Guidelines for Using Bedload Traps in 

Coarse-Bedded Mountain Streams:

Construction, Installation, Operation, 

and Sample Processing

Kristin Bunte, Kurt W. Swingle, and Steven R. Abt

United States

Department of

Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-191

May 2007
Bunte, Kristin; Swingle, Kurt W.; Abt, Steven R. 2007. Guidelines for using 

bedload traps in coarse-bedded mountain streams: Construction, 

installation, operation, and sample processing. Gen. Tech. Rep. RMRSGTR-191. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station. 91 p.

Abstract

A bedload trap is a portable sampler designed specifically for collecting gravel 

and cobble bedload (4 to 180 mm in diameter) in wadeable streams. Bedload 

traps consist of an aluminum frame with a 12 by 8 inch (0.3 by 0.2 m) opening 

to which a 3- to 5.5-ft (0.9 to 1.65 m) long trailing net is attached. Bedload 

traps are installed on ground plates that are anchored to the stream bottom 

with metal stakes. Traps do not have to be hand-held while sampling and have 

a large volumetric capacity. This permits collection of bedload over relatively 

long intervals, typically one hour per sample. In this document, we provide 

detailed guidelines for bedload trap construction and operation. We describe 

component parts and offer instructions for making the nets and assembling the 

sampler. Appropriate site selection and preparation are discussed as well as 

bedload trap installation, use, and maintenance. These guidelines also show 

how to process the collected bedload samples in the field and how to perform 

some of the typical calculations used in bedload evaluation.

Key words: Bedload traps, bedload transport, sampling, field work, mountain 

gravel-bed rivers

Authors

Kristin Bunte is a Fluvial Geomorphologist and Research Scientist at the 

Engineering Research Center, Department of Civil Engineering, Colorado State 

University. She received M.S. and Ph.D. degrees in physical geography from 

the Freie Universität Berlin in Germany.

Kurt W. Swingle is an Environmental Scientist from Boulder, Colorado. He 

received an M.S. degree in biology from Montana State University.

Steven R. Abt is a Professor in the College of Engineering at Colorado State 

University, and a Professional Engineer. He received M.S. and Ph.D. degrees 

in civil engineering from Colorado State University.

You may order additional copies of this publication by sending your mailing information in label form through one of the following media. Please 

specify the publication title and series number.

Fort Collins Service Center

Telephone: (970) 498-1392

FAX: (970) 498-1122

E-mail: rschneider@fs.fed.us

Web site: http://www.fs.fed.us/rm/publications

Mailing address: Publications Distribution

Rocky Mountain Research Station

240 West Prospect Road

Fort Collins, CO 80526
cover photos: Top: Six bedload traps and a footbridge 

installed at Hayden Creek at the beginning of the highflow 

season. Bottom left: Two ground plates—one with a 

bedload trap ready for sampling. Bottom center: Operators 

untying the trap net to retrieve the bedload sample. Bottom 

right: Two operators emptying bedload traps at a site 

without a footbridge.

Guidelines for Using Bedload Traps in 

Coarse-Bedded Mountain Streams:

Construction, Installation, Operation, 

and Sample Processing

Kristin Bunte, Kurt W. Swingle, and Steven R. Abt

United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

General Technical Report RMRS-GTR-191
Preface

This document is the product of 7 years of field experience with bedload 

traps during highflow events. Over this period the design of bedload traps 

has evolved, and the techniques of operation and sample processing were 

continuously modified and refined. We recognize that the techniques described 

in these guidelines are adapted to the conditions in Rocky Mountain streams 

during snowmelt highflows. Streams in other climatic and hydrologic regimes 

undoubtedly will pose different problems requiring creative solutions from the 

bedload investigator. Nevertheless, in the hope that others will not have to repeat 

our mistakes, we strongly suggest first trying the procedures as described in 

these guidelines and only then, as necessary, alter them as seems fit.

At times, the guidelines use non-standard units of sample volume such as “a 

cupful of sediment,” a “household pail,” or “5-gal bucket” filled with debris. This 

was done to provide the reader with a quick, visual image of a sample volume, 

not an exact measurement.

If there are further questions regarding the construction, installation, and use 

of bedload traps, please do not hesitate to contact the authors (kbunte@engr.

colostate.edu).

Acknowledgments

The shop of the Engineering Research Center (CSU) built the bedload trap 

frames and had the idea of using shaft collars. The Engineering Research 

Center Hydraulics Lab (CSU) provided field gear, advice, and support (Chris 

Thornton, Chad Lipscomb, Mike Robeson, Andy Darrow).

CSU students (Sean McCoy, Jill Holladay, Dan Armstrong, Rhett Travis, Brian 

Smith, Dan Mastburgen, Simon Stachnik, Brenton Swift) and Janice Barman 

(staff) as well as Forest Service personnel (Paul Bakke, Mike McNamara, Walt 

Lucas, Tim Sullivan, Scott Richards, Missy Shuey, Richard Ford, Terry Smith, 

Wes Smith, Randy Davis, Manuel Martinez, Mark Dixon, Kate Dwire, Megan 

Kabele and her co-workers, Dan Cenderelli, Mark Weinhold and others) helped 

us with field and lab measurements or provided other support.

The Fraser Experimental Forest (CO), Bridger-Teton National Forest (Jackson 

Hole, WY), Winema National Forest (Klamath Falls, OR), and Pike and San 

Isabel National Forest (Leadville, CO) provided general support for our field 

studies. John Potyondy (USDA Forest Service, Stream System Technology 

Center) helped us with guidance, insight, and support all along the way. The 

U.S. Forest Service Stream Systems Technology Center (Stream Team) funded 

the development of the bedload traps and the subsequent field studies. Sue 

Hilton (Forest Service, PSW Research Station), Mark Dixon (Forest Service, 

Rocky Mountain Research Station), and Brod Davies (Federal Interagency 

Sedimentation Program) provided insightful reviews. We are grateful to all 

people who helped with the bedload trap project.

ii
Contents

Preface...................................................................................... ii

Acknowledgments................................................................... ii

Chapter 1. Introduction ........................................................ 1

Properties of Gravel Bedload Transport in Gravel-Bed Streams .... 1

Desirable Sampler Attributes........................................................... 2

Bedload Trap Overview................................................................... 2

Purpose of the Guidelines............................................................... 4

Chapter 2. Construction of Bedload Traps......................... 5

Bedload Traps and Their Parts ....................................................... 5

Net Materials and Fabrication ....................................................... 12

Attaching the Straps...................................................................... 20

Numbering Bedload Traps ............................................................ 21

Chapter 3. Installation of Bedload Traps .......................... 23

Selecting a Sampling Site ............................................................. 23

Positioning Bedload Traps Within the Cross-Section.................... 30

Setting and Anchoring Ground Plates ........................................... 31

Protecting the Streambed ............................................................. 35

Chapter 4. Operation of Bedload Traps ............................ 39

Cleaning and Setting the Traps..................................................... 39

Tying The Nets—Sample Interval Start......................................... 42

Emptying Bedload Traps—Sample Interval End........................... 43

Sampling Time .............................................................................. 48

Ground Plate Readjustment During Sampling and 

Removal After the Highflow Season........................................... 52

Ancillary Measurements................................................................ 54

Sampling Schemes and Required Operator Hours ...................... 58

Operator Safety and Comfort........................................................ 63

Chapter 5. Sample Processing and Computation of 

 Transport Rates.............................................................. 65

Removing Coarse Organic Material.............................................. 65

Drying, Sieving, Weighing, and Removing Mud Flakes ................ 69

Computation of Transport Rates From Bedload Trap Samples .... 72

References ............................................................................. 74

Appendix A: Sampling Intensity of Bedload Traps 

 Compared to Typical Helley-Smith Sampling.............. 78

Appendix B: List of Components for Making 

 Bedload Traps................................................................. 80

iii
Appendix C: Details of Ordering Netting Material.............. 81

Appendix D: Wading Safety, Use of Safety Devices, 

 and Wading Risk ............................................................ 82

Appendix E: Estimated Work Time for One 

 Field Season ................................................................... 85

Appendix F: Equipment and Operator Field Gear When

 Using Bedload Traps in Mountain Gravel/Cobble-Bed 

 Streams ........................................................................... 86

iv
USDA Forest Service RMRS-GTR-191. 2007 1

Chapter 1.

Introduction

Properties of Gravel Bedload Transport in 

Gravel-Bed Streams

Gravel and cobble transport is a stochastic process in which discrete particles ranging in size from 2 to 256 mm hop, roll, slide, and 

bounce over an uneven streambed surface. At any given flow, particles will move fitfully and infrequently, particularly the largest ones. 

Although sampling these particles to determine transport capacity and 

flow competence is often a primary study goal, doing it without bias is 

a challenge.

Due to the irregular particle movement, transport rates fluctuate over 

time. Consecutive short-term (for example, one-minute) samples collected at near constant flow typically comprise transport rates that range 

from almost zero (for example, less than 10 percent of the long-term 

average rate) to four or more times the mean transport rate.

Gravel and cobble transport measured over a highflow event extends 

over a wide range of particle sizes and transport rates. At the very beginning of a highflow event, only a single pea-sized particle might be 

collected over a sampling period lasting several hours. At flows exceeding bankfull, several 10-liter buckets of gravel and cobbles can be 

collected within only a few minutes. Described in terms of mass transport rates, this difference ranges from 0.00001 g/s to 100 g/s and covers 

seven orders of magnitude.

Most samplers are designed to collect only a relatively narrow range 

of particle sizes or transport rates. Accurate measurements of gravel and 

cobble transport rates are difficult to obtain given the wide spectrum of 

particle sizes and transport rates, the infrequent movement of larger particles, and the fluctuating nature of transport. This problem has limited 

our understanding of bedload transport processes in gravel-bed streams. 

In order to address these specific challenges, bedload traps were designed to facilitate sampling irregularly and infrequently moving gravel 

and small cobble particles over a wide range of transport rates in wadeable mountain streams. The development was a joint effort between the 

Colorado State University (CSU) Engineering Research Center, and the 

U.S. Forest Service (FS) Stream Systems Technology Center.
2 USDA Forest Service RMRS-GTR-191. 2007

Desirable Sampler Attributes

Sampler properties desirable for collecting representative samples of 

gravel and cobble bedload in coarse-bedded wadeable streams are:

A large opening that permits large particles to enter the sampler,

A deployment technique that permits long sampling times and thus 

integration over fluctuating transport rates,

A large capacity that permits collection of large volumes of gravel 

and cobble bedload,

Satisfactory hydraulic efficiency (good through-flow rate with little 

retardation or acceleration of flow),

Satisfactory sampler efficiency (neither involuntary particle pick-up 

nor hindrance of particle entry),

Coverage of a large percentage of the stream width,

Portability for use at remote sites,

Relative low cost,

Ease of installation, and

Usable by a two-person team in wadeable flow.

The CSU/FS bedload traps shown in figure 1 were designed to have 

these attributes. The basics of the system are briefly described, with detailed construction and operation guidelines in the following sections.

Bedload Trap Overview

Bedload traps are portable samplers that provide sievable samples 

of gravel and cobble-sized bedload material. They are easy to install in 

the stream, operable at wadeable flow, and not prohibitively expensive. 

Several traps are typically installed across the stream, covering much 

of the stream width. The combination of these properties (esp. the large 

opening, the large sampler capacity, installation on ground plates, and 

long sampling times) are essential for obtaining representative samples 

of gravel and cobble bedload transport. These attributes are more typical of a “trap,” which collects all sediment supplied to it until filled to 

capacity, than a “sampler,” which provides a small sample of what is 

currently being transported. The term “bedload trap” is therefore more 

characteristic of our device in most sampling situations, even though 

bedload traps are not installed below the bed surface.

Bedload traps have an aluminum frame 1ft (0.3 m) wide at the sampler 

opening. This width allows coarse gravel and small cobble particles to 

enter the trap. The frame has a trailing net with a 3.6 mm mesh opening 

that stores the collected gravel bedload. The net, 3 ft (0.9 m) in length 

•

•

•

•

•

•

•

•

•

•
USDA Forest Service RMRS-GTR-191. 2007 3

or longer, can be opened, emptied, and closed from the back without 

removing the frame from the streambed. Bedload traps are placed on 12 

by 16-inch (0.3 by 0.41 m) ground plates that are anchored to the stream 

bottom with metal stakes. Adjustable nylon webbing straps are used to 

fasten the frame to the stakes. Ground plates not only prevent involuntary particle entrainment at the sampler entrance, but their smoothness 

increases the near-bottom flow velocity, which ensures that all particles 

that have moved onto the ground plate will enter the trap without much 

delay. The large-capacity nets and fixed position of the traps on ground 

plates permit a long sampling time, usually an hour. During this time, 

an operator does not need to attend to the sampler. Long sampling times 

allow infrequently moving large particles (in other words, particles of 

the size near the threshold of motion) to be collected, which averages 

Figure 1—A bedload trap installed on a ground plate (right). A ground plate with 

no trap installed (left).
4 USDA Forest Service RMRS-GTR-191. 2007

out short-term temporal variations in transport rates. This reduces variability in sampled transport rates and avoids sampling bias when the 

distribution of transport rates is skewed1. Hour-long sample times also 

allow for sampling a much larger proportion of the total bedload than a 

1 to 2 minute deployment typical of a Helley-Smith type sampler. See 

Appendix A for details.

The guidelines presented here refer to bedload traps that are 1 ft 

(0.3 m) wide, have 3- to 5.5-ft (0.9 to 1.65 m) long nets, and a 3.6 mm 

mesh opening width. Although bedload traps can be constructed with 

different dimensions, these devices should not be radically scaled up or 

down without considering the effects these changes would produce.

Purpose of the Guidelines

These guidelines were assembled in response to the many inquiries 

received about how to make and use bedload traps. Instructions on how 

to construct traps are provided in Chapter 2 of this report, and installation in the stream is described in Chapter 3. Chapter 4 explains how to 

operate bedload traps to obtain accurate samples of gravel and cobble 

bedload, while Chapter 5 shows sample processing and computation 

of transport rates from trap samples. The guidelines are geared toward 

project managers and graduate students, as well as research hydrologists and fluvial geomorphologists who plan to use bedload traps in 

their field studies.

Guidelines for using bedload traps cannot be strictly limited to the 

mechanics of installing and operating them. External factors such as 

stream wadeability, site selection, operator clothing, and sample processing are so closely related to the success of bedload trap operation 

that they need to be discussed as well. The use of trade or firm names in 

this publication is for reader information and does not imply endorsement of any product or service by the U.S. Forest Service or Colorado 

State University.

1

 Transport rates in mountain gravel-bed streams are often not symmetrically distributed 

around a mean rate but skewed toward large rates (see Chapter 4).
USDA Forest Service RMRS-GTR-191. 2007 5

Chapter 2.

Construction of Bedload Traps

Bedload Traps and Their Parts

Each bedload trap consists of:

1 aluminum frame,

1 nylon net,

1 piece of cotton-covered clothesline, 2 to 3 ft long,

1 metal ground plate,

2 cold-rolled steel stakes,

4 nylon straps with metal friction buckles,

4 plastic tension buckles, and

2 shaft collars with thumb screws.

A schematic overview of a bedload trap and its parts is presented in 

figure 2, while components are discussed in more detail in the following 

chapters. A list of components to purchase for making bedload traps is 

provided in Appendix B.

•

•

•

•

•

•

•

•

Figure 2—Schematic diagram of a bedload trap and its parts.
6 USDA Forest Service RMRS-GTR-191. 2007

The Frame

The aluminum frame has the shape of a large shoe box without a 

bottom. It is made from aluminum stock 0.25 inches thick and 4 inches 

wide (6.3 by 101 mm) that is welded together at the corners or along 

the edges (fig. 3). The inside dimensions of the frame are 12 inches 

(0.3 m) wide and 8 inches (0.2 m) high. A slightly different frame size 

should not affect the sampling properties of the bedload traps. The bottom front edge of the bedload trap frame is beveled at an angle of 30°

so particles can easily enter the trap. The frame has four vertical slits 

cut into the sides near the top and the bottom. The slits are 1.25 inches 

(32 mm) long and 0.375 inches (10 mm) wide with rounded tops and 

bottoms. Nylon webbing straps that hold the traps to the stakes and onto 

the ground plates go through these openings. All edges of the frame, 

but particularly the slits, need to be smooth so that they don’t cut the 

Figure 3—Detail of the frame with slits and beveled front edge (all measures in 

inches; oblique view).
USDA Forest Service RMRS-GTR-191. 2007 7

webbing, the nets, or the operator’s hands and gloves. A deburring tool 

works well for smoothing, but sandpaper can be used as well.

The Ground Plate

The bedload trap frame is mounted onto a metal ground plate (fig. 

4) when in the stream. The ground plate extends a few inches upstream 

of the trap and prevents inadvertent particle pick-up at the sampler entrance (Bunte and others 2005). The smoothness of the ground plate 

accelerates the near-bottom flow velocity by 25 to 50 percent, which 

ensures that particles that have moved onto the ground plate will 

Figure 4—Top view of a ground plate and frame outline with dimensions (all 

measurements in inches).
8 USDA Forest Service RMRS-GTR-191. 2007

proceed into the trap. Measurements to determine whether this accelerated flow extends onto the streambed in front of the traps have been 

inconclusive (Bunte and Swingle 2004).

