U.S. Department of the Interior

U.S. Geological Survey

Techniques and Methods 3–A8

Discharge Measurements at Gaging Stations

Chapter 8 of Book 3, Section A
Front cover: On the left, photograph of a USGS hydrographer making a FlowTracker acoustic 

Doppler velocimeter discharge measurement at Little Eagle Creek at Speedway, Ind. (USGS station 

no. 03353600); on the right, photograph of a USGS hydrographer making a tethered boat acoustic 

Doppler current profiler discharge measurement from a cableway at the Snake River near Moran, 

Wyo. (USGS station no. 13011000). 

Back cover: In the background, 1890 black and white photograph of USGS hydrographers making 

a mechanical current meter discharge measurement from a cableway at the Arkansas River near 

Canyon City, Colo. (USGS station no. 07096000); and on the right, 1958 black and white photograph 

of a USGS hydrographer making a mechanical current meter discharge measurement at the Cedar 

River at Belgrade, Nev. (USGS station no. 06791800).
Discharge Measurements at Gaging 

Stations

By D. Phil Turnipseed and Vernon B. Sauer

Techniques and Methods 3–A8

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

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Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Turnipseed, D.P., and Sauer, V.B., 2010, Discharge measurements at gaging stations: U.S. Geological Survey 

Techniques and Methods book 3, chap. A8, 87 p. (Also available at http://pubs.usgs.gov/tm/tm3-a8/.)

ISBN 978–1–4113–2969–0
iii

Preface

This series of manuals on techniques and methods (TM) describes approved scientific and 

data-collection procedures and standard methods for planning and executing studies and laboratory analyses. The material is grouped under major subject headings called “books” and further 

subdivided into sections and chapters. Section A of book 3 is on surface-water techniques.

The unit of publication, the chapter, is limited to a narrow field of subject matter. These 

publications are subject to revision because of experience in use or because of advancement in 

knowledge, techniques, or equipment, and this format permits flexibility in revision and publication as the need arises. Chapter A8 of book 3 (TM 3–A8) deals with discharge measurements 

at gaging stations. The original version of this chapter was published in 1969 as U.S. Geological Survey (USGS) Techniques for Water-Resources Investigations, chapter A8 of book 3. New 

and improved equipment, as well as some procedural changes, have resulted in this revised 

second edition of “Discharge measurements at gaging stations.”

This edition supersedes USGS Techniques of Water-Resources Investigations 3A–8, 

1969, “Discharge measurements at gaging stations,” by T.J. Buchanan and W.P. Somers, available at http://pubs.usgs.gov/twri/twri3a8/, and supplements USGS Water-Supply Paper 2175, 

volume 1, 1982, “Measurement and computation of streamflow: Measurement of stage and 

discharge,” by S.E. Rantz and others, available at http://pubs.usgs.gov/wsp/wsp2175/html/

WSP2175_vol1.html. 

This revised second edition of “Discharge measurements at gaging stations” is published 

online at http://pubs.usgs.gov/tm/tm3-a8/ and is for sale by the U.S. Geological Survey,

Science Information Delivery, Box 25286, Federal Center, Denver, CO 80225.

v

Contents

Preface ...........................................................................................................................................................iii

Abstract...........................................................................................................................................................1

Purpose and Scope .......................................................................................................................................1

Definition of Streamflow...............................................................................................................................1

Discharge Measurements at Gaging Stations..........................................................................................2

Velocity-Area Method..........................................................................................................................2

Midsection Method ..............................................................................................................................2

Site Selection.........................................................................................................................................8

Layout and Stationing of Partial Sections and Verticals in a Midsection Current-Meter 

Discharge Measurement .......................................................................................................8

Measurement of Width.................................................................................................................................9

Tapes and Tag Lines .............................................................................................................................9

Surveying Methods of Width Measurement, Surveying Level, and Electronic Total Station10

Global Positioning System With Differential Corrections ............................................................11

Measurement of Depth...............................................................................................................................12

Use of Wading Rod .............................................................................................................................12

Use of Sounding Lines and Weights................................................................................................12

Use of Sounding Reels.......................................................................................................................12

Use of a Handline................................................................................................................................13

Depth Corrections for Downstream Drift of Current Meter and Weight ...................................13

Use of Sonic Sounder ........................................................................................................................19

Measurement of Velocity ...........................................................................................................................20

Vertical-Velocity Curve Method .......................................................................................................20

Two-Point Method ..............................................................................................................................21

Six-Tenths-Depth Method .................................................................................................................21

Price AA Current Meter............................................................................................................21

Pygmy Current Meter................................................................................................................23

Acoustic Doppler Velocimeter.................................................................................................23

Two-Tenths-Depth Method ...............................................................................................................23

Three-Point Method ...........................................................................................................................23

Surface and Subsurface Methods...................................................................................................23

Direction of Flow Measurements..............................................................................................................24

Current-Meter Measurements by Wading ..............................................................................................26

Current-Meter Measurements From Cableways ...................................................................................27

Current-Meter Measurements From Bridges .........................................................................................28

Current-Meter Measurements From Ice Cover......................................................................................29

Current-Meter Measurements From Stationary Boats .........................................................................32

Moving-Boat Measurements of Discharge.............................................................................................34

Manual Method, Using a Mechanical Current Meter..................................................................34

Automatic Computerized Method, Using a Mechanical Current Meter....................................34

Moving-Boat Method, Using an ADCP............................................................................................34

Networks of Current Meters......................................................................................................................35
vi

Discharge Measurement of Deep, Swift Streams With a Mechanical Current Meter....................35

Possible To Sound, but Weight and Meter Drift Downstream ....................................................35

Not Possible To Sound, but Standard Cross Section Available ..................................................35

Not Possible To Sound, and Standard Cross Section Not Available..........................................36

Not Possible To Put the Weight and Meter in Water....................................................................36

Recording Field Notes.................................................................................................................................37

Standard Paper Note Keeping for a Mechanical Current-Meter Discharge Measurement.37

Mechanical Current-Meter Inside Notes .......................................................................................38

Standard Paper Note Keeping for an ADV Discharge Measurement .......................................38

ADV Inside Notes................................................................................................................................38

Mean Gage Height of Discharge Measurements ..................................................................................39

Discharge Measurements During Rapidly Changing Stage.................................................................41

Measurements of Large Streams With Rapidly Changing Stage ...............................................41

Measurements of Small Streams With Rapidly Changing Stage ...............................................41

Procedure 1—Depth Is Computed From Predetermined Streambed Elevations at Each 

Vertical ...........................................................................................................................42

Procedure 2—Depth Is Measured by Sounding at Each Vertical.....................................42

Correction of Discharge for Storage During Measurement.................................................................43

Instruments and Equipment .......................................................................................................................44

Current Meters....................................................................................................................................44

Current Meters—Mechanical and Vertical Axis...........................................................................44

Price AA Meter...........................................................................................................................44

Price AA Meter (Slow Velocity)...............................................................................................46

Price Pygmy Meter....................................................................................................................46

Price AA Winter Meter.............................................................................................................47

Current Meters—Mechanical and Horizontal Axis ......................................................................47

Current-Meter Contact Heads (Vertical Axis)................................................................................48

Cat’s Whisker..............................................................................................................................48

Magnetic Switch........................................................................................................................48

Optical Head ...............................................................................................................................49

Vertical-Axis Current-Meter Timers and Counters .......................................................................49

Stopwatch and Headset ...........................................................................................................49

Electronic Counters...................................................................................................................50

Current-Meter Digitizer (CMD) .......................................................................................50

Other Electronic Counters, Electronic Field Notebooks, and Personal Digital 

Assistants .............................................................................................................50

Care of the Vertical-Axis Mechanical Current Meter...................................................................50

Recommended Procedure Before, During, and After Each Discharge Measurement..51

Recommended Procedure After a Day of Use in the Field .................................................51

Recommended Procedure After Each Field Trip ..................................................................51

Inactive Current Meters ...........................................................................................................51

Spin Tests ....................................................................................................................................51

Recordkeeping ...........................................................................................................................51

Rating of Mechanical Current Meters ............................................................................................53

Electromagnetic Current Meters .....................................................................................................56

Marsh-McBirney 2000...............................................................................................................56
vii

Ott Electromagnetic Current Meter........................................................................................56

Acoustic Current Meters...................................................................................................................56

Acoustic Doppler Velocimeter (ADV).....................................................................................56

Signal-to-Noise Ratio (SNR) ...........................................................................................58

Speed of Sound.................................................................................................................58

Maintenance and Care ....................................................................................................58

QCTests and BeamChecks ..............................................................................................58

Acoustic Digital Current Meter (ADC)....................................................................................59

Acoustic Doppler Current Profiler (ADCP) ............................................................................60

Midsection Method With an ADCP................................................................................60

Optical Current Meters ......................................................................................................................63

Sounding Equipment ..........................................................................................................................63

Top-Setting Wading Rods.........................................................................................................63

Round Wading Rods ..................................................................................................................64

Winter-Style Suspension Wading Rods.................................................................................65

Sounding Weights......................................................................................................................65

Hanger Bars................................................................................................................................65

Sounding Reels ..........................................................................................................................66

Sounding Cable ..........................................................................................................................67

Connectors..................................................................................................................................68

Depth Indicators ........................................................................................................................68

Handlines.....................................................................................................................................68

Power Unit...................................................................................................................................69

Sonic Sounder............................................................................................................................69

Cableway Equipment.................................................................................................................69

Bridge Equipment................................................................................................................................73

Handlines and Bridge Boards..................................................................................................73

Portable Cranes .........................................................................................................................73

Power-Driven Cranes................................................................................................................74

Tethered Craft.............................................................................................................................74

Boat Equipment...................................................................................................................................75

Manual Stationary Boat............................................................................................................75

Manual Moving Boat.................................................................................................................75

Automatic Moving Boat............................................................................................................75

ADCP Moving Boat....................................................................................................................76

Remote-Controlled ADCP Moving Boat .................................................................................76

Electronic Field Notebooks and Personal Digital Assistants (PDAs)................................77

Miscellaneous Equipment and Personal Items ....................................................................78

Accuracy of Current-Meter Discharge Measurements........................................................................79

Qualitative Evaluation.........................................................................................................................79

Quantitative Evaluation......................................................................................................................80

ADCP Discharge-Measurement Accuracy ....................................................................................80

Uncertainties in Discharge Measurements ...................................................................................80

Quality Assurance and Quality Control ....................................................................................................81

Safety Requirements...................................................................................................................................82
viii

Figures

1. Definition sketch of the current-meter midsection method of computing 

cross-section area for discharge measurements...................................................................3

2. Computation notes of a current-meter measurement by the midsection method; 

an ADV discharge measurement; from the Surface Water Measurement and 

Inspection form; and an ADCP discharge measurement.......................................................4

3. Type-A reels.................................................................................................................................10

4. Kevlar tag-line reel .....................................................................................................................10

5. Horizontal-axis boat tag-line reel without a brake................................................................10

6. Vertical-axis boat tag-line reel.................................................................................................10

7. Surveying level and tripod.........................................................................................................10

8. Total station surveying equipment...........................................................................................11

9. Global positioning system with real-time kinematic base station and radio 

transmitter....................................................................................................................................11

10. Position of sounding weight and line in deep, swift water..................................................14

11. Sketch of geometric relationship of actual to measured vertical angle when 

flow direction is not normal to the measuring section.........................................................18

12. Typical vertical-velocity curve .................................................................................................21

13. Schematic showing the FlowTracker sampling volume and the proximity to a fixed 

boundary in the stream..............................................................................................................22

14. Velocity components when flow is not normal to measuring section...............................24

15. Measurement of horizontal angle with measurement note sheet .....................................25

16. Measurement of horizontal angle with a folding rule ..........................................................25

17. Wading measurement using a top-setting rod ......................................................................26

18. Ice drill being used to cut holes and ice rod being used to support current 

meter for a discharge measurement.......................................................................................29

19. Method of computing meter settings for measurements under ice cover.......................30

20. Typical vertical-velocity curve under ice cover....................................................................30

21. Part of note sheet for discharge measurement under ice cover.......................................30

22. Collapsible hanger assembly for used with 30- and 50-pound C-type weights, for 

measurements under ice...........................................................................................................31

23. Determining the position in a cross section using the stadia method ..............................33

24. Determining the position in a cross section using the angular method............................33

25. Graph of readings used to compute a weighted mean gage height..................................40

26. Assembly drawing of the Price AA and Price pygmy current meter.................................46

28. Price AA meter with winter-style yoke and polymer rotor..................................................47

27. Price AA and Price pygmy current meters.............................................................................47

Portable Weir-Plate Measurements.........................................................................................................82

Portable Parshall-Flume Measurements.................................................................................................83

Volumetric Measurements.........................................................................................................................85

Float Measurements....................................................................................................................................85

Indirect Discharge Measurements...........................................................................................................86

Tracer Discharge Measurements .............................................................................................................86

References Cited..........................................................................................................................................86
ix

29. Ott current meter ........................................................................................................................48

30. Hoff current meter......................................................................................................................48

31. Valeport current meter ..............................................................................................................48

32. Price AA magnetic head current meter contact chamber; single count binding 

post; and shaft with magnet for magnetic heads..................................................................49

33. Analog stopwatch and current-meter headset.....................................................................49

34. Current-meter digitizer...............................................................................................................50

35. Example of a current-meter log (suggested format) ............................................................52

36. Example of a standard current-meter rating table No. 2 for Price pygmy 

current meters with cat’s-whisker contact chamber...........................................................54

37. Example of a standard current-meter rating table No. 2 for Price AA current 

meters with cat’s-whisker and magnetic contact chambers..............................................55

38. Marsh-McBirney Model 2000 electromagnetic flowmeter and display meter................56

39. Ott electromagnetic current meter..........................................................................................56

40. SonTek/YSI FlowTracker acoustic Doppler velocimeter mounted on a 

standard top-setting wading rod and closer view of transmitting and receiving 

transducers and offset-mounting bracket..............................................................................57

41. Ott acoustic digital current meter mounted on a standard top-setting wading rod; 

closer view of transmitting and receiving transducers; ADC datalogger; and 

schematic of the sampling volume and transducers............................................................59

42. Screen capture of an ADCP measurement of discharge for the Pearl River near 

Columbia, MS (02489000)...........................................................................................................61

43. Examples of acoustic Doppler current profilers....................................................................62

44. Hydrographer using an optical current meter to measure surface velocity....................63

45. Top-setting wading rod with current-meter digitizer attached...........................................63

46. Closeup view of setting scale on handle of top-setting wading rod..................................64

47. Parts for the round wading rod ................................................................................................64

48. Round wading rod with Price AA current meter attached..................................................64

49. Water Survey Canada winter-style round 1-inch suspension rod and meter..................65

50. Columbus C-type sounding weights (15 through 300 pounds) ............................................65

51. Sounding weight hanger bars and hanger pins ....................................................................66

52. A-pack reel ..................................................................................................................................66

53. Canfield reel.................................................................................................................................67

54. A–55 reel and B–56 reel.............................................................................................................67

55. E–53 reel.......................................................................................................................................67

56. Connectors for attaching sounding cable to sounding-weight hanger ............................68

57. Computing depth indicator........................................................................................................68

58. Handline .......................................................................................................................................68

59. Power units for sounding reels ................................................................................................69

60. Sit-down cable car.....................................................................................................................70

61. Stand-up cable car.....................................................................................................................70

62. Cable-car pullers ........................................................................................................................71

63. Battery-powered cable car.......................................................................................................71

64. Remotely operated Hydrological Services Hornet cable carriage with ADCP 

and trimaran used by the USGS...............................................................................................72

65. Bank-operated cableway..........................................................................................................72

66. Measuring from a bridge with a bridge board.......................................................................73
x

Tables

1. Standard markings for steel tag lines .......................................................................................9

2. Air correction table (in feet), giving differences between vertical length 

and slant length of sounding line above water surface for selected 

vertical angles.............................................................................................................................16

3. Wet-line table (in feet) giving differences between wet-line length and 

vertical depth for selected vertical angles.............................................................................17

4. Degrees to be added to observed vertical angle, P, to obtain true vertical 

angle when flow direction is not normal to measurement section....................................19

5. Summary table for setting the meter at 0.8-depth position in deep, swift streams.........19

6. Current meter and velocity-measurement method for various depths .............................26

7. Velocity-measurement method for various suspensions and depths ...............................27

8. Breaking loads for Ellsworth stranded cable.........................................................................27

9. Comparison of discharge measurement error for standard and shortcut 

methods........................................................................................................................................41

10. Price current meter and SonTek ADV configurations, usages, and 

recommended ranges of depth (without boundary interference) and velocity ...............45

11. Sounding reel data .....................................................................................................................66

