H51E-0786
Complexities of the Sediment Fingerprinting Approach -- With Examples from the Chesapeake Bay Watershed and Lake Tahoe
Sediment fingerprinting is an approach used to determine the relative magnitude of contributions from the important sources of fine-grained sediment in a watershed. Sediment-fingerprinting results show the contribution from each source over specific time periods -- seasons, individual flow events, and across a given storm hydrograph. Although the gross contribution of watershed sources can be identified for a given time period, the precise location within the watershed and the relative contributions of a given source of sediment (i.e., streambanks) must be identified through another process, such as using a sediment budget approach. Important research obstacles in the sediment-fingerprinting approach that still need to be evaluated include the determination of the: (1) appropriate watershed scales for this approach, (2) effects of grain size and organic content on fingerprint properties, (3) suitable physical and chemical tracers for fingerprinting, (4) conservativeness of fingerprint properties through the erosion cycle, (5) appropriate statistical approaches to determine the significant fingerprints and relative contributions of each source, and (6) effects of storage and delivery times on sediment sources for a particular event. This poster presents the sediment fingerprinting approach that was used in watersheds draining to the Chesapeake Bay and Lake Tahoe with a discussion of the problems encountered.
H51E-0787
Methods for Identifying Ravines and Quantifying Their Contribution to Sediment Loads in the Minnesota River Basin
The Minnesota River Basin generates a disproportionately high amount of total suspended sediments to the Upper Mississippi River Basin. Many reaches in the Minnesota River Basin have impaired water quality due to turbidity. The primary sources of sediment in the Minnesota River Basin include upland erosion from agricultural land, streambluff slumping, and downcutting of ravines. No research has been conducted to date to quantify sediment contributions to the Minnesota River Basin from ravines. Geographic Information Systems (GIS) and terrain analysis were conducted using 3 and 30 m resolution digital elevation models in an attempt to identify the locations and geomorphic characteristics of ravines in the Minnesota River Basin. A combination of the streampower index (SPI) and profile curvature were used to identify the location of ravines. Field verification showed that this method was highly successful. GIS was used to quantify the length, width and volume of each ravine, and these attributes were used to classify ravines according to their sediment loss potential. Field measurements of total suspended sediment and detailed repetitive topographic surveys were made in ravines at several locations to quantify sediment losses from ravines. Field measurements were combined with GIS based ravine attributes to estimate the contribution of ravines to sediment load in the Minnesota River Basin.
H51E-0788
Decadal to Centennial Variability of Erosion Rates in a Rapidly Degrading Channel Network
Quantifying short-term spatial variation of rates of vertical land surface change is a challenge in net degradational systems, but this information has the potential to add significantly to our understanding of transport-limited erosional processes acting over long timescales. We have undertaken a dendrogeomorphic approach to assessing denudation rates in a young channel network on the Florida panhandle where unlithified sands and muds are being incised by water supplied from groundwater seepage. The measured exposure of roots for over 500 trees, combined with tree ages, supply surface change rates along a 2 km reach of channel. This number of samples represents all trees greater than 2" diameter from ~1/3 of the valley bottom area within the study section. Tree rings were counted for cores from 33 trees ranging up to ~200 years in age. From these trees an age- diameter relation was determined to estimate ages for the remaining ~500 trees with ~12% standard error. Mean and maximal lengths of root exposure covaried with tree age resulting in characteristic erosion rates valid over decadal to centennial timescales. Three types of surface change rates were measured: vertical motions of the channel bottoms, vertical motions of the valley bottoms, and lateral motions of the channel sidewalls. Their means are 2.8, 1.7, and 7.3 mm/yr, respectively, and support field observations that suggest the valley bottom is denuding by a process independent of the channel lowering. The vertical erosion rates also covary with tree ages in such a way that mean rates decrease by half an order of magnitude for an order of magnitude increase of the averaging timescale. This result is direct evidence for the unsteadiness of sediment transport within the network. Based upon Central-Limit theory, it also suggests that the decorrelation timescale of the sediment transport process is within the range of decades to centuries. The timing of inception for this network is not well constrained. A rough estimate can be made using the total channel length and the erosion rates measured here by assuming that the steephead propagation rate is related to the valley lowering rate through the streamwise valley slope. For a mean valley lowering rate of 2 mm/yr and a slope of 25 m/km, the whole network could have been created in approximately ~100,000 years. This age can be refined with the development of a local erosion rate-erosion duration relationship.
H51E-0789
Geomorphic Channel Network Analysis of a Headwater Basin in the Italian Alps
This study analyzed the variation of headwater channel geometry under bankfull conditions, and the most likely link among morphological variables in the upper Cordevole basin located in the Eastern Italian Alps. The basin is characterized by a developed ephemeral colluvial network and by a main alluvial channel dominated by rapids and step pool morphologies. The research considered the relationships between the morphological parameters of headwater channels (channel width at bankfull stage, B and local slope, S) and some variables as contributing drainage area (A) and stream power (equal to SA, where A is considered as a surrogate of the discharge). Several field surveys were carried out along the main channel and in some small tributaries in order to measure geometric (B) and morphometric (S) parameters. The hydrological variables (A, stream power) were derived from high resolution DTM. In this work we used airborne laser altimetry elevation data with an accuracy estimated to be better than 0.2 m. Furthermore, field data served as the basis to validate the results. Starting from the DTM, the relationships between morphological and hydrological variables have been investigated and then applied to the whole network. The geomorphic network analysis, obtained by the combination between detailed field surveys and an high resolution topographic representation of landscape, can be considered as an useful tool for the knowledge of morphodynamic relationships.
H51E-0790
Sediment Fluxes from California Coastal Rivers: The Influences of Climate, Geology and Topography
Annual suspended, bedload and total sediment fluxes were determined for 17 river basins draining the west flank of the California Coast ranges. The river basins have hydrologic records covering all or most of the period, 1950- 2006, and were selected to be relatively unaffected by flow storage, regulation, and depletion that alter the downstream movement of water and sediment. Relatively large annual sediment fluxes, Sf, are strongly influenced by the El Niño-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO). The frequency of relatively large Sf, during La Niña (El Niño) phases decreases (increases) from north-to- south. The influence of ENSO is modulated over a period of decades by the PDO, such that relatively large Sf, during La Niña (El Niño) phases are more frequent in conjunction with cool PDO (warm PDO). Mean annual Sf were regressed against basin and climatic characteristics. Basin area, bedrock erodibility, basin relief, and precipitation explain 95.5 percent of the variation in mean annual Sf from the 17 river basins. Bedrock erodibility is the most significant characteristic influencing mean annual Sf. Basin relief is a superior predictor of mean annual Sf compared to basin slope. Mean annual Sf is nearly proportional to basin area and increases with increasing precipitation. For a given percentage change, basin relief has a 2.3 fold greater effect on mean annual Sf than a similar change in precipitation. The estimated mean annual Sf from all California coastal rivers for the period 1950-2006 would have been approximately 130 million without flow storage, regulation, and depletion.
H51E-0791
Prediction of Sediment Yield in Small Mountainous Watersheds Using by GeoWEPP and Design Rainfall
To predict the sediment yield in small mountainous watersheds Water Erosion Prediction Project (WEPP) based on physics and design rainfall were applied. The result using by GeoWEPP from January, 2004 to January, 2005 was compared with observed total sediment yield over the same period, and the sediment yield computed by GeoWEPP slightly overestimated. To compute design rainfall rainfall data from 1970 to 2004 was used and the rainfalls of reoccurrence periods (20, 50, 100 and 200 years) were analyzed with four probability distributions (Normal, Gamma, Gumbel and Lognormal). The optimum design rainfall(Lognormal) was applied to GeoWEPP and the sediment yields computed by GeoWEPP exhibited a good relationship with rainfall change. However, we suggest that the prudent caution needs to apply the models to real world and the many applications to various area needs to obtain more proper results.