The ground plate is 12 by 16 inches (0.3 by 0.41 m) in size and made 

of 1/8 inch (3.2 mm) aluminum stock. Steel could be used, but is heavy, 

rusts under water, and should be painted (preferably in some bright color). The leading edge of the ground plate is bent downward slightly (by 

about 10°) along a fold line 3 inches (76 mm) from the front of the plate 

(fig. 5). This allows the upstream end of the ground plate to be pushed 

slightly into the streambed sediment. Sharp edges on the ground plates 

should be smoothed.

Stakes, Stake Driver, PVC Pipes, and Capping

Stakes are used to anchor the ground plates on the streambed (the process is described in Chapter 3). Stakes are made of 0.5 inch (12.7 mm) 

diameter cold-rolled steel and are available in hardware stores. Rebar 

is not a suitable alternative because the stakes need to be smooth. The 

bottom end of the stake should be ground to a pointed tip to facilitate penetration through the streambed material. The top end should be 

slightly beveled. Stakes of different lengths, between 2.5 and 4 ft (0.75 

to 1.2 m), are useful to have on hand. Short stakes suffice at locations 

where flow depths and velocities are expected to be low, and stakes need 

only to be driven into the bed about 1 ft deep. Short stakes can also be 

used when the bed is difficult to penetrate. Longer stakes are necessary 

when the bed is loose and deeper anchoring is needed. Stakes should 

not extend above the plate more than about 2 ft (0.6 m) as placing and 

Figure 5—Ground plate with inclined front edge, pieces of PVC 

pipe, and pieces of garden hose.
USDA Forest Service RMRS-GTR-191. 2007 9

removing the traps becomes awkward. Stakes do bend occasionally and 

should be replaced if this happens. Stake tops may need to be re-shaped 

after a few seasons of use, and the tips may need to be re-sharpened.

Stake driver

We recommend using a stake driver to pound stakes into the streambed. A stake driver is a metal pipe approximately 1 ft (0.3 m) long with 

a 1-inch inside diameter onto which a 0.5-ft (0.15 m) long piece of solid 

steel rod, with the same outside diameter as the pipe, is welded as a cap. 

A metal shop can easily make this device. The open pipe end is set over 

the stake and the steel cap is pounded with a hammer. The stake driver 

prevents (much of the) mushrooming of the stake head during repeated 

hammer blows. A mushroomed stake head should be avoided because it 

makes removal of the traps difficult.

PVC pipes

At times when a bedload trap is not placed on the ground plate, 

pieces of PVC pipe (fig. 5) hold the ground plate on the stream bottom. 

These pieces are about 8 inches (0.2 m) long with a 3⁄4-inch (19 mm) 

inside diameter that slides easily over the stakes. The bottom end of the 

pipe rests on the ground plate. A shaft collar is slid over the stake and 

screwed tight to hold the pipe pieces in place.

Stake capping

Pieces of garden hose about a foot long (figs. 5 and 6a) or mushroomshaped plastic rebar caps (fig. 6b) are placed onto the ends of the stakes 

as a safety device to protect the operators in case they fall. Attaching 

garden hose to the stake tops has the added benefit of identifying the trap 

location when traps and stakes are submerged in deep flows (fig. 6c).

Webbing Straps

The frame is attached to the ground plate by four adjustable webbing straps that connect the frame to the stakes (fig. 7). An adjustable 

connection between the traps and the stakes is needed because, in practice, stakes are not exactly parallel to the frame, and each bedload trap 

plate configuration requires individual adjustment. Plastic friction-lock 

buckles (like those found on a backpack) tend to allow the webbing to 

slip and are not adequate to maintain the frame in position. Webbing 

straps with attached metal spring-loaded friction buckles are preferable 

because they resist slippage. Such straps are available in hardware or 

sporting goods stores. Select a sturdy strap, 1 inch (25 mm) wide, about 

2 ft (0.6 m) long with a mid-sized buckle. The buckle should operate 

smoothly as it needs to be tightened under water when one is wearing 

neoprene gloves. Note that each trap requires four straps. See the section “Attaching the straps” in this chapter for details on how to attach 

webbing straps to the frame.
10 USDA Forest Service RMRS-GTR-191. 2007

Figure 6—Stakes capped with pieces of garden hose (a) and rebar caps (b). 

Trap locations are only visible from the hose pieces sticking out of the flow 

(Little Granite Creek, 1999) (c). Note the tongue of fine gravel moving onto 

the ground plates (a).

a b

c
USDA Forest Service RMRS-GTR-191. 2007 11

Shaft Collars to Hold the Frame Down

The force of flow on the frame and the net can make the frame tilt 

backward, which disrupts the contact between the frame and the ground 

plate. If not corrected, this will cause an inaccurate sampling result. 

To prevent the frame from tilting or moving up the stakes, the webbing straps need to be held in place. We accomplished this by sliding 

shaft collars over the stakes and fastening them right above the webbing straps. Shaft collars are thick metal rings with an inside diameter 

slightly larger than the outside diameter of the stakes (fig. 8).

When buying shaft collars, select an inside diameter of 5/8 inches 

(16 mm) for 0.5-inch (13 mm) diameter stakes. Shaft collars come with 

Allen set-screws. Replace the small Allen set-screw with a thumbscrew 

so that an operator wearing neoprene gloves can fasten and unfasten 

Figure 7—Four webbing straps with sturdy, metal spring-loaded 

friction buckles are attached to the frame.

Figure 8—Shaft collars 

with thumb screws to 

hold the webbing straps 

in place.
12 USDA Forest Service RMRS-GTR-191. 2007

shaft collars under water. Use a center punch to deform the last thread at 

the tip of the thumb screw after it has been inserted into the shaft collar 

so it can’t come lose and get lost in the stream. Both shaft collars and 

thumb screws are available in the specialty section of hardware stores. 

Lightly oiling the shaft collars and thumb screws after the field season 

reduces rust.

Net Materials and Fabrication

Gravel collected by a bedload trap is stored in an attached trailing net 

3 ft (0.9 m) long or longer (figs. 1 and 7). Bedload trap nets are handsewn from netting material2

.

Netting Material

Bulk nylon Raschel knotless netting is used to make the bedload 

trap nets. This material is sturdy, abrasion resistant, and knitted of thin 

nylon yarn. It has proved remarkably strong and durable—able to hold 

samples of 20 kg—and requires little maintenance other than cleaning. 

It is commercially available from aquaculture suppliers in several mesh 

sizes and yarn thicknesses (Appendix C).

Recommended mesh size

The authors used a 3.6-mm mesh width netting. This mesh width 

seems to be a good compromise between permissible retardation of 

flow through the net and collection of small gravel particles (fig. 9). 

Using this net reduced the flow in the center of the trap entrance (4 

inches above the plate) by about 10 percent, an effect considered too 

small to compromise accurate sampling (Bunte and Swingle 2004). A 

mesh width of 3.6 mm generally collects particles 4 mm and larger. 

However, flat particles with a b-axis slightly larger than 4 mm can escape because the mesh openings form parallelograms instead of perfect 

squares when the net is under load. To get a mesh opening of 3.6 mm, 

order a nominal mesh size of 3/16 inches (4.7 mm) (see Appendix C for 

further information).

Finer mesh sizes

We have experimented with attaching a 0.5 mm mesh-width net to the 

bedload traps (fig. 10a) and do not recommend using this configuration. 

The 0.5 mm mesh seriously retarded the rate of flow through the bedload 

traps. This caused the net to bulge and water to pond excessively upstream 

2

 We have experimented with machine sewing. Using a zigzag stretch stitch and stretchable nylon thread provides a stretchable seam. However, we have not tested the 

durability of such seams during a field season.
USDA Forest Service RMRS-GTR-191. 2007 13

Figure 9—Detail of Raschel nylon 

knotless netting with 3/16 inch 

mesh size (3.6 mm mesh opening) used for bedload traps. The 

netting is shown stretched such 

that the mesh openings are approximately square.

Figure 10—Net with 0.5 mm mesh 

(a) bulging in the flow and severely ponding the water level 

upstream of the traps even at 

low flow (b).

a

b
14 USDA Forest Service RMRS-GTR-191. 2007

of the bedload traps (fig. 10b). Algae and fine debris caught in the net 

clogged the mesh and further exacerbated the hydraulic impedance. 

Unlike the flared entry of a pressure-difference Helley-Smith sampler 

that sucks water into the sampler entrance in order to compensate for the 

reduction of flow velocity caused by the fine 0.25 mm mesh, the bedload 

trap frame is unflared and therefore not designed for nets much finer than 

3.6 mm3.

Mesh sizes larger than 3.6 mm

Some studies are particularly interested in the transport of large 

gravel and cobbles that typically move together with a lot of finer-sized 

bedload during only the highest flows. In order to focus on the collection of very large particles at high flows, one could use a larger mesh 

size that lets the abundantly moving finer gravel, sand, and some of the 

coarse organic material pass through while leaving space in the sampling 

nets for the collection of the less abundant and less frequently moving 

coarse gravel and cobbles. Although we have not used a net with a mesh 

larger than 3.6 mm for bedload traps, larger mesh widths have been 

used successfully with a similarly designed net frame sampler to which 

Bunte (1996) applied a 0.39-inch (10 mm) mesh. Whitaker (1997) and 

Whitaker and Potts (1996) used a 1.25-inch (32 mm) mesh in a gravel 

bed stream with relatively few fines.

Net lengths

Nets should be at least 3 ft (0.9 m) long behind the trap to provide a 

sufficiently large sampling capacity and to permit the nets to be emptied 

while the traps remain in place on the ground plates. Shorter nets are not 

recommended because they fill quickly, and the sampling efficiency of 

the trap decreases if the nets fill beyond a capacity that is generally assumed to be 40 percent of the total sampler bag volume (Bunte and Abt 

2005; Bunte and others 2006a; Emmett 1981). With a total net volume 

of about 6.4 gal (25 liters), the capacity of the 3-ft long net is about 2.7 

gal (10 liter) of sediment and/or organic material or 44 lb (20 kg) of 

gravel. Nets need to be 5 to 6 ft long (1.5 to 1.8 m) when they are emptied from a footbridge (Chapter 3) in order to provide sufficient space 

for the operator to lift them (fig. 11, see also fig. 41 in Chapter 4).

Nets up to 8 or 9 ft long (2.4 m) can be used for bedload traps installed in parts of the stream that are unwadeable at high flow (fig. 12). 

With the help of a hook, the downstream end of the net is pulled from 

deep to shallower water where it can be emptied more easily. Long nets, 

3

 The performance of nets with mesh width slightly smaller than 3.6 mm, for example, 2 

and 3 mm, has not been evaluated.
USDA Forest Service RMRS-GTR-191. 2007 15

Figure 11—3-ft long nets proved too short for emptying the net from a bridge. 

Note the cramped workspace for the operator in the stream (a) (Photo courtesy of J. Potyondy, USFS Stream System Technology Center); 5-to 6-ft long 

nets are more suitable (b).

Figure 12—Two 8.4 ft (2.6 m) and one 8 ft (2.4 m) nets are installed in the 

outside bend in the deepest part of the stream. Two 3.6 ft (1.1 m) nets are 

installed in shallower water.

a

b
16 USDA Forest Service RMRS-GTR-191. 2007

however, have a higher drag, and traps become more difficult to install 

on the ground plates as the flow pulls harder on the net. Long nets also 

have a tendency to be compressed by the flow. A few stiff hoops could 

be sewn on the outside of long nets to keep them open along their entire 

length. The exact length of the net might be best decided at the field site. 

An extra piece of netting can quickly be sewn on the net in the field. 

Long nets, however, should not be used for collecting samples (much) 

larger than is possible with 3-ft nets because the sample mass becomes 

unmanageably large.

Amount of netting material to order

The netting material is knitted to a width of 8 ft (2.44 m) when 

stretched (in aquaculture, this width is referred to as a hanging depth of 

8 ft; see “depth” in table 1C, Appendix C). The material can be ordered 

to any length (in full feet).

A 4-ft (1.22 m) long section of this netting provides enough material 

to make two short nets, each about 3 ft (0.91 m) long behind the bedload 

trap frame (see “Cutting Patterns” in the next section). Nets longer than 

about 3 ft require a 4-ft long piece of material to build one net. Ordering 

an additional 4 to 8 ft of netting provides extra material should the nets 

have to be extended later. A piece of netting can easily be sewn on the 

end of a net to extend it.

Cutting the Netting Material 

and Sewing the Net

Cutting patterns

Measure the outside circumference of the bedload trap frame. For 

the frame 12.5 inches wide and 8.5 inches high shown in figure 3, the 

outside circumference is 42 inches (107 cm). Add 1.5 to 2 inches to the 

circumference plus 2 inches for the seam for a total length of 46 inches 

(117 cm) of material needed for one net. The distance from A to D in 

figure 13 indicates this length. Cut along the dashed line from A to B’. 

For a net 3 ft (0.91 m) long behind the frame, measure 4 ft (from A to 

B)—while the net is stretched—and cut along the line BC. For a 5-ft 

(1.52 m) long net, cut along the line B’C’. Use the entire width of the 

material (from A to B’’) for a net that is approximately 7 ft (2.13 m) 

long behind the frame. Use scissors to cut along the same mesh row 

to ensure that all cut pieces will be of the same size. A piece of netting 

12 ft long provides material for either six nets, each 3 ft long, or three 

nets longer than 3 ft.
USDA Forest Service RMRS-GTR-191. 2007 17

Sewing the nets

Needle and thread—To hand sew the nets, use tightly twisted nylon 

yarn4, about 1 mm in diameter, and a carpet needle with an eye large 

enough for 1 mm thread. Both can be purchased in a sewing or hardware store. A carpet needle is about 2.5 inches (6.4 cm) long, slightly 

curved, flattened near the tip and only slightly pointed. For a 4-ft long 

seam, thread the needle with a piece of yarn about 6 ft (1.8 m) long. 

Prevent the yarn from unraveling by using a butane lighter to sear both 

thread ends lightly after the needle is threaded. While it is still hot, 

squeeze a seared end quickly between your wetted thumb and index 

finger to flatten it (careful, hot). Avoid forming a big blob that will be in 

the way when sewing.

4

 Yarn used for tying purposes; un-stretchable and more tightly twisted than used for knitting or crocheting.

Figure 13—A piece of netting with a knitted width of 8 ft 

(in other words, with an 8 ft hanging depth) and 12 ft 

long. The corner points A, B, C, and D refer to the corner points in figure 14.
18 USDA Forest Service RMRS-GTR-191. 2007

Figure 14—Step 1 to 4: Sewing the nets for the bedload traps. The lettering of 

the corners corresponds to the lettering in figure 13.

Sewing the tube—A cut piece of netting material has a tendency 

to pull together into a rectangular shape (fig. 14; step 1). Stretch the 

material to open the mesh holes to see the size that the netting will assume when deployed in the stream. The direction of the stretch is in 

the upstream-downstream direction of the net (fig. 14; step 2). Fold the 

netting into a tube that stretches along its lengthwise direction. To make 

a flat seam, lay the netting edges on top of each other so they overlap 

by about 1.5 inches (4 cm) (fig. 14; step 3). Position the netting so that 

the mesh holes of the upper and lower net line up exactly. Leave about 

a foot of thread hanging at the beginning of the seam (to be secured 

later). Using a simple forward up-and-down stitch, sew through the 

mesh openings, not through the netting material itself. For a straight 

seam, keep the stitches within a row of mesh holes (fig. 15). Sew two 

parallel seams in two separate mesh rows.

It is important to stretch the netting while sewing it because the netting material will have a tendency to pull together and away from the 

direction of stretch. Hold part of the material tight between your knees 

while stretching the part that is being sewn, or press the netting against 

your knee with your sewing hand and stretch the material with your 

other hand. Repeatedly stretch the piece of seam that was just sewn. 
USDA Forest Service RMRS-GTR-191. 2007 19

After the entire seam is completed, stretch it to its fullest extent. Ensure 

that the piece of thread hanging at the end of the net is not pulled entirely into the seam when stretching the net. Then secure the thread 

ends, either by sewing through a few inches of netting or by tying a few 

knots. If the seam is not stretched while sewing, it will be shorter than 

the length of the net as soon as the net is stretched in the water during 

sampling. Nets that have a seam along the top that is too short or shriveled are awkward to operate.

Sewing the tube around the frame—After the tube is sewn, push 

one end of the tube through the inside of the frame and fold it outward 

around the frame (fig. 16, fig. 14; step 4). The beveled edge of the frame 

Figure 15—Two parallel seams sewn through the mesh openings using a 

simple forward up-and-down stitch. The two layers of netting are positioned 

such that mesh holes exactly overlay each other.

Figure 16—The sewn netting tube is folded around the frame.
20 USDA Forest Service RMRS-GTR-191. 2007

must point toward the front of the sampler; the net points to the back 

of the sampler. The seam along the length of the tube should be placed 

along the middle of the top of the frame. The nylon yarn used for sewing the tube abrades and becomes nappy if the seam is placed on the 

underside of the trap where it can rub against the gravel bed.

The fold of the net tube around the frame should have some slack. 

The netting can be temporarily secured into its folded position with a 

few twist ties, tooth picks, or paper clips. To sew the netting around 

the frame, stitch through the mesh holes, staying within the same row 

of holes. It may be difficult to line up the mesh holes of the upper and 

lower layers of netting. Loosely stitch through the upper layer of the 

net, the lower layer, and the upper layer again. Hold the two layers apart 

so that mesh holes are clearly visible (fig. 17). After a few stitches, pull 

the thread tight to connect the upper and lower layers. Place the second 

seam parallel to the first so that none of the seams is more than about 

1 inch away from the metal of the frame.

