Pebble Count Methods - California

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Pebble Count MethodsThe composition of the streambed and banks isan important facet of stream character,influencing channel form and hydraulics,erosion rates, sediment supply, and otherparameters. Each permanent reference siteincludes a basic characterization of bed andbank material. For studies of fish habitat,riparian ecosystems or stream hydraulics, thecharacterization of substrates and bankmaterials may require greater detail than can becovered here.Observations tell us that steep mountainstreams with beds of boulders and cobbles actdifferently from low-gradient streams with bedsof sand or silt. You can document thisdifference by collecting representative samplesof the bed materials using a procedure called apebble count.The most efficient basic technique is theWolman Pebble Count. This requires anobserver with a metric ruler who wades thestream and a note taker who wades or remainson the bank with the field book. Particles aretallied by using size classes or categoriessimilar to the ones shown in table 1.Pebble counts can be made using grids,transects, or a random step-toe procedure. Astep-toe procedure is described here and azigzag pattern is shown in the illustration.Collection ProcedureSelect a reach on or near the cross-section andindicate it on your site map. For streamcharacterization, sample pools, runs and rifflesin the same proportions as they occur in thestudy reach. For other purposes, it may beappropriate to sample these separately.Measure a minimum of 100 particles to obtaina valid count. Use a data sheet to record thecount.Table 1. Pebble count size classesSize classSandVery fine gravelFine gravelMedium gravelCoarse gravelVery coarse gravelSmall cobbleMedium cobbleLarge cobbleSmall boulderMedium boulderLarge boulderSize range (mm) 22-45-89 - 1617 - 3233 - 6465 - 9091 - 128129 - 256257 - 512513 - 1024 1025The above scale has been modified slightlyStart the transect at a randomly selected pointat one of the bankfull elevations (notnecessarily the present water level). Avertingyour gaze, pick up the first particle touched bythe tip of your index finger at the toe of yourwader.Measure the intermediate axis (neither thelongest nor shortest of the three mutuallyperpendicular sides of each particle picked up)(Figure 1). Measure embedded particles orthose too large to be moved in place. For these,measure the smaller of the two exposed axes.Call out the measurement. The note taker talliesit by size class and repeats it back forconfirmation.Take one step across the channel in thedirection of the opposite bank and repeat theprocess, continuing to pick up particles untilyou have the requisite number (100 or more) ofmeasurements. The note taker keeps count.Traverse across the stream perpendicular to theflow or in a zigzag pattern (Figure 2).Examples of data sheets are provided on pagessix and seven.Information provided here was taken from Section 11 of the manual Stream Channel Reference Sites: An IllustrateGuide to Field Techniques.1

Pebble Count MethodsFigure 1. Axes of a pebbleReferencesHarrelson, Cheryl C; Rawlins, C. L.; Potyondy,John P. 1994. Stream Channel Reference Sites:An Illustrated Guide to Field Technique. Gen.Tech. Rep. RM-245. Fort Collins, CO: U.S.Department of Agriculture, Forest Service,Rocky Mountain Forest and Range ExperimentStation. 61 p.A. Long axisB. Intermediate axisC. Short axisContinue your traverse of the cross-sectionuntil you reach an indicator of bankfull stageon the opposite bank so that all areas betweenthe bankfull elevations are representativelysampled. You may have to duck under banktop vegetation or reach down through brush toget an accurate count. Move upstream ordownstream randomly or at a predetermineddistance and make additional transects to samplea total of at least 100 particles.Leopold, L. B., M. Wolman, and J. Miller,1964. Fluvial Processes in Geomorphology.W. H. Freeman, San Francisco, CA, 522 pp.G.S. Bevenger and R.M. King. 1995. A PebbleCount Procedure for Assessing WatershedCumulative Effects. Res. Pap. RM-RP-319.Fort Collins, CO: U.S. Department ofAgriculture, Forest Service, Rocky MountainForest and Range Experiment Station. 17 p.Bankfull physical features include the top (levelsurface) of adjacent point bars, change in slope,change in bank composition, limit of woodyvegetation and in some cases debris and scourlines. A minimum of 10% of your pebble countshould be collected from bankfull features.The red line drawn onthis image indicates theapproximate path thestudents chose whileconducting their pebblecount within a 100-meterreach of Skaggs Run.ResultsSand (1); Fine gravel (20);Coarse gravel (27); Cobble(20); Boulder (8)Index 3.38D50 23Information provided here was taken from Section 11 of the manual Stream Channel Reference Sites: An IllustrateGuide to Field Techniques.2

