H53C-1253 1340h
Differential sediment entrainment theory for the maintenance of pool-riffle sequences: evidence from in-situ measurements of friction angle
Theories for the maintenance of the pool-riffle sequences have evolved to include consideration of the nature of the bed sedimentology and how this responds to the different low-flow hydraulic characteristics. According to this theory, high frequency turbulence experienced over riffle surfaces during low discharge result in a distinct packing and surface sediment structure that restricts the entrainment of particles. In contrast, pool sediments do not experience such high frequency turbulent flow, and the sediments are effectively "overloose" in comparison. As a result, the difference in critical shear stresses developed between pool and riffle bed sediments results in greater entrainment rates and transport of sediments from pools, whilst riffle surfaces essentially remain immobile. Evidence for the existence of sedimentological contrasts is relatively well established, but explicit links to the entrainment of particles is still untested. This poster reports a new method for testing the in-situ friction angle distribution of water-worked sediments from submerged areas of the river bed. This permits the quantitative analysis of friction angles and particle exposure from pool and riffle bed surfaces. A physically-based method for calculating critical shears stress is applied and the contrasts between pool and riffle identified. The data confirms the existence of variations between un-worked and water worked sediments and between pool and riffle sediments. The magnitudes of these differences vary over time according to the sediment transport dynamics experienced. The theory of differential sediment entrainment therefore provides a partial explanation for the maintenance of riffle pools in this study.
H53C-1254 1340h
A conceptual model of the formation of the meso-scale geomorphology of lowland forested floodplains
Within forested floodplains geomorphological processes are strongly modified by live and dead vegetation. However, interactions between the vegetation, water and sediments are poorly understood. Observations made along semi-natural rivers in the New Forest, UK suggest the presence of particular suites of landforms on the floodplain that owe their formation to such interactions. For example extensive networks of floodplain channels and the development of discrete areas of sand shadows. These features are only found in semi-natural reaches which have a high sinuosity, high frequencies of debris dams, longer sediment and organic matter residence times, and therefore experience relatively high frequencies of overbank flows during the flood season. This research explores the occurrence of such features and proposes a conceptual model for their formation. This model integrates the development of in-channel debris dams with the processes operating on the floodplain. The conceptual model is then applied in order to help set monitoring targets for a habitat restoration programme aimed at restoring wet woodlands along degraded lowland watercourses.
http://www.newforestlife.org.uk
H53C-1255 1340h
Experimental Channel Response to Tectonic Uplift
We performed a series of experiments to investigate the relationship between channel geometry and tectonic forcing in steady state landscapes at various uplift rates. The experiment consists of uniformly uplifted silica paste eroded by artificial rainfall. The landscape evolves by growth and amalgamation of incisions and organises into a drainage network. High precision (0.5mm pixel size) digital elevation models were taken by a stereometric camera system. Channel bed slope is found to be independent of discharge and position on the experimental surface and increases linearly with uplift rate. This suggests it is controlled by global conditions rather than local channel processes. Using hydraulic assumptions the channel parameters width, depth, cross-sectional area, wetted perimeter and hydraulic radius were derived. We find that the channel width, cross-sectional area and wetted perimeter decay exponentially with uplift rate. The hydraulic radius is approximately constant and the channel depth rises linearly with uplift rate. These results are qualitatively consistent with recent field surveys.
H53C-1256 1340h
River Bank Erosion at the Intra-Event Timescale: Implications for Bank Sediment Delivery
Bank erosion is often a significant contributor to catchment sediment yield. However, it has proved difficult to gain insight into the relationship between erosion events that supply bank material to river systems, and associated variations in fluvial sediment flux. One of the difficulties has been that there is a mismatch in our capability to quantify processes at relevant timescales. Thus, while fluvial flux can readily be monitored quasi-continuously during competent flow events, studies of bank erosion processes have largely been focused at the event timescale. A lack of sub-event scale data is understandable given the difficulties involved in accessing river banks during competent flows. Nevertheless, bank sediment delivery processes involve a combination of (quasi-continuous) fluvial erosion and (quasi-discrete) mass-wasting processes. Moreover, relatively little research has been focused on the dynamic interactions between these two groups of processes, especially in terms of quantifying the extent to which different bank erosion processes might dominate in different environments. To address these issues, we have been investigating bank sediment delivery processes at two study sites. The Fiume Cecina (central Italy) is an actively migrating (c. 10 m/yr) gravel-bed river, which constrasts with the River Asker (southern UK), a gravel-bed river with relatively slow (c. 0.1 m/yr) retreat rates. At each site we have developed simulation models to predict modes of bank sediment delivery during observed flow events. Our approach follows previous investigations in that event hydrographs are initially discretised into a series of time steps. Finite-element seepage analysis and slope-stability modelling software packages are then used to simulate bank pore water pressure and mass-failure conditions at each time step, to quantify their evolution throughout the flow event. Where our approach differs from previous studies is that we have used Computational Fluid Dynamics (CFD) simulations to estimate the boundary shear stress distribution exerted on the banks in each time step. Consequently, the bank profile is updated in response to any deformation caused by fluvial erosion. As a result, our simulations are able to highlight two contrasting roles that fluvial erosion has in triggering bank failures during flood hydrographs. First, fluvial erosion destabilises the bank with respect to mass failure by steepening the bank profile. However, this bank deformation also results in a modified distribution of bank pore-water pressures relative to cases where there is no fluvial erosion. A possible implication of our results is that conceptual models of bank sediment delivery processes founded on event-scale analyses may be misleading. Previous modelling studies have emphasised mass-failures as quasi-catastrophic events timed to occur on the falling limb of event hydrographs. In contrast, our simulations suggest that mass-wasting might also occur as a series of much smaller-scale episodes, timed at frequent intervals throughout the event hydrograph. It follows that the residence time of bank material debris delivered to the bank-toe may be much shorter than suggested by approaches that do not account for bank deformation processes at the intra-event time scale. Further research is required to investigate this hypothesis.