H51E-0792
Measurement of Mountain River Flow Characteristics from Formosat II Optical Satellite Imagery
A number of recent field observations and model results have shown that the shape of an incising mountain river's planflorm is closely tied to the shape of its discharge distribution. However, due to logistical challenges and limited economic and societal needs, mountain rivers are rarely gauged, and the characteristics of their flow which control the styles and rates of erosion remain poorly understood. Using newly available high temporal and spatial resolution satellite imagery coupled with a key set of field measurements along a river in Taiwan's Central Mountains, we have developed a methodology to produce a set of rating curves based on flow width. With these rating curves we are able to use satellite observations of inundation extent to estimate flow depth, velocity, shear stress, and discharge along and across this mountain river. Our results demonstrate a new application for high resolution satellite imagery and provide data that gives new insight into the processes at work along bedrock rivers as well as important constraints for landscape evolution models and flood hazard assessments.
H51E-0793
Spatial and Continuous Observations of Stream Hydrodynamics and Bedload Transport From Spectral Analysis of River Induced Seismic Noise
Analysis of continuous seismic data recorded by a passive seismological network (Hi-CLIMB) installed across the Himalayas reveals strong spatial and temporal variations in the ambient seismic energy produced at high frequencies (> 1 Hz). During the summer 2003, an increase of high-frequency seismic noise is observed for all the stations located along a steep narrow and deeply-incised channel of the Trisuli River, a major trans- Himalayan river. The summer growth in high-frequency energy is modulated by a 24-hour periodicity with minimum amplitude around noon and maximum late in the evening. The comparison of seismic noise amplitude with both regional meteorological and hydrological data along the river reveals clear correlations. Seasonal increase in ambient noise coincides with the strong Monsoon rainfall and a period of rapid melting of snow and ice in the high elevations. The observed 24-hour cyclicity is consistent with the daily fluctuation of the precipitation and river-discharge in the region. The observed river seismic noise is partly generated by stream turbulence but this mechanism fails to explain the noticed clockwise hysteresis of seismic noise amplitude versus water level. This pattern is better explained if a significant part of the observed seismic noise is caused by ground vibrations generated by bedload transport. Monitoring river bedload is essential to understand erosion processes and the evolution of bedload transport are valuable data difficult to acquire, especially during periods of flood crisis. Spectral analysis of background seismic noise could offer a great potential to safely quantify in continuous river bedload and monitor its spatial variations.
H51E-0794
Coarse sediment transport dynamics at three spatial scales of bedrock channel bed complexity
Rivers incised into bedrock in fold-dominated terrain display a complex bed topography that strongly interacts with local hydraulics to produce spatial differences in bed sediment flux. We used painted tracer clasts to investigate how this complex bed topography influences coarse sediment transport at three spatial scales (reach, cross- section and grain). The study was conducted along the Ocoee River gorge, Tennessee between the TVA Ocoee #3 dam and the 1996 Olympic whitewater course. The bed topography consists of undulating bedrock ribs, which are formed at a consistent strike to the bedding and cleavage of the metagreywake and phyllite substrate. Ribs vary in their orientation to flow (from parallel to oblique) and amplitude among three study reaches. These bedrock ribs create a rough bed topography that substantially alters the local flow field and influences reach- scale roughness. In each reach, 300 tracer clasts were randomly selected from the existing bed material. Tracer clasts were surveyed and transport distances were calculated after five scheduled summer releases and a suite of slightly larger but sporadic winter releases. Transport distances were examined as a function of rib orientation and amplitude (reach scale), spatial proximity to bedrock ribs and standard deviation of the bed elevation (cross- section scale), and whether clasts were hydraulically shielded by surrounding clasts, incorporated in the armour layer, imbricated, and/or existed in a pothole, in addition to size and angularity. At the reach scale, where ribs are parallel to flow, lower reach-scale roughness leads to greater sediment transport capacity, sediment flux and transport distances because transport is uninhibited in the downstream direction. Preliminary results indicate that cross section scale characteristics of bed topography exert a greater control on transport distances than grain size.
H51E-0795
Relative contributions of sand and gravel bedload transport to acoustic Doppler bedload- velocity magnitudes in the Trinity River, California
Apparent bedload velocities measured using the bottom-track feature of acoustic Doppler current profilers (ADCPs) have received attention over the past few years as potential surrogate technique for estimating bedload transport rates and for investigating bedload dynamics. This poster reports findings from perhaps the first use of ADCP bedload velocity measurements in an applied sediment monitoring program. Sediment transport data reported here were collected under the auspices of the Trinity River Restoration Program as part of an intensive sediment monitoring effort to assess the effects of the 2006 flow release in the Trinity River of Northern California. A 1200-kHz ADCP was deployed for a subset of bedload samples collected during the release to evaluate whether acoustic bedload velocities can be used to aid interpolation between less-frequent physical samples. Paired conventional bedload samples and acoustic bedload velocity samples supplemented by underwater video showed that the instrument used in this study is sensitive primarily to the motion of sand-sized particles at the bed, but comparatively insensitive to the motion of gravel- and cobble-sized particles. High bed velocities were measured at times and in locations where sand transport rates at the bed were high, as determined by both physical samples and video. Low bed velocities were measured where both the video and bedload samples indicated that little or no bedload was being transported, irrespective of the persistence of fast-moving sand particles in the near-bed water column. To the extent that suspended or saltating particles influence the bottom- track signal, they are near enough to the bed to be captured in the physical sampler. Thus, contamination of the bottom-track signal by suspended particles (commonly referred to as water bias) is not a significant problem with this instrument in streams with low to moderate suspended sediment concentrations. These results demonstrate that acoustic bedload velocity can be a useful supplement to bedload sampling in sand-bed streams or in mixed-bed streams when information regarding the fractional transport of the sand-sized load is desired. However, ADCP bedload velocity appears to be inappropriate for monitoring bedload transport in cases where the transport of gravel sizes is important.
H51E-0796
Fine sediment infiltration into gravel bed: theory and experiments
We derived partial differential equations (PDEs) that describe the process of fine sediment infiltration into coarse sediment deposits based on mass conservation and our intuitive understanding of the physical processes involved. We reasoned that fine sediment trapping efficiency (FSTE), defined as the volumetric fraction of fine sediment trapped in the deposit per unit downward distance traveled, is either independent of fine sediment fraction (FSF) within the deposit or increases monotonically as the FSF increases. Solution to the PDEs under the assumption that FSTE is independent of FSF indicates that the equilibrium FSF decreases exponentially with depth into the deposit following fine sediment infiltration into a deposit initially void of fine sediment. Solutions to the PDEs under the assumption that FSTE is a function of FSF results in non-exponential decay function for fine sediment fraction that decreases faster into the depth compared to the exponential profile under the assumption that FSTE is independent of FSF. We also conducted flume experiments independent of the theoretical analysis to examine fine sediment infiltration into clean coarse sediment deposits. Experimental results illustrate that an exponential decay function with depth into the deposit adequately describes the FSF profile following infiltration, indicating that FSTE is at most weakly dependent on FSF in the deposit. Results from the experiments also provide quantifications to the coefficients within the theory as functions of the grain size distributions of the infiltrating fine sediment and the coarse deposit. Quantitatively comparing the theoretical solution under the assumption that FSTE is independent of FSF with the experimental data indicates that the root-mean-square- error between theoretical prediction and weighted-averaged experimental data is 7.3 percent of the predicted saturated FSF value (i.e., predicted maximum FSF once fine sediment can no longer infiltrates through the near- surface layer to reach the deeper part of the deposit), indicating a good agreement. http://www.stillwatersci.com/PubUnderReview
H51E-0797
Suspended Sediment Erosion in Laboratory Flume Experiments
Laboratory flume experiments were used to examine the role of suspended sediment abrasion in bedrock channel erosion. A range of topographies was used, from a planar bed to a sinuous and scalloped inner channel. Experiments were run separately with bedload (used to form topography) and suspended load at a variety of water flows and sediment fluxes. Sediment samples were collected to determine mass flux and concentration profiles. Erosion was measured between each timestep and erosion rate determined for a variety of conditions. Rouse, Froude, and Stokes numbers were calculated from measured data for various timesteps to determine mode of sediment transport and flow characteristics. Flow was supercritical, and sediment was in suspension. Erosion patterns around imposed topography perturbations (a rock protrusion and a drilled pothole) were briefly examined. A hydraulic jump was used in one timestep to see the effect of the transition from supercritical to subcritical flow. Suspended sediment causes erosion in all bed morphologies. The amount and pattern of erosion are coupled to topography, but are not constrained by it to the same degree as bedload. As in the case of bedload, suspended sediment erosion is strongly coupled to sediment flux.