Attaching the Straps

Webbing straps are attached to the frame through the vertical slits. 

The webbing is fastened behind the slits with a plastic tension buckle 

that is placed inside the frame and acts as a crossbar. The first step to 

attaching the webbing straps is to cut open about three of the mesh 

Figure 17—Stitching loosely through the upper and lower layers 

of netting while the layers are apart and the mesh holes are 

visible. The thread is pulled tight after a few stitches to close 

the seam.
USDA Forest Service RMRS-GTR-191. 2007 21

holes covering the slits. Insert the stiff front end of the strap (with the 

friction buckle on the trailing end) into a slit on the frame from the 

outside of the trap (fig. 18; step 1) and pull it through so that about 2 

inches of strap remain before the buckle. Insert the stiff front end into 

the right slit of the plastic tension buckle (fig. 18; step 2) and bring it 

back through the left slit (fig. 18; step 3). Place the tension buckle right 

behind the slit inside the trap frame. Lead the stiff end of the strap from 

the inside to the outside of the slit of the frame (fig. 18; step 4), and pull 

the strap tight. A close-up view of a bedload trap with netting and straps 

in place is shown in figure 7.

Numbering Bedload Traps

Each trap should be numbered on its top with a permanent marker 

(for example, 1 through 4 or A through F) depending on the number of 

traps to be used at a site. Numbering identifies which trap goes with 

which ground plate. This match is important because the straps on each 

trap are specifically adjusted to the position of the stakes at each plate. 

Bedload traps and collection buckets should be labeled with the same 

number to ensure samples correspond with their location in the stream 

cross-section.

Figure 18—Four steps of fastening the webbing straps to the frame. The numbers refer to the steps described in the text (see fig. 7).
22 USDA Forest Service RMRS-GTR-191. 2007
USDA Forest Service RMRS-GTR-191. 2007 23

Chapter 3.

Installation of Bedload Traps

Installation of bedload traps on the stream bed involves four steps: 

1) Selecting a site, considering both the stream reach and the crosssection; 2) Positioning the traps within the selected cross-section; 3) 

Setting and anchoring the ground plates; and 4) Preparing the area next 

to the traps. The four steps are described as follows.

Selecting a Sampling Site

Careful selection of the study location is important in order to 

adequately sample bedload, meet study objectives, and avoid undesirable site-specific results. Two main criteria need to be considered: 

Appropriateness of the site to answer the study question and suitability 

of the site to deploy bedload traps.

Appropriateness of the Site to Answer 

the Study Question

Several criteria may be involved in the selection process depending 

on the study aim and local stream conditions:

Distance from upstream gravel sources and sinks,

Selection of a representative site versus sampling at a specific stream 

location, and

Proximity to infrastructure (gauging station, road, work area).

Distance from upstream gravel sources or sinks

Proximity to upstream gravel sources or sinks affects measured 

bedload transport rates and influences the shape of the bedload transport–discharge relationship. In general, it is better to avoid sites just 

downstream from either source or sink locations. If gravel bedload is 

collected just downstream from a gravel source, measured transport 

rates may be generally higher than at other stream locations. Transport 

may also increase markedly on the first rising limb of seasonal flow, 

and the same rates may not be reached during the falling limb of flow, 

resulting in a clockwise hysteresis loop in a graph of transport versus 

flow. This is particularly true if the sediment source becomes depleted 

over the course of the highflow event. If gravel bedload is collected at 

a site below a sediment sink, transport rates are generally lower there 

•

•

•
24 USDA Forest Service RMRS-GTR-191. 2007

than at other stream locations and there may be a delayed response of 

bedload transport to flow, causing a counterclockwise hysteresis loop.

Sources of gravel to the stream may include accumulations of hillslope sediment at the stream banks supplied over periods of low flow, 

easily erodible material from bank collapse, gravel supplied from recently destroyed log jams or beaver dams, bedload waves traveling 

downstream, and material from degrading reaches upstream. Complete 

gravel sinks include beaver dams, new log dams or log jams, some flow 

diversion structures, or sediment retention ponds. Partial sediment retention may occur below a partially filled log dam or when sediment 

deposits upstream and downstream from a partial stream blockage, 

such as a log protruding into flow.

Selection of a representative site versus sampling at a 

specific location

One criterion for selecting a study site might be that the data collected must be suitable for making generalizations regarding transport over 

the entire stream reach. This means that bedload transport rates and 

particle sizes measured at the sampling site should be representative 

of a study reach and not be excessively influenced by local conditions. 

By contrast, a study aim might be tied to a specific stream location. For 

example, one might want to compare bedload transport rates or particle 

sizes upstream and downstream from a tributary or a site that received 

gravel augmentation or was subject to gravel mining. Similarly, a study 

may want to compare transport rates collected between riffles and pools, 

over a series of downstream cross-sections, or at the same location over 

time. The site location may also be influenced by proximity to a gauging station, a road, or other infrastructure.

Suitability of the Site to Deploy Bedload Traps

Using bedload traps poses additional criteria for site selection. These 

site criteria are:

Wadeability at high flow,

Sufficient dry workspace on the banks, and

Absence of boulders from the sampling site.

Further desirable features are:

Suitability for placement of a footbridge (if one is to be used) and

Vehicle accessibility.

Wadeability at high flow

Wadeability of the cross-section, preferably up to bankfull flow, is a 

prerequisite when using bedload traps. There are several considerations 

involved in identifying wadeable locations and making marginal areas 

more wadeable. These include:

•

•

•

•

•
USDA Forest Service RMRS-GTR-191. 2007 25

Operator stature,

Magnitude of flow and stream geometry,

Sampling across the head of a point bar,

Applying long nets to the bedload traps, and

Installation of a footbridge.

Operator physique—Whether a specific flow condition is wadeable 

for a person depends on the wading person’s physique. Based on flume 

experiments with tethered people holding their footing against increasing flow, Abt and others (1989) assessed wadeability by establishing 

a relationship between the wading person’s physique and flow conditions. The person’s physique was determined as the product of body 

weight (lb) and height (inches). Flow conditions were quantified by the 

product of mean flow velocity (in units of feet/second) and flow depth 

(in units of feet), also called the product number. The critical product 

number, PNcrit, for a tethered operator when footing was lost increased 

with the product of the person’s weight and height. The critical product 

number for wadeability can be predicted from a linear regression

PNcrit = 0.8 (wt * ht/1000) + 6. (1)

Based on Eq. 1, the average point at which footing was lost occurred 

at a product number of about 10 for a petite (5 ft; 90 lb [1.52 m; 41 kg]) 

person and at 18 for a tall (6 ft 3 inches [1.9 m]) and heavy (200 lb 

[91 kg]) person (fig. 19).

•

•

•

•

•

5 

10

15

20

25

4000 6000 8000 10000 12000 14000 16000

Weight (lbs) · Height (inches)

70% PN

PN = 0.8 (wt·ht/1000) + 6

r? = 0.41, n = 66

Product Number [Depth(ft)·Vel.(ft/s)] Figure 19—Limit of wadeability quantified by the product number 

for persons of different weight and height. Data plotted from 

Abt and others (1989) and fitted with a linear equation (thin 

line). The thick line indicates 70 percent of the critical product 

number, a more practical limit of wadeability in coarse-bedded 

mountain streams.
26 USDA Forest Service RMRS-GTR-191. 2007

Based on our experience, the practical limit of wadeability in the 

field is substantially lower than predicted from Eq. 1, perhaps at 60 to 

80 percent of the flume value, because a wading person usually does 

not push wadeability to the limit of falling. Factors such as bottom slipperiness, bulkiness of waders, objects carried, and work near the stream 

bottom further decrease the wadeability of a particular flow, while a 

person’s athletic capability, safe wading practices, and use of safety devices increase it. However, a person needs to consider not only the risk 

of slipping, but also the risk associated with the consequences of lost 

footing. See Appendix D for more information on safety issues while 

wading.

Wadeable flows in Rocky Mountain streams—Whether flow conditions are wadeable at high flow depends on the discharge and on 

the channel geometry. Even small streams with moderate highflow 

discharges may not be wadeable at all locations in the stream. Crosssections wider than average have the best chance of being wadeable 

at bankfull (or higher) flow and are preferred study sites when using 

bedload traps (fig. 6c). However, the selected site should not scour or 

aggrade during a highflow.

To determine whether a specific discharge is wadeable at bankfull flow would require fixed relationships between discharge, stream 

morphology, and wadeability for streams in different climatic regions. 

Without fixed relationships, one can resort to a rule of thumb: coarsebedded Rocky Mountain streams with mainly plane-bed morphology 

tend to remain wadeable at the widest cross-sections in flows up to 

about 200 cfs (6.2 m3/s). A bankfull flow of 200 cfs may be expected in 

watershed sizes of approximately 20 to 40 square miles (50 to 100 km2), 

depending on mean annual precipitation and watershed conditions.

Bedload sampling across the head of a point bar—If a cross-section 

expected to remain wadeable at high flow cannot be located, sampling 

only across the head of a point bar may be another option. Our experience at several streams has shown that gravel transport follows a 

particular downstream path in a coarse-bedded stream with a meandering thalweg (Bunte and others 2004a, 2006b; fig. 20)5

. For a specified 

cross-section within the reach, the lateral position of most gravel bedload transport is generally predictable. In a transect across the head 

of a point bar, most of the gravel is transported over the submerged 

(and likely wadeable) point bar, while almost none of the gravel moves 

through the thalweg (at least not in flows below bankfull). The lateral location of maximum gravel transport even appears to shift further 

5

 The path is similar to the path of the coarsest particles in sand-bedded streams (Anthony 

and Harvey 1991; Bridge and Jarvis 1982; Dietrich and Whiting 1989; Julien and 

Anthony 2002).
USDA Forest Service RMRS-GTR-191. 2007 27

toward the bar side (and away from the thalweg) with increasing flow 

(Bunte and Swingle 2005, Bunte and others 2006b). Thus, little of the 

gravel transport is missed if the thalweg of that cross-section is not 

sampled in flow below bankfull.

Long nets—In order to extend sampling as far as possible into the 

deeper flows of the thalweg area, the authors have attached nets 6 to 8 ft 

(1.8 to 2.4 m) long to the bedload traps (fig. 12). The end of a long net 

can be pulled aside into shallow flow to empty it. Note, however, that 

a daily period of lower flows is needed to set and retrieve the bedload 

traps from their ground plates when operating under these conditions.

Adding a footbridge—The best option for using bedload traps in 

marginally wadeable flows (and flows of all depths) is to install a footbridge positioned a few inches above the water surface. The operator 

can then hold on to the bridge when wading through the water to service 

the bedload traps. In addition, the amount of bank-to-bank wading is 

greatly reduced because the second operator does not enter the water, 

but assists from the bridge surface, carrying buckets, gravel samples, 

and support equipment across the bridge. Footbridge use is described 

ahead in this chapter. If a footbridge is used, the selected site should 

have a stable bed near the banks to support the cinderblocks on which 

the bridge rests (or an opportunely placed boulder), and the stream 

should be somewhat incised.

Adequate dry workspace on the banks

Another practical aspect to consider when selecting a site is ensuring 

adequate workspace on the banks for handling the bedload traps and 

the samples collected in them. Open space on the banks is needed for 

washing and bagging samples, as well as for storing field gear. A work 

area of about 400 ft2 (37 m2) is minimal for two people. Water seepage 

and rain can quickly turn a vegetated work area into a morass. Areas 

Figure 20—Approximate path of major gravel transport in a meanderingthalweg, coarse gravel-bed stream (yellow dottes band). Darkest blues indicate deepest flows (from Bunte and others 2006 b, slightly modified).
28 USDA Forest Service RMRS-GTR-191. 2007

receiving high foot traffic can be protected by placing permeable foot 

mats on the ground 6 (fig. 21).

Absence of boulders in the stream bed

Avoid sites dominated by large cobbles and boulders in the streambed when using bedload traps. Buried large clasts make it difficult to 

drive the stakes into the bed, while protruding boulders direct the flow, 

cause scour jets, and lead to irregular spatial patterns of transport and 

deposition of bedload within the cross-section. Note that the influence 

of boulders on the lateral distribution of transport can extend downstream for a distance several times larger than the boulder diameter.

Accessibility by vehicle

Bedload traps are portable, so two or three people with backpacks 

can carry into remote areas all of the equipment necessary for a bedload 

trap study. This allows studies to be performed independent of direct 

road access and at undisturbed sites. Transporting the majority of the 

gear can be limited to the beginning and end of the field season. Most 

of the gear can be left out overnight, covered with a tarp, and tied to a 

tree with a long plastic-mantled cable and a padlock. Other advantages 

of a remote site study are lower probability of site disturbance by curious explorers and reduced potential for vandalism. What speaks for a 

Figure 21—Foot mats placed at the end of a footbridge to protect the grass from trampling and to avoid creating a mud 

puddle.

6

 Foot mats that are made from cut tire pieces have worked well and are available in 

hardware stores.
USDA Forest Service RMRS-GTR-191. 2007 29

vehicle-accessible site is its convenience and support for the often wet 

and cold operators. If a footbridge will be hauled to the site, vehicle accessibility to within a few hundred feet is required.

Final Site Selection

Before selecting potential study sites, the authors suggest ranking the 

selection criteria according to their importance and walking along the 

stream for a mile or two at low flow. Walking provides an overview of 

gravel source and sink locations, channel width and wadeability, stream 

morphology, stream accessibility, etc. Selecting a sampling site is often 

a competition between factors that speak for or against a specific location. Listing pros and cons of potential sites and discussing them can 

help the selection process. One also needs to consider that appropriate 

locations need to be found for ancillary measurements such as stage 

and discharge, and that a space conflict might arise at the sampling site 

(Chapter 4).

Once the site is selected for the bedload traps, the cross-section should 

be flagged and marked with rebar stakes for the duration of the study. If 

the site is not remote, signs informing passers-by and anglers about the 

study site and asking people to stay off may be helpful (fig. 22).

Figure 22—Sign set up at a bedload trap study site near well-frequented 

camp sites at Halfmoon Cr., CO.
30 USDA Forest Service RMRS-GTR-191. 2007

Positioning Bedload Traps Within 

the Cross-Section

Several bedload traps should be deployed per cross-section to cover 

the majority of the stream width. In narrow and high gradient streams, 

we have spaced bedload traps as close as 2 ft (0.6 m) apart. In wider 

streams, the maximum trap spacing was 6 ft (1.8 m). Traps may be 

spaced evenly or unevenly across the stream. Use an even spacing 

along a transect if the study objective is to document the lateral variability of transport or if the lateral distribution of gravel transport is 

unknown. Perfectly even spacing (such as the equal-width spacing typical of Helley-Smith sampling) is usually not possible because some of 

the ground plates may need to be shifted laterally in order to find a location where the stakes holding the ground plates can be driven in.

If the study objective is to provide the most accurate prediction of 

the cross-sectional transport rate, trap spacing should coincide with 

width increments of approximately equal transport. This requires predicting the locations of the highest transport rates, either from patterns 

of stream morphology or from pilot measurements. However, the user 

must weigh the advantage of increased accuracy in measured crosssectional transport rates against the risk that the selected spacing may 

become unrepresentative, because transport locations vary to some degree over time and with flow. A major change in transport paths is likely 

to occur after changes in the local stream morphology, in response to 

high flows, with a change in sediment supply, or with high bed mobility. 

In practice, the bedload trap spacing is a compromise between equalwidth spacing, equal-transport spacing, and finding the location where 

the stakes can be driven into the bed. Ground plates can be laid out and 

arranged on the streambed in low flow to help visualize the best position of the bedload traps in the cross-section (fig. 23).

Figure 23—Ground plates are laid out in a cross-section at low flow.
USDA Forest Service RMRS-GTR-191. 2007 31

Setting and Anchoring Ground Plates

Placing Ground Plates at the Right Height on the Bed

Position ground plates flush with the mean surface level of the bed 

without protruding above, or lying below, this level (fig. 24 a through c). 

Two methods can be used to obtain this position: 1) When flows are low, 

remove large surface particles from the area where the ground plate is 

to be placed and replace them with smaller ones. Place the ground plate 

onto the streambed (fig. 25) and check it for the appropriate height. 

Remove large stones, replacing some with smaller ones, and continue to 

check the ground plate height until it is flush with the mean level of the 

bed. Fill gaps along the sides of the ground plate with a few stones large 

enough to fill the gap. 2) If flow is already high, ground plates are best 

Ground plate

Mean bed

height

a) When no particles are removed,

 ground plates protrude above the mean bed

 level (avoid).

b) When all large particles are removed,

 ground plates lie below the mean bed

 level (avoid).

c) To place ground plates at the average

 height of the bed, large particles need to be

 replaced by smaller ones (desired).

Figure 24—Ground plate 

positions in relation to 

the mean bed level 

(side view). Avoid positions a) and b).
32 USDA Forest Service RMRS-GTR-191. 2007

installed by shoving the bent front edge of the plate about an inch deep 

into the bed surface. Hold the back end of the ground plate above the 

bed and remove all large surface particles below the plate. Lower the 

plate onto the bed and check for the appropriate elevation relative to the 

mean height of the bed. Remove or replace larger stones with smaller 

ones until a satisfactory position for the ground plate is reached.

