between rock uplift rate and channel morphology (e.g. width, bed state). Detailed field surveys in summer 2007 conducted with a laser rangefinder provide georeferenced data on channel geometry, bedrock exposure, median and maximum grain size, and valley width for ~25 km of 7 channels, spanning millennial-scale erosion rates from 0.05 to 0.4 mm/yr. Preliminary observations focus mainly on channel response across knickpoints recording landscape response to a temporal increase in rock uplift rate, and detail the partitioning of bedrock and alluvial reaches in over-steepened zones. Much of the relief across knickpoints is expressed in waterfalls and steep bedrock reaches that interrupt alluvial reaches (likely transport-limited). In addition, though channel width appears to decrease with increasing channel steepness index, there is a tendency for over-narrowing in knickzones that is not matched in adjusted high uplift zones. Indeed, knickpoints, especially the tectonically induced knickpoints along the Big Tujunga, consistently produced steep, narrow, inner gorges that are uncommon elsewhere in the landscape, suggestive of a sediment-flux control on channel width. More field data is needed to answer the question of whether this over-narrowing is a transient condition or if it is simply the result of increasing rates of incision.
H41D-0761
Modeling Fluvial Incision and Transient Landscape Evolution: Influence of Dynamic Channel Adjustment
Channel geometry exerts a fundamental control on fluvial processes. Recent work has shown that bedrock channel width (W) depends on a number of parameters, including channel slope, and is not only a function of drainage area (A) as is commonly assumed. The present work represents the first attempt to investigate the consequences, for landscape evolution, of using a static expression of channel width (W ~ A0.5) versus a relationship that allows channels to dynamically adjust to changes in slope. We consider different models for the evolution of the channel geometry, including constant width-to-depth ratio (after Finnegan et al., Geology, v. 33, no. 3, 2005), and width-to-depth ratio varying as a function of slope (after Whittaker et al., Geology, v. 35, no. 2, 2007). We use the Channel-Hillslope Integrated Landscape Development (CHILD) model to analyze the response of a catchment to a given tectonic disturbance. The topography of a catchment in the footwall of an active normal fault in the Apennines (Italy) is used as a template for the study. We show that, for this catchment, the transient response can be fairly well reproduced using a simple detachment-limited fluvial incision law. We also show that, depending on the relationship used to express channel width, initial steady-state topographies differ, as do transient channel width, slope, and the response time of the fluvial system. These differences lead to contrasting landscape morphologies when integrated at the scale of a whole catchment. Our results emphasize the importance of channel width in controlling fluvial processes and landscape evolution. They stress the need for using a dynamic hydraulic scaling law when modeling landscape evolution, particularly when the uplift field is non-uniform.
H41D-0762
Structural Controls on Channel Geometry and Dynamics in the Peikang River, Central Taiwan
The Western Foothills of central Taiwan contain numerous rivers flowing nearly orthogonal to a series of west- verging thrust sheets. This relationship offers an exceptional opportunity to document channel dynamics across active faults. The study focuses on a ~45 km stretch of the Wu River and its principal tributary, the Peikang River. Geomorphic data, including channel width, slope, bed cover, and grain size, are derived from a combination of field mapping and DEM analysis, and are used to estimate unit stream power along the channel. Correlation and dating of strath terraces provide estimates of late Quaternary incision rates. Incision rates, terrace deformation, and estimated stream power all correlate with mapped structures. Estimated incision rates are fairly high but spatially variable (0.35-4 mm/yr). Normalizing channel width by contributing drainage area exhibits a strong correlation between channel width and mapped structures, revealing channel width as the first-order control on variations in stream power. Regions where thrust faults have been mapped show significant narrowing in contrast to anomalously wide reaches where synclines have been proposed. Analysis of the longitudinal profile and normalized steepness indices does not show a strong correlation between gradient and incision rate suggesting that channel bed elevation is readily adjusted to seismic events through incision and/or changes in alluvial cover. We combine this interpretation with field observations of stream response to recent seismic events along the Cho Shui River to formulate a working hypothesis explaining the long term narrowing of streams over active thrust faults. This work underscores the necessity to understand the processes and rates of channel widening for interpreting tectonics from channel geometry.