H51E-0798
Simulating the Fluvial Erosion of Fine-Grained River Banks
River bank erosion is the product of a suite of specific processes that together contribute significantly to the sediment yielded from river catchments. Many studies have emphasised that hydraulic erosion of bank-toe materials may exert a dominant influence on the long term rate of river bank retreat. Fluvial bank erosion rates are normally quantified using an excess shear stress model of the form E = k(τb-τc)a, where E is the erosion rate per unit time and unit bank area, τb is the boundary shear stress applied by the flow, k and τc are erodibility parameters (erodibility coefficient, k, and critical shear stress, τc), and a is an empirically derived exponent (equated to unity in bank erosion studies). This model has the advantage of simplicity, but in practice difficulties in estimating the values of the erodibility and shear stress parameters seriously inhibit its accuracy. We are seeking to improve the parameterization of the excess shear stress model through the use of field measurements and analytical modelling, at field sites on the Mekong River in Laos. Specifically, τb is estimated using a new model [Kean and Smith, 2006, J. Geophys. Res., 111(4), F04009, doi:10.1029/2006JF000467] of flow over irregular bank topography. Data from our study sites indicate that the form roughness induced by natural topographic bank features (slumps, embayments, etc) is a major component of the spatially-averaged total shear stress, with the skin friction component (i.e, τb) typically an order of magnitude less than the total stress. This indicates that previous bank erosion investigations, that employ estimates of the total shear stress, may grossly misparameterize the true value of τb. To estimate τc, we have employed a Cohesive Strength Meter [CSM, Tolhurst et al., 1999, Estuarine, Coastal & Shelf Sci., 49, 281-294], a jet-testing device that is normally used in studies of the stability of cohesive sediments on inter-tidal flats, but which has not previously been employed in the context of river bank studies. Our data show that values of τc are typically of the order of 1 Pa, again indicating that previous studies may have over-estimated the true critical stress. Moreover, compared to conventional jet-testing devices, the portability and small size of the CSM's sampling chamber, together with the speed of individual tests, allows the collection of large numbers of replicate samples within discrete sedimentary horizons, such that the inherent natural variability of each bank material horizon can be defined. This has allowed us to modify the excess shear stress erosion model to include a probabilistic component associated with the measured statistical distribution of τc for a specific bank-toe material. We have used our data to estimate bank erosion rates for a range of flow discharges, with annual erosion rates subsequently determined by integration across the predictable annual (monsoonal) flow regime. To assess the predictive ability of our model we have compared annual rates of bank-toe retreat (assumed to be a good proxy of bank retreat rates) with estimates of bank retreat derived from analysis of aerial photographs and satellite imagery.
H51E-0799
Reach-Scale Retrieval of Alluvial Bed Roughness
System-scale patterns of bed roughness exert a fundamental control on flow resistance, sediment transport and river ecological processes. While field techniques for the measurement of roughness are well established they are often invasive, labor intensive and difficult to generalize over space. Recent research has demonstrated that analysis of low-altitude imagery can be used to map grain-size over large spatial scales with tolerable precision. However, although this approach provides a route to the parameterization of particle roughness, such an implicitly 2D method provides little insight into the stage-dependence of particle and form resistance arising in mixed grain-size, water-worked contexts. By contrast patch-scale experiments with hand-held laser scanners, have indicated that an explicitly topographical approach to roughness, involving statistical analysis of elevations, may hold more potential to capture these complex effects (e.g., Aberle and Nikora, WRR, 42, W11414). Upscaling this approach to acquire system-scale data however remains unresolved. In an attempt to address this, we outline a methodology to retrieve local patterning of topographical roughness from detailed 3D point cloud data, acquired using a terrestrial laser scanner (TLS) in a 1 km study reach of the River Feshie. Unlike hand-held scanners, a ruggidized TLS can be tripod mounted and acquire data over ranges exceeding 100 m, and with high angular resolution and temporal sampling, achieve data densities well above 1000 points/m2. Here, a Leica ScanStation was deployed to acquire a point cloud comprising over 200 million points, with RMS errors of 2-11 mm. An experimental design using a combination of grain-size counts, physical profiling, aerial photography and detailed patch-scanning, was developed to test a range of algorithms designed to retrieve patch-scale (0.5-5 m) roughness metrics from the 3D point cloud. Results indicate that after local detrending using a low-frequency DTM, the local standard deviation of elevations can be successfully correlated to ground mapping and offer potential for improved parameterization of hydraulic models.
H51E-0800
Quantifying and Partitioning Roughness in Mountain Streams, Fraser Experimental Forest, Colorado USA
Flow resistance is recognized as an important component of hydraulics in mountain streams, yet researchers are unable to accurately quantify values for mountain channels. Manning and Darcy-Weisbach equations used for indirect flow estimates are based on the assumption of steady, uniform flow, and the quantification of flow resistance in a channel. Steep channels are characterized by unsteady, rapidly varied flow in which grain size is comparable to flow depth. To improve predictions of resistance and subsequently discharge approximations in mountain channels, we need to obtain a better understanding of resistance and the parameters that control variations in total resistance. This study investigates the interactions of bed, bank, and form resistance in cascade, step-pool, and plane bed channels of headwater streams. We hypothesize that magnitude and variability of individual components of resistance vary systematically by channel type. Detailed surveys were conducted on 15 stream reaches on East St. Louis and Fool Creek in Fraser Experimental Forest, Colorado. Both streams have long term discharge records from US Forest Service gages placed just downstream of the study sites between 1941 and 1943. Each reach was measured under at least three different discharges: low flow, intermediate flow, and peak flow. The detailed surveys included directly measuring all variables needed to calculate discharge, Froude number, total flow resistance, and grain resistance. A total station and LIDAR (Light Detection and Ranging) unit were used to survey channel geometry. These data are used to determine planform geometry, bankfull width, depth, bed gradient, bedform geometry where present, relative submergence and wood size and orientation where present. Flourometers were attached to rebar at 0.6 flow depth, and a rhodamine tracer was used to determine reach-averaged mean velocity at the three stages at which measurements were made. The total flow resistance varied significantly over the three measured flows and between channel types. The most significant factor for variations in flow resistance is gradient.
H51E-0801
Stepped Hydraulic Geometry in Stepped Channels
Steep mountain streams typically present a stepped longitudinal profile. Such stepped channels feature tumbling flow, where hydraulic jumps represent an important source of channel roughness (spill resistance). However, the extent to which spill resistance persists up to high flows has not been ascertained yet, such that a faster, skimming flow has been envisaged to begin at those conditions. In order to analyze the relationship between flow resistance and bed morphology, a mobile bed physical model was developed at Colorado State University (Fort Collins, USA). An 8 m-long, 0.6 m-wide flume tilted at a constant 14% slope was used, testing 2 grain-size mixtures differing only for the largest fraction. Experiments were conducted under clear water conditions. Reach-averaged flow velocity was measured using salt tracers, bed morphology and flow depth by a point gage, and surface grain size using commercial image-analysis software. Starting from an initial plane bed, progressively higher flow rates were used to create different bed structures. After each bed morphology was stable with its forming discharge, lower-than-forming flows were run to build a hydraulic geometry curve. Results show that even though equilibrium slopes ranged from 8.5% to 14%, the reach-averaged flow was always sub-critical. Steps formed through a variety of mechanisms, with immobile clasts playing a dominant role by causing local scouring and/or trapping moving smaller particles. Overall, step height, step pool steepness, relative pool area and volume increased with discharge up to the threshold when the bed approached fully- mobilized conditions. For bed morphologies surpassing a minimum profile roughness, a stepped velocity- discharge relationship is evident, with sharp rises in velocity correlated with the disappearance of rollers in pools at flows approaching the formative discharge for each morphology. Flow resistance exhibits an opposite pattern, with drops in resistance being a function of the height of the drowned steps. Step formation seems to occur under a hydraulic regime different from the lower flows, because spill resistance begins below step-forming flows.