Water flow should be as low as possible as the operator must crouch 

on the stream bottom to install the ground plates. The best time for setup 

is shortly before the onset of the snowmelt highflow season. However, 

installation may be hampered in early spring by snow drifts that cover 

roads and stream banks, making them inaccessible and complicating 

logistics. On the other hand, if you wait until the snow has melted, 

you may find yourself in a sudden warming spell that quickly increases 

flows. Installation of bedload traps at near bankfull flow is possible in 

some streams, but may result in poor placement of the ground plates 

and is a rather wet and cold enterprise. Ground plate heights may need 

to be adjusted occasionally during highflows to match the bed plane in 

actively degrading or aggrading beds (Chapter 4).

Driving in the Stakes

Once a ground plate is properly positioned, anchor it to the streambed by pounding the stakes through each hole at the sides of the ground 

plate 1 to 1.6 ft (0.3 to 0.5 m) deep into the bed (if necessary). Step on 

the plate to hold it in place on the bed. Use a driver (fig. 26) to pound 

the stake to prevent the head from mushrooming. The first few inches 

of penetration into the bed can usually be obtained by sliding the stake 

Figure 25—Setting a ground plate onto the stream bottom.
USDA Forest Service RMRS-GTR-191. 2007 33

driver down on the rod. Driving the stakes with a hammer is generally 

a two person operation. One person holds the stake driver with both 

hands, keeping the orientation of the stake as vertical as possible, while 

the other person pounds the stake driver using an engineers’ hammer or 

a lightweight sledge hammer. The pounding must be controlled and deliberate so that if the stake hits a rock and vibrates heavily, quick blows 

with the hammer do not miss the stake and hit the hands of the person 

holding the driver.

If a stake hits an impenetrable rock, which happens more frequently 

in cobble than gravel beds, try a new location a few inches away. Go 

through the procedure described previously for positioning the plates 

relative to the streambed. Ideally, the stakes holding the plate are parallel and at right angles to the plate. In practice, one or both of the stakes 

will be off vertical to some degree. Adjust the length of the webbing 

straps on the traps to compensate for this imperfection in stake orientation (Chapter 2). It is important to make the strap adjustments and 

actually position the trap over the stakes and onto the plate before deciding that the installation is adequate and that the stakes will not have 

to be removed and re-driven.

Restoring the Streambed Area Around 

the Ground Plates

If the installation process disturbed the streambed at the upstream 

side of the traps, the bed must be restored to avoid artificially increased 

or decreased particle entrainment conditions near the bedload trap 

Figure 26—Starting to drive the stakes in by sliding the stake 

driver up and down.
34 USDA Forest Service RMRS-GTR-191. 2007

entrance. Replace the large particles that were removed at the upstream 

side of the ground plates with smaller ones along the front edge of the 

ground plate that sticks slightly into the bed. Gather finer bed material 

from a nearby location and carefully place it over the replacement stones 

to refill interstitial spaces (figs. 27 and 28). To ensure that particles at the 

remodeled streambed have settled into place, do not commence sample 

collection for at least several hours after the ground plate installation.

Figure 27—Ground plates installed on the stream bed during 

low flow (view upstream).

Figure 28—Ground plates with PVC pipe pieces fastened with 

shaft collars.
USDA Forest Service RMRS-GTR-191. 2007 35

Finishing Plate Installation

When bedload traps are not installed for sampling, the ground plates 

must be held down on the streambed to prevent them from being lifted 

by flow and presenting an obstacle that might cause bed scour. Ground 

plates are held in place by pieces of PVC pipe and shaft collars. The 

pipe acts as an extender, such that the shaft collar does not need to be 

fastened deep under water directly on the ground plate (fig. 28). Place 

foot-long pieces of garden hose on the top of the stakes (figs. 6a and b) 

so they are more visible, particularly if flow starts to overtop the stakes, 

and to add a safety feature should a person slip and fall onto a stake.

Once the ground plates are installed, confirm they are still positioned 

within the originally selected cross-sections and adjust the crosssection markers on the bank (rebar stakes) if necessary. Next, measure 

the ground plate locations along a tape strung between the two rebar 

stakes. Accurate measures of trap spacing along the cross-section are 

necessary because transport rates are individually computed for each 

trap and the stream section that it represents (Chapter 5).

Protecting the Streambed

Foot Traffic, Bed Armor, and Stepping Stones

Walking repeatedly on the bed can shift surface particles—even cobbles—and loosen their interlock with other bed particles. All foot traffic 

upstream of the trap cross-section should therefore be strictly avoided whether or not bedload transport is taking place. Walking behind 

the traps (unavoidable) disturbs the armor layer and can form a scour 

trough. The scour increases with the amount of foot traffic, strength of 

flow, and erodibility of the bed. If scour advances upstream between 

the ground plates, the traps will become pedestalled, which greatly reduces their sampling efficiency and requires resetting the ground plates 

(Chapter 4). In a relatively fine-grained stream bed or high flow velocities, stepping stones are useful to prevent tread impacts on the bed and 

subsequent scour. Barbell weights with a diameter of about 15 inches 

and a weight of 35 lb make good stepping stones (Bunte and Swingle 

2007). Stepping stones that are of lighter weight (for example, 1-foot 

diameter concrete garden foot path stones) are useful only at moderate flows (fig. 29). At high flow velocity, they become entrained, leave 

scour holes, and become obstacles to the operators (Bunte and Swingle 

2003a). If scour does occur behind the traps, it can be reduced by strategically placing and embedding a few cobbles (collected downstream) 

more or less flush with the bed surface.
36 USDA Forest Service RMRS-GTR-191. 2007

Use of a Footbridge

A footbridge greatly reduces the amount of treading and subsequent 

bed scouring. It also extends wadeability to higher flows and is generally useful when operating bedload traps (figs. 29, 30; see also fig. 41 

in Chapter 4). Although stepping on the bed is required when servicing 

the bedload traps, standing next to the footbridge provides support to 

the operator and allows him/her to deposit the traps and the collected 

sample onto the bridge before and after deployment in the stream. A 

second person on the bridge assists the operation (Chapter 4). Using a 

footbridge is highly recommended with the bedload traps, particularly 

if the bed is erodible, flows are expected to be high, or large numbers of 

samples are to be collected.

The footbridge rests on cinder blocks at each side of the stream (fig. 

31). As flow rises, additional cinder blocks may be added to elevate the 

bridge above the water surface. The goal is to keep the underside of 

Figure 29—Stepping stones placed behind ground plates in relatively fine gravel bed material at the East St. Louis Creek sampling site. Note the impractical, unnecessarily long stake lengths at the two ground plates near the left 

bank. The 16-ft long footbridge in the back is placed too close to the ground 

plates causing a cramped work space for the trap operator.
USDA Forest Service RMRS-GTR-191. 2007 37

the bridge as close to the water surface as is safely possible—approximately 0.5 ft (0.15 m). If the site is left over night, it may be necessary 

to prop the bridge up higher to ensure that floating debris will pass under the bridge and to allow enough space for an unexpected rise in flow. 

We recommend that one end of the bridge be tied to a tree or post in the 

bank so that if the bridge is swept off its cinder block supports, it will 

swing around and come to rest parallel to flow along the bank.

Figure 30—A 32-ft long footbridge installed at Halfmoon Creek at low flow (a) 

and at 70 percent of bankfull flow (b).

Figure 31—Details of the bankward side of the bridge resting on cinder blocks. 

Note the foot mats placed on the banks to protect vegetation at the location 

of repeated treading (a). The slack rope in (b) ties the bridge to a nearby 

tree.

a b

a b
38 USDA Forest Service RMRS-GTR-191. 2007

The footbridge shown in figures 30 and 31 was constructed using 

floor joists, 2 by 4 lumber, and plywood decking following the plans by 

Pitlick and Miller (1987) and Martinez and Ryan (2000). Two people 

assembled this footbridge in less than two days using handheld power tools (waterproofing took most of this time). Construction details 

for this bridge (fig. 32) are described in Bunte and Swingle (2005). 

Transportation of a 32 ft bridge requires a large truck or a trailer. The 

decking is installed after the bridge is in place across the stream (and 

removed before the bridge is pulled from the stream). Without the decking, two people can carry this bridge over a short distance.

Figure 32—Close up of the footbridge used at Halfmoon Creek without decking (to show construction).
USDA Forest Service RMRS-GTR-191. 2007 39

Chapter 4.

Operation of Bedload Traps

Cleaning and Setting the Traps

Before they are deployed in the stream, bedload traps should be thoroughly cleaned each morning of organic debris adhering to the net from 

previous samples (fig. 33). Lay the bedload traps out on the bridge and, 

after they are thawed and partially dried, vigorously shake and brush 

them by hand, with a rag or a brush.

Figure 33—Laying out traps to be deployed for drying and 

cleaning out organic debris.
40 USDA Forest Service RMRS-GTR-191. 2007

To set the traps on the ground plates, remove hose pieces, shaft collars, and the PVC pipes at the bottom of the stakes (fig. 34 a and b). Set 

the webbing loops on the frame to their approximate lengths and slide 

them over the stakes (fig. 34c). Slide the trap, complete with straps, 

down the stakes and set it on the ground plate (fig. 34d). Adjust the strap 

length for each trap such that the trap opening faces squarely into the 

Figure 34—Six steps of setting the bedload traps onto the ground plates: Removing the hose pieces (a), removing the shaft collar (b), sliding the webbing loops over the stake (c), pushing the trap down onto the ground plate 

(d), checking the trap position (e), and fastening the shaft collar (f).

a b c

d e f
USDA Forest Service RMRS-GTR-191. 2007 41

direction of flow, even if the ground plate and the stakes do not. Ensure 

that the entire bottom face of the frame is flush with the ground plate. 

Because each of the ground plate-and-stake assemblies differ slightly 

from each other, adjust the webbing strap loop length of a specific trap 

to fit a specific ground plate and stake pair. Traps should be numbered 

to coordinate placement with “their” ground plate.

Despite the individual fitting, ensure traps and straps are properly positioned on the ground plate before each sample is taken and re-adjust 

if necessary. Check the trap/plate contact by running a finger along the 

beveled edge of the trap and ground plate (fig. 34e). A gap between the 

frame and the ground plate must be corrected to prevent small particles 

from passing under the trap frame. Similarly, larger particles can bypass 

the trap laterally if the transition between ground plate and frame is not 

smooth, or if the frame is not oriented straight into the direction of flow.

After the trap is positioned satisfactorily, slide the shaft collars over 

the stakes, push them down to the top pair of webbing straps, and screw 

them tight to keep the straps in place (fig. 34f). Leave the ends of the 

nets untied until all traps are set. Figure 35 shows a trap properly seated 

on a ground plate. Depending on the distance between the traps and the 

force of flow, with practice, it takes only 1 to 2 minutes to install and 

secure a trap to a ground plate.

Figure 35—Detail of bedload trap mounted on a ground plate. Note that the 

net on this trap is not sewn onto the frame as is described in Chapter 2, but 

clamped around the frame with a metal band, a method not entirely successful.
42 USDA Forest Service RMRS-GTR-191. 2007

Tying The Nets—Sample Interval Start

Tie the nets once all the traps are in place. Typically, two operators 

are employed to accomplish this task. Operator 1 should grasp the net 

and, if the trap has been sitting with an open net for some time, shake it 

to remove any bedload or debris already collected in the net. Operator 

1 should then gather up the end and hold it between his/her hands while 

operator 2 ties the net with a cotton-covered clothesline. Two tight 

wraps of the cord around the net end, secured with a simple overhand 

knot and bow tie (like shoe laces), are sufficient to hold an entire net full 

of gravel. Two people tying the net take a little less than a minute per 

trap (fig. 36). Operator 2 should read the tying time for each trap from 

a watch worn on a string around the neck (a wrist watch interferes with 

neoprene gloves). The time at which traps were tied off (and later the 

time at which traps were emptied) must be recorded in order to compute 

sampling times which, in turn, are needed to compute transport rates.

Figure 36—Tying off a bedload trap at East St. Louis Creek 

(Photo courtesy of J. Potyondy, USFS Stream System Technology Center).

Recording Sampling Times

Written notes are difficult to take since both operators will have 

wet hands and may be wearing neoprene gloves when tying the nets. 

Therefore, operator 2 must remember the tying times for all traps and 

record them immediately after tying off the last trap. For a 60-minute 
USDA Forest Service RMRS-GTR-191. 2007 43

sampling period, recording the time to the nearest minute generally 

provides sufficient accuracy (a maximum of 1.67 percent time error). 

Tying the nets takes approximately one minute per trap, so one usually 

needs to remember just four to six sequential numbers. We recommend 

having a table prepared in the field book for entering the tie-off times 

and having a field towel handy. Developing a routine can help. For 

example, always start at the same trap and refrain from conversation 

and distraction until the times are recorded in the field book. Alternative 

solutions for recording the tie-off times include having a dry assistant 

on land or using a voice recorder worn around the neck.

Bedload Traps Ready for Sampling

The authors have used bedload traps in a variety of settings. In figure 

37, we provide an overview of different sampling sites with bedload 

traps installed in the streams at the beginning of the highflow season, 

ready for sampling.

Emptying Bedload Traps—Sample Interval End

After the sampling time interval is complete (generally one hour), 

empty the traps. Typically, samples are retrieved quickly from the 

back of the nets while the traps remain installed on their ground plates. 

Emptying the traps in the stream and retying them for the next sample 

allows continuous sampling throughout a day. Bedload traps may be 

removed from their ground plates and emptied on the bank if it becomes 

too dark to see or when large samples are collected at a site without a 

footbridge.

Without Footbridge

If traps are emptied in the stream, operator 1 grabs the net with 

gloved hands and strokes and shakes it to ensure that all material inside 

the net accumulates at the end (this usually happens by itself even when 

using long nets). The operator should also rub off any fine debris that is 

caught in the net, particularly close to the frame where most of the water exits the net. Long pieces of woody debris inside the net may need 

to be broken in place. Operator 1 holds the end of the net closed with 

one hand below the clothesline knot, and with the other hand, grabs 

around the net on the frameward side of the collected bedload (fig. 38). 

The bedload sample is thus contained between the two points of the 

pinched-off net. Operator 2 checks the time (the minutes of the hour) 

and unties the cotton cord. Both operators then maneuver the sample 

into a bucket brought by operator 2. Operator 1 releases the end of the 

net and shakes the contents into the bucket. The net is briefly released 
44 USDA Forest Service RMRS-GTR-191. 2007

Figure 37—Four bedload traps installed in relatively small streams: East St. 

Louis Creek, CO (a) and Little Granite Creek, WY (b). Six bedload traps 

installed at a pool exit/riffle on the upstream side of the footbridge at Halfmoon Creek, CO (d), and six bedload traps installed across the head of a 

point-bar (e). Five bedload traps installed in a pool exit at Cherry Creek, OR 

(e), where one of the traps had to be installed further upstream than the 

others because a large rock made installing that trap in line with the others 

impossible. Six bedload traps installed at the upstream side of a footbridge 

at Hayden Creek, CO (f).

a b

c d

e f
USDA Forest Service RMRS-GTR-191. 2007 45

into the current (untied) to flush organic debris out of the trap before 

being retied and tossed back into the stream to start another sample. To 

reduce wading and to speed up the process, operator 2 can bring two 

buckets along to empty two neighboring traps in the same trip. The 

sequence in figures 39 a through d shows several steps to emptying 

bedload traps. The process described above is repeated until all traps 

are emptied and retied. It is important to use buckets with numbers that 

match the trap numbers.

Because the entire process of untying, emptying, and retying typically takes only about one minute per trap, the end of a sample and the 

start of the next sample are assumed to occur at the same time. Thus, 

only one value is recorded to denote the time of both untying and retying. However, if the process of untying, emptying, and retying cannot 

be done within a minute, the times for untying and retying need to be 

recorded separately. During the highest flows, sampling times may last 

for only a few several minutes, thus both tying and untying times should 

be recorded to the second for higher accuracy.

With Footbridge

We recommend using a footbridge together with bedload traps that 

have 5.5-ft long nets. The footbridge not only reduces the amount of 

treading and scouring on the bed, but is also a safer means of operating 

the traps at high flow when the wadeability limits are approached. A set 

of buckets appropriately labeled for each trap should be set out on the 

bridge next to the traps (fig. 40). We often put a few inches of water in 

the bottom of each bucket to help prevent the wind from blowing them 

off the bridge. Operator 1 stands in the stream upstream of the bridge, 

Figure 38—Operator 1 holds the end of the net above and below the accumulated debris while operator 2 unties the net.
46 USDA Forest Service RMRS-GTR-191. 2007

Figure 39—Emptying bedload traps in the stream without a footbridge: grabbing the net (a), untying it (b), emptying the net into a bucket (c), and retying 

the net (d). Note: the persons in the right of each photo should be wearing 

wading belts!

Figure 40—Footbridge with buckets ready for emptying bedload traps.

a b

c d
USDA Forest Service RMRS-GTR-191. 2007 47

grabs the net, and ensures the entire catch has moved toward the rear. 

He/she then grabs the end of the net above and below the sample mass 

and lifts the net end out of the water and onto the bridge. Operator 2 

crouches on the footbridge, unties the net, and shakes the contents of 

the net into the bucket held slightly above the water by operator 1. The 

bucket is lifted onto the bridge and the net is retied (see the sequence of 

tasks shown in fig. 41). As the net is released back into the flow, operator 1 checks the traps for good contact with the ground plates. Operator 

2 checks and remembers the tie-off time to record it later. When all 

traps are emptied, the buckets are removed from the bridge for sample 

processing.