12. Rating table for 3-inch modified Parshall flume. ...................................................................84

67. Type A crane mounted on a three-wheel base .....................................................................73

68. Type E crane mounted on a four-wheel base ........................................................................73

69. Protractor used for measuring vertical angles......................................................................74

70. Vehicle-mounted, power-driven crane...................................................................................74

71. Measuring with a tethered ADCP, DGPS, and trimaran .......................................................75

72. Manual stationary-boat equipment assembly.......................................................................75

73. ADCP equipment mounted and operated on a moving boat and a tethered 

platform ........................................................................................................................................76

74. Personal digital assistants and electronic field notebooks ................................................77

75. Ice chains for boots and waders .............................................................................................78

76. Portable weir plate.....................................................................................................................82

77. Modified 3-inch Parshall flume ................................................................................................84
xi

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.4047 hectare (ha)

acre 0.4047 square hectometer (hm2

) 

acre 0.004047 square kilometer (km2

)

square foot (ft2

) 929.0 square centimeter (cm2)

square foot (ft2

) 0.09290 square meter (m2)

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

gallon (gal) 3.785 cubic decimeter (dm3

) 

million gallons (Mgal) 3,785 cubic meter (m3

)

cubic foot (ft3

) 28.32 cubic decimeter (dm3) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

acre-foot (acre-ft) 1,233 cubic meter (m3

)

acre-foot (acre-ft) 0.001233 cubic hectometer (hm3

) 

Flow rate

acre-foot per day (acre-ft/d) 0.01427 cubic meter per second (m3

/s)

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

acre-foot per year (acre-ft/yr) 0.001233 cubic hectometer per year (hm3

/yr)

foot per second (ft/s) 0.3048 meter per second (m/s)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

cubic foot per second per square mile 

[(ft3

/s)/mi2]

 0.01093 cubic meter per second per square kilometer [(m3

/s)/km2]

cubic foot per day (ft3

/d) 0.02832 cubic meter per day (m3/d)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

gallon per day per square mile 

[(gal/d)/mi2

]

 0.001461 cubic meter per day per square

kilometer [(m3/d)/km2]

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

million gallons per day per square mile 

[(Mgal/d)/mi2

]

1,461 cubic meter per day per square kilometer [(m3

/d)/km2]

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 
xii

Multiply By To obtain

Pressure

atmosphere, standard (atm) 101.3 kilopascal (kPa)

pound-force per square inch 

(lbf/in2

)

6.895 kilopascal (kPa)

pound per square foot (lb/ft2

) 0.04788 kilopascal (kPa) 

pound per square inch (lb/in2

) 6.895 kilopascal (kPa) 

Density

pound per cubic foot (lb/ft3

) 16.02 kilogram per cubic meter (kg/m3)

pound per cubic foot (lb/ft3

) 0.01602 gram per cubic centimeter (g/cm3)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the insert datum name (and abbreviation) here 

for instance, “North American Vertical Datum of 1988 (NAVD 88).”

Horizontal coordinate information is referenced to the insert datum name (and abbreviation) 

here for instance, “North American Datum of 1983 (NAD 83).”

Elevation, as used in this report, refers to distance above sea level. Because this report is based 

on a large number of previously published scientific investigations, “sea level” is not referenced 

to a single vertical datum. “Mean sea level” also is not used with reference to a single datum; 

where used, the phrase means the average surface of the ocean as determined by calibration 

of measurements at tidal stations.

Abbreviations and Acronyms

AA Price AA current meter

ADCP acoustic Doppler current profiler

ADC acoustic digital current meter

ADV acoustic Doppler velocimeters

Aquacalc JBS Instruments Aquacalc Pro Discharge Measurement Computer

C1 C1 connector

CMD current meter digitizer

CMCsp Hydrological Services Current Meter Counter signal processor

CSG crest stage gage

CST Central Standard Time

C type Columbus type

DCP Data Collection Platform

DGPS global positioning system with differential corrections

dB decibels

EDT Eastern Daylight Time
xiii

EFN electronic field notebook

EST Eastern Standard Time

FlowTracker SonTek/YSI FlowTracker Handheld acoustic Doppler velocimeter

GPS global positioning system

HIF U.S. Geological Survey Hydrologic Instrumentation Facility

HWM high water mark

Hz hertz

ISO International Organization for Standardization

kHz kilohertz

LEW left edge of water

MHz megahertz

NAD 83 North American Datum of 1983

NAVD 88 North American Vertical Datum of 1988

NWIS U.S. Geological Survey National Water Information System

OFR U.S. Geological Survey Open-File Report

PDA portable digital assistant

PFD personal flotation device

QA/QC quality assurance and quality control

RD Teledyne RD Instruments

REW right edge of water

RP reference point

RTK real-time kinematic

RTK-GPS real-time kinematic global positioning system

SNR signal-to-noise ratio

SWAMI surface water measurement and inspection

TM U.S. Geological Survey Techniques and Methods

TRDI Teledyne RD Instruments

TWRI U.S. Geological Survey Techniques of Water-Resources Investigations

USGS U.S. Geological Survey

WAAS Wide Area Augmentation System

WRD Water Resources Division/Discipline

WSC Water Science Center

WSCan Environment Canada Ministry Water Survey of Canada Agency

Definitions of Symbols

ai cross-section area for the ith segment of the n segments into which the cross 

section is divided

arctan arctangent

b width of the throat section (in units of distance)

bi distance from initial point to location i

b(i-1)_ distance from initial point to preceding location

b(i+1) distance from initial point to next location

di depth of water at location i

C coefficient of discharge
xiv

c speed of sound (in distance per unit time)

cos cosine of the angle alpha

°C temperature in degrees Celsius

CORRwl correction (in units of distance) to subtract from the wet-line depth to obtain the 

vertical depth

Dwl wet-line depth (in units of distance)

Dv vertical depth (in units of distance)

average change in stage in the reach L during the measurement (in units of 

distance)

elapsed time during measurement

computed phase difference

°F temperature in degrees Farenheit

H weighted mean gage height (in units of distance)

h static head, head, elevation, or gage height (in units of distance)

IQR interquartile range

L length of reach between measuring section and control (in units of distance)

m difference (in feet) between the air-line correction for the sounding position and that 

for the 0.8 position

n difference (in feet) between the wet-line correction for the sounding position and 

that for the 0.8 position, if the depths are greater than 40 feet and the change in 

vertical angle is more than 5 percent

P vertical angle (in degrees)

ratio of the circumference to the diameter of a circle; approximately equal to 3.14159

Q discharge (in volume per unit time)

Q1 25 percent of samples are less than this value

Q3 75 percent of samples are less than this value

qi discharge through partial section i, (in volume per unit time)

R number of rotor revolutions per second

T total time for the measurement

t

1 duration of time intervals between breaks in the slop of the gage height graph

tan tangent

time lag between pulses

V velocity (in distance per unit time)

Vx velocity (in distance per unit time) in the x direction (perpendicular to the tag line)

Vy velocity (in distance per unit time) in the y direction (parallel to the tag line)

V volume of water in container

vi mean velocity (in units of distance per unit time) of the flow normal to the ith 

segment, or vertical

W average width of stream between measuring section and control (in units of 

distance)

W1 weight of empty container (in units of mass)

W2 weight of container with water (in units of mass)

w unit weight of water (in units of mass per volume)

x x-direction (perpendicular to the tag line) or x-component of velocity
Discharge Measurements at Gaging Stations

By D. Phil Turnipseed and Vernon B. Sauer

the use of a moving boat with the ADCP (Mueller and Wagner, 

2009), the wading rod mounted ADV, electromagnetic current 

meters, electronic field notebooks, personal digital assistants 

(PDAs), and various procedural changes.

The original version of USGS Techniques of WaterResources Investigations book 3, chapter A8 (TWRI 3–A8), 

by Buchanan and Somers (1969), was used as an extensive 

resource in the preparation of this publication because much 

of the equipment and techniques described by Buchanan and 

Somers are still applicable to current streamgaging methods. 

The USGS publications “Measurement and Computation of 

Streamflow, volumes 1 and 2,” by Rantz and others (1982); 

“Discharge measurements using a broad-band acoustic Doppler current profiler,” by Simpson (2002); “Quality-assurance 

plan for discharge measurements using acoustic Doppler 

current profiler,” by Oberg and others (2005); and “Measuring discharge with acoustic Doppler current profilers from a 

moving boat,” by Mueller and Wagner (2009), were also used 

extensively in the preparation of this publication. Numerous parts of this chapter were taken verbatim from Buchanan 

and Somers (1969), Rantz (1982), Simpson (2002), Oberg 

and others (2005), and Mueller and Wagner (2009), and even 

though some of these parts are not specifically denoted, credit 

is hereby given to these authors.

Definition of Streamflow

Streamflow, or discharge, is defined as the volumetric rate 

of flow of water (volume per unit time) in an open channel, 

including any sediment or other solids that may be dissolved 

or mixed with it that adhere to the Newtonian physics of openchannel hydraulics of water. The definition of streamflow in 

this chapter does not include non-Newtonian flow events such 

as debris flows and lahars (an avalanche of volcanic mud and 

water down the slopes of a volcano). Streamflow in the USGS 

is usually expressed in English dimensions of cubic feet per 

second (ft3

/s). Other common units are million gallons per day 

(Mgal/d) and acre-feet per day (ac-ft/d). Streamflow cannot be 

measured directly but must be computed from variables that 

can be measured directly, such as stream width, stream depth, 

and streamflow velocity. Even though streamflow is computed 

from measurements of other variables, the term “streamflow 

measurement” or “discharge measurement” is generally 

applied to the final result of the calculations.

Abstract

The techniques and standards for making discharge measurements at streamflow gaging stations are described in this 

publication. The vertical axis rotating-element current meter, 

principally the Price current meter, has been traditionally used 

for most measurements of discharge; however, advancements 

in acoustic technology have led to important developments in 

the use of acoustic Doppler current profilers, acoustic Doppler velocimeters, and other emerging technologies for the 

measurement of discharge. These new instruments, based on 

acoustic Doppler theory, have the advantage of no moving 

parts, and in the case of the acoustic Doppler current profiler, 

quickly and easily provide three-dimensional stream-velocity 

profile data through much of the vertical water column. For 

much of the discussion of acoustic Doppler current profiler 

moving-boat methodology, the reader is referred to U.S. Geological Survey Techniques and Methods 3–A22 (Mueller and 

Wagner, 2009).

Personal digital assistants (PDAs), electronic field 

notebooks, and other personal computers provide fast and 

efficient data-collection methods that are more error-free 

than traditional hand methods. The use of portable weirs and 

flumes, floats, volumetric tanks, indirect methods, and tracers 

in measuring discharge are briefly described.

Purpose and Scope

The U.S. Geological Survey (USGS) makes tens of thousands of streamflow measurements each year across the United 

States and its territories. Measured discharges range from a 

trickle in a small ditch or stream [less than 0.01 cubic foot 

per second (ft3/s)], to a flood on the Mississippi River (greater 

than 1,800,000 ft3/s). Several methods are used by the USGS 

to make streamflow measurements. Principally, the USGS 

uses mechanical current meters and hydroacoustic meters [for 

example, acoustic Doppler current profilers (ADCPs) and 

acoustic Doppler velocimeters (ADVs)]. The purpose of this 

report is to describe the equipment and procedures used by the 

USGS and others for making discharge measurements, and 

to describe new developments in equipment and procedures. 

Other traditional methods of measuring streamflow include 

portable weirs and flumes, and volumetric, float, indirect, and 

tracer measurements. Relatively new developments include 
2 Discharge Measurements at Gaging Stations

Discharge Measurements at Gaging 

Stations

Procedures for making most types of current-meter 

[mechanical meters, electromagnetic meters, ADV meters, 

acoustic digital current meters (ADCs), and so forth)], 

moving-boat ADCP, and ADCP midsection measurements are 

described in the following sections. For much of the discussion of moving-boat ADCP, the reader is referenced to Mueller 

and Wagoner (2009). The chapter includes discussions on the 

selection of a measuring section, laying out the stationing for 

subsection verticals, width measurements, depth measurements, velocity measurements, direction of flow measurements, and recording of field notes. Additional details that 

pertain to instrumentation and specific types of measurements, 

such as wading, cableway, bridge, boat, and ice, are described 

in subsequent sections. Special procedures such as networks of 

current meters, measurement of deep, swift streams, and measurements during rapidly changing stage are also described.

Velocity-Area Method

The most practical method of measuring the discharge of 

a stream is the velocity-area method. Discharge is computed as 

the product of the area and velocity. The measurement is made 

by subdividing a stream cross section into segments (sometimes referred to as partial areas, sections, subareas, verticals, 

stations, profiles, panels, or ensembles), and by measuring the 

depth and velocity in a vertical within each segment. The total 

discharge is the summation of the products of the partial areas 

of the stream cross section and their respective average velocities. This computation is classically expressed by the equation

, (1)

where Q total discharge, in cubic feet per second,

ai cross-section area, in square feet, for the ith 

segment of the n segments into which the cross 

section is divided, and

vi the corresponding mean velocity, in feet per 

second of the flow normal to the ith segment, or 

vertical.

Midsection Method

The current-meter midsection method of making a 

current-meter discharge measurement is used by the USGS 

and others. The method assumes that the mean velocity in 

each vertical represents the mean velocity in a partial rectangular area (segment). The mean velocity in each vertical is 

determined by measuring the velocity at one or more selected 

points in that vertical, as described in a later section of this 

chapter. The cross-section area for a segment extends laterally from half the distance from the preceding vertical to half 

the distance to the next vertical, and vertically, from the water 

surface to the sounded depth as shown in figure 1.

The cross section in figure 1 is defined by depths at 

locations 1, 2, 3, 4, . . . , n. At each location, the velocities are 

sampled by current meter to obtain the mean of the vertical 

distribution of velocity. The partial discharge is now computed 

for any partial section (segment) at location i as

, or (2)

, (3)

where qi discharge through partial section i,

vi mean velocity at location i,

bi distance from initial point to location i,

b(i-1) distance from initial point to preceding location,

b(i+1) distance from initial point to next location, and

di depth of water at location i.

Thus, for example, the discharge through partial section 4 

(heavily outlined in figure 1) is

. (4)

The procedure is similar when i is at an end section. The 

“preceding location” at the beginning of the cross section is 

considered coincident with location 1; the “next location” at 

the end of the cross section is considered coincident with location n. Thus,

, and (5)

. (6)

For the example shown in figure 1, q1 is zero because the 

depth at observation point 1 is zero. However, when the crosssection boundary is a vertical line at the edge of the water as at 

location n, the depth is not zero and velocity at the end section 

may or may not be zero. Equations 5 and 6 are used whenever 

there is water only on one side of an observation point, such 

as at the edge of the stream, piers, abutments, and islands. It 

usually is necessary to estimate the velocity at an end section because it normally is impossible to measure the velocity 

accurately with the current meter close to a boundary. There 

also is the possibility of damage to the equipment if the flow is 

turbulent. The estimated velocity is usually made as a percentage of the adjacent section.

The summation of the discharges for all the partial sections 

is the total discharge of the stream. An example of the measurement notes is shown in figure 2A. In the hydraulic properties 

reported, the summation of discharges from an ADV discharge 
Discharge Measurements at Gaging Stations 3

measurement (fig. 2B) is similar to that of a current meter; however, it is designed to report the properties inherent to the ADV 

software and signal processing necessary to compute discharge 

using acoustic Doppler technology. A program written by staff 

in the USGS Maine Water Science Center entitled Surface 

Water Measurements and Inspections (SWAMI) has become 

common in use in the USGS with a PDA, and may be used to 

record discharge measurements, inspections, differential level 

surveys, and other field measurements (fig. 2C).

Included here for convenience is a typical, well-documented ADCP discharge measurement (fig. 2D). This measurement serves as an example of how an ADCP measurement note 

should be kept in the field. Further discussion of ADCP measurements can be found in subsequent sections of this chapter.

Figure 1. Definition sketch of the current-meter midsection method of computing cross-section area for discharge measurements.