H41D-0763
Lithologic Control on the Form of Amphitheater-headed Channels and the Influence of Seepage Erosion vs. Downstream Incision on Rates of Waterfall Retreat
The use of amphitheater-headed channels as indicators for groundwater sapping on Earth and Mars was recently challenged by Lamb et al., (2006, 2007) who demonstrated that this form-process relation is not unique. A field study of 27 channels with amphitheater-headed valleys along the Dead Sea western tectonic escarpment identified seepage indications only at 7 channels and seepage-related sapping at 2 of these 7 channels. These findings support the idea that amphitheater-headed valleys can form across waterfalls regardless of seepage erosion. Major controls on the amphitheater morphology of the studied channels include waterfall height and especially the height of the waterfall erodible subcaprock face ( Hscap), which dictates the length of talus slopes along the canyon walls adjacent to the waterfall. The characteristic width of the amphitheater can be approximated by: 2 Hscap/tanα + dpp where α is the talus angle of repose and ( dpp) is the plunge pool diameter. Amphitheatre morphology is less pronounced and valley plan form is V- shaped across waterfalls with low Hscap. Utilizing the downstream rate of change in valley width ( dw/dx) we define a V-plan form as a condition where dw/dx is uniform and a U-plan form where dw/dx decreases downstream. We demonstrate that dw/dx is a positive function of channel gradient ( dz/dx) and argue that rapid downstream decrease in channel gradient can contribute to a U-plan form. Commonly found debris-induced oversteepened reaches below waterfalls are therefore another possible trigger to amphitheater morphology. Waterfalls within two of the escarpment stretches we have studied have quasi uniform subcaprock face height (i.e., similar toe stratigraphic position) independent of drainage area and retreat distance from the escarpment outlet. This indicates that their retreat rate and the rate of downstream incision are tightly interdependent. Retreat rates of these waterfalls are probably set by base level lowering and incision wave velocity at a downstream transition to a resistant formation. This velocity influences the efficiency of coarse debris evacuation (transportation and weathering) through its effect on the length and the gradients of the reach between the resistant formation and the waterfall. Under such conditions a theoretical onset of seepage along the contact marked by the upper end of the subcaprock talus slope at the waterfall face will not affect retreat rates assuming coverage of this contact by talus debris can suppress seepage-induced erosion. We demonstrate, however, that there are plausible theoretical cases where Hscap can vary over time and seepage can influence waterfall retreat rates for Myrs. We also show that groundwater sapping observed in two of the waterfalls we have studied probably still effects their retreat rate.
H41D-0764
Valley Width Variation Controls on Riffle Location and Persistence on a Gravel Bed River
Numerous studies have examined the effect of constrictions on the formation of riffles and pools in rivers. Low velocity backwater effects are created above the constriction, with steep water surface slopes and flow acceleration after it. Such flow conditions cause varying sediment transport capacity, influencing the formation of riffles and pools near constrictions. Studies have shown that local bedrock outcrops, alluvial fans, sediment bars, and wood are capable of functioning as constrictions, but few address channel-confining valley walls as potential constrictions. Can variations in valley width influence riffle location and persistence? To address this question, research was performed on a valley-confined wandering gravel-bed reach of the regulated lower Yuba River from the Narrows Pool to the Highway 20 bridge. This reach has an average valley width of 164 m (range of 102-314 m). During 1853-1884, hydraulic gold mining filled this 7-km reach with ~26 m of mixed coarse sediment (totaling ~9-18 million cubic meters), creating a dynamic fluvial landscape comparable to recently deglaciated wandering gravel-bed streams. The hypothesis of this study was that