Pebble Counts Research Articles

Adequacy of Visually Classified Particle Count Statistics From Regional Stream Habitat Surveys1

Abstract: Streamlined sampling procedures must be used to achieve a sufficient sample size with limited resources in studies undertaken to evaluate habitat status and potential management‐related habitat degradation at a regional scale. At the same time, these sampling procedures must achieve sufficient precision to answer science and policy‐relevant questions with an acceptable and statistically quantifiable level of uncertainty. In this paper, we examine precision and sources of error in streambed substrate characterization using data from the Environmental Monitoring and Assessment Program (EMAP) of the U.S. Environmental Protection Agency, which uses a modified “pebble count” method in which particle sizes are visually estimated rather than measured. While the coarse (2ϕ) size classes used in EMAP have little effect on the precision of estimated geometric mean (Dgm) or median (D50) particle diameter, variable classification bias among observers can contribute as much as 0.3ϕ, or about 15‐20%, to the root‐mean‐square error (RMSE) of Dgm or D50 estimates. Dgm and D50 estimates based on EMAP data are nearly equal when fine sediments (<2 mm) are excluded, but otherwise can differ by up to a factor of 2 or more, with Dgm < D50 for gravel‐bed streams. The RMSE of reach‐scale particle size estimates based on visually classified particle count data from EMAP surveys, including variability associated with reoccupying unmarked sample reaches during revisits, is up to five to seven times higher than that reported for traditional measured pebble counts by multiple observers at a plot scale. Nonetheless, a variance partitioning analysis shows that the ratio of among site to revisit variance for several EMAP substrate metrics exceeds 8 for many potential regions of interest, suggesting that the data have adequate precision to be useful in regional assessments of channel morphology, habitat quality, or ecological condition.

Assessment of Coarse Sediment Mobility in the Black Canyon of the Gunnison River, Colorado

The Gunnison River in the Black Canyon of the Gunnison National Park (BCNP) near Montrose, Colorado is a mixed gravel and bedrock river with ephemeral side tributaries. Flow rates are controlled immediately upstream by a diversion tunnel and three reservoirs. The management of the hydraulic control structures has decreased low-frequency, high-stage flows, which are the dominant geomorphic force in bedrock channel systems. We developed a simple model to estimate the extent of sediment mobilization at a given flow in the BCNP and to evaluate changes in the extent and frequency of sediment mobilization for flow regimes before and after flow regulation in 1966. Our methodology provides a screening process for identifying and prioritizing areas in terms of sediment mobility criteria when more precise systematic field data are unavailable. The model uses the ratio between reach-averaged bed shear stress and critical shear stress to estimate when a particular grain size is mobilized for a given reach. We used aerial photography from 1992, digital elevation models, and field surveys to identify individual reaches and estimate reach-averaged hydraulic geometry. Pebble counts of talus and debris fan deposits were used to estimate regional colluvial grain-size distributions. Our results show that the frequency of flows mobilizing river bank sediment along a majority of the Gunnison River in the BCNP has significantly declined since 1966. The model results correspond well to those obtained from more detailed, site-specific field studies carried out by other investigators. Decreases in the frequency of significant sediment-mobilizing flows were more pronounced for regions within the BCNP where the channel gradient is lower. Implications of these results for management include increased risk of encroachment of vegetation on the active channel and long-term channel narrowing by colluvial deposits. It must be recognized that our methodology represents a screening of regional differences in sediment mobility. More precise estimates of hydraulic and sediment parameters would likely be required for dictating quantitative management objectives within the context of sediment mobility and sensitivity to changes in the flow regime.