Initial

point

bn

Water surface 

n

 

d d(n-1) 2 

d3

 d4

d5

 d6

6 Streambed 

b(n-1)

b6

b5

b4

b3

b2

b1



1

2

3

4

5

(n-1)

EXPLANATION

1, 2, 3,............... n 

b1

, b2, b3,........... bn

d1, d2, d3,........... dn

- - - - - - - - - - - -

Observation points

Distance, in feet, from the initial point to the observation point

Depth of water, in feet, at the observation point

Boundary of partial sections; one heavily outlined discussed in text
4 Discharge Measurements at Gaging Stations

Figure 2. Computation 

notes of: A, a currentmeter measurement by the 

midsection method; B, an 

ADV discharge measurement; 

C, from the Surface Water 

Measurement and Inspection 

(SWAMI) form; and D, an ADCP 

discharge measurement.
Discharge Measurements at Gaging Stations 5

SEM
6 Discharge Measurements at Gaging Stations
Discharge Measurements at Gaging Stations 7
8 Discharge Measurements at Gaging Stations

The mean-section method used by the USGS prior to 

1950 differs from the midsection method in computation 

procedure. Partial discharges are computed for partial sections 

between successive verticals. The velocities and depths at 

successive verticals are each averaged, and each partial section 

extends laterally from one vertical to the next. Discharge is 

the product of the average of two mean velocities, the average 

of two depths, and the distance between verticals. A study by 

Young (1950) concluded that the midsection method is simpler 

to compute and is a slightly more accurate procedure than the 

mean-section method.

Site Selection

The first and most critical step in making a midsection 

current-meter or ADV measurement, or an ADCP measurement is to select a measurement cross section of desirable 

qualities. If the stream cannot be waded, nor high-water 

measurements made from a bridge, moving or tethered boat, 

or cableway, the hydrographer may have little or no choice 

in selecting a measurement cross section. If the stream can 

be waded or the measurement can be made from a boat, the 

hydrographer should look for a cross section with the following characteristics:

• There is a reasonably straight channel with streamlines parallel to each other; a stable streambed free 

of large rocks, weeds, and obstructions that would 

create eddies, slack water, and turbulence; and desirable measurement sections that are roughly parabolic, 

trapezoidal, or rectangular. These conditions are obviously not always possible, but remember that most 

current meters are rated in a still water tank by towing 

them through the tank at a known speed. With that 

in mind, these are conditions a hydrographer should 

seek in the field: a smooth, mirror-like water surface 

with steady, uniform, nonvarying flow conditions in 

the stream reach where the discharge measurement 

will be taken.

• The velocities are, for the most part, greater than 

0.5 ft/s, and depths that are greater than about 0.5 ft. 

These conditions are not always possible to find in the 

field.

• The measurement section is relatively close to the 

gaging station control to avoid the effect of tributary 

and (or) intervening drainage area inflows between the 

measurement section and the control, and to avoid the 

effect of channel storage between the measurement 

section and the control during periods of changing 

stage.

It is usually not possible to satisfy all of these conditions. 

Select the best possible reach using these criteria and then 

select a cross section. For a further discussion regarding site 

selection when using a mechanical or other point-velocity current meter refer to Rantz and others (1982).

For convenience, special site-selection considerations for 

an ADCP discharge measurement are presented as follows, 

and further discussion of ADCP methods and instruments is 

presented in subsequent sections of this chapter:

• The minimum depth near the left and right edges of 

water at the measurement site should allow for the 

measurement of velocity in two or more depth cells 

while being close enough to minimize the estimated 

edge discharges.

• Make sure velocities are, for the most part, greater than 

0.5 ft/s, and depths are greater than the minimum depth 

required by the ADCP. Although measurements can 

be made in low velocities, keep boat speeds extremely 

slow (if possible, less than or equal to the average 

water velocity), which requires special techniques for 

boat control (Simpson, 2002).

• Avoid measurement sections having local magnetic 

fields, especially if a moving bed is present and a 

Global Positioning System with differential corrections 

(DGPS) or the Loop Method (Mueller and Wagoner, 

2006) is used. For example, during measuring, avoid 

overhead truss bridges, low steel-beam spans, power 

lines, and other sources of magnetic fields. Just as with 

ADCP mounts and boats, the presence of ferrous metals will result in ADCP compass errors.

• If possible, avoid asymmetric channel geometries (for 

example, deep on one side and shallow on the other; 

Simpson, 2002) and avoid cross sections with abrupt 

changes in channel-bottom slope. The streambed cross 

section should be as uniform as possible and free from 

debris and vegetation or plant growth.

• When using DGPS with an ADCP, avoid cross-section 

locations where multipath interference, such as riparian vegetation (low-hanging trees and large bushes on 

river or stream banks), buildings at or near the river 

banks, bridges, and other flow-control structures, could 

impede or block signals from GPS satellites.

It is usually not possible to attain all of these conditions, 

but site selection cannot be understated as a critical part of a 

discharge measurement. Select the best possible reach using 

these criteria and then select a cross section. For more discussion regarding site selection when using an ADCP, refer to 

Mueller and Wagner (2009).

Layout and Stationing of Partial Sections 

and Verticals in a Midsection Current-Meter 

Discharge Measurement

After the cross section has been selected, determine the 

width of the stream. For a mechanical current-meter or other 

point-velocity measurement, string a tag line or measuring tape 

for measurements made by wading, from a boat, from ice cover, 

or from an unmarked bridge. Except for bridges, string the line 
Measurement of Width 9

at right angles to the direction of flow to avoid horizontal angles 

in the cross section. For cableway or bridge measurements, 

use the graduations painted on the cable or bridge rail. Next, 

determine the spacing of the verticals, generally using about 

25 to 30 partial sections. With a smooth cross section and even 

velocity distribution, fewer partial sections may be used. Space 

the partial sections so that no partial section has more than 10 

percent of the total discharge in it. The ideal measurement is one 

in which no partial section has more than 5 percent of the total 

discharge in it; this can be challenging when only 25 partial sections are used. For example, the discharge measurement shown 

in figure 2A had 6.5 percent of the total discharge in the partial 

section with the greatest discharge. Equal widths of partial sections across the entire cross section are not recommended unless 

the discharge is evenly distributed. Lessen the width of the partial sections as depths and velocities become greater. Usually an 

approximate or expected total discharge can be obtained from 

the stage-discharge curve. Space the verticals so the discharge 

in each partial section is about 5 percent of the expected total 

discharge from the rating curve. When using an electronic field 

notebook [such as the JBS Instruments Aquacalc Pro Discharge 

Measurement Computer (Aquacalc), a PDA with the Hydrological Services Current Meter Counter signal processor (CMCsp), 

or the SonTek FlowTracker], the expected total discharge can be 

entered prior to starting the discharge measurement. During the 

measurement, a warning message will be displayed if a partial 

discharge exceeds 10 percent of the expected total discharge. 

When using an ADV or other acoustic point-velocity instrument, make sure the instrument is appropriately aligned and 

plumbed to the tag line because slight variations in the alignment of the instrument can result in large errors in the measurement of point velocity. See further discussion of the use of 

acoustic point-velocity instruments in this chapter.

For a standard mechanical current-meter discharge measurement, the usual procedure, after selecting and laying out the 

section, is to measure and record at each vertical (1) the distance 

from the initial point, (2) the depth, (3) the meter position, (4) 

the number of revolutions, (5) the time interval, and (6) the horizontal angle of flow. The starting point can be either bank. The 

edge of water, which may have a depth of zero, is considered 

to be the first vertical. The hydrographer should move to each 

of the verticals in succession and repeat the procedure until the 

measurement is completed at the opposite bank.

Measurement of Width

The first measurement made in a discharge measurement 

is usually the determination of horizontal stationing (width) in 

the cross section being measured. Width needs to be measured 

using the proper equipment and procedures that apply to the 

type of measurement being made (that is, wading, bridge, 

cableway, boat, or ice). Details of measuring width using a 

variety of equipment, and under different flow conditions, are 

described in subsequent sections of this chapter.

The horizontal distance to any vertical in a cross section 

is measured from an initial point on the bank. Cableways and 

bridges used regularly for making discharge measurements 

are commonly marked at 2-, 5-, 10-, and (or) 20-ft intervals 

by paint marks. Distance between markings is interpolated, or 

measured with a rule or pocket tape. Steel or Kevlar tag lines 

and metallic tapes are used for measurements made by wading, from boats, or from unmarked bridges. For wide streams 

of about 2,500 ft or more, where conventional measuring 

methods cannot be used, surveying methods and Global Positioning Systems (GPS) can be used.

Tapes and Tag Lines

Tag lines used for wading measurements are usually 

made of either galvanized steel aircraft cord with solder beads 

at measured intervals, or Kevlar, which is marked with black 

ink and waxed to resist abrasion. A Kevlar tag line consists of 

a Kevlar core with a nylon jacket.

The standard arrangement of solder beads on steel tag 

lines is shown in table 1. The standard markings for Kevlar tag 

lines is one mark every 2 ft, two marks every 10 ft, and three 

marks every 100 ft. The standard lengths of tag lines are 300, 

400, and 500 ft, but other sizes are available.

Four types of tag-line reels typically used for the steel tag 

lines are the Lee-Au, Pakron, Columbus type A, and the USGS 

Stainless Steel Tag line as shown in figure 3. The reel used for 

the Kevlar tag line is shown in figure 4.

Larger reels, used for boat measurements, are designed 

to hold up to 3,000 ft of 1⁄8-inch (in.) diameter steel tag line. 

These reels and boat measurement methods have largely been 

replaced by the ADCP technology. Two different types of reels 

still available are as follows:

• A heavy-duty, horizontal-axis reel without a brake, and with 

a capacity of 5,000 ft of 1⁄8-in. beaded tag line or 3,000 ft of 3

/

16-in. Kevlar boat tag line, as shown in figure 5.

• A vertical-axis reel without a brake (fig. 6), and with a 

capacity of 1,500 ft of 1⁄8-in diameter steel tag line (800 ft 

tag lines are standard) or up to 900 ft of 3/16-in. Kevlar 

boat tag-line cable.

Table 1. Standard markings for steel tag lines.

Distance from initial 

point (zero mark), 

in feet

Distance between 

marks, in feet

Number of solder 

beads, or tags

 0 to 50 2 1 (single bead)

 50 to 100 5 1 

150 to 500 10 1 

 0 to 50 10 2 (double bead)

 50 to 450 100 2 

 0 to 500 100 3 (triple bead)
10 Discharge Measurements at Gaging Stations

of a surveying level, as shown in figure 7, or electronic total 

station instrumentation, as shown in figure 8. The procedure 

to determine width with a transit is used less frequently, but is 

described in the section of this chapter on boat measurements.

With the advent of electronic total station surveying 

instruments and digital surveying levels, a direct reading of the 

distance can be made from the digital surveying level or from 

the total station instrument setup point to the boat. One example 

of a commercially available total station instrument is shown in 

figure 8. Most of these instruments require a reflector target at 

the point where a measurement is desired (in this case the boat); 

however, total station instruments are also available that provide 

accurate measurements of distance without a reflector target. 

Accurate distance measurements can be made with total station 

instruments over distances of 1 mi or more, provided the boat 

can be seen and not obstructed by intervening objects.

Figure 3. Type-A reels: A, Lee-Au; B, Pakron; C, Columbus; and D, USGS stainless steel tag line.

A B C D

Figure 4. Kevlar tag-line 

reel.

Figure 5. Horizontal-axis boat tagline reel without a brake.

Figure 6. Vertical-axis boat 

tag-line reel.

Figure 7. Surveying level and tripod.

Surveying Methods of Width Measurement, 

Surveying Level, and Electronic Total Station

For wide streams where it is not practical to string a 

tag line for discharge measurements from a boat, surveying 

methods can be used to measure stream width and stationing 

for measurement points. Surveying methods require the use 
Measurement of Width 11

satellite telemetry from an array of satellites, and use radio 

triangulation to compute positions for any point on the Earth. In 

order to obtain the accuracy necessary for a discharge measurement, the raw GPS positions must have differential corrections 

applied on the basis of simultaneous readings at a base station. 

Most DGPS units contain built-in differential correction receivers that make automatic and instantaneous corrections. Other 

GPS units may use a separate receiver that attaches to the GPS 

unit with a cable. In either case, base station data are received 

by radio signal from nearby ground base stations or via communications satellites from a network of ground stations, such as 

the Wide Area Augmentation System (WAAS).

A surveying grade DGPS unit with capability of storing 

and recalling data is preferred. These units may or may not 

have built-in or attached differential correction receivers. If 

instantaneous differential corrections are not made, then use 

correction data obtained after the fact from a separate GPS 

base station to postprocess coordinate data. Various agencies collect and provide the base station data via the Internet. 

Coordinate data for the measurement points are downloaded 

from the GPS unit to a computer for postprocessing. Software 

is available to make the differential corrections, to compute 

corrected coordinates of the measurement points, and to automatically compute distances between and to plot a map of the 

measurement points.

Global Positioning System With Differential 

Corrections

Measurement points can be stationed for wide streams, 

such as flood plains that may be several miles wide, or large 

tidal estuaries, with a global positioning system with differential 

corrections (DGPS) instrument, such as that shown in figure 9, 

which is a real-time kinematic (RTK) GPS base station with 

radio transmitter. RTK-GPS and other DGPS instruments utilize 

A

B

Figure 8. Total station surveying equipment: A, instrument and 

tripod and B, instrument reflector targets and surveying tape.

Figure 9. Global positioning system with real-time kinematic 

(RTK–GPS) base station and radio transmitter.
12 Discharge Measurements at Gaging Stations

Accuracy of GPS coordinates will vary depending on the 

type of GPS unit used and whether or not differential corrections are made. Coordinates without differential corrections 

can be in error by as much as ±300 ft because of various errors 

in the system. Obviously, this is not acceptable for discharge 

measurements. However, if care is taken in making observations, and then making differential corrections, errors can be 

reduced to as little as ±3 ft, and even less in ideal conditions. 

This method is acceptable for wide flood plains and inaccessible estuaries with open skies and minimal reflective surfaces, 

which can result in multipath errors.

Measurement of Depth

The second measurement normally made at a vertical is 

the stream depth. Depth should be measured using the proper 

equipment and procedures that apply to the type of measurement being made (that is, wading, bridge, cableway, boat, or 

ice). Details of measuring depth using various equipment and 

under different flow conditions are described in the following sections of this chapter. The water depth of a stream at a 

selected vertical can be measured in several ways, depending on the type of measurement being made, the total depth 

of the stream, and the velocity of the stream. Stream depth is 

usually measured by use of a wading rod, sounding lines and 

weights, acoustic Doppler sensor, or another sonic sounder, as 

described in the following sections of this chapter.

Use of Wading Rod

Use a wading rod for measuring stream depth when depth 

is shallow enough, or when measuring from a low footbridge 

or other supportive structure over the stream. Likewise, use 

the wading rod for measuring from ice cover for shallow 

depths. Wading rods can even be used from a boat if depths 

are not too great. The top-setting wading rod can be used for 

depths up to 4 ft, but greater depths can be measured with 6-, 

8-, and 10-ft top-setting wading rods. The round wading rod, 

which is assembled with 1-ft sections, can be made up into any 

length, but generally is not used for depths greater than about 

10 ft. Velocity of flow is also a consideration because high 

velocity may not allow for keeping a long wading rod in place.

Wading rods have a small foot on the bottom to allow the 

rod to be placed firmly on the streambed, and yet not sink into 

the streambed under most conditions. In sand-bottom streams, 

or in soft muck, it is sometimes difficult to keep the wading 

rod from sinking into the streambed as the weight of the rod 

and meter and the eroding power of the flowing water cause 

the foot of the wading rod to sink. The hydrographer must use 

care in these conditions to be sure the measured water depth, 

as well as the depth of the current-meter placements, are accurately based on the surface of the streambed. In some cases, 

the wading rod may need to be supported in some manner 

other than resting on the streambed.

When using a wading rod in streams with moderate-tohigh velocity, there will be a velocity-head build-up of water 

on the wading rod. The stream depth should be based on 

where the surface of the stream intersects the wading rod, and 

not on the top of the velocity-head build-up. Wading rods are 

graduated in tenths-of-a-foot, and stream depths are generally 

measured or estimated and recorded to the nearest 0.01 ft.

Use of Sounding Lines and Weights

Water depth is measured with sounding lines and weights 

when the depth is too great to use a wading rod, and when 

measuring conditions require measuring from a bridge, 

cableway, or boat. This section will describe the measurement 

of depth when using sounding reels and handlines. It also 

discusses the procedures used to correct observed depths when 

high velocity causes the weight and meter to drift downstream.

Use of Sounding Reels

When using one of the sounding reels described in a subsequent section of this chapter, a counter or dial is used to determine 

the length of cable that has been dispensed. Depths are measured 

to the nearest 0.1 ft when using a sounding line and weight.

The size of the sounding weight used in current-meter 

measurements depends on the maximum depth and velocity in 

a cross section. A rule of thumb is that the size of the weight in 

pounds should be greater than the maximum product of velocity and depth in the cross section. If insufficient weight is used, 

the sounding line will be dragged at an angle downstream. If 

debris or ice is flowing or if the stream is shallow and swift, a 

heavier weight can be used than the rule designates. The rule 

is not rigid but it does provide a starting point for deciding on 

the size of the weight that is needed. If available, notes can be 

examined of previous measurements at a site to help determine 

the size of the weight needed at various stages.