H53C-1257 1340h
Modeling pool response to sediment pulses
Distinct sediment pulses following wildfires, landslides and releases from dam reservoirs exert a strong influence on channel morphology. Recent theoretical, experimental and field studies have found that large sediment inputs are often transported as a single pulse that is either translated or dispersed, depending on the grain size of the ambient bed material and the pulse itself. Our ability to route the sediment pulse down the channel requires an understanding of the dominant transport mechanism (translation, dispersion) as well as the reach-scale patterns of erosion and deposition due to pools, bars and riffles. The goal of this work was to determine the processes involved in reach-scale channel adjustment following a pulse of sand over a coarse bed, using detailed measurements of three-dimensional flow and sediment transport patterns. We designed experiments on flume channels at the Sierra Nevada Aquatic Research Laboratory that included alternate pool-riffle sequences, thus greatly improving our approximation of natural conditions. Results found substantial translation as well as dispersion of the topographic high point of the pulse. On a reach-scale the patterns of erosion and deposition closely follow the hydraulics. The pulse was transported through the riffles and the bar-pool topography showed the most significant channel change. As the pulse was mobilized, the pool thalweg was reestablished and significant lateral dispersion occurred, resulting in substantial bar growth in areas of flow separation. The bars became relatively stable zones of sediment storage within the channel that did not undergo subsequent erosion, while the pool head, center and tail were gradually eroded to ambient conditions. These experimental results will be used to evaluate current theoretical models and develop a process-based understanding of pool response to sediment pulses. Findings from this work will significantly improve our ability to include reach-scale patterns of erosion and deposition when routing sediment pulses through river systems.
H53C-1258 1340h
Can Springs Cut Valleys Into Bedrock?
Valleys formed from groundwater sapping are thought to have a characteristic form including steep walls, flat floors, and amphitheatre-like heads. Observations of these features on Earth and Mars have led to the morphologic-based interpretation that groundwater sapping is an important valley forming process. This interpretation has significant implications for Mars in particular because it has been used to constrain Martian hydrology and the associated prospects for life. However, a mechanistic understanding of sapping erosion has only been demonstrated for granular mediums (i.e. sand). Many of the "sapping" valleys on Earth and (likely) Mars have been carved into bedrock, and the extension of previous work to bedrock erosion is unclear. To our knowledge, a process-based understanding of seepage erosion in bedrock does not exist, even though it is thought to be a first order geomorphic process on Earth and Mars. In order to address this knowledge gap, we are currently investigating Box Canyon, Idaho. Box Canyon, developed in the Snake River Plain, has many of the morphologic features often associated with sapping valleys. In addition, it was carved into basaltic bedrock and has a large spring emanating from its amphitheatre-like head, making it an ideal candidate for a sapping origin. There is currently no overland flow contribution to the canyon; however, based on mapping bedrock scours, a paleo-flood from an unknown upslope source did enter the canyon (and perhaps carved it). We present some first order hydraulic measurements, sediment transport calculations, and field observations to try and constrain the types of flows needed to carve Box Canyon. These flows could conceivably be derived from expansion of the current spring. Direct observation at the head of the canyon has not yet indicated how sapping could be responsible for the erosion of the headwall. We are using various dating techniques to constrain the timing and rate of headwall migration to constrain the possible mechanisms responsible for canyon development.
H53C-1259 1340h
Discrimination between mountain stream channel types using independent control variables
We use a large and diverse dataset from mountain streams around the world to explore our ability to classify reach-scale channel morphology using easily measurable control variables. The dataset includes 136 step-pool reaches, 44 plane-bed reaches, and 93 pool-riffle reaches from streams in the western United States, Panama, and New Zealand. We used stepwise discriminant analysis to select the most parsimonious subset of variables for classifying channel type. A 3-variable discriminant function using slope, D84, and channel width produced a classification error rate of 24% (103 reaches correctly classified). Seventy percent of plane-bed reaches were correctly classified (16% incorrectly classified as pool-riffle, 14% incorrectly classified as step-pool). Sixty-seven percent of pool-riffle channels were correctly classified (31% incorrectly classified as plane-bed, 2% as step-pool). Eighty-nine percent of step-pool reaches were correctly classified (9% incorrectly classified as plane-bed, 2% as pool-riffle). The partial R2-values indicate that slope is by far the most significant single explanatory variable. The ability to accurately classify channel type in other regions using the elegant 3-variable discriminant function developed from the entire dataset has important implications for water-resources management and for understanding relationships between process and form in mountain streams.
H53C-1260 1340h
The Development of Gullies on the Landscape: a Model of Headcut Retreat Resulting From Plunge Pool Erosion
Head advance due to plunge-pool erosion is a common process in gullies incising resistant soils. A model of headcut retreat resulting from plunge-pool erosion is developed and implemented in CHILD, an existing 3D landscape evolution modeling framework. The model estimates horizontal headcut retreat as a function of discharge, height of the headcut, upstream slope and relevant land surface and soil properties for soil erosion. The physical model results compare well with the published data from flume experiments. We analyzed the sensitivity of headcut retreat to flow discharge, upstream slope and surface roughness, and headcut height. CHILD simulations indicate that headcut retreat is most significant in zones with either gentle slopes or large headcut heights. Model parameters have contrasting effects on the retreat rates depending on the size and depth of the pool beneath the headcut, and upstream flow hydraulics, making the process difficult to predict as a function of topographic thresholds and simple geomorphic transport laws.
H53C-1261 1340h
Bed Sediment Characteristics and Sediment Movement in a Sub-arctic Watershed
The project on bed sediment characteristics and bed sediment displacement is being conducted in the Caribou-Poker Creeks Research Watershed (CPCRW), which is reserved only for scientific research. The watershed is located about fifty miles northeast of Fairbanks, Alaska. The fieldwork started in the summer of 2003 and was continued during late spring and summer of 2004. The sediment samples were taken from fifteen different locations in five streams: Poker Creek, Caribou Creek and its tributaries (C2, C3, C4). The rocks were analyzed in labs at the University of Alaska Fairbanks. Zingg diagram was used to obtain the stones' shape. The results indicate a high percentage of blades and discs in the rocks analyzed. The particles were returned to their initial locations after they were painted and numbered. The sampling sites were located in seven different groups in the watershed, namely: Caribou, Poker, C2, C3, C4, CB, and weather station. Travel distance was measured during the warm season. Measured displacements, in meters, are in the order of a single digit. The maximum recorded movement was 9.70 m and it was registered in the Poker Creek. Data collection will be continued in the future along the streams to obtain the bed sediment variation.