riffles that persist over decades on wandering gravel-bed rivers are controlled by valley width variations. The study objectives were to (1) assess planform and elevation changes over 22 years and (2) correlate locations of persistent riffles with valley- wall constrictions. ArcGIS was used to measure valley width and changes in planform wetted area in georectified photo sets of this reach for 1984, 1986, 1991, 1996, 2002, 2004, 2005, and 2006. Detailed river valley DEMs were also available for 1999 and 2006 for measuring the recent incision rate, and pattern of the valley fill. Riffle crest locations for each year were plotted along with the valley width versus distance upstream, and all riffles crests that persisted for the entire duration were identified. The persistent riffle crests were statistically analyzed to various metrics of their associated downstream valley wall constriction. The aerial photo analysis revealed that over the past 22 years, the river has experienced significant lateral change in response to blocked sediment input, and frequent floods. River valley DEM differencing revealed that over the 7-year period, 463,000 cubic meters of sediment were scoured out of the reach, with an overall lowering of valley fill elevation. Despite the dramatic changes that have occurred over the past two decades, 7 riffles persisted in the same location. Among the metrics analyzed, a strong linear correlation was found between the riffle crest location and the midpoint between downstream and upstream constrictions. The seven riffles crests, that persisted, were within an average of 0.9 channel widths (standard deviation of 0.6 channel widths) of the midpoint between the downstream and upstream constriction. Wandering gravel-bed rivers provide complex aquatic habitat, and identifying controls on riffle locations and persistence, such as valley constrictions, will aid in managing aquatic species.
H41D-0765
Lithologic feedbacks between valley and channel form in canyons of southeast Utah
Does the slope of an incising channel decrease, increase or stay the same when cutting across a lithologic contact from a stronger to a weaker rock unit? It depends--all of these patterns of incision occur at a particular lithologic contact along bedrock channels in southeast Utah, near the Henry Mountains and Navajo Mountain. We interpret that changes in longitudinal slope at the contact result from (1) valley-channel coupling and local sediment supply, in which coarse sediment from the more resistant unit above mantles the channels below, and (2) variations in the amount of bedload transported from upstream, which influences the incision of both rock units. We compare channels near the lithologic contact between the Wingate sandstone above (a well cemented aeolian cliff-former that is resistant to weathering and is jointed at the scale of meters to several meters) and the Chinle formation below (a mudstone which contains occasional resistant sandy and carbonate-rich lenses but overall is a slope-former that weathers easily). Along many channels (e.g. Fourmile Canyon), longitudinal slope increases through the weaker Chinle mudstone: once the lower unit is exposed, the overlying Wingate sandstone is undermined by Chinle slope weathering which in turn dumps resistant colluvial boulders of Wingate into the channel and armors the bed. A second key variable that influences slope at the lithologic contact is the amount of bedload supplied from upstream (i.e. transportable sediment that is not locally derived). Channels with high coarse sediment supplies from upstream are incisional but effectively alluvial and show little to no break in slope associated with the lithologic contact (e.g. Trachyte Creek, Swett Creek). In contrast, channels with negligible bedload supply appear to have difficulty incising through the Wingate sandstone, leaving very steep channels or cliffs at the Wingate-Chinle contact (e.g. Trail Canyon at Navajo Mountain). Channels with moderate bedload supply cut relatively narrow slot canyons through the Wingate sandstone and then steepen at the Chinle contact, as described above. Together, these field observations demonstrate ways that bedrock lithology can influence patterns of channel incision.