Some sounding reels are equipped with a computing depth 

indicator, or spiral. To use the computing spiral, the dial pointer 

must be set at zero when the center of the current-meter rotor 

is at the water surface. After the sounding weight and meter 

are lowered until the weight touches the streambed, and the 

indicated depth should be read. The distance that the meter is 

mounted above the bottom of the weight should be added. For 

example, if a 30 C .5 (that is, a 30-pound Columbus weight is 

being used and the center of the meter cups is 0.5 ft above the 

bottom of the weight) suspension is used and the dial pointer 

reads 18.5 ft when the sounding weight touches the streambed, 

the depth would be 19.0 ft (18.5 + 0.5). To move the meter to 

the 0.8-depth position, merely raise the weight and the meter 

until the pointer is at the 19-ft mark on the graduated spiral, 

which will correspond to 15.2 ft on the main dial (0.8 × 19.0). 

To set the meter at the 0.2-depth position, raise the weight and 

meter until the pointer is at 3.8 ft on the main dial (0.2 × 19.0).

Tags can be placed on the sounding line a known 

distance above the center of the meter cups as an aid in 

determining depth. The tags, which are usually streamers of 
Measurement of Depth 13

different-colored binding tape, are fastened to the sounding 

line by solder beads or by small cable clips. Tags are used for 

determining depth in two ways; the following is the preferred 

procedure:

1. Set the tag at the water surface and then set the depth 

indicator to read the distance of that tag above the center 

of the meter cups.

2. Continue as if the meter cups themselves have been set at 

the water surface.

3. When the weight touches the streambed, read the depth 

indicator and add the distance that the meter is above the 

bottom of the weight to obtain the total depth.

4. Use the spiral indicator, as described above, for setting the 

0.8-depth meter position. If debris or ice is flowing, this 

method keeps the meter below the water surface and helps 

to prevent damage to the meter.

This is an alternate method that is sometimes used with 

handlines and sounding reels: With the sounding weight on the 

streambed, raise the weight until the first tag below the water surface appears at the surface. If using a reel, determine the distance 

the weight was raised by subtracting before and after readings of 

the depth indicator; if using a handline, use a tape or measuring 

stick. The total stream depth is the sum of (a) the distance the 

weight was raised to bring the tag to the water surface, (b) the 

distance the tag is above the center of the meter cups, and (c) the 

distance from the bottom of the weight to the center of the cups.

Use of a Handline

Although rarely used in the USGS, handlines still provide a 

viable means of measuring discharge from bridges. When using a 

handline, unwind enough cable from the handline reel to keep the 

reel out of water when the sounding weight is on the streambed at 

the deepest part of the cross section. If the bridge is high enough 

above the water surface, raise and lower the weight and meter by 

the rubber-covered cable rather than by the bare cable.

The usual procedure for determining depths is to set the 

meter cups at the water surface and then lower the sounding 

weight to the streambed while measuring the amount of line 

needed to reach the streambed. Measure along the rubbercovered service cord with a steel or metallic tape or a graduated rod to determine the distance the weight is lowered. This 

measured distance, plus the distance from the bottom of the 

sounding weight to the meter cups, is the depth of water. When 

the meter is set for the velocity observation, stand on the 

rubber-covered cable or tie it to the handrail to hold the meter 

in place. This arrangement frees the hands to record the data.

Another method of determining depth when using a handline includes the use of tags set at a known distance above the 

meter. Lower the sounding weight to the streambed, and then 

raise the weight until one of the tags is at the water surface. 

Measure along the rubber-covered service cord with a steel 

or metallic tape or a graduated rod to determine the distance 

the weight is raised. The total depth of water is then the 

summation of (1) the distance the particular tag is above the 

meter cups, (2) the measured distance the meter and weight 

was raised, and (3) the distance from the bottom of the weight 

to the meter cups.

Depth Corrections for Downstream Drift of 

Current Meter and Weight

Where it is possible to sound but the weight and meter drift 

downstream, the depths measured by the usual methods are too 

great. Figure 10 graphically illustrates this condition. The correction for this error has two parts, the air correction and the wet-line 

correction. The air correction is shown in figure 10 as the distance 

cd. The wet-line correction in figure 10 is shown as the difference 

between the wet-line depth de and the vertical depth dg.

As shown in figure 10, the air correction depends on the 

vertical angle P and the distance ab. The correction is computed as follows:

 , (7)

where ab = ac

The air correction for even-numbered angles between 4 

degrees and 36 degrees and vertical lengths between 10 and 100 

ft is shown in table 2. The correction is applied to the nearest 

tenth of a foot; hundredths are given to aid in interpolation.

Use of an air correction table may be nearly eliminated 

by using tags at selected intervals on the sounding line and 

using the tags to refer to the water surface. This practice is 

almost equivalent to moving the reel to a position just above 

the water surface.

The correction for excess length of line below the water 

surface is obtained by using an elementary principle of 

mechanics. If a known horizontal force is applied to a weight 

suspended on a cord, the cord takes a position of rest at some 

angle with the vertical. The tangent of the vertical angle of 

the cord is equal to the horizontal force divided by the vertical 

force owing to the weight. If several additional horizontal and 

vertical forces are applied to the cord, the tangent of the angle 

in the cord above any point is equal to a summation of the 

horizontal forces below that point, divided by the summation 

of the vertical forces below the point.

The distribution of total horizontal drag on the sounding 

line is in accordance with the variation of velocity with depth. 

The excess in length of the curved line over the vertical depth 

is the sum of the products of each tenth of depth and the function [(1/cosP) – 1] of the corresponding angles. The function 

is derived for each tenth of depth by means of the tangent relation of the forces acting below any point.
14 Discharge Measurements at Gaging Stations

Figure 10. Position of sounding weight and line in deep, swift water.

Water surface

Flow

h

b

a

Sounding

line

d

c

Apex of vertical angle

in sounding line

P

P

f g e

Streambed



Bridge
Measurement of Depth 15

The wet-line correction for even-numbered angles between 4 degrees and 36 degrees and 

wet-line depths between 10 and 100 ft is shown in table 3. The correction is applied to the 

nearest tenth of a foot. The wet-line correction cannot be determined until the air correction has 

been deducted from the observed depth.

The following assumptions were used in deriving the wet-line correction table:

1. The weight will go to the bottom despite the force of the current.

2. The sounding is made when the weight is at the bottom but entirely supported by the line.

3. Drag on the streamlined weight in the sounding position is neglected.

4. The air-line/wet-line table is generic and can be used for any size sounding weight or line, 

provided they are designed to offer little resistance to the current.

Wet-line corrections can also be computed with the equation below. This polynomial equation was derived by Kenneth L. Wahl (U.S. Geological Survey [ret.], Regional Surface Water 

Specialist, written commun., 2000) from the data in table 3, and reproduces the table values 

to within a few hundredths of a foot. It can be used in a field computer to quickly and easily 

compute wet-line corrections. With additional programming, depths and depth settings can also 

be computed.

, (8)

where CORRwl the correction, in feet, to subtract from the wet-line depth to obtain the 

vertical depth,

Dwl the wet-line depth, in feet,

Dv the vertical depth, in feet, and

P the vertical angle, in degrees.

If the direction of flow is not perpendicular to the measuring section, the observed angle 

in the measuring line as indicated by a protractor will be less than the true angle of the line. 

The air correction and wet-line correction will then be too small. To correct for this, measure 

the horizontal angle between the direction of flow and a perpendicular to the measuring section 

with a protractor, or determine the horizontal angle coefficient as described in a subsequent section of this publication. The geometry of this condition is illustrated in figure 11.

If the horizontal angle of the direction of flow is called H, the observed vertical angle P, 

and the true vertical angle X, the relation between the angles is expressed by the equation

. (9)

Table 4 gives the amounts in tenths of degrees. Add these to observed vertical angles to 

obtain the true vertical angles for a range of horizontal angles between 8 degrees and 28 degrees.

The conditions that cause error in sounding the depth also cause error in the placement of the 

meter at selected depths. The correction tables are not strictly applicable to the problem of placing 

the meter because of the increased pressure placed on the sounding weight by higher velocities when 

it is raised from the streambed. A meter placed in deep, swift water by the ordinary methods for 

observations at selected percentages of the depth will be too high in the water. The use of tables 2 

and 3 will tend to eliminate this error in placement of the meter, and although not strictly applicable, 

their use for this purpose has become general.

For the 0.2-depth position, the curvature of the wet line is assumed to be negligible and the 

length of sounding line from the apex of the vertical angle to the weight is considered to be a 

straight line. The method used to place the meter at the 0.2-depth position is as follows:

1. Compute the 0.2 value of the vertical depth.

2. Lower the meter to this depth into the water and read the vertical angle.

3. Obtain the air correction from table 2. The vertical length used to obtain the air correction 

is the sum of (a) 0.2 of the vertical depth, (b) the distance from the water surface to the 

apex of the angle, and (c) the distance from the bottom of the weight to the meter.

4. Let out an additional amount of line equal to the air correction.
16 Discharge Measurements at Gaging Stations

Table 2. Air correction table (in feet), giving differences between vertical length and slant length of sounding line above water surface 

for selected vertical angles.

Vertical 

length 

(feet)

Vertical angle of sounding line at protractor Vertical 

length 

(feet) 4° 6° 8° 10° 12° 14° 16° 18° 20° 22° 24° 26° 28° 30° 32° 34° 36°

10 0.02 0.06 0.10 0.15 0.22 0.31 0.40 0.51 0.64 0.79 0.95 1.13 1.33 1.55 1.79 2.06 2.36 10

12 0.03 0.07 0.12 0.19 0.27 0.37 0.48 0.62 0.77 0.94 1.14 1.35 1.59 1.86 2.15 2.47 2.83 12

14 0.03 0.08 0.14 0.22 0.31 0.43 0.56 0.72 0.90 1.10 1.32 1.58 1.86 2.17 2.51 2.89 3.30 14

16 0.04 0.09 0.16 0.25 0.36 0.49 0.64 0.82 1.03 1.26 1.51 1.80 2.12 2.48 2.87 3.30 3.78 16

18 0.04 0.10 0.18 0.28 0.40 0.55 0.73 0.93 1.16 1.41 1.70 2.03 2.39 2.78 3.23 3.71 4.25 18

20 0.05 0.11 0.20 0.31 0.45 0.61 0.81 1.03 1.28 1.57 1.89 2.25 2.65 3.09 3.58 4.12 4.72 20

22 0.05 0.12 0.22 0.34 0.49 0.67 0.89 1.13 1.41 1.73 2.08 2.48 2.92 3.40 3.94 4.54 5.19 22

24 0.06 0.13 0.24 0.37 0.54 0.73 0.97 1.24 1.54 1.88 2.27 2.70 3.18 3.71 4.30 4.95 5.67 24

26 0.06 0.14 0.26 0.40 0.58 0.80 1.05 1.34 1.67 2.04 2.46 2.93 3.45 4.02 4.66 5.36 6.14 26

28 0.07 0.15 0.28 0.43 0.63 0.86 1.13 1.44 1.80 2.20 2.65 3.15 3.71 4.33 5.02 5.77 6.61 28

30 0.07 0.17 0.29 0.46 0.67 0.92 1.21 1.54 1.93 2.36 2.84 3.38 3.98 4.64 5.38 6.19 7.08 30

32 0.08 0.18 0.31 0.49 0.71 0.98 1.29 1.65 2.05 2.51 3.03 3.60 4.24 4.95 5.73 6.60 7.55 32

34 0.08 0.19 0.33 0.52 0.76 1.04 1.37 1.75 2.18 2.67 3.22 3.83 4.51 5.26 6.09 7.01 8.03 34

36 0.09 0.20 0.35 0.56 0.80 1.10 1.45 1.85 2.31 2.83 3.41 4.05 4.77 5.57 6.45 7.42 8.50 36

38 0.09 0.21 0.37 0.59 0.85 1.16 1.53 1.96 2.44 2.98 3.60 4.28 5.04 5.88 6.81 7.84 8.97 38

40 0.10 0.22 0.39 0.62 0.89 1.22 1.61 2.06 2.57 3.14 3.79 4.50 5.30 6.19 7.17 8.25 9.44 40

42 0.10 0.23 0.41 0.65 0.94 1.29 1.69 2.16 2.70 3.30 3.97 4.73 5.57 6.50 7.53 8.66 9.91 42

44 0.11 0.24 0.43 0.68 0.98 1.35 1.77 2.26 2.82 3.46 4.16 4.95 5.83 6.81 7.88 9.07 10.39 44

46 0.11 0.25 0.45 0.71 1.03 1.41 1.85 2.37 2.95 3.61 4.35 5.18 6.10 7.12 8.24 9.49 10.86 46

48 0.12 0.26 0.47 0.74 1.07 1.47 1.93 2.47 3.08 3.77 4.54 5.40 6.36 7.43 8.60 9.90 11.33 48

50 0.12 0.28 0.49 0.77 1.12 1.53 2.02 2.57 3.21 3.93 4.73 5.63 6.63 7.74 8.96 10.31 11.80 50

52 0.13 0.29 0.51 0.80 1.16 1.59 2.10 2.68 3.34 4.08 4.92 5.86 6.89 8.04 9.32 10.72 12.28 52

54 0.13 0.30 0.53 0.83 1.21 1.65 2.18 2.78 3.47 4.24 5.11 6.08 7.16 8.35 9.68 11.14 12.75 54

56 0.14 0.31 0.55 0.86 1.25 1.71 2.26 2.88 3.59 4.40 5.30 6.31 7.42 8.66 10.03 11.55 13.22 56

58 0.14 0.32 0.57 0.89 1.30 1.78 2.34 2.98 3.72 4.55 5.49 6.53 7.69 8.97 10.39 11.96 13.69 58

60 0.15 0.33 0.59 0.93 1.34 1.84 2.42 3.09 3.85 4.71 5.68 6.76 7.95 9.28 10.75 12.37 14.16 60

62 0.15 0.34 0.61 0.96 1.39 1.90 2.50 3.19 3.98 4.87 5.87 6.98 8.22 9.59 11.11 12.79 14.64 62

64 0.16 0.35 0.63 0.99 1.43 1.96 2.58 3.29 4.11 5.03 6.06 7.21 8.48 9.90 11.47 13.20 15.11 64

66 0.16 0.36 0.65 1.02 1.47 2.02 2.66 3.40 4.24 5.18 6.25 7.43 8.75 10.21 11.83 13.61 15.58 66

68 0.17 0.37 0.67 1.05 1.52 2.08 2.74 3.50 4.36 5.34 6.44 7.66 9.01 10.52 12.18 14.02 16.05 68

70 0.17 0.39 0.69 1.08 1.56 2.14 2.82 3.60 4.49 5.50 6.62 7.88 9.28 10.83 12.54 14.44 16.52 70

72 0.18 0.40 0.71 1.11 1.61 2.20 2.90 3.71 4.62 5.65 6.81 8.11 9.55 11.14 12.90 14.85 17.00 72

74 0.18 0.41 0.73 1.14 1.65 2.27 2.98 3.81 4.75 5.81 7.00 8.33 9.81 11.45 13.26 15.26 17.47 74

76 0.19 0.42 0.75 1.17 1.70 2.33 3.06 3.91 4.88 5.97 7.19 8.56 10.08 11.76 13.62 15.67 17.94 76

78 0.19 0.43 0.77 1.20 1.74 2.39 3.14 4.01 5.01 6.13 7.38 8.78 10.34 12.07 13.98 16.09 18.41 78

80 0.20 0.44 0.79 1.23 1.79 2.45 3.22 4.12 5.13 6.28 7.57 9.01 10.61 12.38 14.33 16.50 18.89 80

82 0.20 0.45 0.81 1.27 1.83 2.51 3.30 4.22 5.26 6.44 7.76 9.23 10.87 12.69 14.69 16.91 19.36 82

84 0.20 0.46 0.83 1.30 1.88 2.57 3.39 4.32 5.39 6.60 7.95 9.46 11.14 12.99 15.05 17.32 19.83 84

86 0.21 0.47 0.85 1.33 1.92 2.63 3.47 4.43 5.52 6.75 8.14 9.68 11.40 13.30 15.41 17.73 20.30 86

88 0.21 0.48 0.87 1.36 1.97 2.69 3.55 4.53 5.65 6.91 8.33 9.91 11.67 13.61 15.77 18.15 20.77 88

90 0.22 0.50 0.88 1.39 2.01 2.75 3.63 4.63 5.78 7.07 8.52 10.13 11.93 13.92 16.13 18.56 21.25 90

92 0.22 0.51 0.90 1.42 2.06 2.82 3.71 4.73 5.90 7.22 8.71 10.36 12.20 14.23 16.48 18.97 21.72 92

94 0.23 0.52 0.92 1.45 2.10 2.88 3.79 4.84 6.03 7.38 8.90 10.58 12.46 14.54 16.84 19.38 22.19 94

96 0.23 0.53 0.94 1.48 2.14 2.94 3.87 4.94 6.16 7.54 9.09 10.81 12.73 14.85 17.20 19.80 22.66 96

98 0.24 0.54 0.96 1.51 2.19 3.00 3.95 5.04 6.29 7.70 9.27 11.03 12.99 15.16 17.56 20.21 23.13 98

100 0.24 0.55 0.98 1.54 2.23 3.06 4.03 5.15 6.42 7.85 9.46 11.26 13.26 15.47 17.92 20.62 23.61 100
Measurement of Depth 17

Table 3. Wet-line table (in feet) giving differences between wet-line length and vertical depth for selected vertical angles.