H53C-1262 1340h
Suspended-Sediment Transport Where Rivers Become an Estuary: Sacramento-San Joaquin River Delta, Water Years 1999-2002
Tidal influence from San Francisco Bay extends landward into the Sacramento-San Joaquin River Delta at the confluence of the Sacramento and San Joaquin Rivers. The interplay between the riverine and estuary forcing has created a complex network of channels and tidal wetlands which has been significantly modified by humans over the past 150 years. Interest in restoring tidal action and sedimentation to some diked and subsided former tidal wetlands lead the U.S. Geological Survey to begin collecting data in July 1998 in order to describe sediment transport in the Delta. The data collection program consisted primarily of a network of optical backscatter sensors supported by standard suspended-sediment sampling techniques and flow measurements in order to develop continuous, 15-minute, records of sediment flux at several sites. These data were combined with other available sediment transport data in order to develop a sediment budget with error estimates. Over the four-year period, water years 1999-2002, 6.6$\pm$0.9 million metric tons of sediment entered the Delta and 2.2$\pm$0.7 million metric tons exited, resulting in 4.4$\pm$1.1 million metric tons of deposition. This mass of deposited sediment corresponds to approximately 2.1$\pm$0.5 cm of deposition averaged over the entire open water and wetland area of the Delta over the four-year period (or 0.52$\pm$0.13 cm/yr). The deposition rate from our sediment flux calculations is virtually identical to the recent deposition rate from analysis of sediment cores. Over the four-year period, 85% of the suspended-sediment came from the Sacramento River, 13% came from the San Joaquin River and 2% came from other sources. Analysis of tidally averaged sediment flux and sediment transport pathways indicates that the San Joaquin riverine signal attenuates more rapidly in the downstream direction (i.e. tidal effects increase) than for the Sacramento River. At least 82% of the sediment entering the Delta from the Sacramento River watershed either deposits along the Sacramento River or moves past Mallard Island and into San Francisco Bay, leaving no more than 18% to move through the complex network of channels toward the San Joaquin River.
H53C-1263 1340h
Turbulent flow over an evolving gravel bed
There is a growing literature on bedforms developed in gravel bedded streams such as clusters, cells, bedload sheets, imbrications and a variety of coarse surface layers. In particular, clusters, cells and imbrications tend to stabilize the bed by providing structural integrity and thereby reducing the transport rate. It has also been suggested in the literature that these features increase flow resistance based on laboratory studies with high-relief clusters on fixed beds but, these high relief forms may not have a natural analogue. This poster reports on a series of experiments undertaken to examine turbulent flow over a gravel bed as cluster and cellular structures develop through time. The experiments were conducted in a 0.5 m deep, 1 m wide and 9 m long flume with a sediment feed. Flow depths ranged between 0.05 and 0.10 m and bulk mean shear stresses ranged between 0.9 and 4.4 Pa. Measurements of velocity were made at a sampling rate of 25 Hz using an acoustic Doppler velocimeter at 8 to 15 points in each of three velocity profiles separated by 0.5 m in the along stream direction. Bed sediment was well mixed at the beginning of the experiments and low-relief gravel clusters and cellular structures developed, generally increasing in density with time. Profiles were obtained after a coarse surface layer had developed at 2, 4, 8, 16, 32, 40, 70 hours from the beginning of the experiments. Shear stress estimates are made based on the Law of the Wall and Reynolds stress profiles and compared to structure type and density. The results suggest that the low relief clusters and cellular structures commonly observed in nature may not have as strong an effect on flow resistance as previously thought. Rather, the first order effect on flow resistance is derived from the development of the coarse surface layer and not its organization into clusters and cells.
H53C-1264 1340h
Modelling Fluvial Sediment Budgets Under Uncertainty
The last decade has witnessed a resurgence of interest in morphological estimation of fluvial sediment budgets, facilitated by major advances in survey technology, including airborne lidar and photogrammetry and ground-based GPS. The reliability of such morphologically inferred budgets is controlled by: a) uncertainty in the flux boundary conditions; b) survey frequency; and c) DEM quality. A body of research has sought to identify the sensitivity of derived parameters to these controls and establish a methodological framework for data quality control and assurance. To date, most interest has focused on evaluating the uncertainty in budget estimates due to DEM errors. These arise as a largely unknown function of survey point quality, sampling strategy and interpolation methods. A commonly adopted procedure for managing these uncertainties involves specifying a minimum level of detection threshold (LOD) to distinguish actual surface changes from inherent noise. Determining the LOD requires both a theory of change detection and a metric of DEM quality. Typically this is achieved by applying the classical statistical theory of errors and a measure of DEM precision derived from check data or point precision estimates. Research presented here aims to demonstrate that simple thresholding of DEMs of difference may, however, significantly underestimate the information that can be optimally retrieved through DEM differencing. Analyses are based on a series of annual surveys of the River Feshie in the Scottish Highlands using high-quality rtkGPS and lower precision airborne photogrammetric data. A new methodology for change detection is presented which incorporates: (i) a stepwise analysis of errors arising during DEM construction; (ii) the development of a spatial filter to group areas of scour and fill; and (iii) alternative methods for analysing change data which relax the assumptions of the LOD approach. These latter strategies explicitly incorporate uncertainties in DEM data and permit sediment budget calculations to be presented in a stochastic framework. The results suggest that enhanced data retrievals are possible and have the potential to recast the geomorphic interpretation of process rates.
H53C-1265 1340h
Fingerprinting suspended sediment sources using fallout and in-situ radionuclides in forested watershed in Japan
In Japan, forest plantations have became unmanaged because of the lowering of the log price. In unmanaged Hinoki ({\itJapanese cypress, Chamaecyparis obtusa Sieb. et Zucc.}) plantations, understory vegetation decreases because of low light conditions, and forest surface become a bare land. The occurrence of Hortonean overlandflow and soil erosion has been reported. Fine surface materials removal from forest floor due to Hortonean overlandflow will affect the downstream environment and fish habitat. However, no studies have been conducted fingerprinting the suspended sediment from unmanaged forest plantations. To study the sources of suspended sediment in forested watershed in Shikoku Island, Japan, the concentration of Cs-137 and Pb-210ex, U and Th decay series radionuclides were analyzed. Soil sampling was conducted in hillslopes in various locations such as landslide scar, surface erosion area in unmanaged Hinoki plantation and forest road, and sampling of deposited sediment in the channel was also conducted in several tributaries. The activities of Cs-137, Pb-210ex, Bi-214 and Tl-208 of soils and fluvial sediments were determined by gamma spectroscopy. We also sampled the suspended sediment by pumping up from the streams, and the time-integrated sediment sampler (Phillips et al, 2000) was also used to collect the various storm event samples. The study area is a 0.33 km$^{2}$ watershed, upstream of the Shimanto river basin, located about 700 km southwest of Tokyo. High concentrations of Cs-137 and Pb-210ex in suspended sediment were detected, which suggest that the suspended sediments are mostly derived from surface soil of the forests. The concentrations of Cs-137 and Pb-210ex vary between events to events, suggesting that the different sources among the storm events.