H51E-0802
Modeling Step-pool Sequences in Mountain Channels
Mountain channels are characterized by a preponderance of steps and pools that produce a striking, repetitive staircase-like longitudinal profile. Although the rhythmic character of the step-pool streambed has long attracted the attention of scientists, few studies have successfully quantified the nature and significance of this spatial rhythmicity. This paper develops a statistical model to describe and predict the rhythmic step-pool morphology in a range of environmental settings. Time series analytic techniques applied to streambed profiles from California, Oregon, Washington, and Italy indicate that an ARIMA (1, 1, 0) model is capable of describing the step-pool sequences. These successful applications suggest the utility of modeling step-pool sequences with autoregressive moving average techniques, and more generally with time-series analysis. Such models are potentially useful in the design, management, and restoration of steep channels. Because mountain streams are important habitats for sensitive aquatic species, results of this study can also have implications for ecological management of mountain watersheds.
H51E-0803
Understanding the structure of step-pool mountain streams through wavelet analysis
The periodicity of structures, like step-pool spacing, in the long profiles of mountain streams, has been debated in the literature. Some authors claim to have established the existence of periodicity in step-pool sequences through the use of the Fourier transform. Other authors claim that statistical models, based on renewal processes, adequately explain the placement of the steps. In the latter case, the implication is that step placement is random rather than periodic. In fact, the Fourier transform is best suited to the analysis of stationary signals: signals for which the spectral (frequency or wavelength) content does not evolve through time or space. In the context of long profiles, this implies that one or more dominant wavelengths persist down the entire reach. Given the complexity of river systems, it is not unreasonable to assume that spacing of steps varies along the reach. As such, straightforward Fourier analysis may provide an incomplete or misleading picture of the spectral content of long profiles. On the other hand, the wavelet transform has been successfully applied to analysis of non-stationary signals. Unlike the Fourier transform, which supplies spectral information for the entire signal record, the wavelet transform produces a time-scale or space-scale (i.e. space-wavelength) map that preserves the local spectral content of the signal. Consequently, for long profiles, the wavelet transform has the capacity to detect periodic structures, which persist along the entire reach, as well as "transient" structures that occur at isolated locations along the reach. In this paper, we investigate the applicability of the continuous wavelet transform for the analysis of long profiles associated with mountain streams. We begin by demonstrating how the wavelet transform differentiates prototypical signals that cannot easily be differentiated with the Fourier transform. We then show how the transform can be applied to reveal the space-wavelength structure of a number of mountain stream long profiles.
H51E-0804
Evidence for knickzone propagation and landscape disequilibrium along the James River, central Virginia Piedmont
Several lines of evidence indicate landscape disequilibrium in the James River basin in the central Virginia Piedmont. The river longitudinal profile possesses pronounced convexities, and the James flows in a narrow inner valley incised into a discontinuous, low-relief upland. We hypothesize that this disequilibrium is generated by upstream propagation of several pronounced knickzones along the James profile, and we focus here on a major ~five km long knickzone within the western Virginia Piedmont to determine if it is a translational feature. We combine bedrock and surficial mapping, topographic analysis, and cosmogenic radionuclide dating of terraces along the James River to constrain the role of lithology, reconstruct past river profiles, and estimate rates of river erosion and knickzone propagation. Bedrock mapping at 1:12000 scale reveals the knickzone reach to be underlain primarily by moderately resistant schists and phyllites, with narrow bands of more resistant greenstone and diabase, within a complexly deformed melange. Mapping has revealed no clear relationship between rock resistance and river gradient, suggesting minimal lithologic control on knickzone location. We have identified numerous, disconnected terrace deposits, and reconstruction of past longitudinal profiles from these deposits suggests at least three separate terrace levels and hints at terrace generation by knickzone movement. Longitudinal profiles of minor tributaries entering the mainstem within and downstream of the knickzone possess pronounced convexities whose upper elevations generally match that of the mainstem knickzone. We have dated high terraces near but outside the knickzone reach with in-situ 10Be profiles and a depth integration technique. Dates from three locations along ~100 km of the river are ~1 m.yr., suggesting rapid river incision rates of ~55 m/my. This dating suggests disequilibrium erosion was initiated and has persisted here during the late Quaternary. We hypothesize this recent incision was induced by the shift to more rapid climate fluctuations in the early Pleistocene, leading to base level lowering and knickzone generation
H51E-0805
Amphitheater-Headed Valleys: Unique or Non-Unique Origin? The Hawaiian Laboratory
While the formation and migration of amphitheaters (knickpoints) in unconsolidated sediment is fairly well understood the same cannot be said for amphitheaters formed in a variety of bedrock types, climate settings, and even planetary location. The question remains, is there a single knickpoint forming process at work or do multiple landscape forming processes have a convergence tendency because of cybernetic feedback? A simple example would be a waterfall creating a micro-environment that enhances and focuses specific landscape forming processes like weathering, microbial action, and vegetation growth aiding in the continuation of the knickpoint form. It should be noted that weathering, microbial growth and vegetation growth would have cybernetic feedback among them. In fact it may be difficult to determine the controlling process, if any. If one considers that knickpoints likely intercept the regional groundwater flow system there is an additional focused source of water supply which further contributes to the micro-environment of the knickpoint. Groundwater discharge has significant cybernetic feedbacks with landscape forming processes. The nature and composition of the bedrock and climatic factors may determine rates of knickpoint migration but the resulting morphologic features in different settings would likely be similar with cybernetic feedback. It should be noted that cybernetic feedback can either be damping or amplifying. Amphitheater-headed valleys have developed in many locations on the Hawaiian Islands. The islands have formed in a time sequence as the supporting oceanic plate moves over a focused mantle source for the basalts. The amphitheaters of Hawaii occur in "fresh" and "older" basaltic rock depending on island location. Weathering processes have acted longer on some islands. Because of the topography and its affect on trade winds the main islands have focused rainfall, significant recharge, and active groundwater flow systems. While climate is uniform overall, the wet and dry sides of the islands coupled with topography represent multiple climatic zones. This affords the opportunity to use the Hawaiian Islands as a laboratory to study the cybernetic feedbacks among knickpoint forming processes. Feedback examples will be presented for several Hawaiian knickpoints.
H51E-0806
Convergent Hydraulics and Knickpoint Migration in an Incising Gravel-Cobble River
Regulated gravel-cobble rivers are known to incise, but the mechanism of incision is not well documented by process-based research. Widespread use of simple "stream power law" equations assumes that incision is caused by continuous downcutting that increases with discharge. To provide an alternate explanation that recognizes the inherent non-uniformity of natural channels, we hypothesize that regulated rivers experience waves of migrating knickpoints that retreat through riffles during low flow when riffle crests function as supercritical weirs and are rejuvenated by floods that downcut the intervening width-constricted pools. To test this new hypothesis, monitoring was performed on the rapidly incising 7-km Timbuctoo Bend of the lower Yuba River, CA, where 463,000 cubic meters of sediment have been scoured out in just the last seven years alone. Direct field measurements of velocity and depth fields were obtained at three migrating, horseshoe-shaped knickpoints in this reach. Also, detailed channel DEMs were obtained at different stages of knickpoint migration to track geomorphic change over seven years. At one of the knickpoints, a special torque sensor was deployed to map near-bed lift and drag stress components. Velocity fields reveal convergent hydraulics controlled by the horseshoe morphology that focus scour in the upstream center of the U-shape. At a relatively low discharge, supercritical flow and near-critical standing waves were observed. Furthermore, the peak near-bed drag stress that was directly measured exceeded 1000 Pa during this relatively low discharge regime, which explains why the bedforms are retreating so rapidly. These direct measurements were compared to similar measurements previously reported for horseshoe waterfalls analyzed in a flume, and will aid in determining the real mechanism for knickpoint migration and channel incision in regulated gravel-cobble rivers.
