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Analyzing Pebble Count Data Collected By Size Classes

Prepared by John Potyondy and Kristin Bunte

Version 1.0a February 6, 2002

(Based upon a similar spreadsheet developed by Greg Bevenger and Rudy King)

Introduction

e ave eveope t s wor oo to ass st you w t t e stat st ca ana ys s o oman pe e count ata ta e

by size class. Sampling should be geomorphically stratified based on the natural sorting of grain sizes into

distinct channel features to sample homogeneous populations from the river bed. The intermediate axis of

individual particles may be measured with a ruler or by using a gravel template (gravel-o-meter). We strongly

recommend the use of templates because they avoid incorrectly identifying the intermediate axis and have been

shown to reduce measurement errors among observers. Classifying particle sizes on the basis of square

o enin s in the tem late has the further advanta e of rovidin a measure of size that is com atible witho scourage ser a corre at on n t e samp e, t e stance etween success ve y samp e part c es s ou e

chosen so that successive particles are at least several grain diameters apart. This can typically be achieved by

setting the spacing between sampled particles to a distance larger than the intermediate axis of the largest

particles in the reach or the population being sampled.

When conducting a pebble count, we recommend sampling geomorphic features or habitat units (riffles, pools, or

combinations of pools and riffles) depending on study objectives. For example, pebble counting done to monitor

changes in the particle-size distribution of a habitat feature of biological significance usually collect particles from

riffles alone. This approach represents a spatially segregated, stratified sample that provides a size distribution

for a specific geomorphic feature or habitat unit.

Pebble counts done for general stream classification usually collect particles from an entire stream reach.

Rosgen suggests sampling a reach two meander wavelengths long (four riffle-pool sequences). Transects are

placed such that the percent stream length occupied by riffles (incl. rapids and runs) and by pools matches the

percent of the transect located in riffles and pools (e.g., if riffles occupy 70% of the stream length, 70 % of all

pebbles are collected on riffles). Since particles were collected on a spatially proportional basis, all data may be

combined to obtain a reach-averaged particle-size distribution.

, ,analyses and produce graphs, and 4) take notes. Also provided are case studies that may assist you in

developing your study plan and interpreting your analysis.

Sample-size estimation and statistical analysis are explained in greater detail and were derived from: Bevenger,

Gregory S. and Rudy M. King. 1995. A Pebble Count Procedure for Assessing Watershed Cumulative Effects.

Rocky Mountain Forest and Range Experiment Station Research Paper RM-RP-319, 17 pages. We encourage

you to become thoroughly familiar with the statistical discussions in RM-RP-319 before proceeding.

Additional information about pebble counts is also available in Bunte, Kristin and Steven R. Abt. 2001. Sampling

Surface and Subsurface Particle-Size Distributions in Wadeable Gravel- and Cobble-Bed Streams for Analyses in

Sediment Transport, Hydraulics, and Streambed Monitoring. Rocky Mountain Research Station RMRS-GTR-74,

428 pages.You may order these documents from the Rocky Mountain Research Station, 240 West Prospect Road, Fort

Collins, Colorado 80526.

How to Use this Workbook

Step 1.

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Before initiating field work, you should ask "How many pebbles need to be counted to detect a real difference

between reference and study conditions?" To answer this question, the nature of the reference condition, the size

of change that might occur from a management action, the biological or physical significance (if any) of the

induced change, and acceptable levels of risk must be specified. The necessary information may be obtained

during a reconnaissance field trip and/or from information available in the office. The Sample-Size Worksheet is

en n e e , e ques on o as s, ere exac y s ou e pe e coun s e one ssume, or

example, the geomorphic feature or habitat units to be sampled are riffles. Depending on the homogeneity of

(A) Find that all riffles in the reach look pretty much the same. In this case, select one riffle that is most

representative and do a pebble count of the specified sample size;

one r e s oo sma or co ec ng e requ re samp e s ze, or one r e a one oes no cover e s g

variability encountered between the riffles of the reach, select several riffles. They need to be similar enough,

however, to characterize one population. Do one pebble count and collect particles from all riffles, distributing the

(C) If individual riffles depart in their appearance from each other more than you consider appropriate for

members of one population, do an individual pebble count of the specified sample size on each of the selected

riffles. In this complex situation you may need to employ a statistical test to evaluate whether the degree of

statistical dissimilarit amon the riffles is acce table and erha s re-evaluate our entire sam lin strate .

Step 2.

After you have completed your reference and study reach field work enter your pebble count data into the Data

Input Worksheet. Direction on how to enter your data is provided in that worksheet.

Step 3.

Navigate to the Analysis Worksheet to see your output. Guidance on data interpretation is provided in that

worksheet. If necessary, review the case studies for further thoughts on data interpretation. Add notes to the

Step 4.

If you need printed copies of your analysis, highlight the appropriate area, then select File, Print Area, Set Print

Area. Next, select Print Preview and make adjustments accordingly (for example, switch from portrait tolandscape view or add headers and footers). Once you are satisified with the print preview, select Print. Then, if

necessary, save your work for future reference by selecting File, Save As and specifying a new file name.

Spreadsheet Assistance:If you have problems using this spreadsheet contact:

John Potyondy, Rocky Mountain Research Station, Stream Systems Technology Center (970) 295-5986;

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, ,

between percent fines in the reference (control or unimpacted) and the study (impacted) reach?" To answer this

question, the nature of the reference condition, the size of change that might occur from a management action,

The potential risk(s) or cost(s) associated with possible testing errors should be considered. Type I error is the

risk of falsely concluding that there is a difference between reference and study conditoins. Type II error is the

risk of falsely concluding that there is no difference between reference and study conditions. In typical situations,Type II error is allowed to be larger than Type I error, but the reverse might be the case if the situation is highly

sensitive. Typical values are Type I error = 0.05 and Type II error = 0.20.

, .

applications using stratified sampling, e.g., riffles as habitat units. If the reference sample size will be greater

than the study sample size, then the sample size factor should be less than 1. For instance, if double the number

of reference samples will be taken, the sample size factor should be set to 0.5. Conversely, if double the number

.

the difference between reference and study proportions should reflect a "real difference" usually interpreted as

one that has some biological significance. The hypothesis tested in this framework is that the study proportion is

For example, suppose you have data indicating that the proportion of particles

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Class NameParticle Size

Class (mm)

Reference

Total

Study

Total

Reference

Cumulative %

Study

Cumulative %

Sand 4096 0 0 100.0 100.0

Totals 101 95

Data InpuEnter size class pebble count d

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STEPS

1. Clear BOTHthe reference and study reach data columns by pressing Ctrl-b; OR

clear ONLYthe reference reach data column by pressing Ctrl-r; OR

clear ONLYthe study reach data column by pressing Ctrl-s.

2. Enter or paste data in the white columns; output appears in yellow cells on other sheets.

3. Navigate to the Analysis worksheet to view the output.

Note:

The data entry table is set up for 1/2 phi-size intervals.

If you have phi-size interval data, leave the intervening intervals blank.

ta in this worksheet.

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User-selected size criterion:

< > or = Total < > or = Total

Reference 0 101 101 Reference 2 99 101 Reference

Study 0 95 95 Study 10 85 95 Study

Total 0 196 196 Total 12 184 196 Total

Reference < Study < Average < Average >= Reference < Study < Average < Average >= Reference