H53C-1266 1340h
Sediment Production from Forest Roads During Different Rainfall Conditions
Sediment productions from storm-proofed forest roads were investigated at four locations in the managed forested land of Northwestern California (annual rainfall @1000 mm), where rocks have been characterized as Wildcat group consisting of mudstone, siltstone, claystone, and minor conglomerate. Rocked road (storm-proofed road) constitutes a part of the effective road management practices in the study area where other management practices include disconnection of roads from watercourses, reduction in distance between culverts, frequent road maintenance, and seasonal (or during rainfall) restrictions of log traffic. Specifically, this study examines the influence of rainstorm and road characteristics on sediment production from storm-proofed road in the context of varieties of road management practices in place. Continuous ditch flow during the rainstorm events was monitored using an UNIDATA float device and data logger. An ISCO sampler collected suspended sediment at temporal resolution of 2-h during the rainstorm events, and collected sediment samples were analyzed for turbidity and suspended sediment concentration in the laboratory. Sediment samples were also collected for particle size distribution analysis. Two tipping bucket rain gages located in the vicinity of road sites collected rainfall. Using an electronic total station, road surface was surveyed in detail, and a road Digital Elevation Model (DEM) was generated. Thus produced DEM was processed to delineate the road surface area contributing to ditch flow. The analysis of collected data for rainstorm events of hydrological year 2003/2004 reveals that the suspended sediment concentration conforms closely to rainfall hyetograph and ditch flow hydrograph with rapid flushing of road sediments for about 15-h following the onset of rainstorm. In general, ditch flow and suspended sediment concentration relation illustrates that the sediment transport is evidently "supply limited". For this reason, suspended sediment concentration decreases to a low level after about 15-h of onset of rainstorm even when ditch flow as well as rainfall are considerably high. In addition to rainstorm characteristics, road characteristics such as slope gradient, and road use (traffic volume) caused differences in the short-term sediment concentration and total sediment yield among road segments during rainstorm events. Temporal distribution of suspended sediment concentration and short-term peak suspended sediment concentration dependent on the traffic volume prior to the onset of rainstorm event since truck traffic was ceased during rainstorms. During the same storm, road segments with high traffic volume produced peak suspended sediment concentration of up to 30-fold compared to road segments with low traffic volume; however, the total sediment yield during the rainstorm did not vary that much among road segments.
H53C-1267 1340h
Performance of Bedload Transport Equations Relative to Geomorphic Significance
Bedload transport is a fundamental process for alluvial rivers, creating bedform topography that stabilizes channel form and provides diverse physical habitat for aquatic organisms. It also controls the storage and mobilization of heavy minerals and coarse-grained contaminants, and is an important aspect of channel migration that both revitalizes river floodplains and can threaten human infrastructure within river valleys. Numerous studies over the past two decades have assessed the performance of various equations for predicting bedload transport. However, these studies considered formula performance statistically based on paired observations of measured and predicted bedload transport, the majority of which were taken at low flows. Consequently, formula performance is weighted toward low discharges which may not have geomorphic significance. We define geomorphic significance in terms of both the effective discharge (that which transports the most sediment over time) and the total bedload yield (the product of discharge and bedload transport rate integrated over time). Accurate prediction of the effective discharge depends on how representative both the flow record and the flood frequency distribution are, and on the validity of the transport equation. The latter specifically depends on how well the transport equation represents the exponent of the observed bedload rating curve which is, in turn, a function of supply-related channel armoring (transport capacity relative to sediment supply). Poorly-armored, fine-grained channels exhibit lower thresholds for bedload transport and thus lower rating-curve exponents compared to well-armored, coarse-grained channels. In contrast, accurate prediction of the total bedload yield depends on the overall performance of the transport equation over the range of observed flows, and may be sensitive to a variety of factors, including performance of transport threshold functions embedded within the equation, roughness correction, and degree of equation calibration to site-specific conditions over the range of channel discharges. We consider the performance of 8 different formulations of 5 bedload transport equations at 41 gravel-bed rivers in mountain basins of the western United States. Performance is assessed in terms of the accuracy with which the equations are able to predict 1) the effective discharge, 2) the bedload transport rate for the effective discharge, and 3) the total bedload yield for a given discharge record.
H53C-1268 1340h
Mountain river meanders and typhoon strike frequency in the western Pacific
Bedrock-floored mountain rivers are shaped by erosion processes that ultimately control the evolution of the landscape on geological time scales. In mountains across the western Pacific, meanders in bedrock channels are common and often emerge during incision rather than inherit their sinuosity from a past alluvial form. Incising emergent meanders are important because they reveal a process of lateral channel erosion at least as fast as the vertical rate erosion. Here we report a remarkable link between incised meander development and typhoon strike frequency, a good proxy for extreme rainfall and flood discharge. Using satellite imagery, shuttle-radar topographic data and a 58~year inventory of typhoon tracks, we mapped meander abundance and quantified regional densities of mountain river sinuosity and typhoon strikes. Our analysis shows that eroding meanders are most common in the typhoon-prone islands of Japan, Taiwan and the Philippines, and in rivers incising weak lithologies. One might expect that the faster the erosion rate, the greater the meandering, but we have found that monthly mean rainfall - and therefore mean discharge - correlates very poorly with sinuosity. Instead, the variability of rainfall, and presumably discharge, about the mean explains bedrock meander development much better. Mountain river sinuosity, for geologically similar bedrock, increases in a roughly linear fashion with typhoon strike frequency. The coefficient of variation of monthly rainfall (standard deviation normalized by the mean) exhibits a similar trend. We deduce that extreme flood discharge, e.g. driven by typhoon rainfall, accelerates lateral erosion rates and spurs meander development in mountain rivers.
H53C-1269 1340h
Steepness and Concavity Controls on the Expression of Reach-Scale Channel Morphology, Debris Flow Deposition, and the Spatial Distribution of Salmonids in the Pacific Northwest
Steepness and concavity indexes derived from the power function relationship between drainage area and channel slope provide a first-order control on (1) the expression of reach-scale channel morphology, (2) runout potential of debris flows, and (3) the spatial distribution of anadromous fish in the Pacific Northwest. Channels steeper than about 10% are typically dominated by the effects of periodic debris flow scour and subsequent accumulation of coarse sediment. Downstream of this area, channels with slopes between 3 to 10% represent a transition from debris flow to fluvial process dominance. In this transitional region of the network, debris flow deposits often form fill deposits that are subsequently incised by fluvial re-working that leads to the formation of step-pool sequences. Such reaches have restricted salmonid access, generally being most favorable to steelhead and cutthroat trout. The stronger the concavity of a channel profile, the shorter the length of this transitional reach. In the Oregon Coast Range, steepness and concavity values are high and the spatial extent of transitional channels is greatly restricted (typically only occurring in reaches with draining areas between 0.5 and 1.5 km$^{2}$). The abrupt change in slope from steep debris flow prone channels to low-gradient pool-riffle and bedrock channels promotes debris flow deposition and fan formation at tributary junctions. In these highly concave basins, a relatively large proportion of the fluvial channel network have gradients below 3% and are accessible to salmonids, resulting in a broad spatial distribution. This broad distribution allows for a spreading of risk that may enhance a population's ability to persist during severe disturbance. In contrast, many catchments in the Klamath Mountains of northern California have high steepness values but low concavity. In this region, the portion of the network occupied by transitional reaches is greatly expanded. Step-pool channels dominate the majority of the fluvial channel network, and occur in reaches with drainage areas ranging from 5 to 70 km$^{2}$. With low concavity the change in slope at tributary junctions is less pronounced and debris flows rarely form discrete fans. Instead, these mass flows continue to travel down steep mainstem channels and alter aquatic and riparian habitats for many kilometers. Because of the high steepness and low concavity in the Klamath Mountains, the spatial distribution of salmonids is severely restricted to a small portion of the network, limiting their resilience to disturbance. We propose that steepness and concavity indexes provide a useful context for classifying basins that express different reach morphologies, fish habitat capacity, and responses to episodic disturbance.