H51E-0807
Post-Mazama Break-out Flood on the Sycan River, southern Oregon
The Sycan River is the largest tributary to the Sprague River watershed in the semi-arid Upper Klamath Basin of south-central Oregon. Only 56 kilometers (35 miles) from Crater Lake, Mt Mazama's&premnant volcanic crater, the headwaters of the Sycan watershed has an abundance of aeolian and fluvially distributed Mazama pumice. The upper Sycan River flows through a wide basin 17 km long (Sycan Marsh), then southward through 40 kms of narrow basalt canyon. The lower16 river kilometers occupy a low gradient unconfined southward-widening valley at the Sprague River confluence. Field investigation of cut banks and auger holes in the lower valley reveal an extensive 1 to 2-m thick lobe-shaped deposit dominated by Mazama pumice, and fining upward from granules to medium sand. This deposit is interpreted as resulting from a single or set of break-out flood events relatively soon after the pyroclastic eruptions of Mt Mazama 7,700 years ago. The flood event buried a fine-grained floodplain with a thick layer of pumiceous sands across the river's unconfined lower valley section. In the upstream canyon section, boulder bars and boulder terraces as well as terraces of Mazama tephra indicate that the event was a relatively large system-wide flood. Distribution and stratigraphy of the resultant flood terraces of the lower Sycan River suggest that the event was a break-out flood caused by blockage in the confined canyon section. The location and mechanism of blockage are not definitely known; it may be either landslide(s) from the canyon walls, or accumulation(s) of Mazama tephra and other sediments in the canyon. Submitted radiocarbon samples from or near the base of the flood deposits will date the break-out flood event. Additional analysis and field investigation is underway to determine the mechanism and location of the canyon blockage, the hydraulic dynamics of the flood and the extent of the flood deposit lobe.
H51E-0808
Comparison of Two Landslides and Related Outburst Flood Deposits and Their Effects on River Evolution, Owyhee River, Oregon
Abundant channel-encroaching landslides and lava flows on the Owyhee River in southeastern Oregon have the capacity to both inhibit incision by altering channel slope, width, and bed character, and burying valley-bottom bedrock under exogenous material, and promote incision by generating cataclysmic floods through natural dam failures. We hypothesize that these extrafluvial events play a significant role in creating and maintaining the geomorphic features of river canyons in uplifted volcanic terraces that comprise a significant part of the western U.S. Numerous landslides have entered the Owyhee River canyon north of Rome, Oregon. As the river flows through different lithologic units, the style of mass wasting changes from large slump events to large debris flows. These differences seem to be related to the composition and thickness of the underlying, exposed Tertiary sediments relative to the basalt cap. The largest exemplary mass wasting events of each morphologic category in two reaches were examined to compare the effects of these events on the river channel. Multiple slump events ranging in age from approximately 104 to 105 ka have impacted the river channel near Artillery Rapid (River km 33 from Rome, OR). At least one of these appears to have dammed the channel and failed catastrophically, creating a flood bar immediately downstream, with boulders up to 3 m in diameter that decrease in size with distance from the landslide dam. Greeley Debris Flow (River km 70), the largest debris flow in the Hole in the Ground reach, blocked the channel, creating extensive fill terraces behind the landslide dam and a large depositional flood bar downstream, with an age possibly as young as early Holocene. Downstream of both landslide reaches, outburst flood deposits of large boulders armoring the channel were measured and used to determine the velocity of the outburst flood that entrained the boulders. Based on this evidence, it is clear that landslides have affected the evolution of both the stream channel and hillslopes. Comparing the different landslide types shows differences regarding slip surface geology and mass wasting stage. Approximately 270ft of the sediment package underneath the basalt cap at Artillery Landslide is exposed and shows coarser grained units with a mixture of lithics and volcaniclastic sediments, indicating fluvial emplacement. This specific reach has undergone only one generation of slump events. In contrast, the Greeley Debris Flow is located in an area where approximately 900ft of underlying sediment has been exposed by many undifferentiated mass wasting events. The sediments that create Greeley Debris Flow are composed of bentonitic clay lacustrine deposits. The presence of a well exposed and thicker sequence of silicic bentonitic clays at Greeley Debris Flow suggests (1) these large debris flow events are more common in areas where basalt overlies silicic lacustrine deposits and (2) there has been a longer and greater frequency of mass wasting at the Hole in the Ground reach resulting in secondary debris flows created from initial slump events.
H51E-0809
Near surface geophysics and sediment analysis to precise the outbreak of glacial Lake Devlin, Front Range Colorado, USA
Beside the geomorphological setting and the associated lithologic sphere the distribution and genesis of glacial, periglacial and paraglacial sediments is affected also significantly by climatic parameters. The time of deglaciation of the last glacial period is especially valuable to investigate the correlation between climatic shifts and possible geomorphological response. Ice-dammed lakes and their outbreaks are ideal archives to study this climate-geomorphological link. Glacial Lake Devlin is a key area of Pinedale deglaciation in the Front Range of Colorado, USA, because it catastrophically drained after the retreat of North-Boulder-Creek-Glacier. The breaching of the moraine is determined by Radiocarbon data from disperse organic matter within the uppermost lake sediments to about 13 ka 14C BP, an important date of the Front Range deglaciation (Madole 1986). A reinvestigation of the site allows a 3-dimensional mapping of the distribution and the depth of different glacial, periglacial, fluvial and lacustrine sediments by using various near surface geophysical methods such as seismic refraction, ground penetrating radar and electric tomography. Results of shallow geophysics help to determine positions to dig for sediment exposures. Detailed images of the different sediment units enable reconstructing a stratigraphic and genetic model of lake sediments, deltaic deposits, glacial till and fluvial sediments from the outbreak of the lake. Together with that 3-dimensional sedimentary model, new AMS-radiocarbon data combined with data derived from optical stimulates luminescence (OSL) will precise the date of the outbreak. MADOLE R.F. (1986): Lake Devlin and Pinedale glacial history, Front Range, Colorado. - Quaternary Research 25: 43–54.
H51E-0810
Constraining the Timescales of Sediment Sequestration Associated With Large Woody Debris Using Cosmogenic Beryllium-7
The overabundance of fine sediment (<2 mm) in fluvial environments engenders various detrimental impacts on aquatic ecosystems, such as embedded spawning gravels. Conversely, large woody debris (LWD) has been shown to provide an array of positive geomorphic and ecogeomorphic functions at multiple scales in the fluvial environment. In this study along a 9 km stretch of the Ducktrap River of coastal Maine we use a novel application of cosmogenic 7Be to calculate the storage times of fine-grain sediment sequestration by in-channel obstructions (LWD and boulders). Using a constant initial activity (CIA) sediment storage aging model, the majority of in-channel sediment bars associated with channel obstructions in the transport dominated reach were found to be young (< 106 days), while those in the transport limited reach were found to be older (> 200 days) as evidenced by the lack of detectable 7Be activity in the majority of samples. Emergent channel margin bar sediment storage times indicate longer-term storage on the order of one year to several years, as evidenced by stratigraphic and geochemical data. These findings underscore the importance of wood frequency and size in offsetting relatively short residence times of sediment sequestration and have implications for assessing the benefits of wood in channel restoration efforts and the potential impacts of wood removal to sediment regimes in disturbed landscapes.
H51E-0811
Trapping of fine-grained sediment in a gravel-bed river by large woody debris: a physical modeling study
We used a physical model to investigate the processes of erosion, transport, and storage of fine-grained sediment in a gravel-bed river with abundant large woody debris (LWD) accumulations. Fine-grained sediment storage specifically refers to deposits of mud and sand located in the channel margins and near-bank regions. We have termed these fine-grained channel margin (FGCM) deposits. Extensive field surveys of the study area indicate that these near-bank deposits are principally controlled by the presence of LWD, which obstructs a portion of the main flow and creates regions of reduced velocity, allowing for the deposition of mud and sand. The specific experimental question we addressed in these experiments is: what is the behavior of water and sediment behind a porous obstruction in the nearbank region of an open channel? We focused on the relative roles of discharge, slope, suspended sediment concentration, and the configuration of the LWD, which are the primary variables controlling the quantity and distribution of fine sediment in the study area. We conducted several types of experiments: 1) a series of experiments to define sediment transport behavior in the absence of simulated LWD in a straight, prismatic channel, 2) experiments to explore most favorable configuration of simulated LWD that conform to field observations, 3) experiments to investigate the formations and geometry of the deposit as controlled by sediment supply with constant discharge and LWD, and 4) a series to investigate the erosion and deposition of the features after they have been formed. We developed quantitative, predictive relationships between the volume of mud stored and the governing variables (i.e. hydraulics, sediment supply, and characteristics of the LWD).