Using pebble lithology and roundness to interpret gravel provenance in piedmont fluvial systems of the Rocky Mountains, USA

Clast populations in piedmont fluvial systems are products of complex histories that complicate provenance interpretation. Although pebble counts of lithology are widely used, the information provided by a pebble count has been filtered by a potentially large number of processes and circumstances. Counts of pebble lithology and roundness together offer more power than lithology alone for the interpretation of provenance. In this study we analyze pebble counts of lithology and roundness in two contrasting fluvial systems of Pleistocene age to see how provenance varies with drainage size. The two systems are 1) a group of small high-gradient incised streams that formed alluvial fans and terraces and 2) a piedmont river that formed terraces in response to climate-driven cycles of aggradation and incision. We first analyze the data from these systems within their geographic and geologic context. After this is done, we employ contingency table analysis to complete the interpretation of pebble provenance. Small tributary streams that drain rugged mountains on both sides of the Santa Cruz River, southeast Arizona, deposited gravel in fan and terrace deposits of Pleistocene age. Volcanic, plutonic and, to a lesser extent, sedimentary rocks are the predominant pebble lithologies. Large contrasts in gravel lithology are evident among adjacent fans. Subangular to subrounded pebbles predominate. Contingency table analysis shows that hard volcanic rocks tend to remain angular and, even though transport distances have been short, soft tuff and sedimentary rocks tend to become rounded. The Wind River, a major piedmont stream in Wyoming, drains rugged mountains surrounding the northwest part of the Wind River basin. Under the influence of climate change and glaciation during the Pleistocene, the river deposited an extensive series of terrace gravels. In contrast to Santa Cruz tributary gravel, most of the Wind River gravel is relatively homogenous in lithology and is rounded to well-rounded. Detailed analysis reveals a multitude of sources in the headwaters and the basin itself, but lithologies from these sources are combined downstream. Well-rounded volcanic and recycled quartzite clasts were derived from the headwaters. Precambrian igneous and metamorphic clasts were brought down tributary valleys to the Wind River by glaciers, and sandstone was added where the river enters the Wind River structural basin.

Effects of sediment pulses on channel morphology in a gravel-bed river

Research Article| January 01, 2007 Effects of sediment pulses on channel morphology in a gravel-bed river Daniel F. Hoffman; Daniel F. Hoffman 1Department of Geosciences, University of Montana, Missoula, Montana 59802, USA Search for other works by this author on: GSW Google Scholar Emmanuel J. Gabet Emmanuel J. Gabet 1Department of Geosciences, University of Montana, Missoula, Montana 59802, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (1-2): 116–125. https://doi.org/10.1130/B25982.1 Article history received: 06 Feb 2006 rev-recd: 06 Jun 2006 accepted: 03 Aug 2006 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Daniel F. Hoffman, Emmanuel J. Gabet; Effects of sediment pulses on channel morphology in a gravel-bed river. GSA Bulletin 2007;; 119 (1-2): 116–125. doi: https://doi.org/10.1130/B25982.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Sediment delivery to stream channels in mountainous basins is strongly episodic, with large pulses of sediment typically delivered by infrequent landslides and debris flows. Identifying the role of large but rare sediment delivery events in the evolution of channel morphology and fluvial sediment transport is crucial to an understanding of the development of mountain basins. In July 2001, intense rainfall triggered numerous debris flows in a severely burnt watershed in the Sapphire Mountains of Montana. Ten large debris flow fans were deposited on the valley floor, and investigations focused on the channel response to these sediment pulses. The channel has aggraded immediately upstream of each fan, and braided in reaches immediately downstream. Channel incision through the fans has created sets of coarse-grained terraces. The deposition upstream of the pulses consists almost exclusively of fine material, resulting in a median bed material size (D50) 1–2 orders of magnitude lower than the ambient channel material. The volume of sand being transported is so great that these aggrading reaches can extend hundreds of meters upstream of the fans, with 1–2 m of sand deposited across the entire valley floor. Along a 10 km study reach, cross section surveys, longitudinal profiles, and pebble counts chronicle channel response to a punctuated increase in sediment supply and provide insight on the processes of sediment wave dispersal. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Trends of grain sizes on gravel bars in the Rio Chagres, Panama