H53C-1270 1340h
Stream Succession: Channel Changes After Wildfire Disturbance
One paradigm in geomorphology is that vegetation is a fundamental control on sediment and water supplies to streams, and therefore on downstream geomorphology. Within this paradigm, wildfire has been implicated as a major driving force behind landscape erosion and changes to stream channels, periodically yielding pulses of sediment from upland basins, which, in turn, hypothetically, drive cyclical changes to stream channels. Within the context of management for ecologically valuable aquatic species across a landscape, biologists have envisioned available stream habitats cycling on long time scales, with some habitats increasing or decreasing in productivity, while others are temporarily taken out of production by severe disturbances related to fire. Some hypothesize that stream habitats may benefit from disturbance after the initial reorganization, increasing in quality over time, until the disturbance-supplied materials, gravel and wood, eventually become scarce, reducing habitat quality until the next disturbance. Systematic observations of actual channel "succession", however, are rare. We examined the long-term effects of wildfire disturbance on channel characteristics in moderate-gradient (2.3-3.9%), unconfined, mountain streams. Selection of this stream type excludes direct impacts from post-fire debris flows and allows us to focus on post-fire changes in basin hydrology, sediment supply and proximal riparian characteristics (supply of wood debris, bank strength from roots, etc.). The study was designed using a space-for-time substitution within the Idaho batholith. We considered three different forest age classes, corresponding with three different times since fire: recent (15-20 yrs), mid (90-130 yrs), and old (>150 yrs). Variables independent of fire and with potentially confounding effects (elevation, drainage area, land use, lithology, valley slope) were controlled to isolate the effect of fire on channel characteristics. Characteristics of interest included measures of channel morphology (e.g. channel geometry, pool spacing, residual depth, substrate size) and large woody debris (e.g. amount, location, function, size). Multi-response permutation procedures were used to measure between age-class variability (/alpha= 0.10). Only one out of sixteen channel morphology characteristics varied: average variance in maximum residual depth. Five out of twenty-six large woody debris characteristics varied: the proportion of pieces above bankfull, below bankfull, between the channel edge and floodplain, armoring the bed, and damming sediment. The lack of morphologic variability between age classes implies that wildfire disturbance does not have a long-term effect on channels of this stream type, suggesting that low- to moderate -gradient, unconfined channels act as relatively stable, potentially productive, refugia relative to a larger scale disturbance. Additional studies are needed in other stream types and geomorphic settings to examine variability in channel changes after wildfire disturbance. Compilation of such studies would greatly assist in development of risk-assessment models for stream and riparian ecosystem response to wildfire and forest management.
http://www.fs.fed.us/rm/boise/
H53C-1271 1340h
Impacts of Fire and Mass Wasting on Channel Morphology and Stream Temperature in Mountain Rivers of Central Idaho
Debris flows and hyperconcentrated flows immediately impact streams by changing channel morphology, grain size, sediment storage and transport, amount of incision, riparian vegetation, large woody debris dynamics, and extirpating fish, amphibian, and insect populations. In central Idaho, these disturbances are commonly triggered by intense thunderstorms or rain-on-snow events, and are exacerbated by wildfires which alter basin hydrology and sediment supply by removing vegetation and creating hydrophobic soils. While the immediate effect of these flows is dramatic, the time to recovery of the physical habitat is poorly understood and the long-term significance of these disturbances to aquatic organisms is unknown. Stream temperature is a key variable of stream ecosystems and has been shown to control the distribution of salmonids in our study area of the Idaho Batholith. Previous research in 10 recently disturbed streams shows a systematic increase in stream temperature across three stream types representing progressively greater disturbance: undisturbed; burned; and those impacted by both fire and mass-wasting events. Here, we test the hypothesis that the observed pattern of warming is due to increased solar radiation loading caused by wider, shallower streams and the removal of vegetative shade by fires and mass-wasting events. We examine channel conditions across several treatment classes (undisturbed, post-fire debris flow, debris flow without fire) and time since disturbance (1964 to present). In 32 streams, 200-600 meter reaches were surveyed and upstream and downstream temperatures were monitored throughout the summer, the solar load was estimated as a function of shading (measured with hemispherical photo analysis), stream width and depth, and average velocity estimated with salt tracers. Preliminary results indicate that while recent disturbances (1995-2003) significantly increase the solar load and stream temperatures, older disturbances (1964) are similar to undisturbed streams.
H53C-1272 1340h
Braided Channel Pattern Morphodynamics: The Physical Controls on Braiding Intensity in Gravel Bed Rivers
Braiding occurs in alluvial non cohesive river beds where stream power is high relative to sediment size. Previous research has clarified the local mechanisms (e.g. deposition of bars) causing braiding and the overall landscape conditions associated with braided rivers. Surprisingly there is not yet a complete explanation for one of the most important, yet basic characteristic of braided channel pattern - the degree of braiding and its variation spatially and temporally. The goal of this research is to understand the physical controls on braiding intensity (BI) during changes in stage in a particular river and to explain differences in BI at channel-forming conditions in different rivers. Analysis of variation in BI at varying stage is based on field results from Sunwapta River, Alberta, Canada, where changes in bed elevation, flow rate and river morphology were monitored for 12 consecutive days during a glacial runoff season. Daily discharge cycles were also reproduced in physical model experiments in a tilting flume, 18 m long, 3 m wide and 0.2 m deep in which the extent of braiding activity (sediment movement) can be directly observed and compared with field results.
H53C-1273 1340h
Streambed Particle Size From Pebble Counts Using Visually Estimated Size Classes: Junk or Useful Data?