H51E-0812
Channel Motion as a Random Walk: Erosion Probabilities and Implications for Sediment Residence Time
Stochastic models of fluvial systems require an estimate of the erosion probability of sedimentary deposits in order to predict particle trajectories, the distribution of sediment residence time, the time a grain takes to transit the system, and the time to overturn all the sediment in a valley. We can use this information to test the assumptions made in detrital geochronology, to predict the transport and dispersion of solid-phase contaminants, and to predict the fate of sudden inputs of sediment. Unfortunately, estimating the erosion probability is difficult. It is possible to determine the average erosion probability if the mean residence time of the sediment in a deposit is known or if the mass of the deposit and the sediment flux out of it can be determined. However, even if these things are known, the average erosion probability does not capture the potential variability in the system. It is more desirable to know the distribution of erosion probability. We examine a simplified fluvial system with a one-dimensional, single-threaded, meandering channel that migrates by point bar deposition and cut bank erosion. There is no net aggradation or incision and over-bank deposition is neglected. In this simple model, the probability that a sediment grain in the valley is eroded is equal to the probability that the channel occupies the grain's location. The probability density function (PDF) of erosion is identical to the PDF of channel position. We treat the channel motion as a random walk, allowing us to make predictions about the PDF of channel position for two end-member cases of meandering behavior. When the channel takes small steps relative to the valley width and the motion is symmetrical about the valley axis, the PDF of channel position is a Gaussian centered on the valley axis. Alternately, if the motion of the channel is dominated by meander cutoffs, then the channel location can change by increments that that are similar to the valley size. In this case, the channel position approaches a uniform distribution where all positions in the valley are equally probable. We define the residence time of a sediment grain at a location in the valley as the interval between successive occupations of that location by the channel. This allows us to treat the system as a first passage process and predict the PDF of residence time from the PDF of channel position. For the two end members described above, the results are very different. When the PDF of channel position is a Gaussian, the residence time PDF is a heavy- tailed power law. When the PDF of channel position is uniform, the PDF of residence time is exponential.
H51E-0813
Long-Term Evolution of Meandering Rivers
Natural rivers are self-formed features whose shapes are the result of interaction between erosion, deposition and transport of sediments. The study of their morphodynamics and the characterization of related sedimentary processes are of great interest not only to environmental engineers but also to hydrology and historical geology, contributing to the interpretation of stratigraphic records. In the present contribution we focus our attention on the long-term behaviour of meandering rivers, a very common pattern in nature, which belongs to a class of dynamical systems occurring at the spatial scale of the channel width and driven by the coexistence of complex linear and non-linear processes. On the short term time scale, the formation of meandering patterns can be suitably explained as an instability process, driven by bank erosion (bend instability). The planar development of the river is described by a non-linear integro-differential bend evolution equation, complemented with a suitable model for flow and bed topography in sinuous channels with cohesionless bed. On the long-term timescale, a further highly non-linear process must be accounted for, namely channel shortening via cutoff processes. Depending on the description adopted for the flow field, various mathematical models allowing the description of the temporal evolution of the channel axis can be developed. The problem then arise to compare the morphologic characteristics of the planimetric configurations obtained using the different flow field models as well as the differences/analogies between calculated patterns and those observed in the field. The goal of this contribute is to test the prediction capabilities of numerical models simulating the long-term evolution of meandering rivers planform. To this end the morphological characteristics of the planar configurations resulting from different numerical models and those extracted from Landsat mosaic images will be compared through suitable statistical methods. In particular, various morphological variables (e.g channel axis curvature, cartesian wavelength, sinuosity, asymmetry coefficient) will be considered in order to ascertain the ability of synthetical patterns to reproduce the complex morphological features (e.g. compound bend, multi-bend loop) usually observed in nature.
H51E-0814
Field Investigation of Flow Structure and Channel Morphology at Confluent-Meander Bends
The movement of water and sediment through drainage networks is inevitably influenced by the convergence of streams and rivers at channel confluences. These focal components of fluvial systems produce a complex hydrodynamic environment, where rapid changes in flow structure and sediment transport occur to accommodate the merging of separate channel flows. The inherent geometric and hydraulic change at confluences also initiates the development of distinct geomorphic features, reflected in the bedform and shape of the channel. An underlying assumption of previous experimental and theoretical models of confluence dynamics has been that converging streams have straight channels with angular configurations. This generalized conceptualization was necessary to establish confluence planform as symmetrical or asymmetrical and to describe subsequent flow structure and geomorphic features at confluences. However, natural channels, particularly those of meandering rivers, curve and bend. This property and observation of channel curvature at natural junctions have led to the hypothesis that natural stream and river confluences tend to occur on the concave outer bank of meander bends. The resulting confluence planform, referred to as a confluent-meander bend, was observed over a century ago but has received little scientific attention. This paper examines preliminary data on three-dimensional flow structure and channel morphology at two natural confluent-meander bends of varying size and with differing tributary entrance locations. The large river confluence of the Vermilion River and Wabash River in west central Indiana and the comparatively small junction of the Little Wabash River and Big Muddy Creek in southeastern Illinois are the location of study sites for field investigation. Measurements of time-averaged three-dimensional velocity components were obtained at these confluences with an acoustic Doppler current profiler for flow events with differing momentum ratios. Bed and channel morphology were also surveyed with a digital fathometer to document geomorphic change. Preliminary analysis of the velocity data reveals the presence of a well-defined shear layer between the converging flows and secondary circulation in the main channel. The tributary channel appears to oppose high velocity flow directed toward the outer bank by centrifugal acceleration through the meander bend of the main channel, thereby diminishing erosion along the cut bank and possibly stabilizing the meander bend channel. The flow structure and channel morphology of the study sites are compared to consider the effect of spatial scale and geometric characteristics on confluent-meander bend dynamics.
H51E-0815
Dating Fluvial Floodplains and Quantifying Channel Migration Rates Using Short-Lived Radioisotopes
A variety of methods, including dendrochronolgy and aerial photography, have been used previously to quantify fluvial channel geometry changes and lateral migration rates on time scales of tens to hundreds of years. However, no universally satisfactory technique has emerged due to various limitations of each method. We explore the novel technique of using short-lived radionuclides to date fluvial surfaces with the goal of determining lateral migration rates. In particular, we use 210Pb (t(1/2) ~ 22.3 years) in floodplain sediment to determine the lateral migration rates of regulated reaches on the Genesee River, NY, and unregulated reaches on the Winooski River, VT and the upper Connecticut River, NH. Surface ages and lateral migration rates are compared to those determined using historical aerial photography. Preliminary analyses of sediment cores are consistent with the general trend of surface ages recorded in the aerial photography, but the absolute ages of the surfaces indicate that inheritance of 210Pb within the deposited sediment affects the surface ages. Furthermore, the temporal resolution of the method is much greater in areas with rapid migration over the past 50 years than in those areas with similarly rapid migration occurring in years prior to 1960.