The process of emptying and retying the net for the next sample 

occurs virtually in one step. Thus, bedload traps can collect samples 

back-to-back throughout the day and provide a continuous, hourly 

record of gravel bedload transport over the course of a workday. To 

prevent the nets from overfilling, sampling times must be reduced when 

transport volumes of organic debris and/or bedload are high.

Figure 41—Emptying bedload traps from a footbridge. Operator 1 grabs the net 

above the accumulated bedload (a) and lifts it out of the water and places the 

end of the net with the accumulated bedload onto the bridge (b). Crouching 

on the bridge, operator 2 unties the net and dumps the contents of the traps 

into a 5-gal bucket that operator 1 holds between himself and the bridge (c). 

Operator 2 places the sample bucket onto the bridge and reties the net (d).

a b

c d
48 USDA Forest Service RMRS-GTR-191. 2007

Removing Bedload Traps From Their Ground Plates

In some instances, traps should not be emptied in the stream but 

rather removed from the ground plates, carried to the bank while the 

net is still tied, and emptied into the correspondingly numbered buckets 

on the bank. There are several instances where this may be necessary: 

1) When working at a site without a footbridge, it can be difficult to 

empty large volume samples into buckets while the traps are fixed to 

the ground plate, particularly when using short 3-ft nets; 2) When transporting heavy buckets while wading at high flow, there is less danger of 

losing a sample when carrying traps back to the bank with the net still 

tied; 3) When taking a discharge measurement or Helley-Smith samples 

at the bedload trap cross-section in the middle of the day; and 4) When 

it is too dark to see after the last sample of the day. In this case, samples 

can be kept in the nets on the banks over night.

To remove the trap from the ground plate: 1) Remove the garden 

hose; 2) Unscrew and remove the shaft collars that hold down the straps; 

3) Pull the trap off the ground plate, ensuring that the plate does not lift 

off the bed; 4) Place the PVC pipes over the stakes and fasten them 

with a shaft collar; and 5) Place the garden hose pieces over the stakes. 

Step 3 is shown in figures 42a through c and step 4 in figures 42d and 

e. Note that bedload traps should always be removed from their ground 

plates when operators are absent from the site for any length of time, 

particularly overnight when flows are typically highest in snowmelt 

regimes. Unattended sampling can overfill the traps and large woody 

debris might wedge in front of the traps. Sampled bedload volumes 

would be invalid in both cases, and damage might occur to the traps 

and/or the surrounding stream bed.

Sampling Time

Typical Sampling Time of One Hour or Longer

Gravel bedload particles move irregularly and particle sizes at the 

threshold of motion move infrequently. In an attempt to include irregularly and infrequently moving particles in the sample, bedload traps 

were specifically designed to facilitate long sampling times.

The typical sampling time for bedload traps at low and moderate 

transport rates is 1 hour. During this time, the operators can attend to 

other tasks, such as processing and bagging the previous hour’s samples. 

At the lowest flows, 1-hour samples collected in bedload traps might 

comprise a few small gravel particles plus a few cups of organic debris. 

If transport rates are this low, sampling times can even be extended over 

several hours provided the flow is stable. However, the traps must be 
USDA Forest Service RMRS-GTR-191. 2007 49

checked periodically to confirm the net is neither filling nor clogged 

with organic debris. At moderate flows, 1-hour samples may comprise a 

cupful of gravel plus several quarts (several liters) of organic debris. As 

flows increase further, sample volume can amount to several gallons. One 

needs to check that the collected volume does not exceed the capacity of 

the sampler and reduce sampling times if needed (next section).

Figure 42—Removing a bedload trap from its ground plate (see text for explanation).

a b c

d e
50 USDA Forest Service RMRS-GTR-191. 2007

Shorter Sampling Times

At high transport of organic material or gravel

A 3-ft long net has a total volume of about 6.4 gal (25 liters) or 110 

lb (50 kg) of gravel. Assuming a 40 percent fill level does not compromise the sampler efficiency (Emmett 1981), the bedload trap capacity is 

about 2.7 gal (10 liters) or 44 lb (20 kg) of gravel. Mountain gravel-bed 

streams with forested watersheds carry large amounts of coarse organic 

debris, particularly on the first rising limb of the snowmelt highflow. 

This material is collected in the bedload traps and may fill the net beyond capacity in less than an hour, even if gravel transport is miniscule 

(fig. 43). The largest sample volume collected in a 1-hour sample comprised 4.25 gal (16 liters), of which 0.6 cups (0.15 liters) were gravel. To 

avoid overfilling the net, which causes a reduction in sampling efficiency, sampling time must be shortened (Bunte and Abt 2005). Although 

longer nets can hold larger sample volumes, they are not necessarily 

suited for a longer sampling time because collected sample volumes 

may become unmanageably large.

Figure 43—Inappropriately large volumes of organic debris of 

up to 4 gal (approx. 16 liter) collected during a 1-hour sample 

in the bedload traps (upper left bucket).

Sampling time should also be reduced during times of high gravel 

transport. The highest measured transport rates occurred at flows > 130 

percent of bankfull. At one site, bedload sheets formed during high 

flow and an entire layer of the bed about 10 cm thick was in motion. A 

6-minute sampling time filled the 3-ft net (0.9 m) with 20 kg of gravel 

and cobbles (fig. 44). At another site, a 10-minute sample accumulated 
USDA Forest Service RMRS-GTR-191. 2007 51

24 kg of gravel and cobbles in a 5.5-ft net (1.65 m). Note that while the 

amount of organic debris is more or less evenly distributed across the 

stream, gravel transport rates can vary greatly over the stream width 

(Chapter 3, fig. 20).

In rapidly rising flow

Decisions regarding sampling time must be made in the field in response to current conditions of flow and sample volumes. A 1-hour 

sampling time may be too long when flow is rapidly rising. Discharge 

can easily increase by 10 to 15 percent within an hour on the daily 

rising limbs during snowmelt highflow. If the relationship between 

transport and discharge follows a power function with an exponent of 

eight or greater (typical of sampling results from bedload traps), a 10 

to 15 percent increase in flow causes a doubling or tripling of the transport rate. A fast rise in transport rates during rapidly rising flow speaks 

against collecting full 1-hour samples because correlating transport 

rates with flow may be difficult. However, gravel transport in mountain 

streams often has a delayed response and does not immediately rise 

with increasing flow. In this case, a longer sampling time can be maintained. A compromise is to reduce sampling time to 30 or 45 minutes 

Figure 44—Twenty kg of gravel and cobbles were collected in one of the bedload traps over a 6-minute 

sampling time when bedload sheets moved along 

the stream bed at Little Granite Creek, WY, in 

1999.
52 USDA Forest Service RMRS-GTR-191. 2007

during rapidly rising snowmelt runoff, but shorter sampling times may 

be necessary during flashy rainstorm events.

Effect of Sampling Time on Measured Gravel 

Transport Rates

In coarse-bedded mountain streams, short-term transport rates 

typically fluctuate even during constant flow, and transport rates larger 

than average occur less frequently than those smaller than average (for 

example, Einstein 1937, Hubbell 1987). Due to this skewed distribution 

of short-term transport rates, sampling time has an effect on measured 

transport rates. This effect was quantified in an empirical study in which 

bedload traps were deployed for sampling intervals of 2, 10, and 60 

minutes (Bunte and Abt 2005). At bankfull flow, transport rates estimated 

from 2-minute sampling were only one-sixth of the rates estimated using 

1-hour sampling. The reason is that short-term samples are less likely 

to catch the infrequently occurring large transport incidents. The 

measured transport rates are, therefore, comparatively low and under 

predict the mean rate. The opposite effect occurred at low flows (about 

30 percent of bankfull) when small gravel particles were just starting 

to move. Deployment over 2 minutes produced more samples with zero 

transport than a longer deployment. However, if gravel was contained 

in a 2-minute sample, the computed transport rates were higher than 

for samples obtained from longer deployment because the “catch” is 

assigned to a short sampling duration. Even when zero-values were 

included in the computation, transport rates estimated from 2-minute 

sampling times were four times higher during marginal transport than 

those from 1-hour sampling. Transport rates did not vary markedly 

between 10-minute and 1-hour sampling. Two-minute sampling 

resulted in a statistically significant flatter rating curve compared to 

longer sampling.

Ground Plate Readjustment During Sampling 

and Removal After the Highflow Season

Checking and Readjusting

Over the course of the highflow season, aggradation and/or degradation 

of the bed may occur around ground plates. This presents a problem to 

accurate sampling. If the bed scours, the plate can become pedestalled 

with bedload moving on either side of the trap. Eventually, a gap may 

form underneath the plate that permits sediment to pass unsampled 

below the trap. Aggradation of the bed presents the opposite problem. 

As the bed near the traps aggrades, the bed plane becomes higher than 

the surface of the ground plate. Simply scraping the ground plate free of 
USDA Forest Service RMRS-GTR-191. 2007 53

overlaying material causes the plate to become located in a depression 

that is constantly filled with bed material falling down the inclined 

sides. In this situation, the bedload trap will collect material from an 

area that is larger than the width of the trap opening and overestimate 

the true transport rate. However, if the entire depth of the aggraded 

material remains mobile, and the material is transported in the form of 

bedload sheets, the ground plate should stay in its original position to 

yield accurate results.

In a degrading bed

To prevent incorrect sampling results caused by bed degradation, 

ground plates must be lowered to match the scoured bed plane. To 

lower a plate, pull it off the stakes and remove particles from under 

the plate until the plate matches the lowered bed plane. Reset the plate. 

As was done when the plate was initially placed on the bed, fill the 

interstitial spaces below the plate with smaller gravel. If wading behind 

the bedload traps scoured a trench where headcuts start to migrate 

between the traps, fill the trench with cobbles to prevent headcutting 

that pedestals the plates.

In an aggrading bed

To adjust the ground plate position in an aggrading bed, place a second ground plate on top of the bed material that covers the original 

plate. To do this, simply push the second plate down the stakes of the 

original plate. Bed disturbance is minimized by placing a second plate 

over the first. If the aggraded location experiences degradation at a later 

time in the highflow season, remove the second plate and use the original plate. An aggrading bed may be associated with the movement of 

bedload sheets. In this case, position the plates at the lower boundary of 

the moving bedload to sample the entire moving layer of bedload.

Removing Ground Plates and Stakes Using a Lever

At the end of the study, or possibly during ground plate placement 

(Chapter 3) or readjustment, the stakes driven into the streambed need 

to be removed. Sometimes, the stakes holding the plates can simply be 

removed by hand. At other times, a stake will not budge and a lever may 

be needed to pull it out (for example, use a 6-ft [1.8 m] long board or 

steel pipe). To grab the stake, fasten a 6-mm nylon rope loop to it using 

a prusick knot (see inset of fig. 45). Tie a webbing loop around the lever 

and connect it to the rope loop with a carabineer. Set one end of the 

bar on a rock on the stream bottom 2 to 3 ft away from the stake to be 

pulled. Place the webbing loop a few inches offset from the top of the 

stake such that when the lever is pulled up, the webbing loop sits vertically above the stake where it exerts the strongest pull on the stake.
54 USDA Forest Service RMRS-GTR-191. 2007

Ancillary Measurements

Bedload transport sampling is usually accompanied by a variety of 

field measurements including:

Hydraulic measurements such as stage, flow velocity, flow depth, 

and discharge;

Bedload sampling of sediment smaller than 4 mm using a HelleySmith type sampler;

A site survey, typically requiring longitudinal profiles along the banks 

and the thalweg, and one or several cross-sectional profiles; and

Bedmaterial sampling, typically comprising a surface pebble count 

covering the reach and two or more subsurface samples at representative locations.

For more information on how to conduct these measurements, consult 

the literature on field methods, for example, Guy and Norman (1970); 

Dackombe and Gardiner (1983); Gordon and others (1992); Harrelson 

and others (1994); Ryan and Troendle (1997); Edwards and Glysson 

(1999); Bunte and Abt (2001a, b); and Ryan (1998, 2005).

Some of these measurements, such as surveying and bedmaterial 

sampling, are performed before or after the highflow season and are 

not discussed here. Hydraulic measurements and sampling fine-grained 

•

•

•

•

Wooden or

metal bar

~ 6 ft long

thin, nylon rope with

prusick knot on the stake

webbing loop

and carainer

stake

Figure 45—Using a wooden or metal lever to pull a stake from the stream bottom (left). Prusick (or prusik) knot (right, inset). Copied from http://storrick.

cnchost.com/VerticalDevicesPage/Ascender/KnotPages/KnotPrusik.html
USDA Forest Service RMRS-GTR-191. 2007 55

bedload using a Helley-Smith type sampler occur together with bedload 

trap sampling and can affect transport rates measured with the bedload 

traps. Similarly, measurements of stage, discharge, and Helley-Smith 

samples can be affected by the presence of bedload traps. Although 

mutual interactions between these measurements cannot always be 

completely avoided, operators need to be aware of the interactions and 

try to minimize them.

Stage

Two or more stage recorders and at least two staff gages should 

be installed per site. The water surface elevation often follows different temporal patterns within a reach due to secondary flows, standing 

waves, and local scour and fill of the bed. Locations less than 30 ft (9.1 

m) apart can produce different hydrographs. Recording stage at several 

locations brings attention to this problem and provides an opportunity 

to adjust the measured hydrograph. Stage averaged over right and left 

bank, for example, may be a better indicator of flow than a single stage 

record, or a stage record from a given location may be representative of 

discharge for only a range of flow or part of the highflow season.

Although discharge associated with each sample might eventually 

be computed from the combined hydrographs of all staff gauges and 

water level recorders, at least two staff gages should be read and manually recorded with each bedload trap sample. Stage readings inform 

the operator of current flow conditions and provide a backup record in 

case water level recorders malfunction. For convenience, staff gauges 

should be installed close to the bedload trap site (fig. 46). However, the 

Figure 46—Bedload trap sampling site with footbridge. Staff gauges and electronic water level recorders (inside the white PVC tubes) were installed upstream near both banks, conveniently close to the bridge, but were affected 

by the hydraulic resistance of the bedload traps.
56 USDA Forest Service RMRS-GTR-191. 2007

presence of several bedload traps in a cross-section can pond the water 

upstream of the traps and elevate the water surface. This effect becomes 

more pronounced with increasing flow until bedload traps are inundated. 

Further inundation decreases the ponding effect. The upstream distance 

of ponding is short for steep gradient (step-pool) streams and typically 

does not extend beyond an upstream step. In lower gradient streams 

without steps, ponding of up to 0.5 inches (13 mm) was observed at a 

distance of 15 ft (4.6 m) upstream when bedload traps were just inundated 7

. The presence of ponding requires a trade-off: for convenience 

and ease of access, particularly if dense vegetation makes walking 

along the banks difficult, staff gauges should be close to the bedload 

trap cross-section. However, in order to be unaffected by the elevated 

water surface created by the presence of the bedload traps, staff gauges, 

and particularly water level recorders, should be well out of the way.

Operators should check whether ponding affects stage readings by 

comparing stage readings with and without bedload traps installed. If 

the presence of bedload traps increases the stage readings, a correction 

factor can be developed from the difference in stage readings with and 

without bedload traps in place, and that factor is applied to correct readings of stage for times when bedload traps are installed. Because the 

ponding effect decreases as bedload traps are overtopped by flow, the 

correction factor first increases with flow and then decreases for further 

increasing flow after inundation of the traps.

Ponding increases with the number of traps installed per cross-section. 

However, the trap spacing (Chapter 3) thought necessary to characterize the lateral variability of transport rates should not be increased to 

prevent ponding. The effects of ponding on stage readings can be corrected (see above) or avoided if gages are installed sufficiently far away 

from the bedload traps.

Discharge

Unless the study site is close to an agency-operated gauging station 

that provides a continuous discharge record, discharge measurements 

must be taken at different flows to determine a stage-discharge relationship. Discharge is ideally measured in a symmetrical cross-section. 

Such a cross-section may not be close to the bedload trap site, and the 

cross-section may become difficult to wade at high flow. If the study 

site has a footbridge, it may be used to take discharge measurements, 

7

 The authors have not evaluated if the increased water surface upstream of the bedload 

traps and the associated decrease in the local stream gradient and flow velocity affect 

measured bedload transport rates. If ponding decreased the bottom shear stress in 

front of the traps, a decrease in transport rates of fine gravel particles could be possible.
USDA Forest Service RMRS-GTR-191. 2007 57

particularly if flows are deep and difficult to wade. Taking discharge 

measurements close to the bedload traps provides an opportunity to 

compare local flow hydraulics to local transport rates. However, if 

discharge measurements are taken at the footbridge, remove the traps 

during the measurements because the presence of the traps alters the 

vertical velocity profile. Measure discharge at the downstream side of 

the bridge because the presence of the ground plates increases the nearbottom flow velocity (Chapter 2). A distance of 4 to 5 ft (1.2 to 1.5 m) 

between the ground plates and the velocity verticals should ensure that 

velocity profiles have readjusted to the bed.

The most convenient time to take discharge measurements from the 

bridge is at the start of each work day (before the traps are installed) 

or at the day’s end (after the traps have been removed). However, taking discharge measurements at the beginning or end of the day may 

diminish the range of flows at which samples can be collected with the 

bedload traps.