In large-scale studies, it is often neither feasible nor necessary to obtain the large samples of 400 particles or more advocated by many geomorphologists to adequately quantify streambed surface particle-size distributions. Synoptic surveys such as U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program (EMAP) seek to assess the status and trends of various ecological indicators within populations of streams in regions ranging from a single large watershed to the entire United States. To sample an adequate number of streams, EMAP surveys use a streamlined pebble count procedure with visual size class estimates for 105 particles. To assess the practical applications and limitations of such data, we quantified the precision and sources of error in several measures of streambed particle size distribution from over 800 stream sites in 12 western states surveyed between 2000 and 2003, including geometric mean particle size, D$_{gm}$ (a proxy for median particle size, D$_{50}$, that can be computed from size-class data), and the percentage of the streambed surface covered by fine sediments ($<$2 mm), P$_{sf}$. Sampling precision was quantified using data from randomly selected sites revisited during the same field season (56 same-year revisits total). We examined three measures of precision: the root mean square error (RMSE) of repeat measurements, the coefficient of variation (CV=RMSE/mean), and a "signal-to-noise" (S:N) ratio defined as the ratio of between-site variance to repeat visit variance. RMSE was 0.19 for log D$_{gm}$ (equivalent to a 1.55-fold error in D$_{gm}$) and 5.3% for P$_{sf}$. The CV values for the same metrics were 28% and 15%, respectively, and were between 12% and 30% for each metric in 5 of 6 geographic sub-regions (N = 32 to 442 sites per region). For both D$_{gm}$ and P$_{sf}$ the S:N ratio was $>$30 for the whole dataset, $>$12 for 5 of 6 sub-regions and $>$8 for all sub-regions, indicating adequate precision to characterize within-region variability, including variations that might be associated with land cover alterations or other anthropogenic landscape disturbances.
H53C-1274 1340h
Basin Hydrology and Substrate Controls on Mountain Stream Morphology: Highlands of Southeastern West Virginia
Evolution of mountain drainage basins across a broad spectrum of geologic, tectonic, and climatic conditions is an active area of investigation in the field of fluvial geomorphology. Mountain streams are typified by steep channel gradients ($>$0.002), high channel roughness, rapid changes in drainage area, and high spatial and low temporal variability in channel morphology, leading to complexities in landscape modeling relative to their lowland counterparts. Factors driving this recent investigative trend are the refinement and generation of digital topographic data and terrain analysis software, and more importantly, the demand for a multidiscipline approach to the assessment, restoration, and management of entire watersheds. A significant volume of research has been conducted in mountain drainage basins of the western United States, with particular attention paid to tectonically active regions of the Pacific Northwest, which also contain federally listed threatened and endangered salmonid populations. Brook trout (Salvelinus fontinalis), native to the highlands of the eastern margin of the Appalachian Plateau are impacted by acid rain deposition; however, geomorphic research into landscape modeling, applicable to restoration and management of lotic ecosystems of the eastern United States, is comparatively lacking. This current research explores the potential for modeling channel morphology in mountain streams; specifically, how downstream trends in channel substrate resistance and unit stream power effect the partitioning of mountain stream morphology along and downstream of the fluvial/colluvial transition. In order to address this issue, two mountain drainage basins in the headwaters of the Gauley River watershed on the Appalachian Plateau of southeastern West Virginia were chosen. The westerly flowing Cranberry (250 sqkm) and Cherry (429 sqkm) rivers incise gently northwestward dipping Carboniferous-aged strata (shale, minor coal, siltstone, sandstone, and conglomerate), with a large percentage of both drainages managed as the Monongahela National Forest. A total of 68 reach-scale (10-20 channel widths) channel surveys were completed in which reach gradient, average bankfull channel widths, and bed surface grain size data were determined. This information was synthesized with data extracted from 10-meter digital elevation models using both RiverTools v. 2.4 and ArcGIS Desktop 8.3 terrain analysis software packages. Surveyed channel reach gradients range from (0.002-0.150 m/m) and are characterized by pool-riffle to cascade and step-pool morphologies, though observed morphology succession is atypical of an equilibrated system. Partitioning in channel morphology succession correlates with both changes in lithology (e.g. siltstone to conglomerate) and the extent of headwater debris flow activity, which reflects a shift in the balance between driving and resisting forces as stream size increases.
H53C-1275 1340h
Linking channel morphology, stream flow and micro-scale hydraulics in habitats utilized by spawning Atlantic salmon
The study examines how instream hydraulics respond to varying flow in differing channel geometries and how this corresponds to habitat utilization by spawning Atlantic salmon. Data on the location, discharge and channel morphology associated with 474 incidences of spawning activity were obtained at the Allt a' Ghlinne Bhig, Scotland. Six study sites were selected that represented the range of channel size and morphology utilized by spawning fish in the stream. Repeat hydraulic surveys were carried out at these sites over the range of stream flows observed to have been utilized by spawners. The hydraulic surveys provided data on how depth, velocity and Froude number distributions varied with stream flow at each of the 6 study sites. Hydraulic geometry-type relationships were produced that related discharge to each hydraulic parameter at each site. These relationships were applied to the discharges associated with specific spawning observations, describing the meso-scale channel hydraulics of each site at the flows under which utilization by fish was observed. This showed that, although the upper limit to the discharges used by spawning fish varied considerably between the 6 study sites, the associated hydraulic characteristics exhibited much less variation, implying a channel morphology control. Indices of hydraulic variability for each site correlated with the associated number of spawning incidents and the discharge range at which they occurred. Habitat suitability criteria applied to the raw hydraulic survey data produced relationships between discharge and habitat availability that were compared to specific spawning discharge observations at each site. This showed that higher flows predicted as providing suitable habitat were not utilized by spawning fish at any of the sites. Although higher flows provide suitable habitat, the flashy hydrological regime of the stream means that these conditions do not persist long enough to permit completion of spawning. It is suggested that in systems with a dynamic hydrological regime, the rate of change of discharge and its effect on channel hydraulics should be considered in habitat modeling exercises.