We examined the trends of grain sizes along the upper 414 km 2 of the mountainous Rio Chagres drainage basin in Panama. Gravel bars were sampled along 40 km of the Rio Chagres and five major tributary streams using a transect pebble count of median diameter, lithology, and clast rounding. Although previous investigators have found that downstream fining can be obscured by inputs of colluvial sediment and other local controls in mountain drainages, we decided to examine the trends of grain sizes along a tropical mountain river where rapid weathering and high capability of transport might be capable of overriding the input effects of colluvium. Specifically, we tested the hypotheses that downstream fining would be present as a result of selective sorting, and that weak felsic particles would decrease in size preferentially to strong mafic particles because of abrasion. Statistical analyses reveal a weak downstream decrease of sediment size on gravel bars along the study reach of the Rio Chagres, with a Sternberg diminution coefficient ( α) for felsic and mafic grains of − 0.013 and − 0.017, respectively. Felsic clasts have thicker weathering rinds and become rounded downstream faster than mafic particles, but tumbling-mill tests of abrasion show no significant differences in rate of mass loss in relation to lithology, and downstream decreases in grain size are similar between lithologies. Dividing the study reach into six sub-reaches bounded by major tributary junctions, we further tested the hypothesis that downstream trends in fining might be obscured at the basin scale by sediment input from tributaries, but that trends in grain sizes might be more visible at the reach scale between tributaries. We did not find any consistent trends in grain size between tributaries. Stream width appears to assert a local control on grain size; coarse particles are associated with narrow channel reaches, whereas smaller particles are associated with wide channel reaches.

Debris dams and the relief of headwater streams

In forested, mountain landscapes where debris flows are common, their deposition commonly forms valley-spanning dams of wood, boulders, or complex mixtures of both in headwater valleys. Sediment impoundment behind these dams causes alluviation in what would otherwise be bedrock channels. In this paper, the effects of debris dams on the evolution of headwater valley profiles over geologic time are examined. In the Oregon Coast Range, USA, longitudinal profiles of three headwater channels (approximately 2 km 2 maximum contributing area), two of which are nominally bedrock, and all debris dams were surveyed. Channel and valley widths were measured, and surface bed material measurements (pebble counts) were taken at several locations along one stream. Cumulative relief in debris dams is highly variable within and among streams, reaching a maximum of 58% of profile relief at one location. The surveyed dams comprise 9.8%, 19%, and 6.4% of total basin relief in the three corresponding drainage basins, respectively. A model of bedrock erosion and channel profile evolution is derived that accounts for (a) fractional coverage by the channel of total valley width at any given time and (b) shielding by sediment impounded behind debris dams. The equation for incision rate, assumed constant, is solved for valley gradient as a function of contributing area, and parameter values are estimated from or supplied by field data. Results are compared to stream-gradient–contributing-area relationships obtained for the field sites. These comparisons suggest a strong effect of network structure, which varies significantly among the sites, on profile shape and relief because of the different susceptibilities to debris-flow deposition and, therefore, debris-dam formation. Longitudinal profiles with and without the effects of debris dams are modeled for one forced-alluvial, i.e., nominally bedrock, channel. For these model profiles, which extend only as far as the surveyed profile at this site, 55% of the profile relief is due to debris dams and the concomitant shielding of the bed by impounded sediment. The results suggest that a significant fraction of the relief of such forested, mountain landscapes is due to the effects of relatively immobile wood and boulders deposited in the valleys by debris flows.

Relative Ages of Pleistocene Moraines Discerned from Pebble Counts: Eastern Sierra Nevada, California

A modified Wolman pebble count, traditionally used in fluvial science, is used to assess relative ages for glacial moraines at Bloody and Sawmill canyons, eastern California. Results show statistically separable size distributions among surface deposits of five known moraine groups, with finer distributions associated with older deposits. Large variance in clast size prevents robust distinction of younger moraines, but not older moraines that are most difficult to date. Results contribute to ongoing debate about the locale's oldest age groups, with Mono Basin predating Tahoe 1 moraines.

SOURCES OF VARIABILITY IN CONDUCTING PEBBLE COUNTS: THEIR POTENTIAL INFLUENCE ON THE RESULTS OF STREAM MONITORING PROGRAMS

Pebble counts have been used for a variety of monitoring projects and are an important component of stream evaluation efforts throughout the United States. The utility of pebble counts as a monitoring tool is, however, based on the monitoring objectives and the assumption that data are collected with sufficient precision to meet those objectives. Depending upon the objective, sources of variability that can limit the precision of pebble count data include substrate heterogeneity at a site, differences in substrate among sample locations within a stream reach, substrate variability among streams, differences in when the substrate sample is collected, differences in how and where technicians pick up substrate particles, and how consistently technicians measure the intermediate axis of a selected particle. This study found that each of these sources of variability is of sufficient magnitude to affect results of monitoring projects. Therefore, actions such as observer training, increasing the number of pebbles measured, evaluating several riffles within a reach, evaluating permanent sites, and narrowing the time window during which pebble counts are conducted should be considered in order to minimize variability. The failure to account for sources of variability associated with pebble counts within the study design may result in failing to meet monitoring objectives.