Helley-Smith Samples

A Helley-Smith sampler may be used with bedload traps if the study 

is interested in collecting pea gravel and sand, or if samples collected 

with a Helley-Smith sampler and bedload traps are to be compared 8. 

A deployed Helley-Smith sampler from a footbridge may also permit 

sampling at flows that are no longer wadeable. However, finding a time 

and place to deploy the Helley-Smith sampler can pose a challenge. One 

option is to take Helley-Smith samples at the beginning or end of the 

day when bedload traps are not installed. Alternatively, Helley-Smith 

samples can be taken during bedload trap operation. To avoid effects of 

spatial differences in sampler location, as well as destabilization of the 

bed upstream of the sampler, bedload traps and the Helley-Smith sampler could be deployed within the same cross-section. The advantage is 

that both samplers can be deployed at the same time (provided care is 

taken not to step in front of the traps). However, sampling does not occur with the typical equal-width spacing if the Helley-Smith sampler is 

set into the spaces between the bedload traps. One needs to balance the 

possibility that a somewhat uneven spacing causes inaccuracies in the 

computed transport rates against the added labor of separate bagging 

needed for sampling at uneven spacing. Also, given the close proximity 

of the two samplers, they may affect each other’s flow hydraulics and 

thus their sampling efficiencies.

8

 Transport rates computed from a Helley-Smith sampler at all field sites were routinely 

found to be two to four orders of magnitude higher than those for bedload traps at 

moderate flows (about 50 percent Qbkf). At bank full flow, transport rates from both 

samplers become similar (Bunte and others 2004b).
58 USDA Forest Service RMRS-GTR-191. 2007

If a Helley-Smith sampler is deployed in a separate cross-section, 

sampling should be downstream of the bedload traps. Effects of upstream wading (when setting the traps) are avoided if Helley-Smith 

samples are collected before bedload traps are installed upstream. When 

using a footbridge, Helley-Smith samples should be collected at the 

downstream side. The streambed here is not, or only slightly, affected 

by wading (during discharge measurements). Alternatively, one might 

consider deploying the Helley-Smith sampler onto each ground plate 

for several minutes per plate 9.

Sampling Schemes and 

Required Operator Hours

The bedload trap researcher must decide when samples are to be 

collected during a day, how long sampling is to extend over the highflow season, the number of daily and total samples, as well as the flow 

and transport rates to be sampled. These choices define the sampling 

scheme and depend on the study aim, as well as the degree of temporal 

and spatial variability of bedload transport.

Snowmelt regimes typically produce diurnally fluctuating flows. For 

Rocky Mountain streams with basin sizes of 3 to 23 mi2 (8 to 60 km2), 

flow decreases throughout the morning and reaches its daily minimum 

between 12 and 2 p.m. (12:00 to 14:00). Flow increases rapidly during 

the (late) afternoon and peaks between 7 and 10 p.m. (19:00 and 22:00). 

The differences between daily minimum and maximum discharge can 

be large: jumps from 75 to 125 percent or 100 to 150 percent of bankfull are possible within a few hours. Such changes in flow (Q) cause 

not only an immediate response in transport rates, but may also lead 

to high and variable transport throughout peak flow and the falling 

limb. Transport rates (QB) thus follow the hydrograph only approximately. Daily QB vs Q relationships typically have hysteresis patterns 

that change over the course of days and reflect changing conditions of 

sediment supply. Sequential hourly measurements of bedload transport 

between mornings and evenings typically show a rather well-defined 

QB vs Q relationship; however, repeatedly measured transport rates for 

the same discharge can vary over two orders of magnitude between 

days. Generally, the QB vs Q relationship is erratic during the lowest 

flows and becomes better defined during the highest flows.

9

 Placement of the Helley-Smith sampler onto the ground plates with bedload traps removed provides a better sampling result than obtained from setting the Helley-Smith 

sampler directly onto the stream bed (Bunte and others 2005).
USDA Forest Service RMRS-GTR-191. 2007 59

Understanding bedload transport processes and evaluating how 

a stream responded to disturbance or mitigation effects are common 

study aims. Common approaches may involve establishing a bedload 

transport rating curve, analyzing transported particle sizes, computing 

annual load, and comparing measurements over time. Accurately 

determining rating curves, bedload particle sizes, and annual loads 

requires representative sampling that takes into account the daily 

and seasonal variability of bedload transport. Ideally, the complex 

relationship between bedload transport rates and flow in a snowmelt 

highflow regime would be characterized by sampling all flows to 

produce a continuous temporal record of bedload transport rates over 

the highflow season. However, such a sampling approach constitutes 

a large logistical effort with several crews and is usually not feasible. 

A compromise is to sample during daylight hours, use one crew, and 

schedule sampling to include daily low and high flows as well as part 

of the daily rising and falling limbs. The aim is to approximate the 

information obtained from a continuous record as best as possible while 

keeping a logistically feasible sampling scheme.

Intensive 1-Hour Sampling Throughout a Day

Transport rates around peak flows are usually highest and most important for estimating annual load. Daily field work should therefore be 

scheduled to sample those rates. However, including peak flows in the 

evening, daily falling limb of flow in the morning, and low flow around 

noon makes for a long field day that lasts from 9 or 10 a.m. (9:00 or 

10:00) to 8 or 9 p.m. (20:00 or 21:00) when it becomes too dark to work 

without artificial lighting.

Bedload traps that are emptied at 1-hour intervals and collect backto-back samples lend themselves to providing time series records of 

gravel transport. Two people can operate six or more bedload traps 

at one field site, and about eight hourly bedload trap samples can be 

collected in an 11- to 12-hour day, plus a few Helley-Smith and/or discharge measurements. The time between sample collections is used to 

process samples (Chapter 5) collected in the previous hour, take stage 

readings, and take notes.

Collecting eight consecutive 1-hour samples still leaves much of the day 

unsampled, particularly on the early falling limb during which transport 

rates may be high and variable (for example, Bunte 1996, Gottesfeld and 

Tunnicliffe 2003). High transport rates were also reported for the falling 

limbs of storm events (for example, Lisle 1989, Rickenmann 1994). These 

findings suggest that trends and relationships observed during the beginning of an event (or day) may change over time. In order to obtain at least 
60 USDA Forest Service RMRS-GTR-191. 2007

qualitative information of bedload transport during unmeasured 

times, one should consider installing an automated device such as a 

hydrophone.

Covering the Highflow Season

Daily hysteresis patterns of the QB vs Q relationship typically change 

over the highflow season, and relationships between transport rates and 

flow typically differ between the rising and falling limb of the highflow 

season. Sampling should therefore not be limited to a partial season. 

Moreover, extending sampling over (almost) all days of the highflow 

results in a large sample size. Rating curves can be better defined and 

segregated into increments that best describe specific times or flows 

when more data points are available. Bedload samples that are collected 

over the entire highflow season—almost as a time series—are particularly important when using a summation approach to compute annual 

loads (Chapter 5).

Some methodological studies require neither sampling over the entire 

highflow season nor keeping a detailed daily record. For example, sampling simply at different transport rates suffices if the study compares 

two devices that each collect instantaneous transport rates or compares 

the effects of different deployment times. In this case, the stream simply 

serves as an outdoor flume.

Representative Sampling for Rating Curve and 

Annual Load Estimates

Intensive sampling that covers most days of the highflow season still 

may not collect samples at times or flows most crucial for establishing rating curves or computing annual load. In order to improve the 

representativeness of samples, operators should try to understand the 

temporal patterns of flow and transport and on that basis, ensure that 

sampling includes all flows and the highest transport rates.

For example, an understanding of the temporal variability of bedload transport at the field site allows operators to better identify times 

that are most important to sample and times that could be skipped with 

minimal loss of important information. In order to gain some insight 

into the temporal patterns of an ongoing QB vs Q relationship during 

field work, an operator could visually estimate the gravel volume collected at one of the bedload traps and assign it to one of five classes 

that each increase by approximately one order of magnitude: 1) A few 

small particles, 2) 1 tablespoonful, 3) 1⁄2 cup, 4) 1 quart, or 5) 1 pail. 

These volumes could be plotted in the field book versus stage or over 

a hydrograph.
USDA Forest Service RMRS-GTR-191. 2007 61

When establishing a rating curve, operators should also ensure that 

a sufficient number of samples are collected within all increments of 

flow. The distribution of flows and gravel transport rates over a snowmelt highflow season is typically skewed. Low and moderate flows and 

transport rates occur over much longer portions of the highflow season 

than peak flows and the highest transport rates. Sampling only between 

8 a.m. and 4 p.m. misses peak flows in a snowmelt regime. Keeping a 

list of sampled flows and trying to sample all increments of flow evenly 

will help increase the representativeness of the collected data.

Collecting samples at the highest transport rates is crucial when the 

study aim is to compute a rating curve and annual load. The day of peak 

flow (or a few hours of it) may easily transport 50 to 80 percent of all 

gravel transported over the highflow season, although the highest transport rate may not coincide exactly with peak flow. Inaccuracies in the 

prediction of the maximum transport rates cause large errors in annual 

load predictions. Sampling the highest transport rates—those that occur 

infrequently but carry the overwhelming majority of all gravel moved 

within a highflow season—requires special effort. The question “Were 

the highest flow and transport rates measured?” is vexing because one 

normally does not know until a day or two later, when stage has receded, whether the highest flow and transport rates have occurred.

The statistical evaluation of the correlation between bedload transport rates and discharge (in other words, the QB vs Q relationship) 

requires absence of serial correlation between consecutive samples. 

Sampling at a preset daily time, for example, 8 a.m. or 5 p.m., may 

satisfy this requirement but restricts sample collection to either the daily falling or rising limbs of flow. Most likely, morning and afternoon 

samples produce different bedload transport rating curves. Sampling at 

random times throughout the highflow season also satisfies the statistical requirement of regression analysis, but appearing at the site at truly 

random times is logistically infeasible. Even if the randomness referred 

to daylight hours only, taking isolated measurements is not a good use 

of time when using bedload traps that take time to set up and take down, 

while back-to-back sampling easily produces a time series of measurements. In addition to obscuring the complex temporal patterns of the QB

vs Q relationship over a day and the course of the highflow season, the 

collection of isolated samples leads to a number of other problems. The 

lack of a daily time series takes away an opportunity for quality control: 

one cannot evaluate whether a specific sample might be an outlier in 

an otherwise typically well-defined QB vs Q relationship of sequential 

1-hour measurements. Collecting isolated samples also results in a far 

lower sample size which, given the high variability of transport rates, 

leads to larger scatter in the computed bedload transport rating curve 

than semi-continuous sampling. One can always elect not to use certain 
62 USDA Forest Service RMRS-GTR-191. 2007

data for a rating curve if they do not meet some preset requirements, but 

to do this evaluation, one needs to understand the temporal patterns of 

transport as thoroughly as possible.

Effort in Time and Labor for a Field Season

Intensive sampling requires a large work effort—both in the field 

and in the lab—that needs to be budgeted. The most effective use of 

budgeted time is probably achieved by employing a two-person crew 

that works 11- to 12-hour field days. Working repeated long field days 

requires suitable nearby accommodation (for example, a rented house). 

At the highest flows when sample volumes become too large to be 

processed between 1-hour samples, a third person may be needed temporarily to help with sample bagging if the 1-hour sampling interval is 

to be maintained (even though the actual sampling time may need to be 

reduced).

The time and effort needed over the field season increases with the 

duration of the highflow, the magnitude of the highest flows, and the 

size and number of the samples collected. Table 1E in Appendix E 

provides estimates for work hours required for a 2-week field season 

with low highflows and a 4-week field season with large highflows. 

Two person crews working 12-hr days for 6 to 7 days per week over a 

4-week highflow period accrue a total of 624 work hours. A third person 

needed temporarily during the highest transport rates or when installing 

a footbridge adds another 30 to 40 hours. Washing samples that exceed 

the capacity for field processing requires extra time, up to 100 hours, 

after the field work (Chapter 5). Half the field work hours (312 for a two 

person crew) are required for a short, 2-week highflow season with low 

peak flows and no extra time for help during the highest flows or for 

sample washing. The pre-field work hours of scouting, site selection, 

and setting up the field site, as well as the post-field work tasks of taking 

down a site and storing and maintaining gear, do not depend on the duration or magnitude of the highflow and amount to about 170 hours for 

a two-person team. Not included in this budget is the time needed for 

pre and post highflow measurements such as surveying the site, sampling the surface and subsurface bed material, and traveling to the field 

site.

Intensive sampling over a field season can produce a large number of 

samples. A daily average of seven samples adds up to almost 200 samples over a 4-week highflow season. This amounts to 1000 bags if five 

traps were used. About half as many samples are collected in a 2-week 

highflow season. The time for sieving those samples is estimated at 

260 hours for a 4-week season with high discharges and large samples, 

whereas only half of those hours may be needed for a 2-week highflow 
USDA Forest Service RMRS-GTR-191. 2007 63

season with low peak flows and fewer large samples (See Chapter 5 for 

effort and time of lab sieving). Thus, depending on the magnitude and 

duration of the highflow season, the total number of work hours may 

range between 600 and 1200.

Operator Safety and Comfort

Operating bedload traps can be challenging, with a variety of tasks 

to be accomplished quickly and accurately in cold, wet, and sometimes 

dangerous situations. Wading safety was discussed in Chapter 3 and 

Appendix D. Additional suggestions for keeping operators functioning 

at their best are discussed in Appendix F.
64 USDA Forest Service RMRS-GTR-191. 2007
USDA Forest Service RMRS-GTR-191. 2007 65

Chapter 5.

Sample Processing and 

Computation of Transport Rates

Samples collected with the bedload traps need to be processed before 

transport rates can be computed. Processing includes removal of organic debris, bagging, drying, and sieving. Organic removal and bagging 

typically occurs in the field and drying and sieving at home and in the 

lab. The importance and effort required for sample processing should 

not be underestimated. Roughly 1⁄2 to 3⁄4 of the daily sampling effort 

must be dedicated to sample processing.

Removing Coarse Organic Material

Amounts and Composition

In forested watersheds, a large proportion of the sample volume collected with bedload traps may consist of coarse organic material (fig. 47). 

The amount of organic debris increases strongly with flow, particularly 

during the first rising limb of the snowmelt hydrograph, when organic 

debris comprises up to 99 percent of the entire moist bulk sample volume (Bunte and Swingle 2003a). The largest volume of organic debris 

Figure 47—4.5 gal of organic debris 

was collected in a 19-minute bedload trap sample.
66 USDA Forest Service RMRS-GTR-191. 2007

collected during a 19-minute sample in a bedload trap was 4.5 gal (17.5 

liters). This sample volume filled the net beyond capacity. The sampling 

time was too long given the stream conditions (fig. 43).

In Rocky Mountain streams, organic debris includes whole or partial 

cones from conifer trees, conifer tree needles, willow and alder leaves, 

twigs and sticks, wood and bark pieces, grass, moss, algae, elk and deer 

droppings, aquatic insects, and caddis fly cases. The debris, particularly 

at high flows, may also include small fish, drowned bugs, ants, and 

earthworms.

Advantages of Field Processing

Separating the organic debris from the inorganic sediment is quite 

labor intensive, particularly when the organic fraction is large. Ideally, 

this is done by washing the lower density organic material from the 

higher density bedload immediately after the sample is collected during 

the 1-hour interval between trap servicing. Advantages of field processing are that the stream can be used for the large amounts of water needed 

for washing, and the organic material collected can be recycled into the 

stream. Washing at the stream site also avoids having to haul voluminous and heavy samples back to the lab, to clean up a considerable mess 

in the lab (do not try washing samples in a kitchen sink), and deal with 

smelly, decaying organic material after sitting for weeks in the bag.

As a stopgap measure, if it’s raining hard or the amount of organic 

material is too large to be processed during the time between two samples, the collected material can be stored, unprocessed, in labeled Ziploc 

bags. However, bagging unprocessed samples is not recommended as 

the standard procedure because of the considerable effort that goes into 

labeling, filling, transporting, and storing a large number of heavy, leaking Ziploc bags filled with organic debris and gravel. In practice, one 

can wash some samples on the spot and store others. Early evening 

samples collected during rapidly rising flow are likely to contain the 

largest amount of organic material. One can often catch up with washing these samples the next morning when the organic load is relatively 

low on the daily falling limb of flow.

Washing Procedure

Washing a large number of samples requires setting up a work space. 

This is the procedure we use: Sit on a camping stool in front of a 5-gal 

bucket with a metal bowel on top of it. Place three buckets on your side. 

One bucket contains water for washing the samples, one serves as a 

repository for the washed out organic debris, and one holds the bagged 

samples. Place cartons with different sizes of Ziploc bags, a Sharpie 

marker, and a field book for notes within easy reach on top of a plastic 
USDA Forest Service RMRS-GTR-191. 2007 67

tub. Place the buckets containing unwashed samples along your other 

side (fig. 48).

To separate organic material from the inorganic bedload, pour the 

entire sample (organics plus gravel) into the metal bowl placed on top 

of one of the 5-gal buckets (fig. 49). Cover the mixture with water and 

gently stir. Stirring stratifies the sample so that the relatively heavy 

gravel particles fall to the bottom of the bowl while the less heavy 

organic material remains near the top (most of the organic particles 

are too waterlogged to float). With a rubber spatula, carefully push the 

Figure 48—Washing samples in the field: the work space.