Wet-line 

length 

(feet)

Vertical angle of sounding line at protractor Wet-line 

length 

(feet) 4° 6° 8° 10° 12° 14° 16° 18° 20° 22° 24° 26° 28° 30° 32° 34° 36°

10 0.01 0.02 0.03 0.05 0.07 0.10 0.13 0.16 0.20 0.25 0.30 0.35 0.41 0.47 0.54 0.62 0.70 10

12 0.01 0.02 0.04 0.06 0.09 0.12 0.15 0.20 0.24 0.30 0.36 0.42 0.49 0.57 0.65 0.74 0.84 12

14 0.01 0.02 0.04 0.07 0.10 0.14 0.18 0.23 0.29 0.35 0.41 0.49 0.57 0.66 0.76 0.87 0.98 14

16 0.01 0.03 0.05 0.08 0.12 0.16 0.20 0.26 0.33 0.40 0.47 0.56 0.65 0.76 0.87 0.99 1.12 16

18 0.01 0.03 0.06 0.09 0.13 0.18 0.23 0.30 0.37 0.45 0.53 0.63 0.73 0.85 0.98 1.12 1.26 18

20 0.01 0.03 0.06 0.10 0.14 0.20 0.26 0.33 0.41 0.50 0.59 0.70 0.82 0.94 1.09 1.24 1.40 20

22 0.01 0.04 0.07 0.11 0.16 0.22 0.28 0.36 0.45 0.55 0.65 0.77 0.90 1.04 1.20 1.36 1.54 22

24 0.01 0.04 0.08 0.12 0.17 0.24 0.31 0.39 0.49 0.60 0.71 0.84 0.98 1.13 1.31 1.49 1.68 24

26 0.02 0.04 0.08 0.13 0.19 0.25 0.33 0.43 0.53 0.64 0.77 0.91 1.06 1.23 1.41 1.61 1.81 26

28 0.02 0.04 0.09 0.14 0.20 0.27 0.36 0.46 0.57 0.69 0.83 0.98 1.14 1.32 1.52 1.74 1.95 28

30 0.02 0.05 0.10 0.15 0.22 0.29 0.38 0.49 0.61 0.74 0.89 1.05 1.22 1.42 1.63 1.86 2.09 30

32 0.02 0.05 0.10 0.16 0.23 0.31 0.41 0.52 0.65 0.79 0.95 1.12 1.31 1.51 1.74 1.98 2.23 32

34 0.02 0.05 0.11 0.17 0.24 0.33 0.44 0.56 0.69 0.84 1.01 1.19 1.39 1.60 1.85 2.11 2.37 34

36 0.02 0.06 0.12 0.18 0.26 0.35 0.46 0.59 0.73 0.89 1.07 1.26 1.47 1.70 1.96 2.23 2.51 36

38 0.02 0.06 0.12 0.19 0.27 0.37 0.49 0.62 0.78 0.94 1.12 1.33 1.55 1.79 2.07 2.36 2.65 38

40 0.02 0.06 0.13 0.20 0.29 0.39 0.51 0.66 0.82 0.99 1.18 1.40 1.63 1.89 2.18 2.48 2.79 40

42 0.03 0.07 0.13 0.21 0.30 0.41 0.54 0.69 0.86 1.04 1.24 1.47 1.71 1.98 2.28 2.60 2.93 42

44 0.03 0.07 0.14 0.22 0.32 0.43 0.56 0.72 0.90 1.09 1.30 1.54 1.80 2.08 2.39 2.73 3.07 44

46 0.03 0.07 0.15 0.23 0.33 0.45 0.59 0.75 0.94 1.14 1.36 1.61 1.88 2.17 2.50 2.85 3.21 46

48 0.03 0.08 0.15 0.24 0.35 0.47 0.61 0.79 0.98 1.19 1.42 1.68 1.96 2.27 2.61 2.98 3.35 48

50 0.03 0.08 0.16 0.25 0.36 0.49 0.64 0.82 1.02 1.24 1.48 1.75 2.04 2.36 2.72 3.10 3.49 50

52 0.03 0.08 0.17 0.26 0.37 0.51 0.67 0.85 1.06 1.29 1.54 1.82 2.12 2.45 2.83 3.22 3.63 52

54 0.03 0.09 0.17 0.27 0.39 0.53 0.69 0.89 1.10 1.34 1.60 1.89 2.20 2.55 2.94 3.35 3.77 54

56 0.03 0.09 0.18 0.28 0.40 0.55 0.72 0.92 1.14 1.39 1.66 1.96 2.28 2.64 3.05 3.47 3.91 56

58 0.03 0.09 0.19 0.29 0.42 0.57 0.74 0.95 1.18 1.44 1.72 2.03 2.37 2.74 3.16 3.60 4.05 58

60 0.04 0.10 0.19 0.30 0.43 0.59 0.77 0.98 1.22 1.49 1.78 2.10 2.45 2.83 3.26 3.72 4.19 60

62 0.04 0.10 0.20 0.31 0.45 0.61 0.79 1.02 1.26 1.54 1.84 2.17 2.53 2.93 3.37 3.84 4.33 62

64 0.04 0.10 0.20 0.32 0.46 0.63 0.82 1.05 1.31 1.59 1.89 2.24 2.61 3.02 3.48 3.97 4.47 64

66 0.04 0.11 0.21 0.33 0.48 0.65 0.84 1.08 1.35 1.64 1.95 2.31 2.69 3.12 3.59 4.09 4.61 66

68 0.04 0.11 0.22 0.34 0.49 0.67 0.87 1.12 1.39 1.69 2.01 2.38 2.77 3.21 3.70 4.22 4.75 68

70 0.04 0.11 0.22 0.35 0.50 0.69 0.90 1.15 1.43 1.74 2.07 2.45 2.86 3.30 3.81 4.34 4.89 70

72 0.04 0.12 0.23 0.36 0.52 0.71 0.92 1.18 1.47 1.79 2.13 2.52 2.94 3.40 3.92 4.46 5.03 72

74 0.04 0.12 0.24 0.37 0.53 0.73 0.95 1.21 1.51 1.84 2.19 2.59 3.02 3.49 4.03 4.59 5.17 74

76 0.05 0.12 0.24 0.38 0.55 0.74 0.97 1.25 1.55 1.88 2.25 2.66 3.10 3.59 4.13 4.71 5.30 76

78 0.05 0.12 0.25 0.39 0.56 0.76 1.00 1.28 1.59 1.93 2.31 2.73 3.18 3.68 4.24 4.84 5.44 78

80 0.05 0.13 0.25 0.40 0.58 0.78 1.02 1.31 1.63 1.98 2.37 2.80 3.26 3.78 4.35 4.96 5.58 80

82 0.05 0.13 0.26 0.41 0.59 0.80 1.05 1.34 1.67 2.03 2.43 2.87 3.35 3.87 4.46 5.08 5.72 82

84 0.05 0.13 0.27 0.42 0.60 0.82 1.08 1.38 1.71 2.08 2.49 2.94 3.43 3.96 4.57 5.21 5.86 84

86 0.05 0.14 0.28 0.43 0.62 0.84 1.10 1.41 1.75 2.13 2.55 3.01 3.51 4.06 4.68 5.33 6.00 86

88 0.05 0.14 0.28 0.44 0.63 0.86 1.13 1.44 1.80 2.18 2.60 3.08 3.59 4.15 4.79 5.46 6.14 88

90 0.05 0.14 0.29 0.45 0.65 0.88 1.15 1.48 1.84 2.23 2.66 3.15 3.67 4.25 4.90 5.58 6.28 90

92 0.06 0.15 0.29 0.46 0.66 0.90 1.18 1.51 1.88 2.28 2.72 3.22 3.75 4.34 5.00 5.70 6.42 92

94 0.06 0.15 0.30 0.47 0.68 0.92 1.20 1.54 1.92 2.33 2.78 3.29 3.84 4.44 5.11 5.83 6.56 94

96 0.06 0.15 0.31 0.48 0.69 0.94 1.23 1.57 1.96 2.38 2.84 3.36 3.92 4.53 5.22 5.95 6.70 96

98 0.06 0.16 0.31 0.49 0.71 0.96 1.25 1.61 2.00 2.43 2.90 3.43 4.00 4.63 5.33 6.08 6.84 98

100 0.06 0.16 0.32 0.50 0.72 0.98 1.28 1.64 2.04 2.48 2.96 3.50 4.08 4.72 5.44 6.20 6.98 100
18 Discharge Measurements at Gaging Stations

Figure 11. Sketch of 

geometric relationship of 

actual to measured vertical 

angle when flow direction is 

not normal to the measuring 

section.

tan P = fi

af cos H = fifh

H = horizontal angle

P = measured vertical angle

X = actual vertical angle

hfa = 90o

ifa = 90o

fih = 90o

aih = 90o

tan P

cos H

fi

af

fh

fi

fh

af = x = tan X fhaf = tan P

cos H =

Plane of protractor

Measuring

section

Direction

of flow

PLAN VIEW

ELEVATION VIEW

Streambed

Plane of protractor

Measuring

section

Water surface and

direction of flow

a

f

H

e

h

j i

b

X

P

d
Measurement of Depth 19

5. If the angle increases appreciably when the additional line 

is let out, let out more line until the total additional line, 

the angle, and the vertical distance are in agreement with 

values presented in the air-correction table.

To place the meter at the 0.8-depth position, a correction 

to the amount of line reeled in must be made for the difference, 

if any, between the air correction for the sounding position and 

that for the 0.8-depth position. This difference is designated as 

m in table 5. If the angle increases for the 0.8-depth position, the 

meter must be lowered; if it decreases, the meter must be raised.

For the 0.8-depth position of the meter, the wet-line correction may require consideration if the depths are more than 

40 ft and if the change in vertical angle is more than 5 percent. 

If the vertical angle remains the same or decreases, the wet-line 

correction (table 3) for the 0.8-depth position is less than the 

wet-line correction for the sounding position by some difference, designated as n in table 5. If the vertical angle increases, 

the difference in correction n diminishes until the increase in 

angle is about 10 percent; for greater increases in angle, the difference between corrections increases also. Table 5 summarizes 

the effect on air and wet-line corrections caused by raising the 

meter from the sounding position to the 0.8-depth position.

For slight changes in the vertical angle, because of the differences m and n in the air and wet-line corrections, the adjustments 

to the wet-line length of the 0.8-depth position are small and 

usually can be ignored. Table 5 indicates that the meter may be 

placed a little too deep if the adjustments are not made. Because 

of this possibility, the wet-line depth instead of the vertical depth 

is sometimes used as the basis for computing the 0.8-depth position with no adjustments for the differences m and n.

Use of Sonic Sounder

The sonic sounder has been used primarily for measuring 

depth when making a moving boat measurement and in ADCP 

discharge measurements, and is generally not utilized for measurements where sounding weights are used. However, it can be 

used in swift, debris-laden streams, where it is difficult or dangerous to lower the sounding weight and meter into the water. 

The sonic sounder will record the depth when the weight is just 

below the water surface. For moving boat measurements, the 

sonic sounder records a continuous trace of the streambed on a 

digital or analog chart. Details of the setup and use can be found 

in Smoot and Novak (1969). Relevant information on the use of 

acoustic sounders in a riverine environment is discussed in detail 

in Mueller and Landers (1999) and by the International Organization for Standardization (2003). For use of a sonic sounder in 

ADCP measurements, consult Mueller and Wagner (2009).

Table 4. Degrees to be added to observed vertical angle, P, to obtain true vertical angle when flow direction is not normal to 

measurement section.

Observed vertical angle, 

P, in degrees

Horizontal angle, H, in degrees

8

cos = 0.99

12

cos = 0.98

16

cos = 0.96

20

cos = 0.94

24

cos = 0.91

28

cos = 0.88

8 0.1 0.2 0.3 0.5 0.8 1.0

12 0.1 0.3 0.5 0.8 1.1 1.5

16 0.1 0.4 0.6 1.0 1.4 2.0

20 0.2 0.4 0.7 1.2 1.7 2.4

24 0.2 0.5 0.8 1.4 2.0 2.8

28 0.2 0.5 1.0 1.5 2.2 3.0

32 0.2 0.6 1.0 1.6 2.4 3.3

36 0.2 0.6 1.1 1.7 2.5 3.4

Table 5. Summary table for setting the meter at 0.8-depth position in deep, swift streams.

Change in vertical angle

Air correction Wet-line correction

Direction of change Correction to meter position Direction of change Correction to meter position

None None None Decrease Raise meter the distance n

Decrease Decrease Raise meter the distance m Decrease Raise meter the distance n

Increase Increase Lower meter the distance m Decrease, then increase 1

1

Raise meter the distance n unless the increase in angle is greater than about 10 percent, then it is necessary to lower the meter the distance n.
20 Discharge Measurements at Gaging Stations

Measurement of Velocity

With point-velocity meters, after the width and depth at 

a vertical are measured and recorded, determine the method 

of velocity measurement. Normally the two-point method or 

the 0.6-depth method is used. Details of velocity measurement 

methods using various equipment and under different flow 

conditions are described in subsequent sections of this chapter.

Compute the setting of the meter for the particular 

method that will be used at that depth. For the top-setting wading rod, or the spiral-computing dial, some meter settings are 

self-computing. Record the meter position as 0.8, 0.6, 0.2, or 

another setting. After the meter is placed at the proper depth, 

let it adjust to the current before starting the velocity observation. With an ADV, make sure the instrument is located perpendicular to the tag line and the wading rod is plumbed (that 

is, vertical to the channel bed) before beginning measurement. 

The time required for adjustment to the undisturbed stream 

velocity is usually only a few seconds if the velocities are 

greater than 1 ft/s; however, for lower velocities, particularly 

if the current meter is suspended by a cable, a longer period 

of adjustment is needed. After the meter has become adjusted 

to the current, count the number of revolutions made by the 

rotor for a period of 40 to 70 seconds; for an ADV, this time 

period is typically programmed into the instrument’s discharge 

measurement software.

If using a stopwatch to time the revolutions of a mechanical current meter, start the stopwatch simultaneously with the 

first signal or click, counting “zero,” not “one.” End the count 

on a convenient number given in the mechanical current meter 

rating table column heading. Stop the stopwatch on that count 

and read the time to the nearest second. Record the number 

of revolutions and the time interval. If the velocity is to be 

observed at more than one point in the vertical, determine the 

meter setting for the additional observation, set the meter to 

that depth, time the revolutions, and record the data.

When using a current meter digitizer (CMD), a personal 

digital assistant (PDA), or an electronic notebook such as 

the Aquacalc or an ADV, observe the same basic procedure 

for setting the meter, and for providing time for the meter to 

stabilize. With these instruments, however, the counting and 

timing of the rotor revolutions or the acoustic pulse measurement are performed automatically. The number of revolutions, 

time, and velocity displayed by the CMD must be transferred 

manually to paper field notes, whereas, these data are electronically recorded by the Aquacalc, a PDA, or an ADV. When 

using any of these automatic meter counting devices and a 

mechanical current meter, make sure that multiple counts are 

not occurring during measurement of slow velocity. This can 

sometimes be determined by visually observing the rotation of 

the rotor while simultaneously listening to the audible clicks 

or beeps from the counting device. With the ADV, instead of 

audible clicks, acoustic Doppler theory is applied. The ADV 

can sense and measure velocities much smaller than those 

rated and measured by mechanical meters and are not prone to 

the multiple count errors of the mechanical current meter.

Current meters, in general, measure stream velocity at a 

point. One notable exception is the ADCP. This method will be 

discussed in a subsequent section of this chapter; a thorough 

discussion of a moving boat discharge measurement using an 

ADCP is documented in Mueller and Wagner (2009).

The method of making discharge measurements at a cross 

section by using a current meter that measures point velocities requires determination of the mean velocity in each of the 

selected verticals. The mean velocity in a vertical is obtained 

from velocity observations at several points in that vertical. 