Drainage, sediment transport, and denudation rates on the Nanga Parbat Himalaya, Pakistan

The Nanga Parbat Himalaya presents some of the greatest relief on Earth, yet sediment production and denudation rates have only been sporadically addressed. We utilized field measurements and computer models to estimate bank full discharge, sediment transport, and denudation rates for the Raikot and Buldar drainage basins (north slope of Nanga Parbat) and the upper reach of the Rupal drainage basin (south slope). The overall tasks of determining stream flow conditions in such a dynamic geomorphic setting is challenging. No gage data exist for these drainage basins, and the overall character of the drainage basins (high relief, steep flow gradients, and turbulent flow conditions) does not lend itself to either ready access or complete profiling. Cross-sectional profiles were surveyed through selected reaches of these drainage basins. These data were then incorporated into software (WinXSPRO) that aids in the characterization (stage, discharge, velocity, and shear stress) of high altitude, steep mountain stream conditions. Complete field measurements of channel depths were rarely possible (except at several bridges where the middle of the channel could actually be straddled and probed) and, when coupled with velocity measurements, provided discrete points of field-measured discharge calculations. These points were then used to calibrate WinXSPRO results for the same reach and provided a confidence level for computer-generated results. Flow calculations suggest that under near bank full conditions, the upper Raikot drainage basin produces discharges of ∼61 cm and moves about 11,000 tons day −1 (9980 tons day −1) of sediment through its channel. Bank full conditions on the upper portion of the Rupal drainage basin generate discharges of ∼84 cm and moves only about 3800 tons day −1 (3450 tons day −1) of sediment. Although the upper Rupal drainage basin moves more water, the lower slope of the drainage basin (0.03) generates a much smaller shear stress (461 Pa) than does the higher slope (0.12) of the upper Raikot drainage basin (1925 Pa). Dissolved and suspended sediment loads were measured from water/sediment samples collected throughout the day and night over a period of 10 days at the height of the summer melt season but proved to be a minor variable in transport flux. Channel bed loads were measured using a pebble count method of bank material and then used to generate ratings curves of bed loads relative to discharge volumes. When coupled with discharge data and basin area, mean annual sediment yield and denudation rates for Nanga Parbat are produced. Denudation rates calculated in this fashion range from 0.2 mm year −1 in the slower, more sluggish Rupal drainage basin to almost 6 mm year −1 in the steeper, faster flowing Raikot and Buldar drainage basins.

Photographic Techniques for Characterizing Streambed Particle Sizes

We developed photographic techniques to characterize coarse (>2-mm) and fine (≤2-mm) streambed particle sizes in 12 streams in Anchorage, Alaska. Results were compared with current sampling techniques to assess which provided greater sampling efficiency and accuracy. The streams sampled were wadeable and contained gravel—cobble streambeds. Gradients ranged from about 5% at the upstream sites to about 0.25% at the downstream sites. Mean particle sizes and size-frequency distributions resulting from digitized photographs differed significantly from those resulting from Wolman pebble counts for five sites in the analysis. Wolman counts were biased toward selecting larger particles. Photographic analysis also yielded a greater number of measured particles (mean = 989) than did the Wolman counts (mean = 328). Stream embeddedness ratings assigned from field and photographic observations were significantly different at 5 of the 12 sites, although both types of ratings showed a positive relationship with digitized surface fines. Visual estimates of embeddedness and digitized surface fines may both be useful indicators of benthic conditions, but digitizing surface fines produces quantitative rather than qualitative data. Benefits of the photographic techniques include reduced field time, minimal streambed disturbance, convenience of postfield processing, easy sample archiving, and improved accuracy and replication potential.