H51E-0817
Measuring Changes in River Geometry Using Aerial Photographs and Field Surveys: Narraguagus and Sheepscot Rivers, Maine
We study channel geometry of the Sheepscot and Narraguagus rivers in coastal Maine. We work in mainstem reaches, where channel gradient is generally <0.5% and bankfull width is 10-30 m. We used georeferenced historic aerial photographs (from 1940-1946) and recent digital orthophotoquadrangles (DOQs from 1996-1997) to make measurements at 100 m intervals along channel thalwegs. We also surveyed selected reaches of both rivers, at the same locations used in our aerial-photograph comparison, measuring bankfull and wet (active- channel) widths. From aerial-photograph analysis, the Sheepscot River exhibits modest narrowing over time, with 2 of 5 subreaches showing statistically significant change (2-7 m). The West Branch of the Sheepscot River also shows a statistically significant narrowing trend in 4 of 6 subreaches. The subreach with the most pronounced change (mean aerial-photograph width from 42 m in 1940 to 19 m in 1996) resulted from a dam removal. The Narraguagus River shows modest widening from 1944-1946 to 1996, with 2 of 6 subreaches showing statistically significant change (1-8 m), although this trend may be the result of high flow stage in 1996. We are also analyzing the influence of riparian-corridor land-use change on the observed trends in both watersheds. Comparison of field measurements of wet width in 2006-2007 with the 1996-1997 DOQs produced statistically indistinguishable results in 3 of 4 subreaches, in spite of higher flow stage during the aerial surveys. This suggests that aerial-photograph analysis provides a reasonable and inexpensive means to measure active- channel width in these systems. Our ongoing research includes comparing field and aerial-photograph channel geometry measurements in the Maine rivers with those from the Jacquet River in New Brunswick, Canada.
H51E-0818
Persistence and Geomorphology of Clearwater Side Channels in a Braided River: The More Things Change, the More They Stay the Same
Clearwater side channel lifespans and geomorphic changes are being examined to guide research on importance of side channels to salmon habitat quantity and quality at the river-long scale and implications for land management. Dynamic patterns of channel formation in braided rivers can result in rapid abandonment of channels within the braid plain. When these channels fill with water from hyporheic, regional ground water, or upland tributary sources, clearwater side channels result that might create a stable environment for spawning salmon. To determine the annual to decadal persistence of these side channels, clearwater channels in the 120 km-long Matanuska River in southcentral Alaska are being identified from color differences on a 0.3 m pixel 2006 color orthophoto prepared from 1:24,000-scale aerial photography. Channel identification is being calibrated with field observations of selected channels. This inventory of modern clearwater side channels is being compared to historical conditions from a black-and-white orthophoto prepared from 1949 1:40,000-scale aerial photography, and selected aerial photography for the 1960s and 1980s. Where photo quality limits detection of water clarity, side channel presence or absence is noted. Initial results show that while individual side channels may persist for many years, they have rarely persisted more than a few decades. Analysis of selected, wide braid plain areas shows that multiple water sources may exist for supplying abandoned braid plain channels, an indication that some locations may host clearwater side channels regardless of mainstem position. For example, a clearwater side channel that has been historically used as a spawning site is fed by a tributary known locally as Yellow Creek. This channel is presently being captured by the active braids of the Matanuska River. A comparable clearwater side channel has formed from tributary flow on the opposite bank. However, in 1949, this condition was reversed, with Yellow Creek flowing for 2 km within the braid plain, and the mainstem braids flowing along the opposite bank. This study is also examining the importance of periodic resetting, or fundamental disturbance of the sediment, channel form, and aquatic and riparian vegetation, by the mainstem by measuring the physical and hydraulic characteristics of selected channels used for spawning by salmon. Initial measurements of spring- fed channels show coarse gravel substrates, slow velocities in channels less than 0.5 m deep, and limited opportunities for flooding. These conditions indicate that channel form and substrate is likely little modified from the inherited initial condition, although the biologic activity of redd construction can locally modify bedforms and substrate.
H51E-0819
Slow Lateral Migration Rates of the Gravel-Bedded South River, Virginia: the Influence of Bedrock, Riparian Vegetation, and Bend Geometry.
Shorelines mapped on orthorectified aerial photographs from 1937 and 2005 demonstrate that the South River is unusually stable: rates of lateral migration from 1937-2005 range from 0 to 0.3 m/yr with a median of 0.02 m/yr. Shoreline changes are barely resolvable at some locations on aerial photographs despite rectification errors that are typically less than 1m. Field surveys indicate that only 12.4 percent of the entire shoreline is currently actively eroding. Lateral shifting of the channel appears to occur where riverbanks are composed of alluvium, rather than bedrock. In some areas, mid-channel bars and tributary confluences are areas of significant bank erosion, as the bars and tributary-mouth deltas deflect the main flow into the opposite banks. Erodibility values computed using the model of Johanneson and Parker (1989) are distinctly different for bends with different geologic setting and riparian vegetation.
H51E-0820
Channel Morphological Changes in the Yuba River, California, in the Post-Hydraulic Mining Period
Hydraulic gold mining in the Sierra Nevada of California (1853-1884) produced large volumes of sediment from upland placer gravels. The prevailing belief has been that piedmont storage of this sediment is volumetrically negligible or inactive. This study tests the hypothesis that large deposits of historical sediment remaining in the bed, banks and terraces of the lower Yuba River have been remobilized by floods and that erosion has continued over the past few decades. Remote sensing and GIS analyses of topographic and planimetric data from historical maps, surveys, aerial photographs, and LiDAR data document historic changes and the timing of sediment erosion and deposition within the channel and floodplain system. Planimetric and volumetric measurements of channel enlargement, lateral migration, avulsions, and channel filling provide magnitudes of erosion and deposition of historic sediments in the lower Yuba River. In 1906, the California Debris Commission produced a detailed large-scale topographic map of the lower Yuba floodplain showing it as a multi-thread channel system. The paleochannel scars remain evident on air photos, LiDAR images, and in the field. Differencing of topographic data derived from the 1906 topographic maps and 1999 LiDAR data provide volumetric measures of substantial channel morphologic changes including channel shifting, filling, and evolution towards a single- thread channel system. These measures identify processes and rates of sediment production relevant to broader issues of flood hazards in the region. http://people.cas.sc.edu/ajames/sacvalley.data/
H51E-0821
Legacy Sediments and Channel Morphology in the Feather and Yuba Rivers, California
Channel aggradation and morphologic change following 19th century hydraulic gold-mining in the Sierra Nevada, California, differed substantially between the lower Feather and Yuba Rivers. These differences can be explained in part by topographic position in the Sacramento Valley but also by differences in early 20th century engineering structures and management policies. Both rivers experienced extreme aggradation by mining sediment and substantial avulsions but the timing and mechanics of channel adjustments were dissimilar, in part due to varying strategies in river-training and flood control. River engineering and management in the late 19th century identified the lower Yuba River as a repository zone where mining sediment could be sequestered to reduce deliveries to navigable rivers downstream. Levees were set back up to 4 km allowing formation of a multi-thread channel system across a broad floodplain that is now deeply buried by mining sediment. In contrast, levees along the lower Feather were given narrow spacings to encourage self-scouring of channels and promote navigability of channels. The lower Feather River drains a larger basin and has a lower gradient than the Yuba River. Construction of Fremont Weir across the mouth of the Yolo Basin raised flood levels in the lower Feather River and may have reduced transport of bed sediment. This could explain the persistence of large sand sheets at and below the Bear River confluence. Data from historical maps, topographic surveys, aerial photographs, and 1999 LiDAR swath mapping are used to document and contrast channel changes and floodplain evolution between these two rivers. Topographic changes derived by differencing detailed 1906-1909 topographic maps and 1999 LiDAR data indicate substantial channel morphologic changes including channel filling, lateral migration, and evolution towards single-thread channel systems. Modern streambank stratigraphy reflects the differences in channel responses. Sites where channel avulsions moved the channel to its present position often have high banks of pre-historic soil with only a thin cap of historical alluvium at the top. In contrast, sites near original channel positions often have thick deposits of historical alluvium. These differences are key to understanding spatial patterns of storage of historic sediment which contains high concentrations of elemental mercury. http://people.cas.sc.edu/ajames/sacvalley.data/
H51E-0822
Limited Response of Ephemeral Stream Dynamics to Extreme Vegetation Change Related to Historic Overgrazing, Catalina Island, California, U.S.