Pebble Count MethodsFigure 2. Pebble count zigzag patternThe illustration above is from A Pebble Count Procedure for Assessing Watershed Cumulative Impacts.3

Pebble Count ling addressPhone/e-mailDirections to siteDischargeDateWatershedLongitudeOr estimateRiver reachHighNormalLowNoneReach Description and SketchUse the space below to briefly describe the conditions of your reach and provide a bird’s eye viewsketch. Be sure to indicate flow direction and the location of your pebble count stations, bankpins, cross sections, stream structure and other important features.

Pebble Count SurveyLand Uses in the Watershed: Record all known land uses upstream and surrounding yourmonitoring site. Indicate whether they have a High (3), Moderate (2), Slight (1) potential toimpact (I) the quality of the stream. Also, indicate the approximate location (L) of the land useDoes it occurs beside the stream site (S), within ¼ mile of the stream site (M), or within thestream’s watershed (W).Land UsesSingle family homesSuburbanUrbanActive constructionPaved roadsUnpaved roadsBridgesOil and Gas wellsLoggingParks, trails etc.Other recreationImpactLocationLand UsesLandfillTrash dumpAbandoned miningActive miningPasturelandCroplandAnimal FeedlotsOther (describe below)ImpactLocationLand Use CommentsOverall comments – Indicate what you feel are the present and future threats to your stream ormake any additional comments. Feel free to attach any additional information such as topographicmaps, photographs or any other information that you feel is important.Submit the survey to the address below:Citizens Monitoring CoordinatorDivision of Water and Waste Management601 57th StreetCharleston, WV 25304Questions? Send e-mail to tcraddock@wvdep.org or call (304) 926-0499

Pebble Count Data SheetMaterialsSilt/claySize ranges (mm)Fine sand0.126 – 0.25Medium sand0.26 – 0.5Coarse sand0.5 – 1Very coarse sand1-2Very fine gravel2-4Fine gravel5-8Medium gravel9 - 16Coarse gravel17 - 32Very coarse gravel33 - 64Small cobble65 - 90129 - 180Very large cobble181 - 255Small boulder256 - 512Very large boulderStations2345691 - 128Large cobbleLarge boulderPool10.06 – 0.125Medium boulderRun 0.06Very fine sandMedium cobbleCountRiffle78513 - 10241025 – 20489 2048Bedrock10Woody debrisTotalsHabitat Percentages:RifflesRunsIndicate the location of your transects (stations) along your tape measure.Pools

Pebble Count: Collect a minimum of 100-particles from your reach using a zigzag method, percent habitat method or specifictransects throughout the reach (e.g. every 10-metes).Indicate yoursampling methodfrom the choicesbelow.Size Classes (mm)Silt/claySandFine gravelCoarse gravelCobbleBoulderBedrock 0.060.06 – 22 – 2425 – 6465 – 255256 – 1096 1096Zig-Zag%Habitat10-mTransectsTotalsIf a pebble count is not collected, estimate the composition of a representative riffle.Estimate the water levelLowNormalHighPhoto’s: Number and describe the photo’s taken at your stationNote: This data sheet is only designed for broad category pebble counts.SiltSandGravelCobbleBoulderBedrockDryWV Department of Environmental ProtectionWV Save Our Streams Program601 57th Street, S.E.Charleston, WV 25304

of sand or silt. You can document this difference by collecting representative samples of the bed materials using a procedure called a pebble count. The most efficient basic technique is the Wolman Pebble Count. This requires an observer with a metric ruler who wades the stream and a note ta

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