SAMPLING FRAME FOR IMPROVING PEBBLE COUNT ACCURACY IN COARSE GRAVEL‐BED STREAMS1

ABSTRACT: Improved sampling techniques are needed to increase the accuracy of pebble‐count particle‐size distributions used for stream studies in gravel‐bed streams. However, pebble counts are prone to operator errors introduced through subjective particle selection, serial correlation, and inaccurate particle‐size measurements. Errors in particle‐size measurements can be minimized by using a gravel template. Operator influence on particle selection can be minimized by using a sampling frame, 60 by 60 cm, in which sampling points are identified by the cross points of thin elastic bands. Serial correlation can be minimized by adjusting the spacing between the cross points and setting it equal to the dominant large particle size (=D95). In a field test in a cobble‐bed stream, the sampling frame developed in this study produced slightly coarser size distributions, particularly in the cobble range, than the traditional heel‐to‐toe walk that selects particles with a blind touch at the tip of the boot. The sampling frame produced more similar sampling results between two operators than heel‐to‐toe walks. The difference between the two sampling methods is attributed to an unbiased selection of fine and coarse particles when using the sampling frame.

Testing bioassessment metrics: macroinvertebrate, sculpin, and salmonid responses to stream habitat, sediment, and metals.

The purpose of this article is to report on the testing of responses of multimetric macroinvertebrate and habitat indices to common disturbances to streams: stream habitat alteration, excessive sediment, and elevated metals concentrations. Seven macroinvertebrate community metrics were combined into a macroinvertebrate biotic index (MBI), and 11 channel morphology, riparian, and substrate features were combined into a habitat index. Indices were evaluated by comparing the habitat results to fish population surveys and comparing the macroinvertebrate results to habitat ratings, percent fine sediments measured by Wolman pebble counts, and copper concentrations. Macroinvertebrate scores decreased with increasing percentages of fine sediments measured either across the bankfull or instream channel widths. Macroinvertebrate scores decreased with increasing copper. One metric, richness of Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa, was more responsive to both copper and sediment than was the multimetric MBI. Habitat scores corresponded well with the age class structure of salmonids, but not with that of benthic sculpins. Both salmonid and sculpin age classes declined with increasing percentages of fine sediments. The decline was graded with the sculpin age classes, whether fine sediments were measured across the instream or bankfull channel, whereas salmonids consistently responded only to the instream fine sediments.

DETECTING CUMULATIVE EFFECTS ON HEADWATER STREAMS IN THE ROUTT NATIONAL FOREST, COLORADO1

ABSTRACT: This study evaluated the use of a modified pebble count procedure (Bevenger and King, 1995) to detect cumulative watershed effects on headwater streams in the Routt National Forest in northcentral Colorado. The 42 sample reaches were stratified by disturbance (reference or disturbed) and geologic terrene (granitic or mixed sedimentary‐volcanic). Water surface slope was a significant control on the number of fine particles in the reference reaches in both terranes, and the data from the disturbed reaches were adjusted accordingly. The disturbed reaches in the granitic terrene generally had a higher percentage of fine particles, and the adjusted number of fine particles was significantly correlated with the number of road crossings. Disturbed reaches in the sedimentary‐volcanic (s‐v) terrane generally did not have significantly more fine particles, nor were the adjusted numbers of fine particles significantly correlated with any management index. The lack of significant trends in the s‐v streams is probably due to differences in weathering between the two rock types, and the location of the sample reach relative to sedimentary outcrops. Two other procedures were also used to assess cumulative watershed effects, with the Pfankuch channel stability rating yielding stronger and more consistent differences between the reference and the disturbed streams than the Tarzwell substrate ratio. We conclude that it may be difficult to define a standard reference condition, and that the number of road crossings is more strongly correlated with the number of fine particles than equivalent clearcut area.