The role of climatic transition and related changes in vegetation in arroyo development remains a controversial topic. Most studies in the semiarid southwest US suggest that either latest Holocene climate change or overgrazing triggered widespread historic aggradation and incision in ephemeral stream systems, yet a clear consensus has not been reached. The fluvial response question is important when trying to anticipate landscape changes in response to global climate change. A history of extensive vegetation disturbance caused by 150 years of intense grazing on Catalina Island and a record of fluvial erosion and deposition provides clues about ephemeral fluvial system response to extreme vegetation change. Deposits exposed along axial channels contain multiple buried soils that have strong A horizons and weak Bw horizons. Radiocarbon dates indicate that these sediments were deposited between ca. 6-2 ka. New radiocarbon dates and soil stratigraphy of several tributaries along the main valley washes also indicate that widespread deposition occurred episodically up to about 2 ka. The presence of multiple buried soils indicates that several pronounced cycles of deposition, stability, and erosion occurred long before overgrazing and extensive reduction in vegetation cover. Deep (>75 cm), moderately developed soils examined along steep hillslopes also indicate relative slope stability over the past several centuries to several thousand years, although, scattered but localized areas of historic erosion are observed. Holocene deposition and erosion is well represented in the valley bottoms, but little stratigraphic evidence exists for widespread historic hillslope erosion and fluvial deposition in tributaries or trunk streams. The relation implies that an extensive decrease in vegetation alone appears insufficient for triggering a cycle of historic hillslope erosion and valley aggradation. Limited geomorphic response to overgrazing suggests that other response processes such as complex geomorphic response, extensive fires, or an increase in extreme storms may be required to mobilize sediment and trigger extensive arroyo filling and incision.
H51E-0823
Developing Depositional Models for Mercury Contaminated Floodplain Deposits Using Geomorphic Mapping and GIS in South River, Virginia
Mercury was introduced into the South River from the 1930s to the 1950s from an industrial plant in Waynesboro, Virginia. Mercury contamination in fish tissue continues to exceed acceptable levels. The contaminated sediments in the river's floodplains are probably the present source of mercury to the South River ecosystem. Locating and determining the extent and depositional history of these deposits are important for understanding the mercury cycle in the river as well as for remediation plans. The South River is a sinuous, single thread alluvial river with frequent bedrock exposures along its bed and banks. Overbank deposits are discontinuous and thin. Rates of lateral migration by the South River are extremely low, averaging 0.02 m/yr, and the river has been influenced by mill dams along a 19 km study reach. This 19 km section of the 37 km river reach was selected for the study because of its high concentration of Hg. Six different categories of floodplain deposits dating from 1937-2005 have been identified throughout the river using studies of historical aerial photographs in a GIS framework, field mapping, dendro- and radionuclide dating, grain size and Hg analysis. Not surprisingly, traditional depositional models of meandering rivers do not apply. Floodplain depositional units include mill dam deposits, point bar/bench deposits, concave bank bench deposits, islands, cattle deposits, and tributary confluences deposits. The most important deposits for sequestering historic mercury are those that also store the most silt and clay. These include mill dam deposits, point bar/bench deposits, concave bank deposits, and tributary confluence deposits. Many of these deposits represent reservoirs of mercury-contaminated sediments that could supply significant amounts of mercury into the river presently and in the future.
H51E-0824
Floodplain Sedimentation from a 30-year Reccurrence Flood in 2005 on the Ping River, Northern Thailand
This study documents the nature of flood-producing storms and floodplain deposition associated with the September 28-October 2, 2005 30-year-recurrence flood on the Ping River in northern Thailand. The primary purpose of the study is to understand the extent that deposits from summer-monsoon floods can be identified in floodplain stratigraphy A secondary objective is to document the sedimentation processes/patterns associated with a large contemporary flood event on a medium-sized Asian river. Maximum sediment depths of 15 cm were found on the river levee, within 30 m of the main channel, and at 350 m thickness was 4 cm. Sediment depth generally decreased exponentially with distance away from the main channel. The extent of sediment deposition was about 1 km from the river channel. However, 72/% of the sediment was deposited within an oval-shaped area 200-400 m from the main channel and centered on a tributary stream, through which sediment-laden water entered the floodplain, in addition to overtopping the levee of the main channel. Sediment concentration during the flood was estimated at 800-1500 mg L-1; and we believe the sediment was delivered by flows of well-mixed flood water occurring over a 1-2 day period. These data suggest that flood-deposited strata related to 30-year recurrence floods is only likely to be preserved in deposits located relatively close to the main river channel where fine sand and clayey coarse silt deposits have thicknesses of at least 5-10 cm. These relatively thick deposits would survive bioturbation, whereas more distal areas with thin clayey silt deposits would not.
H51E-0825
The Role of Observer Variation in Determining Rosgen Stream Types in Northeastern Oregon Mountain Streams
The ability to consistently determine Rosgen stream types from channel characteristics reported by different observers was evaluated in 12 streams within the John Day Basin, northeastern Oregon. The Rosgen classification system is commonly used in the western United States and is based on the measurement of five stream attributes: entrenchment ratio, width-to-depth ratio, sinuosity, slope, and substrate size. Streams were classified from measurements made by 3 monitoring groups, with each group fielding multiple crews that conducted 2-3 independent surveys of each stream. In only 4 streams (33%) did measurements from all crews in all monitoring groups yield the same stream type. Most differences found among field crews and monitoring groups could be attributed to differences in estimates of the entrenchment ratio. Differences in entrenchment ratio were likely due to small discrepancies in determination of maximum bankfull depth, leading to potentially large differences in determination of Rosgen's flood-prone width and consequent values of entrenchment. The result was considerable measurement variability among crews within a monitoring group, and because entrenchment ratio is the first discriminator in the Rosgen classification, differences in the assessment of this value often resulted in different determination of primary stream types. In contrast, we found that consistently evaluated attributes, such as channel slope, rarely resulted in any differences in classification. We also found that the Rosgen method can yield non-unique solutions (multiple channel types), with no clear guidance for resolving these situations, and we found that some assigned stream types did not match the appearance of the evaluated stream. Based on these observations we caution the use of Rosgen stream classes for communicating conditions of a single stream or as strata when analyzing many streams due to the reliance of the Rosgen approach on bankfull estimates which are inherently uncertain.
H51E-0826
Quantifying the Spatial Distribution of Hill Slope Erosion Using a 3-D Laser Scanner
Soil erosion is a complicated process involving many interdependent variables including rainfall intensity and duration, drop size, soil characteristics, ground cover, and surface slope. The interplay of these variables produces differing spatial patterns of rill versus inter-rill erosion by changing the effective energy from rain drop impacts and the quantities and timing of sheet and shallow, concentrated flow. The objective of this research is to characterize the spatial patterns of rill and inter-rill erosion produced from simulated rainfall on different soil densities and surface slopes using a 3-D laser scanner. The soil used in this study is a sandy loam with bulk density due to compaction ranging from 1.25-1.65 g/cm3. The surface slopes selected for this study are 25, 33, and 50 percent and represent common slopes used for grading on construction sites. The spatial patterns of soil erosion are measured using a Trimble GX DR 200+ 3D Laser Scanner which employs a time of flight calculation averaged over 4 points using a class 2, pulsed, 532 nm, green laser at a distance of 2 to 11 m from the surface. The scanner measures point locations on an approximately 5 mm grid. The pre- and post-erosion scan surfaces are compared to calculate the change in volume and the dimensions of rills and inter-rill areas. The erosion experiments were performed in the Soil Erosion Research Laboratory (SERL), part of the Civil and Environmental Engineering department at San Diego State University. SERL experiments utilize a 3-m by 10-m tilting soil bed with a soil depth of 0.5 meters. Rainfall is applied to the soil surface using two overhead Norton ladder rainfall simulators, which produce realistic rain drop diameters (median = 2.25 mm) and impact velocities. Simulated storm events used in this study consist of rainfall intensities ranging from 5, 10 to 15 cm/hr for durations of 20 to 30 minutes. Preliminary results are presented that illustrate a change in runoff processes and erosion patterns as soil density increases and reduces infiltration characteristics. Total soil loss measured from the bottom of the erosion bed is compared to the volume of soil loss determined using the laser scanner. Due to soil consolidation during the experiment, the accuracy of measured soil loss from the laser scanner increases with increasing soil density. Ratios of rill and inter-rill erosions for each experiment are also presented. URL: http://spatialhydro.sdsu.edu