The mean velocity can be approximated by making a few 

velocity observations and using a known relation between 

those velocities and the mean in the vertical. The various 

methods of measuring velocity are: vertical-velocity curve, 

two-point, 0.6-depth, 0.2-depth, three-point, and surface and 

subsurface.

Vertical-Velocity Curve Method

In the vertical-velocity curve method, a series of velocity observations at points well distributed between the water 

surface and the streambed are made at each of the verticals. If 

there is considerable curvature in the lower part of the verticalvelocity curve, then it is advisable to space the observations 

more closely together in that part of the depth. Normally, the 

observations are taken at 0.1-depth increments between 0.1 

and 0.9 of the depth. Observations are always taken at 0.2, 0.6, 

and 0.8 of the depth so that the results obtained by the verticalvelocity curve method may be compared with the commonly 

used methods of velocity observation. Observations are made at 

least 0.5 ft from the water surface and from the streambed with 

the Price AA meter, at least 0.3 ft from these boundaries with 

the Price pygmy meter, or at least 0.2 ft from these boundaries 

with the FlowTracker ADV. 

The vertical-velocity curve for each vertical is based on 

observed velocities plotted against depth, as shown in figure 12. In order that vertical-velocity curves at different verticals may be readily compared, it is customary to plot depths as 

proportional parts of the total depth. The mean velocity in the 

vertical is obtained by measuring the area between the curve 

and the ordinate axis with a planimeter, or by other means, and 

dividing the area by the length of the ordinate axis.

The vertical-velocity curve method is valuable in determining coefficients for application to the results obtained by 

other methods, but is not generally adapted to routine discharge measurements because of the extra time required to 

collect field data and to compute the mean velocity. A typical 

vertical-velocity curve for the cross section should be measured and evaluated at all new measurement sites, and perhaps 

at new measurement sections if the new section is significantly 

different in hydraulic characteristics than the section normally 

used at a regular measurement site.
Measurement of Velocity 21

Two-Point Method

This is the preferred method for making midsection 

discharge measurements with point velocity meters. In the twopoint method of measuring velocities, observations are made in 

each vertical at 0.2 and 0.8 of the depth below the surface. The 

average of these two observations is used as the mean velocity 

in the vertical. This method is based on many studies of actual 

observation and on mathematical theory. Experience has shown 

that this method gives more consistent and accurate results 

than any of the other methods, except for the vertical-velocity 

curve method. Use the two-point method for depths of 2.5 ft 

or greater, unless using a pygmy current meter or an ADV, in 

which case, this method is used in depths of 1.5 ft or greater.

With an ADV, the actual instrument has much less drag 

or resistance to the flow as compared to a mechanical current 

meter. To prevent boundary interference, avoid placing the 

ADV sample volume [typically 10 centimeters (about 4 in.) 

from the center transmitting transducer] within 2 in. from any 

solid boundary. This boundary condition with the ADV allows 

for measurement of velocity closer to the water surface and 

channel bed than a Price AA or to the pygmy (fig. 13). With 

the Price AA current meter, the two-point method is not used 

at depths less than 2.5 ft because the current meter would be 

too close to the water surface and to the streambed to give 

dependable results.

Six-Tenths-Depth Method

In the 0.6-depth method, an observation of velocity made 

at 0.6 of the depth below the water surface in the vertical is 

used as the mean velocity in the vertical. Actual observation and mathematical theory have shown that the 0.6-depth 

method gives reliable results and is used by the USGS under 

the following conditions:

Price AA Current Meter

1. Whenever the depth is between 0.3 ft and 2.5 ft.

2. When large amounts of slush ice or debris make it impossible to observe the velocity accurately at the 0.2 depth. 

(This condition prevents the use of the two-point method.)

3. When the meter is placed a distance above the sounding 

weight, which makes it impossible to place the meter at 

the 0.8 depth. (This condition prevents the use of the twopoint method.)

4. When the stage in a stream is changing rapidly and you 

must make a quick measurement.

Figure 12. Typical vertical-velocity curve.
22 Discharge Measurements at Gaging Stations

Wading rod

Tag line

Sampling volume

Acoustic

receiver

Wading rod

Sampling volume Acoustic

transmitter

Acoustic

transmitter

Acoustic

receiver

Probe

mounting

bracket

Minimum boundary Line indicating

minimum distance

to any fixed boundary

2” 2” 2”

Stream bed

Rock

Rock

A

B

Figure 13. Schematic showing A, plan vie and B, cross section of the FlowTracker sampling volume and the proximity to a fixed 

boundary in the stream. (Note: Probe mounting bracket not shown.)
Measurement of Velocity 23

Pygmy Current Meter

1. Whenever the depth is between 0.3 ft and 1.5 ft.

2. When large amounts of slush ice or debris make it impossible to observe the velocity accurately at the 0.2 depth. 

(This condition prevents the use of the two-point method.)

3. When the stage in a stream is changing rapidly and a 

measurement must be made quickly.

Acoustic Doppler Velocimeter

1. Whenever the depth is between 0.25 ft and 1.5 ft.

2. When large amounts of slush ice or debris make it impossible to observe the velocity accurately at the 0.2 depth. 

(This condition prevents the use of the two-point method.)

3. When the stage in a stream is changing rapidly and a 

measurement must be made quickly.

Two-Tenths-Depth Method

The 0.2-depth method consists of observing the velocity 

at 0.2 of the depth below the surface and applying a coefficient 

to this observed velocity to obtain the mean in the vertical. It 

is used mainly during times of high water when the velocities 

are great, making it impossible to obtain soundings or to place 

the meter at the 0.8 or the 0.6 depth.

Use a standard cross section or a general knowledge of 

the cross section at a site to compute the 0.2 depth when it 

is impossible to obtain depth soundings. A sizeable error in 

an assumed 0.2 depth is not critical because the slope of the 

vertical-velocity curve at this point is usually nearly vertical. The 0.2 depth is also used in conjunction with the sonic 

sounder for flood measurements. The two-point method and 

the 0.6-depth method are preferred over the 0.2-depth method 

because of their greater accuracy.

The discharge measurement is normally computed by 

using the 0.2-depth velocity observations without coefficients 

as though each were a mean in the vertical. The approximate 

discharge thus obtained divided by the area of the measuring 

section gives the weighted mean value of the 0.2-depth velocity. Studies of many measurements made by the two-point 

method show that for a given measuring section, the relation 

between the mean 0.2-depth velocity and the true mean velocity either remains constant or varies uniformly with stage. 

In either circumstance, this relation may be determined for a 

particular 0.2-depth measurement by recomputing measurements made at the site by the two-point method using only the 

0.2-depth velocity observation as the mean in the vertical. The 

plotting of the true mean velocity versus the mean 0.2-depth 

velocity for each measurement will give a velocity-relation 

curve for use in adjusting the mean velocity for measurements 

made by the 0.2-depth method.

If not enough measurements by the two-point method 

are available at a site to establish a velocity-relation curve, 

vertical-velocity curves are needed to establish a relation 

between the mean velocity and the 0.2-depth velocity. The 

usual coefficient to adjust the 0.2-depth velocity to the mean 

velocity is about 0.88.

Three-Point Method

The three-point method consists of observing the velocity 

at 0.2, 0.6, and 0.8 of the depth, thereby combining the twopoint and 0.6-depth methods. The preferred method of computing the mean velocity is to average the 0.2- and 0.8-depth 

observations and then average this result with the 0.6-depth 

observation. However, when more weight to the 0.2- and 

0.8-depth observations is desired, the arithmetic mean of the 

three observations may be used.

The three-point method is used when the velocities in the 

vertical are abnormally distributed [for example, the 0.2 (top) 

velocity is more than twice the 0.8 (bottom) velocity, or the 

0.8 (bottom) velocity is greater than the 0.2 (top) velocity]. 

It is also used when the 0.8-depth observation is made where 

the velocity is seriously affected by friction or by turbulence 

produced by the streambed or an obstruction in the stream. If 

using a Price AA, the depths must be greater than 2.5 ft to use 

this method. If using a Price pygmy or ADV, the depths must 

be greater than 1.5 ft to use this method.

Surface and Subsurface Methods

Surface and subsurface methods consist of observing the 

velocity at the water surface or some distance below the water 

surface. Surface measurements may be made with the optical current meter, or by observing and timing surface floats. 

Subsurface measurements are made with a current meter at a 

distance of at least 2 ft below the surface to avoid the effect of 

surface disturbances. Surface and subsurface measurements 

are used primarily for deep swift streams where it is impossible or dangerous to obtain depth and velocity soundings at 

the regular 0.2, 0.6, and 0.8 depths.

Coefficients are necessary to convert the surface or 

subsurface velocities to the mean velocity in the vertical. 

Vertical-velocity curves obtained at the particular site are the 

best method to compute these coefficients. However, the coefficients are generally difficult to determine reliably because 

they may vary with stage, depth, and position in the measuring cross section. Experience has shown that the coefficients 

generally range from about 0.84 to about 0.90, depending on 

the shape of the vertical-velocity curve. The higher values 

are usually associated with smooth streambeds and normally 

shaped vertical-velocity curves, whereas the lower values are 

associated with irregular streambeds and irregular verticalvelocity curves.
24 Discharge Measurements at Gaging Stations

Direction of Flow Measurements

Consider the direction of flow because the component 

of velocity normal to the measurement section is that which 

must be determined by both mechanical and acoustic Doppler 

point-velocity current meters. Generally, for the mechanical 

meter, the relation for velocity components not normal to the 

measuring section can be visualized in figure 14, and should 

be corrected using the cosine of alpha.

Flow direction is also critical with respect to an ADV, since 

the ADV assumes a horizontal and perpendicular plane to the 

flow. The hydrographer must pay close attention to the flow 

angle reported by the FlowTracker. Always hold the wading 

rod (with FlowTracker attached) perpendicular to the tag line so 

that the pulse generated by the transmitter is parallel to the tag 

line. Ideally, the tag line should be set up in the cross section to 

be measured so that flow is perpendicular to the tag line. Flow 

angle, as calculated by the FlowTracker, is defined as the direction of flow relative to the x-direction of flow, so that:

, (10)

where Vy is the velocity in the y direction (parallel to the tag 

line), and

Vx is the velocity in the x direction (perpendicular to 

the tag line) used to calculate discharge.

The flow angle calculated by the FlowTracker can result 

from two sources: (1) the flow is not perpendicular to the tag 

line, and (2) the flow is perpendicular to the tag line but the 

wading rod is not held correctly relative to the tag line, as 

described above. Regarding source (1), some small angles and 

variation in the flow angle at a site is not unusual. However, if 

large fluctuations of flow angles are reported, make measurements at another section with more uniform flow. Regarding 

source (2), holding the FlowTracker so that it is skewed at 

any angle relative to the tag line will result in a measurement 

of velocity that is biased low. Small angles do not result in 

significant biases, but because of these biases, users should 

be careful to minimize this error. If the FlowTracker is held 

so that it is skewed at an angle of approximately 8 degrees 

from the tag line, the measured velocity may be in error by as 

much as 1 percent (assuming that flow is perpendicular to the 

tag line). Large variations in flow angles may be indicative of 

poor or inconsistent alignment of the wading rod or poor site 

selection for the measurement.

In a wading measurement, if the meter used is a horizontal-axis meter with a component propeller, such as the Ott 

meter, the propeller should be pointed upstream at right angles 

to the cross section, but only if alpha is less than 45 degrees. 

Such a meter will register the desired component of velocity 

normal to the cross section when alpha is less than 45 degrees. 

The same procedure should be used if an electromagnetic 

component meter is used. These meters also measure the component of velocity normal to the measuring section. Generally, 

for either type of meter, if alpha is greater than 45 degrees, the 

component meter should be pointed directly into the current, 

and the horizontal angle correction should be applied as 

described in the following paragraphs.

Other meters on a wading-rod suspension, such as the vertical-axis Price current meter, should be pointed into the current. 

Any meter on a cable suspension will automatically point into 

the current because of the effect of the meter vanes. When the 

meter is pointed into an oblique current, the measured velocity 

must be multiplied by the cosine of the angle (alpha) between 

the current and a perpendicular to the measurement section in 

order to obtain the desired normal component of the velocity.

Either of two methods may be used to obtain the cosine 

of the angle alpha. In the first method, use the field note sheet 

that has a point of origin (o) printed on the left margin and 

cosine values on the right margin (see figure 2A). Measure the 

cosine of the angle of the current by holding the note sheet in 

a horizontal position with the point of origin on the tag line, 

bridge rail, cable rail, or any other feature parallel to the cross 

section, as shown in figure 15. With the long side of the note 

sheet parallel to the direction of flow, the tag line or bridge rail 

Figure 14. Velocity components when flow is not normal to 

measuring section.

Direction of 

actual flow,

oblique velocity

 (Vmax Normal )

velocity

component

(V)

V = Vmax × cosα

Measuring

Section

α
Direction of Flow Measurements 25

will intersect the value of cosine alpha on the top, bottom, or 

right edge of the note sheet. The direction of the current will be 

apparent from the direction of movement of floating particles. 

If the water is clear of floating material, the edge of the note 

sheet is aligned parallel to the direction of movement. If no 

such material is available, the rather inelegant, but time-honored 

method of spitting into the stream can be used to discern the 

direction of flow. The position of the current meter may also 

be used if it can be seen below the water surface. Multiply the 

measured velocity by the cosine of the angle to determine the 

velocity component normal to the measurement section.

The second method of obtaining the cosine of the angle 

alpha involves the use of a folding rule that folds at 0.5-ft or 1-ft 

intervals. The rule must be graduated in hundredths of a foot and 

jointed every 0.5 ft or 1 ft. Extend the first 2 ft of the rule and 

place the 2.00-ft marker on the tag line or bridge rail, as shown in 

figure 16, with the rule aligned with the direction of flow. Fold the 

rule at the 1-ft mark so that the first foot of the rule is normal to 

the tag line or bridge rail. Make a reading where the 1-ft section 

intersects the tag line or bridge rail. That reading, subtracted from 

1.00, is the cosine of the angle alpha. For example, if the reading 

on the rule is 0.07 ft (fig. 16), the cosine of alpha equals 0.93.

The direction of flow, as observed on the surface of the 

stream, may not always be a reliable indication of the direction of flow at some distance below the surface. For instance, 

when measuring a stream influenced by tidal fluctuations, it is 

possible to have flow moving downstream near the surface of 

the stream, and flow moving upstream near the bottom of the 

stream. Therefore, whenever it is suspected that the direction 

of flow is variable at different depths, other means than those 

already mentioned must be used to determine the direction of 

flow. One such method is to use a rigid rod or pole, with a vane 

attached to the bottom, and an indicator parallel to the vane 

attached to the top. Another method is to use a sounding weight 

with a compass and remote readout, as described in the equipment section of this chapter. If the variation of the direction of 

flow in the vertical is not great, then an average value of the 

cosine of the angle may be used for computing the component 

mean velocity for the vertical. However, if the variation is considerable, then you may need to subdivide the vertical and make 

separate computations for each subdivision. This may require 

additional measurements of velocity in the vertical.

If available, an ADCP from a moving boat can be used to 

quickly discern multidirectional flow in the vertical and across 

the selected cross-section. In a tidal affected reach, an ADCP 

is the preferred method of measuring discharge because of its 

use in the measurement of three dimensional velocity through 

most of the water column.

Figure 15. Measurement of horizontal angle with measurement 

note sheet.

Point

of

origin

Tag line,

edge of cable car,

or bridge rail

Read

angle

coefficient

here

Flow

Form 9-275

Figure 16. Measurement of horizontal angle with a folding rule.

Tag line,

edge of cable car,

or bridge rail

1 ft

Read

this

Folding foot rule,

graduated in 

hundredths of a foot

Direction 

of flow
26 Discharge Measurements at Gaging Stations

Current-Meter Measurements by 

Wading

Current-meter measurements by wading are preferred, if 

conditions permit. Wading measurements offer the advantage 

over measurements from bridges and cableways because the 

hydrographer can usually chose the best of several available 

cross sections for the measurement. Figure 17 shows a wading 

measurement being made with a top-setting rod.

Use the type AA, pygmy, or ADV meter for wading measurements. Table 6 lists the type of meter and velocity method 

to use for wading measurements at various depths.

If a type AA meter is being used in a cross section where 

most of the depths are greater than 1.5 ft, do not change to the 

pygmy meter for a few depths less than 1.5 ft or vice versa. 

The Price AA meter is not recommended for depths of 1.0 ft 

or less because the registration of the meter is affected by its 

proximity to the water surface and to the streambed. However, 

it can be used at depths as shallow as 0.5 ft to avoid changing meters if only a few verticals of this depth are required. 