H41D-0766
Relation Between Width, Water Discharge and Bed Load Concentration
In alluvial rivers, the width of the channel that conveys the majority of the water should also convey the majority of the bed load. Channel width thus scales with both water discharge and sediment discharge. Interactions among the different variables controlling channel width are examined here by coupling the equations for discharge, Q, and total bed load transport rate, Qb, using width as a common variable. A relation for bed load concentration, Qb/Q, is formulated from a set of variables linking excess shear stress, grain size and flow depth. Downstream trends in Qb/Q are then computed for a gravel-bed channel characterized by an exponential profile and an arbitrary but plausible downstream fining relation. The results suggest that the downstream trends in Qb/Q are strongly dependent on the assumed relation for excess shear stress: If the difference between bankfull Shields stress and critical Shields stress is constant, as field data indicate, then bed load concentration decreases slowly downstream. This scenario could reflect conditions where the supply of coarse sediment is decreasing downstream due to lower overall relief, and/or the bed load is being milled into finer sizes that are transported in suspension. If the difference between bankfull shear stress and critical shear stress is allowed to decrease, then bed load concentration decreases very rapidly downstream. This scenario is not particularly realistic but could reflect conditions where sediment supply is diminishing and the bed is becoming increasingly armored downstream. If the difference between bankfull Shields stress and critical Shields stress is allowed to increase downstream, then Qb increases almost in proportion to Q, and thus bed load concentration is nearly constant. This last result suggests that the observed hydraulic geometry relations for width, depth, and velocity may reflect a hidden relation to sediment load, which under conditions of increasing excess shear stress appears to scale almost linearly with discharge.
H41D-0767
Why Rivers Flood: Implications of the Elimination of Overbank Flows Through Channel Widening in a Laboratory Flume
We performed a series of laboratory flume experiments to document the adjustment of straight channels carved from non-cohesive sediment to overbank flows. We first created a stable channel adjusted to a bankfull discharge. Then, the discharge was abruptly increased to create an overbank flow. In all of our experiments, the channel widened during overbank flow until the increased discharge could be accommodated within the channel, suggesting that bank erosion in the absence of deposition will tend to eliminate flooding. We propose two dimensionless numbers to evaluate these processes in nature. The first, termed the Flood Elimination Number (FLENUM), represents the ratio of the duration of flooding to the time required for bank erosion to fully capture overbank flows during a single flood. The second, termed the Channel Stability Number (CHASNUM), evaluates the maximum length of channel that can accommodate the additional sediment supplied by widening without catastrophic deposition or changes in channel planform (such as braiding). Our experiments suggest that a critical value of the FLENUM is ~0.1. For FLENUM values higher than 0.1, rivers will accommodate flooding through channel widening, likely leading to braiding. As expected, braided rivers have high FLENUM values on the order of 10. Rivers with FLENUM values lower than 0.1 should endure overbank flows without significant widening, and indeed single-thread meandering rivers with cohesive banks have FLENUM values around 0.01. In our experiments, the CHASNUM was approximately 50 river widths, approximately equal to the working length of our experimental channel, which explains our ability to successfully maintain a stable, single thread channel despite extensive channel widening. The CHASNUM may provide an interesting explanation for the observation of periodically spaced disturbance zones in some gravel-bed rivers.
H41D-0768
Measuring the Erosion of River Channel Widths Impacted by Watershed Urbanization Using Historic Aerial Photographs and Modern Surveys
Land use in a watershed exerts a strong influence on trunk channel form and process. Land use changes act over human time scales which is short enough to measure their effects directly using historic aerial photographs. We show that high-resolution topographic surveys comparing channel form for paired watersheds in the Lehigh Valley, PA are indistinguishable, but have channel widths that have changed dramatically in the past five decades. The two watersheds, Little Lehigh Creek and Sacony Creek, are similar in all respects except they have different amount of urban land use. Aerial photographs of the urbanized Little Lehigh Creek show that a majority of the measured widths (67 of 85) were statistically wider in 1999 than in 1947. In contrast, the measured widths from the agricultural Sacony Creek are more evenly distributed among those that widened (18), narrowed (28), and those that were statistically unchanged (6) from 1946 to 1999. From 1946 to 1999 the only section of Sacony creek that widened was that reach downstream of the only sizable urban area in the watershed. The current land use in Sacony Creek watershed resembles that of 1946, while the Little Lehigh Creek watershed has more than tripled its urban area. These data suggest that the increase in urban areas that subsequently increases peak discharges is the mechanism behind the widening that occurred in the Little Lehigh Creek. These wider channels can affect water quality, aquatic habitat, suspended sediment loads, and river aesthetics.