Figure 49—The sample is poured into a metal bowl for washing (a). The sample is covered in water and stirred with a rubber spatula (b). Organic material 

is decanted and discarded.

a b
68 USDA Forest Service RMRS-GTR-191. 2007

organic particles to the bowl rim and decant. Decanting into a bowl that 

fits on top of a bucket or a household sieve ensures that an inadvertently 

discarded particle can easily be retrieved without rewashing the entire 

sample. Repeat the process of covering the sample with water, stirring, 

and decanting organic material a few times to remove all organic material from the bedload sample (except for a miniscule residue with a dry 

weight that is insignificant compared to the gravel weight). The gravel 

particles remain in the bottom of the bowl (fig. 50) and are bagged in 

Ziploc bags of suitable size. Bags are labeled and collected for transport 

offsite. Wash the samples that are too large to be washed in one batch 

in smaller portions. Some samples may contain so much organic debris 

that washing in small quantities is inefficient. Place the sample (or a 

portion of it) into another 5-gal bucket, cover it with water, and stir. 

Again, the agitation allows the heavier inorganic material to fall to the 

bottom of the bucket. Remove the top layer of organic material using a 

large wire spoon (Asian kitchen supply) (fig. 51). After the volume of 

the total sample is reduced to a few liters, hand-wash the remainder of 

the sample in a bowl (see above) or bag it for later washing. The organic 

material removed from the sample is either returned to the stream or 

collected for further analysis if it is of interest to fisheries biologists and 

aquatic ecologists.

Washing After the Field Season

At some point, the number and volume of samples collected at high 

flows may exceed the capacity of what can be washed during the time 

period between two samples. Bag these samples to wash them later at 

the field site after the highflow season or in a different location (a backyard usually) using a similar setup as in the field. Washing samples in 

the lab or in a sink is not recommended.

Figure 50—Bedload particles are retained in the bowl, from a small sample (a) 

and a larger sample (b).

a b
USDA Forest Service RMRS-GTR-191. 2007 69

If the sample volume in each of the five traps amounted to four 

1-gal (3.9 liter) bags per sample, collecting seven samples results in 

140 bags for one day. Assuming large sample volumes were produced 

during 3 days, 420 bags would need to be washed. At about 15 minutes 

per 1-gal bag (3.9 liter), washing 420 samples takes a little over 100 

hours.

Drying, Sieving, Weighing, and 

Removing Mud Flakes

A brief description of sieving considerations, referring specifically 

to bedload trap samples, is provided below. See Bunte and Abt (2001a) 

for more details and general information regarding sieving and particlesize analyses for gravel samples.

Air Drying

Samples from the bedload traps contain gravel 4 mm and larger. 

Particles this size can be air-dried in their bags, which is an advantage 

considering that hundreds of bags might be collected over a highflow 

season. Small sample volumes of less than half a cup can dry indoors 

in their opened bags within a few days. If the weather is hot and dry, 

Figure 51—Washing a sample with large amounts of 

organic material using a big wire spoon.
70 USDA Forest Service RMRS-GTR-191. 2007

opened sample bags placed outside will dry within a few days (keep 

samples out of rain and prevent them from being knocked over). The 

drying process can be expedited if the samples are poured onto a flat 

surface and placed in the sun. Dry samples in the lab during cold, rainy, 

or humid weather.

Field Sieving

Large samples may be too heavy to transport from the field to the lab 

when working at a remote site. The weight is greatly reduced by sieving cobbles and large to medium gravel particles in the field. Once air 

dried, pass these particles through a 0.5 ø size template and record their 

number per size class. The weight per size class can be measured at the 

site using plastic shopping bags and an accurate hanging scale (0.1 to 

10 kg range, perhaps a finer scale as well). If field weighing is not possible, estimate the weight of particles in each size class from the number 

of particles per size class using the following regression:

mi = 0.00289 * Di

3.00 (2)

where mi is the average mass (in grams) of a particle in the i

th sieve 

size, and Di is the diameter of the square-hole sieve on which the particles were retained when using 0.5 ø size classes11. For example, the 

average mass (g) of particles retained on the 32 mm sieve is 32 3.00 *

0.00289 = 30,768 * 0.00289 = 94.7 g. Eq. (2) was derived as the mean 

of 11 sets of fluvially transported gravel collected in Rocky Mountain 

streams with basin areas of 3 to 23 square miles (8 to 60 km2). Note 

that the mass of disc-shaped or elongated particles may span a factor 

of up to about ±5 of the average particle mass per 0.5 ø size class. 

Equation (2) applies to gravels and cobbles of mainly ellipsoidal (or 

compact-bladed) shape. Exponents and coefficients of Eq. 2 showed no 

systematic variation with the bedmaterial D50 size, basin area size, or 

predominant rock type12.

Lab Sieving

Samples from the bedload traps are usually sieved in the lab using standard square-mesh sieves in 0.5 ø units13. The smallest 0.5 ø

particle size-class that can be collected with the bedload traps and a 

11 Equation 2 is not applicable for particle diameters measured with a ruler.

12 River gravels containing large amounts of exceptionally light or heavy rocks, such as 

tufa or ores, were not included in the analyses.

13 Round-hole sieves or rulers are not recommended because the resulting size distributions cannot be directly compared to those from square-hole sieves that are typically 

used for fine gravel and sand.
USDA Forest Service RMRS-GTR-191. 2007 71

3.6 mm mesh width net is 4 to 5.6 mm. However, flat particles with a 4 

mm b-axis may escape through the net because the mesh holes tend to 

have a rhomboic shape. Thus, when comparing fractional transport rates, 

the 4 to 5.6 mm class may be slightly underrepresented. Some particles 

of the size class 2.8 to 4 mm are usually present in the sample, but these 

particles are excluded from further analyses because they are not sampled 

representatively. Pour the samples that contain particles of several size 

fractions onto sieve stacks and place the stacks into a sieving machine 

(mechanical shaker) for 15 minutes. Weigh sieved size fractions and 

enter results into prepared forms. Using two shakers and four sieve 

stacks, an experienced two-person team can sieve and weigh two moderately large samples (in other words, the entire sample fits into a sieve 

stack) in 15 minutes, permitting the processing of up to eight samples 

per hour. Small samples of one or a few tablespoons in volume typically 

comprise only a few size classes and can be sieved more quickly by 

hand than by a machine, requiring 1 to 5 minutes per sample depending 

on the volume. If intensive sampling resulted in 1000 bags (7 samples/

day over 28 days = 200 samples times 4 to 6 traps) of which 700 were 

classified as moderately large, sieving would require about 90 hours 

for the large samples, 15 hours for the small samples, plus 15 hours 

for miscellaneous lab tasks. This requires a total of 120 hours each for 

two workers. The lab effort is approximately cut in half for a 2-week 

highflow with relatively low peak flows that might produce 500 sample 

bags of which 200 are moderately large.

Mud Flake Removal

The cobble and gravel bottom of some streams has a crust of hardened mud that flakes off into pieces ranging from several mm2

 to several 

cm2 in size (fig. 52). These may become mobile at the beginning of high 

Figure 52—Bedload sample containing a large quantity of mud 

flakes.
72 USDA Forest Service RMRS-GTR-191. 2007

flow and accumulate in bedload samples. However, crust pieces have 

sedimentary and transport properties different from those of gravel bedload. Whether to include them as part of the measured bedload depends 

on the study goal. We have not found a better way to remove crust 

pieces from the sample than by picking them off the sieve (which is 

painstakingly slow).

Computation of Transport Rates From 

Bedload Trap Samples

Instantaneous Gravel Bedload Transport Rates

Due to the uneven spacing of bedload traps across the stream and the 

potentially different sampling times for each trap, mass transport rates 

(QB) are computed for each bedload trap individually. To compute the 

total transport rate for an array of four traps across the stream, sample 

mass collected at trap 1 (ms1) is divided by the trap width (ws) and the 

sampling time at trap 1 (t

s1) then multiplied by the section of stream 

width represented by trap 1 (wi1). These computations are repeated for 

all four traps, and partial transport rates are summed across the active 

stream width to obtain the total bedload transport rate.

Qb =

ms1 * w1

+

ms2 * w2

+

ms3 * w3

+

ms4 * w4

ws * t

s1 ws * ts2 ws

* t

s3 ws * ts4 (3)

where ms is the sampled bedload mass in each trap, w is the portion of the stream width represented by each trap, ws

 is the width of 

the traps, and t

s

 is the sampling duration. The representative section of 

stream width for each trap typically extends from the trap in question 

halfway toward its left and right neighbors. Near the bank, the width 

boundary depends on the potential for local bedload transport at a specific flow. A stream width section with very shallow flow over grass or 

a still water wake behind a log protruding into flow is not included in 

the representative segment of stream width. Should bedload transport 

occur at such locations at higher flow, they are included in the active 

stream width. The sum of all stream sections equals the active stream 

width. The transport rate per unit width (g/m*s) is obtained by dividing 

the cross-sectional transport rate by the active stream width.

Instantaneous gravel transport rates computed from bedload trap 

samples can range over many orders of magnitude, between the very 

onset of gravel motion to fully developed gravel transport. A sample 

with one 4 mm particle in one of the traps corresponds to a transport 

rate on the order of 0.00001 g/s, and this marginal transport typically 

starts at 25 to 35 percent of bankfull flow. Near bankfull flow, transport 
USDA Forest Service RMRS-GTR-191. 2007 73

rates may range between 0.1 and 100 g/s depending on the size of the 

stream, sediment supply, and the erodibility of the bed.

Annual Gravel Load

Sampling schemes most appropriate for computing annual load 

have been discussed in Chapter 4. Either a rating curve approach or a 

summation of hourly/daily loads can be used to compute annual load. 

Accuracy and precision of both approaches increase not only with the 

number of measurements available, but particularly with an increased 

density of measurements collected during the highest flows. The uses 

and limitations of the two methods when processing bedload trap data 

are explored below.

Rating curve approach

If sampling did not cover most of the highflow season, a rating curve 

approach is likely to be the only option for computing annual load. 

However, if the rating curve is poorly defined, due to high scatter or 

sampling over a small range of observed discharges, a total transport 

estimate is likely to be erroneous and should not be attempted.

Different rating curves may have to be established for portions of the 

hydrograph, particularly for the rising and falling limbs because of 

hysteresis effects.

A rating curve estimate of annual load requires a correction for an 

inevitable statistical bias. Several methods exist to compute a correction factor (Cohn and others 1989; Duan 1983; Ferguson 1986, 1987; 

Koch and Smillie 1986), which are evaluated in Hirsch and others 

(1993). For rating curves obtained from a large number of bedload 

trap samples collected over the entire highflow season, bias correction factors computed from Ferguson (1986, 1987) and Koch and 

Smillie (1986) typically assumed a value between 1.5 and 2.0.

Annual load computed from a rating curve approach is sensitive to 

the length of the time increments used in the analyses when based on 

steep rating curves with exponents of 8 or larger (typical of bedload 

traps). For pronounced daily fluctuations of flow (with approximately 

8 hours of rising and 16 hours of falling flow), annual load computed 

from hourly time increments is larger than the load computed for 

mean daily flows (Bunte 2002).

Summation approach

A summation procedure requires an almost complete time series of 

bedload transport measurements over the highflow season. Transport rates measured (and interpolated) for hourly increments are then 

added over the course of a hydrograph.

•

•

•

•

•
74 USDA Forest Service RMRS-GTR-191. 2007

A summation approach is likely to yield a higher accuracy in annual load estimates than a rating curve approach (Bunte and Swingle 

2003b; Walling and others 1992; Walling and Webb 1981, 1988), 

but again, availability of samples for the highest flows and transport 

rates is essential.

References

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load transport rates in a coarse-bedded stream. Water Resources Research 

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Little Granite Creek, 2002: effect of sampling duration and sampler type on 

bedload transport rates and systematic variability of rating curves with basin 

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Technology Center, U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Research Station, Fort Collins, CO. 110 p.

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•
USDA Forest Service RMRS-GTR-191. 2007 75

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Research Station, Fort Collins, CO. 116 p.

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Collins, CO. 72 p.

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bedload transport between a pool-exit slope and the head of a point-bar. 

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Interagency Sedimentation Project (FISP) Conference, April 2-6, 2006, 

Reno, NV, Session 1B-2 [CD-Rom], 9 p.

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George Allen and Unwin, London, GB, 254 p.

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River Meandering. Water Resources Monograph 12, American Geophysical 

Union, Washington, DC. p. 1-50.
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Duan, N. 1983. Smearing estimate—a non-parametric retransformation method. Journal of the American Statistical Association 87: 605-610.

Edwards, T.K. and Glysson, G.D. 1999. Field methods for measurement of 

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Geological Survey, Reston, Virginia, 89 p.

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Versuchsanstalt für Wasserbau an der Eidgenössischen Technischen 

Hochschule in Zürich, III, p. 3-112, Rascher, Zürich, Switzerland.

Emmett, W.W. 1981. Measurement of bed load in rivers. In: Walling, D. and 

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Ferguson, R.I. 1986. River loads underestimated by rating curves. Water 

Resources Research 22(1): 74-76.

Ferguson, R.I., 1987. Accuracy and precision of methods for estimating river 

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An Introductions for Ecologists. John Wiley and Sons, Hoboken, N.J. 

526 p.

Gottesfeld, A.S. and Tunnicliffe, J. 2003. Bed load measurements with a passive 

magnetic induction device. In: J. Bogen, T. Fergus and D.E. Walling (eds.), 

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Harrelson, C.C., Rawlins, C.L., and Potyondy, J.P. 1994. Stream channel reference sites: an illustrated guide to field technique. Gen. Tech. Rep. RM-245. 

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Mountain Forest and Range Experiment Station. 61 p.

Hirsch, R.M., Helsel, D.R., Cohn T.A., and Gilroy, E.J. 1993. Statistical analysis of hydrologic data. In: D.R. Maidment (ed.), Handbook of Hydrology. 

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Julien, P.Y. and Anthony, D.J. 2002. Bed load motion and grain sorting in a meandering stream. Journal of Hydraulic Research 40(2): 125-133.

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Martinez, M.H. and Ryan, S.E. 2000. Constructing temporary sampling platforms for hydrologic studies. Gen. Tech. Rep. RMRS-GTR-64. Fort Collins, 
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CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain 

Research Station. 10 p.

Pitlick, J. and Miller, D. 1987. A low-cost footbridge for stream measurements. 

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In: P. Ergenzinger and K.-H. Schmidt (eds.), Dynamics and Geomorphology 

of Mountain Rivers. Lecture Notes in Earth Sciences 52: 53-66, Springer 

Verlag, Berlin.

Ryan, S.E. 1998. Sampling bedload transport in coarse-grained mountain channels using portable samplers, in: Proceedings of the Federal Interagency 

Workshop “Sediment Technology for the 21st Century”, St. Petersburg, FL. 

[Online] Available http://water.usgs.gov/osw/techniques/sedtech21/ryan.

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Proceedings of the Federal Interagency Sediment Monitoring Instrument 

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New York.
78 USDA Forest Service RMRS-GTR-191. 2007

Appendix A: Sampling Intensity 

of Bedload Traps Compared to 

Typical Helley-Smith Sampling

Sampling intensity quantifies the percentage of the bedload collected 

in a sample compared to the transport that moves through the cross-section during the time period of sampling. Sampling with bedload traps 

collects a far greater proportion of the gravel passing a site than sampling with a Helley-Smith-type sampler. For bedload traps, all samplers 

are deployed side-by-side and collect samples simultaneously. For a 

Helley-Smith-type sampler, an operator may work for almost 1 hour to 

collect a sample (for example, sampling at 20 verticals for 2 minutes 

each), but the sampling time is only 2 minutes.

Relative sampling intensity Ir can be defined as the ratio of actual to 

potential sampling intensity. Actual sampling intensity Iact is the product 

of actual sampling time (in other words, the time at which the sampler 

is in ground contact t

s

) multiplied by the width and number of samplers (or sampling verticals) used. Potential sampling intensity I

pot is the 

product of either a desirable sampling duration t

tot or the worktime of an 

operator collecting the sample and the active stream width wact.

Ir =

Iact

=

ns * ws * t

s

I

pot wact * ttot (A1)

where ns is the number of samplers or sampling locations per cross-section and ws

 is the sampler width.

For snowmelt highflows and low to moderate gravel transport, a 

1-hour sampling time is desirable for representative measurement of 

infrequently moving particle sizes. For a 1-hour sampling time, relative sampling intensity obtained when deploying six bedload traps at 

Halfmoon Creek was:

Ir =

Iact

=

ns * ws

* t

s

=

6 * 0.3 m * 60 min

= 0.23 = 23 percent I

pot wact * ttot 7.8 m * 60 min (A2)

By contrast, relative sampling intensity for samples collected with 

a 3-inch Helley-Smith sampler (sampling at 15 locations for 2 minutes 

each) was

Ir =

Iact = 15 * 0.076 m * 2 min

= 0.0048 = 0.48 percent I

pot = 8.0 m * 60 min (A3a)
USDA Forest Service RMRS-GTR-191. 2007 79

based on a desirable sampling time of 60 minutes, or

Ir =

Iact = 15 * 0.076 m * 2 min 

= 0.0081 = 0.81 percent I

pot = 8.0 m * 35 min (A3b)

based on the 35-minute work time during which the operator was busy 

collecting the sample. Averaged over all sampling sites to date, sampling intensity obtained by bedload traps 1 ft (0.3 m) wide spaced at 

increments of 2 to 7 ft (0.6 to 2.1 m) within the active stream width and 

when deployed for 1 hour was 24 percent (table 1A). This means that 

24 percent of the bedload passing the site during the time of the operator’s presence was collected. A typical scheme for a 3-inch (0.0762 m) 

Helley-Smith sampler deployed for 2 minutes per location spaced 0.5 m 

apart yields sampling intensities of 0.5 percent based on a desirable 60-

minute sampling time, or 1 percent based on the operator’s worktime. 