H53C-1276 1340h
Periodic Spacing of Channel-Spanning Potholes in Navajo Sandstone, Henry Mountains Utah: Implications for Propagation of Incision Pulses across Tributary Junctions
Incision of the Colorado River at Glen Canyon over the past ~1 Ma triggered pulses of incision that have migrated upstream through tributary drainages. The rate of incision of small tributaries often lags behind that of trunk streams, creatng over-steepened reaches at the tributary junction. In the arid Colorado Plateau, many of these low-order tributaries in massive lithologies have developed sequences of channel-spanning, periodically-spaced pothole bedforms. Although the dynamics of pothole formation and evolution are poorly understood, the occurrence of potholes correlates with super-critical flow and tools-limited conditions. We are exploring the hypothesis that pothole spacing and other profile attributes can be used to reconstruct the propagation of waves of incision through a drainage network. Here we report preliminary results of a field study of potholed reaches in steep, low-order channels draining Navajo Sandstone in the Henry Mountains, Utah. Our survey focused on channels where overlying sedimentary units and pediment deposits have largely been removed from the Navajo Sandstone. We surveyed six tributaries of the middle fork of Trail Canyon, and one tributary of Milk Creek, which are, in turn, minor tributaries of the Colorado River. We surveyed the long profile of each channel using a 5-m rod, 100-m tape, and a clinometer. In bedrock reaches, we focused on quantifying the spacing of alternating pothole steps and bedrock chutes, and the elevation change within and between potholes steps. We also characterized individual potholes by width, depth, maximum sediment size captured, and direction of flow recirculation. In alluviated reaches, we measured the bankfull width and depth to estimate relative differences in bankfull discharge. We also measured channel geometry in higher-order trunk streams at the junctions with the surveyed tributaries. Our preliminary results include the successful prediction of locations where potholed channels were found, by comparing topographic and geologic maps to estimate the character and quantity of coarse sediment supply. Where diorite pediments and overlying sedimentary rocks have been removed by erosion from above the Navajo Sandstone, pothole channels are common. Although the presence of grinders contributes to pothole abrasion, observations suggest that low supply rates of coarse sediment are essential for pothole development. Particularly important is low supply of the relatively hard diorite derived from the laccolithic cores of nearby Mt. Hillers and Mt. Holmes. In many potholes the only sediments present were sand-sized and finer. In each of the surveyed tributary channels we observed regular spacing of pothole bed forms. Preliminary analysis suggests a characteristic spacing of pothole steps that varies systematically with reach gradient, drainage area and other tributary attributes. Further analysis will allow us to explore the relationships between the morphology of potholed channels and the relative incision rates of the tributary and the trunk streams. It may be possible, for example, to assess the relative incisional efficiency of tributaries with differing extents of pothole development.
H53C-1277 1340h
Lithologic Influence and Experimental Variability in Gravel Abrasion: Implications for Predicting Rates of Downstream Fining of River Bed Sediments
The question of what controls the occurrence and rate of downstream fining of bed-material sediments remains a fundamental unsolved problem despite over a century of field, experimental and theoretical investigations. Downstream fining rates are commonly modeled as exponential or power-law functions of travel distance. Much recent work has focused on the relative influence of particle abrasion and differential transport, however, no general method has been developed for explicitly accounting for the influence of rock strength in parameterizing fining models. Here we report preliminary results of laboratory tumbling experiments in which we are investigating the influence of variable rock durability, both between and within distinct lithologic units, on rates of particle abrasion. We consider three separate questions: 1) can rock tensile strength be used to predict differences in bulk fining rates across a wide spectrum of rock types; 2) does variability in rock durability among individual gravel clasts of the same lithologic composition and initial grain size lead to patterns of downstream evolution of grain size distributions that differ significantly from the predictions of simple fining models; and 3) how large is the uncertainty in abrasion coefficients determined by laboratory tumbling, as determined by replicate experiments with identical initial conditions? We use a horizontal axis, 25-cm diameter, steel barrel tumbler, driven by a mechanical transmission with excellent control of rotational velocity. Rock samples were collected from units of the Franciscan Formation in the Redwood Creek Watershed of Marin County, California, and from sedimentary and intrusive volcanic rocks of the Henry Mountains, in southeastern Utah. We collected clasts predominantly from hillslope source areas, to focus our attention on the durability of gravel as it enters the river network. We use the `Brazilian' tensile splitting test to measure the strength of 50-mm diameter core samples obtained from the same localities as gravel source areas. We determine mass loss by weighing individual gravel clasts, and calculate abrasion coefficients after at least five or six runs, with a typical total `travel' distance of 4-6 km. We find a systematic variation in gravel abrasion rate with rock tensile strength, as well as an evolution of the grain size distribution that is quite sensitive to lithology. For initial conditions of both uniform-sized and log-normally distributed size mixtures, we observe development of skewed and bimodal distributions. We interpret consistent development of positively skewed distributions as reflecting differential pre-existing fracture density among otherwise identical clasts. Abrasion of sedimentary rocks tended to produce a large proportion of sand and silt, presumably due to failure between sediment grains. In contrast, microcrystalline rock, including chert and serpentinite, produced more fine gravel by clast splitting and trended toward a bimodal grain size distribution. Finally, we quantify significant uncertainty in experimental abrasion coefficients for replicate runs with the same initial weight distribution and lithology. Although the data are well fit with a power law relationship, the 95% confidence interval for the estimate of the abrasion exponent ranges from plus or minus 20% to 75% of the mean of three replicates. We conclude that predictive models of downstream fining by abrasion need to incorporate the lithologic sources of both systematic and stochastic variability in the rate of bulk mass loss and the resulting evolution of grain size distributions.
H53C-1278 1340h
Fluvial Grade Substantiated in Flume Experiments
A river which conveys sediment downstream without net deposition or erosion through a series of reaches is referred to as being O`gradedO_L or in equilibrium. This fluvial grade concept, from Gilbert (1877), has long been a subject of debate. It is very uncertain if such a graded river can exist in nature and how it could be recognized presently or in ancient strata. This issue has survived centuries of discussion. We now challenge the classical view, by means of theoretical modeling and flume experiments, and suggest a new view of the nature of graded streams. The topographic setting assumed here is of a river delta accreting across a uniform slope of shelf. Theoretical considerations suggest that the alluvial slope attains and sustains a graded condition grade during sea-level fall, and when the rate of fall reaches a particular pattern of deceleration. This prediction was proved with flume experiments in which two-dimensional deltas were constructed. The agreement between the theory and the experimental results suggests the following view of fluvial grade. Fluvial grade is attained only during sea-level fall and sustained only with a particular pattern of decelerating fall. The graded alluvial slope, characterized by a straight profile, is intrinsically unstable under steady basin forcing and does not represent any final stable stage of the river system. Alluvial aggradation (i.e., out of grade) can proceed as long as sea level remains stationary. The conventional notion of graded conditions equating with the stable sea level or base level is therefore incorrect. This has a number of important implications for stratigraphy.
H53C-1279 1340h
Suspended Sediment Dynamics at the Field-scale Channel of an Irrigation-dominated Watershed
Sediment entrainment and transport are major water quality problems that require TMDLs for impaired water bodies. In the Salton Sea watershed (southern California) irrigation is the predominate source of runoff and sediment mainly stems from irrigated agricultural fields. It is therefore essential to control sediment release from the fields in order to meet the requirement for sedimentation TMDLs. Unfortunately, the lack of knowledge on soil erosion and sediment movement hinders the development of best management practices at the field scale. To understand the dynamics of suspended sediment transport, a field-scale drain channel connected to six agricultural fields was monitored for 5-minute average sediment concentration during a 3-month crop season. Three individual irrigation events are selected to represent furrow and sprinkle irrigations at the early stage (i.e. stand establishment) and the late stage (i.e. crop growing), which have different effects on sediment transport. Analysis on discharge and sediment concentration hydrographs and the associated hysteresis loops reveals that at the early stage, sediment is supplied from farmlands and partially deposited in and transported out the field-scale drain channel with higher sediment loads for furrow irrigation. At the late stage, sediment is conveyed from both farmlands and the channel bed due to confined soil erosion in the fields and preceding sediment storage in channel. Sediment dynamics are further complicated by concurrent irrigation of multiple events. Two samples of multiple irrigations disclose that sediment transport is not only controlled by hydraulic forces but also by mixing and routing of water and sediment.