DISCUSSION1

JAWRA Journal of the American Water Resources AssociationVolume 33, Issue 6 p. 1397-1398 DISCUSSION1 “Application of the Pebble Count: Notes on Purpose, Method, and Variants,” by G. Mathias Kondolf2 Panayiotis Diplas, Panayiotis Diplas Respectively, Associate Professor, Department of Civil Engineering, and Assistant Professor, Engineering Fundamentals Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061.Search for more papers by this authorVinod K Lohani, Vinod K Lohani Respectively, Associate Professor, Department of Civil Engineering, and Assistant Professor, Engineering Fundamentals Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061.Search for more papers by this author Panayiotis Diplas, Panayiotis Diplas Respectively, Associate Professor, Department of Civil Engineering, and Assistant Professor, Engineering Fundamentals Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061.Search for more papers by this authorVinod K Lohani, Vinod K Lohani Respectively, Associate Professor, Department of Civil Engineering, and Assistant Professor, Engineering Fundamentals Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061.Search for more papers by this author First published: 08 June 2007 https://doi.org/10.1111/j.1752-1688.1997.tb03564.xCitations: 8 1 Discussion No. 96057D–2 of the Journal of the American Water Resources Association (formerly Water Resources Bulletin). 2 Paper No. 96057 of the Journal of the American Water Resources Association 33(1):79–87. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume33, Issue6December 1997Pages 1397-1398 RelatedInformation

APPLICATION OF THE PEBBLE COUNT NOTES ON PURPOSE, METHOD, AND VARIANTS1

ABSTRACT: The pebble count procedure (Wolman, 1954) is the measurement of 100 randomly selected stones from a homogeneous population on a river bed or bar, which yields reproducible size distribution curves for surficial deposits of gravel and cobbles. The pebble count is widely used in geomorphologr (and increasingly in river engineering) to characterize surficial grain size distributions in lieu of bulk samples, for which adequate sample sizes become enormous for gravels. Variants on the original method have been proposed, one of which, the so‐called ‘zig‐zag’ method (Bevenger and King, 1995), involves sampling along a diagonal line and drawing data points from many different geomorphic units. The method is not reproducible, probably because it incorporates stones from many different populations, and because an inadequate number of grains is sampled from any given population. Sampling of coarse bed material should be geomorphically stratified based on the natural sorting of grain sizes into distinct channel features. If a composite grain size is desired, the areas of the bed occupied by different populations can be mapped, pebble counts conducted on each, and a weighted average distribution computed.

A shear‐diffusion model of till genesis based on the dispersal pattern of indicator rocks in the Grand‐Volume Till of central Gaspésie, Québec, Canada

Dispersal patterns of indicator rocks in central Gaspésie reveal that glacial debris is entrained in a basal debris‐rich zone of shearing where clast diffusion takes place. The Grand‐Volume Till forms a thin till sheet over the high plateaus of Gaspésie Peninsula and resulted from a succession of two Wisconsinan ice flows of distinct orientations (SSE and NE). The lithological composition of this till determined by pebble counts and the three‐dimensional dispersal patterns of indicator rocks in it suggest that debris transport occurred principally by simple shear deformation of glacial debris. In addition, the intermixing of clasts at the intersection of two lithologically distinct dispersal trains of SSE and NE orientations, respectively, suggests that extensive mixing takes place during shearing. Physical interactions among the clasts lead to both upward and downward movements which cause the clasts to diffuse across the zone of shearing. This process of shear‐diffusion results in continuous incorporation and mixing of the newly encountered rock types during glacial transport.

Pebble Counts and The Role of User‐Dependent Bias in Documenting Sediment Size Distributions

Replicate subaqueous pebble counts show that measurement and selection bias can significantly affect sediment size distribution data. The standard deviation about replicate means increases linearly with sediment size and is independent of the sediment percentile (e.g., d50, d84, etc.). For samples collected by one person, sampling error varies from 2 ± 2.6 mm to 300 ± 35.4 mm within one standard deviation. For samples collected by different individuals, sampling bias increases the error from 2 ± 4.2 mm to 300 ± 63.8 mm. Whenever possible, only one individual should select and measure pebbles, which improves the ability of the pebble count to delineate trends between sites or over time (although the absolute size values may be in error due to bias). Bias can be reduced by rigorous training of individuals, by using templates to measure sediment size, and by using grids on the stream bottom to select particles. If two or more individuals collect samples, they should conduct replicate samples to provide a basis for later calibration of results.