The type AA meter or the pygmy meter should not be used in 

velocities less than 0.2 ft/s unless it is absolutely necessary.

It is no longer recommended to use coefficients given by 

Pierce (1941) for the performance of current meters in water 

of shallow depth and low velocities.

When natural conditions for measuring are in the range 

considered undependable, modify the measuring cross section, 

if practical, to provide acceptable conditions. Often it is possible in small streams to build dikes to cut off dead water and 

shallow flows in a cross section, or to improve the cross section by removing the rocks and debris within the section and 

from the reach of stream immediately upstream from it. After 

modifying a cross section, allow the flow to stabilize before 

starting the discharge measurement.

Stand in a position that least affects the velocity of the 

water passing the current meter by facing the bank, with the 

water flowing against the side of the leg. Holding the wading rod at the tag line, stand from 1 to 3 in. downstream from 

the tag line and 18 in. or more from the wading rod. Avoid 

standing in the water if feet and legs would occupy a considerable percentage of the cross section of a narrow stream. In 

small streams where the width permits, stand on a plank or 

other support above the water rather than in the water. Velocity 

bias caused by effects of the hydrographer’s position can be 

significant. Observance of these conditions is important while 

using mechanical meters, ADVs, and any wading measurement where an obstacle could interfere with the natural flow 

conditions of the stream.

When using a Price meter, keep the wading rod in a 

vertical position and the meter parallel to the direction of flow 

while observing the velocity. If the flow is not at right angles 

to the tag line, measure the angle coefficient carefully. When 

using an ADV or other instrument that can measure the x

component velocity, the instrument should be aligned more 

precisely with the tag line. See the discussion of FlowTracker 

use and flow angles in the “Measurement of Velocity” section 

of this chapter.

During measurements of streams with shifting beds, the 

scoured depressions left by the hydrographer’s feet can affect 

soundings or velocities. Generally, place the meter ahead of 

and upstream from the hydrographer’s body and feet. Record 

an accurate description of streambed and water-surface configuration each time a discharge measurement is made in a 

sand-channel stream.

For discharge measurements of flow too small to measure 

with a current meter, use a volumetric method, Parshall flume, 

or weir plate. Those methods are described in subsequent sections of this chapter.

Figure 17. Wading measurement using a top-setting rod.

Table 6. Current meter and velocity-measurement method for 

various depths.

Depth, in feet Current meter Velocity method

2.5 and greater Price Type AA 0.2 and 0.8

1.5 - 2.5 Price Type AA 0.6

0.3 - 1.5 Price Pygmy 0.6

1.5 and greater Price Pygmy 0.2 and 0.8

0.3 - 1.5 ADV 0.6

1.5 and greater ADV 0.2 and 0.8
Current-Meter Measurements From Cableways 27

Current-Meter Measurements From 

Cableways

The Price type-AA current meter is generally used in 

conjunction with sounding weights and a sounding reel when 

measuring discharge from a cableway, although in recent 

years, using an ADCP mounted to a tethered craft has become 

much more widespread. Stationing (for width measurements) 

is usually determined from marks painted on the cableway. 

The velocity is measured by setting the meter at the proper 

position in the vertical, as indicated in table 7. Table 7 is 

designed so that no velocity observations will be made with 

the meter closer than 0.5 ft to the water surface. In the zone 

from the water surface to a depth of 0.5 ft, the current meter is 

known to give erratic results.

One problem found while measuring velocities from a 

cableway is that the movement of the cable car from one station to the next causes the car to oscillate for a short time after 

coming to a stop. Wait until this oscillation has decreased to a 

negligible amount before counting the revolutions.

By using a method of tagging the sounding cable at convenient intervals with streamers of different-colored binding 

tapes, each colored streamer being a known distance above the 

current-meter rotor (known as using tags), the meter can be 

kept under water at all times to prevent it from freezing in cold 

air. Tags are also used in measurements of deep, swift streams. 

See the section of this chapter on “Measurement of depth.”

If large amounts of debris are flowing in the stream, raise 

the meter up to the cable car several times during the measurement to be certain the pivot and rotor of the meter are free of 

debris. However, keep the meter in the water during the measurement if the air temperature is considerably below freezing.

During floods, there is always a danger of catching a 

submerged or floating object, such as a tree or log, which can 

endanger the sounding equipment, meter, and most importantly, the hydrographer. Always be sure that the sounding 

cable has been installed on the sounding reel, according to 

the breaking loads specified in table 8. This assures that the 

sounding cable will break when it reaches its end, thereby 

preventing a potentially serious accident where the cable car 

and hydrographer could be spilled into the stream. Also, for 

added safety, always carry a pair of lineman’s side-cutter pliers 

while making measurements from a cableway. If the sounding cable becomes hopelessly hung and does not break, as it 

should, cut the sounding line to ensure safety. Sometimes the 

cable car can be pulled to the edge of the water and the debris 

can be released.

When measurements are made from cableways where 

the stream is deep and swift, measure the angle that the meter 

suspension cable makes with the vertical due to the drag. The 

vertical angle, measured by protractor, is needed to correct the 

soundings to obtain the actual vertical depth, as described in 

the section on “Depth corrections for downstream drift of current meter and weight.”

Table 7. Velocity-measurement method for various suspensions 

and depths.

Suspension Minimum depth, in feet

0.6 method 0.2 and 0.8 method

15 C .51

, 30 C .5 1.2 2.5

50 C .55 1.4 2.8

50 C .9 2.2 4.5

75 C 1.0, 100 C 1.0, 150 C 1.0 2.5 5.0

200 C 1.5, 300 C 1.5 3.82 7.5

1

15 pound Columbus-type weight, 0.5 above channel bed.

2

Use 0.2 method for depths 2.5 to 3.7 feet with appropriate coefficient (for 

example, 0.87 or 0.88).

Table 8. Breaking loads for Ellsworth stranded cable.

Sounding cable Diameter, in inches Total number of 

strands

Rated total breaking load, in pounds

Recommended 

breaking load, in 

pounds

Number of strands 

to cut

Number of strands 

to remain

Ellsworth 0.084 0.084 36 500 250 15 21

Ellsworth 0.100 0.100 30 1,000 500 15 15

Ellsworth 0.125 0.125 30 1,500 500 20 10
28 Discharge Measurements at Gaging Stations

Current-Meter Measurements From 

Bridges

When a stream cannot be waded, a bridge may be used to 

obtain current-meter measurements. Many measuring sections 

under bridges are satisfactory for current-meter measurements, 

but cableway sections are usually better because they provide 

an unobstructed reach of the channel. In addition, cableways 

usually have no bridges constricting the free flow of the stream 

in the measuring reach.

No set rule can be given for choosing between the 

upstream or downstream side of the bridge while making a 

discharge measurement. The advantages of using the upstream 

side of the bridge are the following:

• Hydraulic characteristics at the upstream side of bridge 

openings usually are more favorable. Flow is usually 

smoother and there is less turbulence than at the downstream side of the bridge.

• Approaching drift can be seen and be more easily 

avoided.

• The streambed at the upstream side of the bridge is not 

likely to scour as much as at the downstream side.

The advantages of using the downstream side of the 

bridge are:

• Vertical angles are more easily measured because the 

sounding line will move away from the bridge.

• The flow lines of the stream may be straightened out 

by passing through a bridge opening with piers.

Using the upstream side or the downstream side of a 

bridge for a current-meter or ADCP measurement should be 

decided based on circumstances for each bridge. Consider the 

factors mentioned above and the physical conditions at the 

bridge, such as location of the walkway, traffic hazards, and 

accumulation of trash on piles and piers.

For an ADCP measurement with a tethered craft, unless 

a special rigid support for deployment or bank-operated 

cableway has been developed for the upstream side of the 

bridge, the downstream side of the bridge is usually where the 

ADCP is most conveniently deployed. Bridge piers can cause 

excessive turbulence during high streamflow, especially if 

debris accumulates on the piers and(or) the piers are skewed 

to the flow. The effect of bridge-pier-induced turbulence may 

be reduced when deploying an ADCP from the downstream 

side of the bridge by lengthening the tether to increase the 

distance between the bridge and the tethered boat. Close 

attention should be paid to the cross section to ensure that no 

large eddies that could cause flow to be nonhomogeneous are 

present. Possible alternatives to measuring off the downstream 

side of the bridges include using a bank-operated cableway, a 

rigid extension that allows the hydrographer to deploy from 

the upstream side of the bridge, or having personnel on each 

bank hold a rope or cord attached to the platform to pull the 

tethered boat back and forth across the river.

For a mechanical current meter measurement, use either 

a handline, or a sounding reel supported by a bridge board or 

a portable crane, to suspend the current meter and sounding 

weight from bridges. Depth measurements should be made 

as described in the section entitled “Measurement of depth.” 

Measure the velocity by setting the meter at the position in 

the vertical as indicated in table 7. Keep equipment several 

feet from piers and abutments if velocities are high. Estimate 

the depth and velocity next to the pier or abutment on the 

basis of the observations at the nearest vertical.

If there are piers in the cross section, it is usually necessary to use more than 25 to 30 partial sections to get results 

as reliable as those from a similar section without piers. Piers 

will often cause horizontal angles that must be carefully measured. Piers also cause rapid changes in the horizontal-velocity 

distribution in the section.

Whether or not to exclude the area of a bridge pier from 

the area of the measurement cross section depends primarily 

on the relative locations of the measurement section and the 

end of the pier. If measurements are made from the upstream 

side of the bridge, it is the relative location of the upstream 

end (nose) of the pier that is relevant; for measurements made 

from the downstream side, it is the location of the downstream 

end (tail) of the pier that is relevant. If any part of the pier 

extends into the measurement cross section, the area of the 

pier is excluded. Bridges quite commonly have cantilevered 

walkways from which discharge measurements are made. In 

these cases, the measurement cross section lies beyond the end 

of the pier (upstream from the pier nose or downstream from 

the pier tail, depending on which side of the bridge is used). In 

that situation, it is the position and direction of the streamlines 

that determines whether or not the pier area is to be excluded. 

If the stationing of the sides of the pier when projected to the 

measurement cross section was not already done, the hydrographer does it at this point. If there is negligible or no downstream flow in that width interval (pier subsection), then the 

pier is excluded. That is, if only stagnation and (or) eddying 

exists upstream from the pier nose or downstream from the 

pier tail, whichever is relevant, the area of the pier is excluded. 

If there is substantial downstream flow in the pier subsection, 

the area of the pier is included in the area of the measurement 

cross section. In that circumstance, the horizontal angles of 

the streamlines in and near the pier subsection will usually be 

quite large.

Footbridges are sometimes used for measuring canals, 

tailraces, and small streams. Rod suspension can be used from 

many footbridges. The procedure for determining depth in low 

velocities is the same as for wading measurements. For higher 

velocities, obtain the depth by the difference in readings at an 

index point on the bridge when the base plate of the rod is at the 

water surface and on the streambed. Measuring the depth in this 

manner will eliminate errors caused by the water piling up on 

the upstream face of the rod. ADCP tethered boats, handlines, 

bridge cranes, and bridge boards are also used from footbridges.

The handline can be disconnected from the headphone 

wire and passed around a truss member with the sounding 
Current-Meter Measurements From Ice Cover 29

in the holes in the ice sometimes causes difficulty in determining the depths. The total depth of water is usually measured 

with an ice rod or with a sounding weight and reel, depending 

on the depth.

Measure the distance from the water surface to the bottom of the ice with an ice-measuring stick. Do not use the icemeasuring stick if there is slush under the solid ice at a hole. In 

order to find the depth at which the slush ice ends, suspend the 

current meter below the slush ice with the meter rotor turning 

freely. Raise the meter slowly until the rotor stops. This point 

is used as the depth of the interface between water and slush. 

After the effective depth of the water has been determined, 

compute the proper position of the meter in the vertical as 

shown in figure 19.

Use the Price winter Water Survey of Canada (WSCan) 

current meter yoke, with a polymer rotor, under ice cover 

when slush ice is present because the cups are solid and cannot 

become filled with slush ice; this is what happens with the 

cups of the regular Price meter. For situations where slush ice 

is not present, use the Price winter WSCan current meter yoke 

weight on the bottom. This eliminates the need for raising 

the weight and meter to the bridge each time a move is made 

from one vertical to another; it is the principal advantage of a 

handline.

Safety is a primary consideration when measuring discharge from bridges. High-speed traffic can present a major 

safety hazard; in fact, it is no longer permissible to make 

discharge measurements from some Interstate route bridges 

without special permission. Observe all safety precautions, 

such as the use of traffic cones, traffic signs, and flag persons, 

that are prescribed in the USGS Water Science Center’s flood 

plan and safety plan.

Observe the same safety precautions regarding the snagging of debris, such as floating or submerged trees or logs, as 

described above for cableways.

Current-Meter Measurements From Ice 

Cover

Discharge measurements under ice cover, as shown in 

figure 18, are made under the most severe conditions, but 

are extremely important because a large part of the discharge 

record during a winter period may depend on one measurement. In recent years ADCPs and ADVs have increasingly 

been used to make measurements from ice cover.

Select the possible locations of the cross section to be 

used for measurement from ice cover during the open-water 

season when channel conditions can be evaluated. Commonly, 

the most desirable measurement section will be just upstream 

from a riffle because slush ice that collects under the ice cover 

is usually thickest at the upstream end of the pools created by 

riffles.

The equipment used for cutting or drilling the holes in the 

ice is described in a previous section of this chapter.

Never underestimate the danger of working on icecovered streams. When crossing, test the strength of the ice 

with solid blows using a sharp ice chisel. Ice thickness may be 

irregular, especially late in the season when a thick snow cover 

may act as an insulator. Water just above freezing can slowly 

melt the underside of the ice, creating thin spots. Ice that is 

bridged above the water may be thick but still be weak.

Cut the first three holes in the selected cross section at the 

quarter points to detect the presence of slush ice or poor distribution of the flow in the measuring section. If poor conditions 

are found, investigate other sections to find one that is free of 

slush ice and that has good distribution of flow. After finding 

a suitable cross section, make at least 20 holes in the ice for a 

current-meter measurement. Space the holes so that no partial 

section contains more than 10 percent of the total discharge. 

On narrow streams, it may be simpler to remove all of the ice 

in the cross section.

The effective depth of the water, as shown in figure 19, 

is the total depth of water minus the distance from the water 

surface to the bottom of the ice. The vertical pulsation of water 

Figure 18. A, Ice drill being used to cut holes and B, ice rod being 

used to support current meter for a discharge measurement.

A

B
30 Discharge Measurements at Gaging Stations

with regular Price metal cups. The old-style vane ice meter is 

no longer recommended, primarily because of its poor performance in slow velocities.

The velocity distribution under ice cover, when the water 

is in contact with the underside of the ice, is similar to that 

in a pipe, with a lower velocity nearer the underside of the 

ice. This is illustrated in figure 20. Use the 0.2- and 0.8-depth 

method for effective depths of 2.5 ft or greater, and the 0.6-

depth method for effective depths of less than 2.5 ft. Define 

two vertical-velocity curves while making ice measurements 

to determine whether any coefficients are necessary to convert 

the velocity (obtained by the 0.2- and 0.8-depth method or the 

0.6-depth method) to the mean velocity. Normally, the average 

of the velocities obtained by the 0.2- and 0.8-depth method 

gives the mean velocity, but a coefficient of about 0.92 usually 

is applicable to the velocity obtained by the 0.6-depth method.

When measuring the velocity, keep the meter as far 

upstream as possible to avoid any effect that the vertical pulsation of water in the hole might have on the meter. Eliminate 

as much as possible the exposure of the meter to the cold air 

during the measurement. The meter must be free of ice when 

the velocity is being measured.

If there is partial ice cover at a cross section, use the 

procedure described above where there is ice cover, and use 

open-water methods elsewhere. Figure 21. Part of note sheet for discharge measurement under 

ice cover.

Figure 20. Typical vertical-velocity curve under ice cover. DISTANCE BELOW WATER SURFACE, IN PERCENTAGE OF TOTAL DEPTH

Ice

VELOCITY, IN FEET PER SECOND

0.0 0.2 0.4 0.6 0.8 1.0 1.2

0

20

40

60

80

100

Figure 19. Method of computing meter settings for 

measurements under ice cover.

a

b

c

Streambed

Water

surface

Ice

a = Water surface to bottom of ice

b = Total depth of water

c = Effective depth (c=b-a)

0.2-depth setting = a + 0.2c

0.8-depth setting = b - 0.2c

0.6-depth setting = b - 0.4c
Current-Meter Measurements From Ice Cover 31

A sample sheet of discharge-measurement notes under ice 

cover is shown in figure 21. In this measurement, the verticalvelocity curves indicate that the 0.2- and 0.8-depth method 

gives the mean velocity and that the 0.6-depth method requires 

a coefficient of 0.92.