H41D-0769
Quantifying River Morphology in Arctic Streams: Remote Sensing and Field Based Measurement of Fluvial Response to Climate Change in Northern Alaska
Though hydraulic geometry in temperate regions has been extensively studied, the remote nature of Arctic rivers has inhibited their characterization. As modern warming progressively impacts arctic hillslope stability, hydrology and network density, it becomes imperative to establish an empirical and mechanistic understanding of the factors that shape northern streams. Toward these ends, we employ DEM analysis (60m and 5m cellsize), satellite (ASTER) and aerial imagery as well as extensive field observations to characterize the form and function of Arctic rivers and their drainage networks. Field and remotely sensed measurements of bed state, discharge, slope, width and drainage area are used to establish scaling relationships that can be compared to channels of similar morphology in temperate regions. Parameters from arctic rivers that are comparable to their temperate equivalents shed light onto which aspects of channel form are insensitive to bed-fast ice, permafrost dominated banks and extreme spring thaw events. Those channel parameters that are unique to the Arctic highlight the aspects of that system that are most susceptible to change. Our measurements, which range over 5 orders of magnitude in drainage area, also establish baseline conditions to which future changes in arctic river geometry can be compared. Changes in arctic river geometry influence not only the chemical, biological and hydrologic components of the Arctic system but also impact native subsistence hunting practices and the logistics of resource extraction.
H41D-0770
Insights About Channel Width Controls Based on Channel Narrowing Below Dams
All channels narrow under conditions of flood reduction, such as downstream from dams, but the magnitude of narrowing is highly variable. Channel width changes on 61 dam-impacted reaches covering more than 4000 km of six large rivers of the western United States (Missouri, Rio Grande, Colorado, Trinity, Snake, Deschutes) were reviewed in an extensive literature search, supplemented by field studies on the Rio Grande, Colorado River and its tributaries, and Snake River. These studies demonstrate that narrowing occurs under conditions of post-dam sediment deficit and surplus, indicating that the primary control on width is not sediment mass balance. However, channel width changes are not related to flood reduction in a simple way, such as would be predicted by downstream hydraulic geometry relations. Narrowing is of larger magnitude on rivers with large suspended sediment loads, and is less obvious on gravel-bed rivers in sediment surplus. Narrowing occurs where beds incise and where this is not the case. Narrowing occurs in the presence or absence of invasive, woody riparian vegetation. Large-scale channel organization, such as distinguishing between meandering and debris-fan affected channels, has a strong effect on the magnitude of narrowing, primarily because the mechanism of narrowing differs between these major channel types. We examined channels whose post-dam 2-yr flood is as little as 0.20 times the pre-dam flood and where channel width narrowed by as much as 0.15 times of the pre- dam width. Field studies were based on extensive analysis of historical data as well an dendrogeomorphic interpretation of floodplain sediments.
H41D-0771
Coupling annual and decadal patterns of sediment flux with channel morphology in gravel- bedded mountain streams
Channel morphologic parameters that may be expected to change in response to changes in driving variables of streamflow and sediment supply include bed material grain size, bed configuration, channel width and depth, and gradient. Determination of which variable is most likely to respond to a particular perturbation depends on the type and magnitude of perturbation and the time scale of interest. Channel width is often of primary interest because it has the potential to adjust on a decadal time frame and because it is a typical criterion in the evaluation of stream health and function. Changes in channel width associated with changes in flow and sediment flux on large sand-bed rivers have been frequently described. We have initiated a project to evaluate the coupling between changes in flow and sediment flux and channel morphologic variables and on a gravel-bedded mountain stream. On the Cub River in southern Idaho, a series of streamflow diversions have caused changes in the average pattern of runoff. Peak flows are lower in magnitude and of shorter duration in segments affected by greater proportions of floodwater extraction. Field measurements of sediment transport and calibrated transport relations indicate these changes in streamflow have significantly altered the longitudinal sediment flux. Annual sediment flux significantly decreases downstream in years of average to high runoff, in contrast to the stable or increasing flux that our models predict for undiverted conditions. However, have these changes affected channel morphology to a degree that can be detected on a gravel-bedded mountain stream over a period of ~100 years? Detailed longitudinal measurements of bed elevation, channel width, bed material size, bed mobility, and channel configuration indicate that with increasing magnitude of flow extraction (1) channel width and channel width variability decreases, (2) bed elevation variability increases, (3) bed mobility increases, and (4) there is a greater abundance of gravel bars that are in-channel sediment storage sites. We compare these patterns of channel characteristics with the sediment mass balance to determine the relative importance of changes in channel width and in-channel sediment storage in accommodating the estimated longitudinal imbalance of sediment supply and export.