USE OF PEBBLE COUNTS TO EVALUATE FINE SEDIMENT INCREASE IN STREAM CHANNELS1

ABSTRACT: The pebble count, a quick and simple technique for characterizing streambed materials, has long been used by geomorphologists, hydrologists, and river engineers. This paper describes how pebble counts have been used to monitor fine sediment (particles less then 6 mm in size) on the Boise National Forest. Data from two watersheds subjected to major wildfires and the failure of a dam are discussed. Following wildfires, pebble count data showed increases in streambed fines followed by improvement of the stream substrate with time as the watersheds recovered. For the dam failure, pebble count data showed an increase in fines in the stream below the failure and were used to track the distance of sediment movement downstream. Pebble counts may be best used where fine sediment on channel substrates are a concern, such as in granitic watersheds where coarse sands are a large component of bedload and land‐disturbing activities introduce fine sediment into streams. Pebble counts are found to be a simple and rapid monitoring method that can be used to help determine whether or not land management activities or land disturbances are introducing fine sediment into streams.

Pleistocene sediment sources, debris transport mechanisms, and depositional environments: a Bering Glacier model applied to northeastern Appalachian Plateau deglaciation, central New York

A modified ice-tongue model suggests that subglacial, saturated, fine sediment derived from local bedrock sources reduced basal shear strength and lowered the ice surface gradient sufficiently to produce ice tongues 20 km long in all major north-south oriented valleys on the northeastern Appalachian Plateau, while adjacent uplands were virtually ice-free. Associated environments of deposition produced two different landform assemblages, one representative of active ice retreat in through valleys and another that depicts widespread stagnation in non-through valleys. Pebble count data indicate that sediment transport by glacial flow was important to the moraine-building process, but the occurrence of isolated kame fields suggests an origin linked to inwash from major upland tributaries. All coarse valley fill (sand and gravel) is derived from two basic sources: (1) re-worked upland drift, and (2) resedimented debris from upvalley sources, including the glacier. Processes common to through valleys favor upvalley sources and active ice landforms, whereas inwash and stagnant ice sedimentation are typical of non-through valleys. Although extensive ice-free uplands served as a source of some fine sediment, a comparison of sediment volume to upland area indicates that inwash processes could not have yielded sufficient fines to account for the volume of fine sand and silt found within the valley fill. Meltwater flow via subglacial tunnels discharged saturated, fine sediment directly into proglacial lakes and served as the major source and transport mechanism for most sand and silt. The Laurentide deglacial environment throughout the upper Susquehanna region was characterized by proglacial lakes, detached remnant ice masses, dead-ice sedimentation and collapsed ice tongues. Stagnation and downwasting in ice-contact lakes peripheral to the eastern Bering Piedmont Glacier, Alaska, serve to depict analog conditions for retreat in central New York.

Pebble Count (Modal) Analysis of Four Outcrops of the Upper Siwalik Conglomerates of the Jammu Region, Northwest India and their Textural Implications

Abstract The pebble count data from the four outcrops of the Upper Siwalik Conglomerates of the Jammu region, northwest India are compared with the results expected from appropriate theoretical models of constant probability of occurrence from point to point. The resulting degree of fit is applied to characterise the textural arrangement of each constituent. The experiment shows that within an outcrop for Jarger pebble counts the Positive Binomial and for lower counts Poisson are the appropriate theoretical models. For the combined data over the four outcrops, the proper models are Lexian and Lexian Poisson. This would suggest at anyone outcrop, 5, 10, or 100 pebble counts obtained from anywhere in the outcrop would yield unbiased sample estimate of the outcrop population parameter. Over many outcrops, however, the results are inconsistent. Combined distributions for the four outcrops indicate heterogeneity in the distribution of the variates. Therefore, it is suggested that no one outcrop or one sample from an outcrop represents the entire target population (Upper Siwalik Conglomerates of the Jammu region). The population has to be sampled regionally over several outcrops preferably over an orthogonal grid spread over the target population. The results also suggest that at anyone outcrop the process responsible for the deposition of the conglomerates is uniform but has varied from place to place thus suggesting more than one episode in the deposition of the conglomerates. One has to explain this multi-episodic deposition by a sedimentation model that would explain adequately the quantitative variability present in the data.