H53C-1280 1340h
Assessing River Channel Change Along Urbanizing Mountain Fronts
Comparatively few studies of channel change following urbanization have been undertaken in steep dryland environments. However, as urbanization encroaches upon mountain fronts in response to population growth, a comprehensive understanding of channel change in these settings is increasingly important for developing successful management strategies. This investigation follows the development of Fountain Hills, a newly urbanizing community flanking the eastern foothills of the McDowell Mountains of Arizona, as a representative example of urban impacts along sensitive physical settings. Surveys conducted since 1987 show that a spatial pattern of channel adjustment has developed in response to road crossings that repeatedly intersect the stream channels, delivering increased runoff and interrupting the flow of water and sediment into such channels. This paper summarizes the channel changes that have occurred since 1987, and suggests a dynamic catchment approach for managing such impacted systems. This approach accounts for spatial variations in channel adjustment and allows consideration of community views. The example of Fountain Hills gives a basis for assessing change and comparing the rates and manner of channel adjustment to those around the world.
H53C-1281 1340h
Planform Dynamics on an Urbanizing New England River
Historical GIS-based analysis was used to quantify channel planform change on the Pomperaug River, Connecticut, over the last seventy years. During this time period, the Pomperaug River watershed has incurred radical land use changes, transitioning from a predominantly agricultural watershed to a combination of re-forestation and urbanization with an increase in population and development. In order to link these land-use transformations to changes within the river system, historical aerial photographs were collected for six years including, 1934, 1941, 1951, 1970, 1990, and 1995. Aerial photographs were digitally scanned and imported into ArcView, where they were georectified to an orthorectified reference base image. This enabled us to accurately create a digitized centerline of the river for each photograph. The channel centerline was then buffered to a distance equal to that of the root mean squared error (RMS error), calculated separately for each photo during the georectifying process. Each buffered centerline was then overlaid to quantify planform changes over time and space. Results show reduction in sinuosity, and variability in rates of channel migration through time.
H53C-1282 1340h
Experimental investigation of the turbulent structures that initiate bedload motion
Knowledge of the coupling between turbulent structures and the subsequent motion of bed grains is important to formulating sediment transport relations in complex turbulent flows. Previous experiments involving measurements of forces on fixed particles have shown that bed grains experience fluctuations in drag and lift that are several times the mean, and these high drag and lift events are well correlated with positive fluctuations in downstream velocity near the particle. In order to extend these experiments to the initial motion of grains that are not fixed, laboratory flume measurements of the initial motion of glass spheres were conducted using a high-resolution (1280X1024 pixels) and high-speed digital video camera at a rate of 400 frames per second. Six experimental runs were conducted on each of four spheres having diameters of 1.1, 1.6, 2.5, and 3.6 cm. Each of these spheres was placed on a bed pocket formed by three other 2.5cm glass spheres that were glued together to form a known bed-pocket angle. All of the spheres were entrained at approximately the same discharge. A 1.3 mm thick laser light sheet aligned parallel to the flume walls illuminated flow seed particles in a two-dimensional plane that included the center of the test sphere. Two-dimensional velocity fields were calculated for all successive pairs of images in the video sequence using particle imaging velocimetry (PIV) algorithms. Results showed that the downstream velocity immediately upstream of the test sphere at the initiation of motion was generally 2-3 standard deviations greater than the local mean velocity. The vertical extent of the volume of high-speed fluid that caused the sphere to begin motion generally extended less than two median bed-grain diameters above the bed. A volume of relatively slow moving fluid was generally found above this volume of near-bed high-speed fluid. Thus, to observe the turbulent structures that cause particle entrainment in a gravel bed requires measurements within a couple grain diameters of the bed.
H53C-1283 1340h
Erosion Processes of Streambed in the Channelized River Draining Into the Kushiro Mire, Hokkaido, Northern Japan
In Japan, the wetlands have shrunk markedly since 1950s due to land-use development from wetland to urban and agricultural land. Rapid sedimentation in the Kushiro Mire, Hokkaido, northern Japan, was caused by extensive land-use development and stream channel rationalization during the 1960s and 1970s. In the Kuchoro River catchment, draining into the Kushiro Mire, the meandering stream was channelized in the mid- and downstream associated with land-use development between 1966 and 1980. Prominent degradation of a streambed due to channelization has occurred over 2 km in the midstream since channelization was finished. Bare slope has occurred due to streambed degradation, and produced fine sediment through the freeze-thaw process in late fall season. Following snowmelt and/or typhoon flood events in spring and summer season could transport fine sediment on the bare slope into the wetland. During a flood event, stream flow eroded the streambed laterally and vertically, resulting in the overhang of riverbank and the dropping down the clods into the stream. These erosion processes has occurred and produced the sediment of 7500 m3/year in average between 2000 and 2003. The upstream portion of a channelized reach is often degraded because of high flow velocities associated with a steeper streambed. On the other hand, the annual sediment production on the streamside bare slopes in the mountain area was measured by erosion pins and estimated as 4500 m3/year. Thus, the reach of streambed degradation is considered a major point-source of suspended sediment in the Kuchoro River catchment for the past 20 years, leading to the recent rapid sedimentation in the marginal area of the wetland.
H53C-1284 1340h
Scaling of Braided River Depths
Braided rivers are dynamic systems of interweaving channels. The properties of these systems are difficult to study in the field and system evolution is difficult to predict. Knowledge of scaling relationships present in these rivers would assist in model testing as well as statistical prediction of channel evolution. In our previous work, scaling relationships have been quantified for braided channel system morphology and evolution and inferences on how the density of channels changes with scale and on the magnitude and frequency of channel shifts have been made. In this work, we study the scaling of hydrological characteristics of braided rivers by analyzing cross-sections from an experimental braided river using global and local multifractal analysis methods. Analysis has been performed separately on depth fluctuations that come from within and outside the braided river system channels and differences in their scaling characteristics have been quantified. The effect of combining the within and outside channel depth fluctuations into a single multifractal analysis has been shown to manifest itself into a \"phase transition\" phenomenon i.e., a cross-over region in the log-log plot of moments of depth increments versus scale, which depends on the order of the moment.