In the first case, sampling intensity of the bedload traps exceeds that of 

the 3-inch Helley-Smith sampler on average by a factor of 52, in the 

second case by a factor of 28.

Table 1A—Relative sampling intensity (I

r

 = I

act/Ipot) for bedload trap and Helley-Smitha

 samples collected in 

different studies.

Sampling intensity (%)

Sampling time Number of

Helley-Smith Bedload traps Active stream HS-verticals Bedload traps

Stream 1 hr work timeb 1 hr width (m) per cross-section

St. Louis Cr. 0.51 1.02 23.8 6.3 13 5

Little Granite Cr. 0.37 0.56 14.5 12.4 18 6

Cherry Cr. 0.46 0.73 25.0 6.0c; 9.4d 17 5

East St. Louis Cr. 0.45 1.35 33.3 3.6c; 5.1d 9 4

Little Granite ’02 0.52 1.24 22.2 5.4 11 4

Halfmoon Creek 0.48 0.81 23.0 8.0 15 6

Hayden Creek 0.58 1.00 25.7 7.0c; 6.5d 15 6

mean for all sites 0.48 0.96 23.9 14 5

a

 3-inch Helley-Smith sampler deployed for 2 minutes per vertical

b

 the time the operator was busy collecting a Helley-Smith sample, in other words, (number of HS verticals + 1)*2

c

 cross-section for bedload traps

d

 nearby cross-section for Helley-Smith samples
80 USDA Forest Service RMRS-GTR-191. 2007

Appendix B: List of Components for 

Making Bedload Traps

A bedload trap costs approximately $200 to 300 to construct, not 

including the time for buying materials or arranging the work to be 

done. Hardware amounts to 25 to 30 percent of the total cost; labor 

costs account for the remaining 70 to 75 percent.

Table 1B—Items to purchase for making one bedload trap.

Aluminum stock, 4 inches wide, 1⁄4 inch thick, 42-inch long strip

Ground plate, aluminum sheet 1/8" thick, 16 inches by 12 inches

2 cold-rolled steel stakes, 4 ft long with sharpened tip

Netting material: depending on length of the net (See Appendix C)

Spool of nylon yarn (thread about 1 mm thick)

Set of carpet/upholstery sewing needles

Cigarette lighter to sear the ends of the nylon yarn

4 webbing straps, 1-inch wide, 2 ft long, with sturdy metal friction buckles

4 plastic tension buckles, 1 inch wide

Cotton-covered clothesline, 2 to 2.5 ft per trap

2 metal shaft collars, 5/8 inch (16 mm) inside diameter, with thumbscrews, 

and

2 pieces of PVC pipe, 8 inches (0.2 m) long (used when frames are not on 

the ground plate)

•

•

•

•

•

•

•

•

•

•

•

•
USDA Forest Service RMRS-GTR-191. 2007 81

Appendix C: Details of Ordering 

Netting Material

Netting material can be purchased from Delta Net and Twine 

Company, Inc. (www.deltanetandtwine.com). Select the netting listed in 

the highlighted row of Table 1C for a 3.6 mm mesh width opening. This 

information is provided for the user’s convenience, not for endorsement 

of this company as the sole distributor.

The price of netting material is computed by weight. With a weight 

of approximately 1 lb per 3 ft of netting material 8 ft deep (wide), a 

16-ft long piece (enough for 4 or 8 traps depending on the length of the 

net) weighs 16/3 = 5.33 lb. At a cost of $8.00/lb, the total price for the 

netting material is 5.33 * $8.00 = $42.64, so the material price for a 3-ft 

long net is $5.33 and $10.66 for longer nets.

Table 1C—Netting sizes of bulk nylon knotless Raschel (from the Delta Net and Twine 2002 

catalog).

Mesh openingb Depthc of Approx. Price per lbd

Twine Mesh sizea size (approx.) netting stretched 1-24 lb 25-100 lb

size (inch) (mm) (mm) (ft) ft to lb ($) ($)

210d/9 3/16 4.7 3.6 8 3.0 8.00 7.50

210d/15 1/4 6.4 5.8 8 3.3 8.00 7.50

210d/18 3/8 9.5 9.0 8 5.5 8.00 7.50

a

 This measure of mesh size includes one of the netting sides and is therefore larger than the mesh opening 

size. b

 Mesh opening size is the open mesh space. This measure is not included in the manufacturer’s catalog.

c

 Depth = knitted width

d

 as of Jan. 2005
82 USDA Forest Service RMRS-GTR-191. 2007

Appendix D: Wading Safety, Use of 

Safety Devices, and Wading Risk

Wading Safety

Wading, especially during high flows, is inherently dangerous. 

Operators must use caution when wading and observe safety measures. 

Field work of this type should only be undertaken by groups of two or 

more people. Wearing a belt with chest waders that prevent water from 

filling the waders in case of a fall is essential even at moderate flows. 

To reduce the risk of slipping, the wading person should move their feet 

close to the stream bottom. Each step should be taken deliberately and 

slowly, one step at a time. Stepping onto slippery rocks or into gaps between large rocks should be avoided. The wading person should move 

sideways, facing upstream, when wading becomes difficult. A sturdy 

pole or staff can help to maintain footing. Two people holding hands 

may be more stable than a single person, especially if only one person 

moves at a time. When encountering unsafe conditions, the wading person should turn back. Walking backwards a few steps until reaching 

shallower or slower flow may be easier than turning in a tricky spot.

Use of Safety Devices

Safety devices and practices not only make wading safer but extend 

“wadeable” conditions to higher flows. Safety practices include wearing a life vest, holding on to a safety line tied across the stream (fig. 

53) and, as an extreme measure, wearing a climbing harness and being 

belayed by a person holding a belay rope on the bank (fig. 54). Several 

practices can be used together. Wearing a dry suit might also be advantageous in flows difficult to wade.

In the event a person has lost their footing and cannot easily get up, 

the person should float on their back, feet downstream, and use their 

arms to steer to a location with shallower water and less flow velocity 

where it is possible to crawl to the bank. The person should not attempt 

to stand up in fast moving water. If a foot gets caught on the stream bottom, fast flow can push the person under water. The safest and fastest 

way out of a stream may not be the closest bank but the nearest shallow 

bank. When floating downstream, locations should be avoided where 

a person may be pinned against an obstacle, such as undercut rocks, 
USDA Forest Service RMRS-GTR-191. 2007 83

Figure 54—Operating bedload traps in poorly wadeable flow. Hazardous downstream flow conditions 

warrant increased safety measures such as life 

vests, climbing harnesses, and ropes for each operator’s belay. Crossing the ropes should be avoided 

and operators should be experienced in handling 

belays. Photo courtesy of Winema National Forest, 

Klamath Falls, OR.

Figure 53—Safety devices used by operators at high 

flow in Little Granite Creek. Notice the life vests and 

the safety rope fastened across the stream.
84 USDA Forest Service RMRS-GTR-191. 2007

strainers, downed trees, and deep holes. For more information, check 

web sites regarding river recreation, fly fishing, and river rescue14.

The Wading Risk

To evaluate whether or not a flow is wadeable, one should not only 

consider the risk of slipping and falling, but also the consequences of 

losing one’s footing. While a swim may be exhilarating in warm weather 

and a safe reach, cold water, bulky waders, the absence of potential 

rescuers, as well as the presence of hazardous objects downstream, such 

as rapid flow, undercurrents, downed trees, snags, sharp metal, and wire 

pose the danger of injury and drowning. Thus, each person and each 

field crew should carefully assess not only the immediate dangers of 

wading (for example, high and fast flow, a slippery bed, a small body 

stature), but also the consequences of lost footing and being swept 

downstream.

14 For example, see the Safety Code of American Whitewater at: http://www.americanwhitewater.org (accessed 12-20-06).
USDA Forest Service RMRS-GTR-191. 2007 85

Appendix E: Estimated Work Time 

for One Field Season

Table 1E—Work effort in total hours for all field personnel combined.

Tasks,

crew size, 2-week highflow season 4-week highflow season

hours with peak flows < Qbkf with peak flow > Qbkf

Scouting for site selection, setting 160 160

up and taking down the site:

2 person crew, 

10 hrs/d, 8 days

1 extra person when setting up 8 8

and taking down a footbridge

Field work: 312 624

2 person crew,

12 hrs/day, 6 to 7 days/week

1 extra person during highest flows 0 36

Washing samples after field work 0 100

Drying and transporting samples 10 20

Lab sieving, 2 person crew 120 240

Storage and maintenance of gear 10 10

Total work hours per season 620 1198
86 USDA Forest Service RMRS-GTR-191. 2007

Appendix F: Field Equipment and 

Operator Gear When Using 

Bedload Traps in Mountain 

Gravel/Cobble-Bed Streams

Work Assignment

Use of bedload traps involves a variety of different tasks, such as 

installing ground plates, setting and emptying the traps in the stream, 

washing and bagging samples, taking notes, etc. While all tasks can 

usually be accomplished by all operators in unchallenging situations, 

cold temperatures, marginally wadeable high flows, time pressures, and 

the requirements of multi-tasking put a stress on everyone. Assigning 

each operator a limited set of tasks based on their skill set is recommended. One operator, for example, may be skilled at sample processing 

or note taking, while the other is better suited to wading at high flows 

or lifting and moving heavy objects. Following an established routine 

helps accomplish the tasks and avoids mistakes, particularly in stressful 

situations.

Field Gear

Using bedload traps requires not only the items immediately associated with the bedload traps (table 1F), but also a relatively large amount 

of field gear (tables 2F and 3F), much of which is necessary for sample processing in the field as well as for personal protection (table 4F) 

due to the relatively cold and high radiation environment of mountain 

streams. The lists provided below attempt to be comprehensive—not 

all items may be needed in every study. Not all of the gear may need to 

be purchased—some of the gear such as buckets, tubs, and ropes may 

be found in the general field gear storage area or in someone’s garage. 

Equipment typically used in fluvial field studies can perhaps be borrowed from a university or government agency (table 2F). Specialty 

personal field gear, such as waders and paddle jackets, were evaluated 

in tables 5F and 6F.

Protective Clothing Suggestions

Water temperature at snowmelt highflows in Rocky Mountain 

streams is typically 32 to 50°F (0 to 10°C). Air temperature fluctuates 
USDA Forest Service RMRS-GTR-191. 2007 87

greatly, freezing at night and rising up to 50 or 60°F (10 to 16°C) during 

the day, occasionally reaching 80°F (27°C). Protection from the cold 

and wet, as well as the sun, is essential for making it through a long day 

of field work.

Gloves

Neoprene gloves are necessary when in contact with the water for 

more than a few seconds at a time, even on relatively warm days. 

Neoprene gloves work well for the operator setting and emptying bedload traps, but are less suitable for the operator tying and untying the 

bedload traps (neoprene gloves limit the operator’s dexterity, are cumbersome to put on and off, and wear out quickly). Partially rubberized 

cotton knit gloves (available in gardening and hardware stores) are better suited for tying the nets. Repeated contact with the cold water and 

handling wet gear all day roughens and dries out the skin. Wearing rubber gloves helps to reduce this problem.

Waders

When working in deep, fast, and cold water during long days in the 

field, waders are an important part of an operator’s outfit and should be 

carefully selected. Table 5F provides the authors’ evaluation of different 

waders with respect to various requirements. As flow and temperature 

conditions change within a highflow season, it may be useful to have 

several waders at hand, at least one pair of hip waders and one pair of 

chest waders per operator. Hang waders upside down and dry them out 

at room temperature each night.

Jackets and Gravel Guards

An operator will need a waterproof jacket for installing ground plates 

and setting and emptying the traps. A nylon paddle jacket is adequate for 

cool weather or short exposure to the flow. A neoprene paddle jacket with 

a front zipper is warmer and easier to take on and off (table 6F). When 

the operator’s arms are under water, water leaks through the neoprene 

cuffs of the paddle jackets. The degree of leakage and coldness to the 

arms can be reduced by wrapping wrists and forearms with neoprene 

gravel guards10. All clothing worn underneath waterproof clothing 

should be moisture wicking and fast drying (for example, polypro).

10 Gravel guards are pieces of neoprene approx. 6 by 18 inches (0.15 by 0.46 m) in size 

with a velcro closure. They are designed to be wrapped around the ankle to prevent 

sand and fine gravel from entering wading boots worn over stocking foot waders.
88 USDA Forest Service RMRS-GTR-191. 2007

Table 1F—Bedload traps and their components.

Bedload traps with netting, straps, and buckles

Cotton cover clothes line to tie off net

Ground plates

Stakes

Stake driver

Shaft collars with thumb screws

•

•

•

•

•

•

Table 2F—General field gear for fluvial studies.

Measuring tape, 50 to 100 ft long

Engineer’s hammer or small sledge hammer

Chest waders

Hip waders and irrigation boots

Life vests

Nylon climbing rope (50 to 100 ft) for safety line

Gravelometer (template) in 0.5 phi increments

Accurate hanging scale

For ancillary measurements:

2 staff gauges

2 stage recorders

4 metal fence posts and fence post driver

Equipment for discharge measurements (for example, current meter and 

top setting rod)

Sampling frame for pebble counts

Surveying equipment

Helley-Smith bedload sampler

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•
USDA Forest Service RMRS-GTR-191. 2007 89

Table 3F—Items needed for operating 4 bedload traps.

Paddle jackets

Rubber and/or rubberized gloves

Work gloves

Neoprene gloves

4 plastic buckets, 5 gal (color 1)

4 plastic buckets, 5 gal (color 2)

4 plastic buckets, 5 gal (any color, 3 for washing samples, 1 spare)

1 mid-sized terry cloth field towel

1 metal bowl, 9 to 10 inch diameter, crimped

1 metal bowl, 12 inch diameter with rim to fit onto a 5-gal bucket

1 clear plastic measuring cup (2-cup size)

1 large wire spoon (Chinese kitchen ware)

Rubber spatulas, large and medium size

Ziploc bags (starter kit: more may be needed)

Snack size, carton of 100 (enough for 25 samples)

Quart size, 2 cartons of 50 (enough for 25 samples)

Gallon size, 2 cartons of 40 (enough for 20 samples)

2 Sharpie markers (new)

1 pack of mechanical pencils (half a dozen or so)

1 water resistant field book

1 note book

2 camping stools

2 camping chairs

2 plastic tubs, 25 to 30 gal, for storing gear

1 plastic mantled cable with end loops, 20- to 25-ft long, and padlock to 

secure gear

Several large- and medium-sized tarps to protect gear and operators from 

rain

Assortment of short ropes and string

Assortment of plastic shopping bags

Surveyors flags and flagging

Several stakes of rebar of different length, always handy

Duct tape, always handy

Silicone gel, for example, Aqua seal, to repair leaky waders, jackets, 

gloves...

Piece of plexi glass, about 1 ft2, for viewing under water

Several homemade signs to mark the study site, with brief explanation

Tool kit, nails and screws, baling wire

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•
90 USDA Forest Service RMRS-GTR-191. 2007

Table 4F—Personal items needed for field work.

Field hat and warm cap

Sun glasses

Sun screen lotion, chap stick

Insect repellent

Hand lotion, liquid bandaid or super glue

First aid kit

Layered clothing, particularly polypro long underwear and other 

quick-drying clothing

Change of clothes (if one set gets wet)

Footwear besides waders (for example, tennis shoes, hiking boots)

Several pairs of thick socks

Rain gear

Head lamps

Easy to read digital sports watch, worn on a string around the neck

Camera

Cooler with food

Several water bottles

Plenty of snacks

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•
USDA Forest Service RMRS-GTR-191. 2007 91

Table 6F—Authors’ evaluation of two types of paddle jackets.

Desired quality Nylon, pullover Neoprene, front zipper

Water tightness along cuffs Neoprene cuffs are easy to put on and off,

but are leaky; arms become wet under water.

Gravel guards worn on forearms reduce 

leakiness and improve cold protection

Water tightness along front Water can enter around the neck and along

the zipper

Protection from cold water Poor Good

Ease of putting on Cumbersome Moderate

or taking off

Table 5F—Authors’ evaluation of wader properties.

Hip waders Chest waders

Thin Heavy weight

Irrigation Gore-Tex rubberized rubberized

boots; (fly fishing) Neoprene or nylon or nylon

Desired quality hip waders bootfoot stocking foot bootfoot bootfoot

Wadeable flow Less than Deeper than thigh high

depth thigh high

Protection from Good Little Little to Moderate Good

cold water moderate

Operator wading Very good Good Good to Moderate Restricted, esp.

agility moderate when oversized

Allowing bending Some Moderately restricted, needs to be Restricted

and squatting restriction oversized

Wear and tear Robust; Delicate; Leaky at Robust; punctures easy

resistance punctures patching stocking foot; to patch with silicone glue

easy to patch with silicone punctures

with silicone glue? easy to patch

glue with silicone

glue

Avoiding sweatiness Moderate Good Poor

Ease of putting on

or taking off Easy Moderate Cumbersome Moderate Moderate
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