H41D-0772
Movable Bed Laboratory Modeling Study for Channel Degradation and Aggradation
Sediment erosion and transporting from upstream basin is one of the main factors for natural alluvial channel bed degradation and aggradation. Downstream of river hydraulic structures crossing a channel such as dams and reservoirs, both the water and sediment supplies can be altered principal to adjustments in the river channel bed morphology. In case of a compounded double-cross sectional channel with floodplain, it is commonly observed that both dynamic responses of alluvial stream refer to channel bed degradation and aggradation. With the limited inflow sediment discharge, channel bed erosion and sediment deposition with vegetation are developed in main channel and in floodplain, respectively. In this study, alluvial channel bed degradation and aggradation due to different inflow sediment discharge were investigated using a movable bed physical model. A 0.6 m wide and 0.8m high rectangular section experimental flume was set with 20m long. A 20m long rectangular section (0.6 m wide and 0.8m high) experimental flume was used for the model. Medium particle diameter 1 mm of sand was covered with 0.15 m depth in the flume. To investigate hydraulic impact of inflow sediment discharge and channel slope on the channel bed elevation changes, three sediment discharges (0 %, 50 %, and 200 %) and three channel slopes (1/400, 1/500, and 1/1000) were tested over three different flow conditions of approaching uniform water depths (0.05 m, 0.10 m, and 0.15 m). Channel bed level changes were measured with surface topologic digital imaging system and generated three dimensional channel surface contour map. The results of the experiments showed that inflow sediment discharge gives a significant impact on alluvial channel bed geometry. Channel bed incising on the main channel was increased in proportion as test run time and for that reason, bed level divergence between the main channel and the floodplain was increased. It was found that alluvial channel dynamic transformation from a compounded double-cross section stream a single-section channel can be controlled with inflow sediment discharge.
H41D-0773
Non linear bend instability theory and finite amplitude evolution of bed deformations in meandering rivers
We develop a three dimensional non linear asymptotic theory for flow and bed topography in meandering channels able to describe finite amplitude perturbations of bottom topography. The model extends a previous analysis on the equilibrium finite bed deformations, accounting here for arbitrary, yet slow, variations of channel curvature. This approach then allows us to formulate a non-linear bend instability theory, which predicts several characteristic features of the actual meandering process and extends results obtained by classical linear bend theories. In agreement with previous weakly non linear findings and consistently with field observations, the bend growth rate turns out to have a peak at some value of the meander wavenumber, typically larger than the resonant value of linear stability theory. Moreover, a feature typical of non linear waves arises: the selected wavenumber depends on the amplitude of the initial perturbation and, in particular, larger wavelengths are associated with larger amplitudes. The picture offered by results obtained through the present theory seems fully satisfactory and consistent with field observations as well as previous theoretical findings. Further substantiation of the model has been achieved by comparing predictions obtained for a test case (a reach of the Cecina river, Italy) with field observations. Finally the model is also extended to follow the evolution of bed deformations in time in order to investigate the morphological response of the river to a sequence of flood events characterized by a slow temporal variation of flow and sediment supply. Such an investigation would possibly provide a rational interpretation of the as yet loosely defined notion of formative discharge of an alluvial river.