Current-meter measurements under ice cover are frequently made with a special winter-style sounding rod, ADV 

or ADCP, as described in this chapter. When depths are too 

deep for rod suspension, use an equipment assembly mounted 

on runners, such as shown in figure 22. For winter conditions, 

the 30-pound C-type weights should be used with a special, 

collapsible hanger assembly (shown in figure 22) that can be 

passed through an 8-in. hole in the ice. A handline can also be 

used for making ice measurements.

Where it is impractical to use a powered ice drill, use 

ice chisels to cut the holes. Ice chisels are usually 4- or 4.5-ft 

long and weigh about 12 pounds. Use the ice chisel when first 

crossing an ice-covered stream to determine whether the ice is 

strong enough to support the hydrographer. If a solid blow of 

the chisel blade does not penetrate the ice, it is safe to walk on, 

providing the ice is in contact with the water.

Some hydrographers supplement the ice chisel with a 

Swedish ice auger. The cutting blade of this auger is a spadelike tool of hardened steel that can cut a hole 6 to 8 in. in 

diameter by turning a brace-like arrangement on top of the 

shaft.

After the hole is made in the ice, water will be forced 

up, owing to the water being under pressure from the weight 

of the ice. In order to determine the effective depth of the 

stream, use ice-measuring sticks to measure the distance 

from the water surface to the bottom of the ice. Measuring 

this distance is done using a bar about 4 ft long, made of 

strap steel or wood, graduated in feet and tenths of a foot and 

having an L-shaped projection at the lower end. Hold the 

horizontal part of the L on the underside of the ice and read 

the depth to that point at the water surface on the graduated 

part of the stick. The horizontal part of the L is at least 4 in. 

long so that it may extend beyond any irregularities on the 

underside of the ice.

Figure 22. Collapsible hanger assembly for used with 30- and 

50-pound C-type weights, for measurements under ice (A, in 

measurement position and B, collapsed).

A

B

A

B
32 Discharge Measurements at Gaging Stations

position the boat. Night measurements by boat are not recommended because of the safety concerns.

When no tag line is used, the boat can be kept in the cross 

section by lining it up with flags positioned on each end of the 

cross section, as illustrated in figure 23. Flags on one bank would 

suffice but it is better to have them on both banks. Determine the 

position of the boat in the cross section by using a transit or total 

station on the shore and a stadia rod held in the boat. Another 

method of determining the position of the boat is by setting a 

transit or total station on one bank a convenient, known distance 

from and at right angles to the cross-section line. The position 

of the boat is determined by measuring the angle α to the boat, 

measuring the distance CE, and computing the distance MC as 

shown in figure 24. A third method of determining the position 

of the boat is done with a sextant read from the boat. Position a 

flag on the cross-section line and another at a known distance 

perpendicular to the line. The boat position can be computed by 

measuring the angle β with the sextant, as shown in figure 24. 

Boat position can also be determined by using a global positioning system with differential corrections (DGPS). This method is 

especially useful on wide streams and in flood plains where other 

methods of determining boat position are not applicable. Unless 

anchoring is more convenient, the motor must hold the boat 

stationary while readings are being taken.

Do not take boat measurements at velocities less than 

1 ft/s when the boat is subject to wave action. The up-anddown movement of the boat (and the meter) seriously affects 

the velocity observations. If the maximum depth in the cross 

section is less than 10 ft and the velocity is low, the hydrographer can use a rod for measuring the depth and for supporting 

the current meter. For greater depths and velocities, use a cable 

suspension with a reel and sounding weight. The procedure 

for measuring from a stationary boat using the boat boom and 

crosspiece is the same as that for measuring from a bridge or a 

cableway, as described in previous sections of this chapter.

Current-Meter Measurements From 

Stationary Boats

Discharge measurements are made from boats where 

no cableways or suitable bridges are available and where the 

stream is too deep to wade, although ADCP discharge measurements from a moving boat, now a USGS standard operating procedure, have largely replaced this method. Personal 

safety is the limiting factor in the use of boats on streams 

having high velocity of flow.

For boat measurements whether using a mechanical meter 

or an ADCP, select a cross section that has attributes similar to 

those described in the previous section “Site selection,” except 

for those listed in items concerning depth and velocity. There 

is no need to consider depth in a boat measurement because 

if the stream is too shallow to float a boat, the stream can 

usually be waded. Velocity, however, is an important concern. 

If velocities are too slow, mechanical current meter registration may be affected by an oscillatory movement of the boat, 

in which the boat (even though fastened to a tag line) moves 

upstream and downstream as a result of wind action. Vertical movement of the boat as a result of wave action may also 

affect a vertical-axis current meter. If velocities are too fast, it 

becomes difficult to string a tag line across the stream.

If it is feasible to use a tag line in making a boat measurement, string it at the measuring section by unreeling the line 

as the boat moves across the stream. After a tag line without a 

brake has been stretched across the stream, take up the slack 

by means of a block and tackle attached to the reel and to an 

anchored support on the bank. If there is traffic on the river, 

one person must be stationed on the bank to lower and raise 

the tag line to allow the river traffic to pass. Place streamers 

on the tag line so that it is visible to boat pilots. If there is a 

continual flow of traffic on the river, or if the width of the river 

is too great to stretch a tag line, other means will be needed to 
Current-Meter Measurements From Stationary Boats 33

Figure 23. Determining the 

position in a cross section 

using the stadia method.

Flow

Right edge

of water

Left edge

of water

Stadia rod

held in boat

Transit

Positioning

flags

Figure 24. Determining the 

position in a cross section 

using the angular method.

β

Direction

of flow

Sextant

on boat

Transit

Positioning

flags

A B D

M

MC = CE × tan α

MC = CE ÷ tan β

E

C

Right edge

of water

Left edge

of water

α
34 Discharge Measurements at Gaging Stations

Moving-Boat Measurements of 

Discharge

On large streams and estuaries, the midsection methods 

of measuring discharge are frequently impractical and involve 

costly and tedious procedures. There may be no facilities at 

remote sites. Where facilities do exist, they may be inundated 

or inaccessible during floods. At some sites, unsteady flow 

conditions require that measurements be made as rapidly as 

possible. Measurements on tide-affected rivers must not only be 

made frequently, but continually, throughout a tidal cycle. The 

moving-boat technique is a method of quickly measuring a large 

stream. It requires no fixed facilities, and it lends itself to the 

use of alternate sites if conditions make this applicable. Brief 

descriptions of three moving-boat methods are given in the following sections. It must be stated here that these methods have 

been almost entirely replaced by the use of ADCP discharge 

measurements. Details of ADCP setup and use can be found in 

Mueller and Wagner (2009) and subsequent section of this chapter discussing the moving boat method using an ADCP.

Manual Method, Using a Mechanical Current 

Meter

Smoot and Novak (1968) describe the manual movingboat technique in detail. It is similar to the mechanical current-meter measurement in that the velocity-area approach 

is used to determine the discharge. The total discharge is the 

summation of the products of the partial areas of the stream 

cross section and their respective average velocities. During 

the traverse of a stream by boat, a sonic sounder records the 

geometry of the cross section, and a continuously operating current meter senses the combined stream and boat 

velocities.

The current meter used for a moving-boat measurement 

is a component propeller type, such as the Ott current meter, 

with a custom body made for mounting on the leading edge 

of a vane. The current meter and vane assembly are attached 

to a vertical rod and bearing assembly that allows them to 

rotate freely. An angle indicator is located at the top of the rod, 

which indicates the crab angle.

It usually takes three people to make a moving-boat 

measurement: one to operate the boat, one to make crabangle observations, and one to take notes. The note keeper is 

responsible for recording the crab-angle observations and the 

current-meter pulses. Depths are recorded automatically on 

the sonic-sounder chart. Special computations and conversions, which are somewhat tedious, are required to determine stationing and normal mean velocities for each vertical. Discharge can then be computed similar to a standard 

velocity-area discharge measurement. Experience has shown 

that measurements obtained by the moving-boat technique 

compare within 5 percent of measurements obtained by conventional means.

Automatic Computerized Method, Using a 

Mechanical Current Meter

The automatic computerized moving-boat method is 

the same basic procedure as the manual moving-boat method 

described above, except that all readings of depth, velocity, 

crab angles, and boat position are automatically fed into an 

onboard computer. A substantial difference is that the automated moving-boat method requires only two crew members—a boat operator and an instrument operator. The manual 

method requires three crew members.

The automated method requires an electronic compass 

that provides automatic input of the crab angle. The current meter and depth sounder likewise provide automatic 

input of pulse rates and depth, respectively. The computer 

is programmed to make the conversions and computations 

that would normally be made manually, thereby speeding up 

the computation process and virtually eliminating arithmetic 

errors. A computed measurement is available immediately 

after completion of the data collection.

Moving-Boat Method, Using an ADCP

ADCPs can be used to measure unsteady, bidirectional, 

and other flows with nonlogarithmic velocity distributions—

problems hydrologists have faced for decades. ADCPs are 

called profilers because they provide measurements of velocity throughout the water column. The ADCP divides the water 

column into depth cells (also referred to by some software and 

references as “bins”) and reports a velocity for each depth cell; 

however, an ADCP cannot measure velocities near the water 

surface or near the bed. The length of the unmeasured zone at 

the water surface is a function of the draft of the instrument 

deployment, the effect of the transducer mechanics, and the flow 

disturbance around the instrument. The length of the unmeasured zone near the streambed is due to side-lobe interference, 

which is a function of the mechanics of transducers and the slant 

angle of the beams. The ADCP must be deployed below the 

water surface; therefore, it cannot measure the water velocity 

above the transducers.

Although ADCPs have no moving parts and typically 

require no calibration, the instruments and associated software 

and firmware are complex. Using quality-assurance procedures 

defined by Oberg and others (2005) and Mueller and Wagner 

(2009) will help identify potential instrument problems.

ADCPs can be mounted on either side of manned boats, 

off the bow, or in a well through the hull. A tethered boat 

can be defined as a small boat (usually less than 6.5 ft long) 

attached to a rope, or tether, that can be deployed from a 

bridge, a fixed cableway, a moving boat, or a temporary or 

permanent bank-operated cableway. The tethered boat should 

be equipped with an ADCP mount that meets all of the specifications for manned boats. Unmanned, remote-control ADCP 

boats are also used and allow the deployment of ADCPs where 

deployment with a manned boat or tethered boat may not be 

feasible or ideal. Similar to (but smaller than) a manned boat, 
Discharge Measurement of Deep, Swift Streams With a Mechanical Current Meter 35

Discharge Measurement of Deep, Swift 

Streams With a Mechanical Current 

Meter

Discharge measurements of deep, swift streams with a 

mechanical current meter usually present no serious problems 

when adequate sounding weights are used and when floating 

drift or ice is not excessive. Normal procedures must sometimes be altered, however, when measuring these streams. The 

four most common circumstances are the following:

1. It is possible to sound, but the weight and meter drift 

downstream.

2. It is not possible to sound, but a standard cross section is 

available.

3. It is not possible to sound, and a standard cross section is 

not available.

4. It is not possible to put the weight and meter in the water.

Procedures are described below for use during measurements made under each of these conditions. Use procedures 

for items 2, 3, and 4 where there is a stable cross section. The 

procedure for unstable channels must be determined by conditions at each location.

Possible To Sound, but Weight and Meter Drift 

Downstream

For some streams, it may be possible to sound the streambed, but because of the force of high velocities, the weight 

and meter are carried downstream. This may be a condition 

for only a few verticals near the center of the stream, or it may 

affect many of the verticals. Make corrections to the observed 

depths and meter settings to account for the downstream drift. 

These corrections are commonly referred to as “vertical angle 

corrections.” The procedure for computing vertical angle 

corrections is described in a previous section of this chapter 

entitled “Depth corrections for downstream drift of current 

meter and weight.” The corrections can be computed manually, or they may be computed automatically through the use 

of an electronic notebook or a PDA.

Not Possible To Sound, but Standard Cross 

Section Available

When it is not possible to sound the streambed, use a 

standard cross section from previous measurements at the 

bridge or cableway for determining depth. Such a cross section is useful only if all discharge measurements use the same 

permanent initial point for the stationing of verticals across the 

width of the stream. There should also be an outside reference gage or reference point on the bank or bridge to which 

the water-surface elevation at the measurement cross section 

a remote-control boat has self-contained motors and a remotecontrol system for maneuvering the boat across the river.

The procedures for predeployment preparation, field data 

collection, and processing of collected data are discussed in 

detail by Mueller and Wagner (2009). A detailed description 

of how an ADCP measures velocity and computes discharge 

and additional details on selected topics are presented in the 

appendices (Mueller and Wagner, 2009).

Networks of Current Meters

In the past, occasional special measurements made by 

USGS hydrographers have required simultaneous velocities at several points in a cross section, distributed either 

laterally or vertically. For example, it may be necessary 

to measure a vertical-velocity profile quickly in unsteady 

flows and to check it frequently in order to determine the 

changes in shape of the vertical profile, as well as the rates 

of these changes. In another example, for the measurement 

of tide-affected streams, it is desirable to measure the total 

discharge continuously during at least a full tidal cycle 

(approximately 13 hours). The need for so many simultaneous velocity determinations (one at each vertical in the cross 

section) for so long a period could be an expensive and 

laborious process using conventional techniques of discharge measurement.

A grouping of 21 current meters and special instrumentation 

has been devised in the past by the USGS to facilitate measurements of the types just described. The 21 meters are connected 

together so that the spacing between any two adjacent meters 

can vary up to 200 ft. In using this method, each meter should be 

uniformly calibrated and have sufficient handline cable to be suspended vertically from a bridge as much as 200 ft. Revolutions 

of the rotors are recorded by electronic counters that are grouped 

compactly in one box at the center of the bank of meters. The 

operator, by flipping one switch, starts all 21 counters simultaneously, and after an interval of several minutes, stops all counters. 

The indicated number of revolutions for the elapsed time interval 

is converted to a velocity for each meter. The distance between 

meters is known, and a record of stage is maintained to evaluate 

depth. Prior information at the site is obtained to convert point 

velocities in the verticals to mean velocities in those verticals. 

All of the information necessary to compute discharge in the 

cross section is available, and is tabulated for easy conversion 

to discharge. If possible a concurrent ADCP discharge measurement should be completed in close proximity and time to this 

measurement to corroborate the results.
36 Discharge Measurements at Gaging Stations

may be referred. If these conditions are met, use the following 

procedure to make a discharge measurement:

1. Determine the depths from the standard cross section, 

based on the water-surface elevation.

2. Measure the velocity at 0.2 of the depth at each vertical.

3. Compute the measurement in the normal manner using the 

measured 0.2-depth velocities as though they were the mean 

velocities in the vertical. Apply horizontal-angle corrections, 

if necessary. Use depths as determined in step 1 above.

4. Determine the coefficient to adjust the 0.2-depth velocity to 

the mean velocity on the basis of previous measurements at 

the site by the two-point method. See a previous section of 

this chapter entitled “Two-tenths-depth method.”

5. Apply the coefficient from step 4 to the computed discharge from step 3.

Not Possible To Sound, and Standard Cross 

Section Not Available

When it is not possible to sound the streambed and a standard cross section is not available, use the following procedure:

1. Reference the water-surface elevation before and after the 

measurement to an elevation reference point on a bridge, 

on a driven stake, or on a tree at the water’s edge. It is 

assumed here that no outside reference gage is available at 

the measurement cross section.

2. Estimate the depth and observe the velocity at 0.2 of the 

estimated depth. The meter should be at least 2.0 ft below 

the water surface. In the notes, record the actual depth the 

meter was placed below the water surface. If an estimate 

of the depth is impossible, place the meter 2.0 ft below the 

water surface and observe the velocity at that point.

3. Make a complete measurement, including some verticalvelocity curves, at a lower stage when you can sound the 

streambed.

4. Use the complete measurement and difference in stage 

between the two measurements to determine the cross 

section of the first measurement. To determine whether 

the streambed has shifted, compare the cross section with 

one taken for a previous measurement at that site.

5. Use vertical-velocity curves, or the relation between mean 

velocity and 0.2-depth velocity, to adjust the velocities 

observed in step 2 to mean velocity. Apply horizontalangle corrections as necessary.

6. Compute the measurement in the normal manner using 

the depths from step 4 and the velocities from step 5.

Not Possible To Put the Weight and Meter in 

Water

If it is impossible to put the sounding weight and 

mechanical current meter in the water because of high velocities and (or) floating drift, use the following procedure:

1. Obtain depths at the measurement verticals from a standard cross section, if one is available. If a standard cross 

section is not available, determine depths by the method 

explained above in the section “Not possible to sound, 

and standard cross section not available.”

2. Measure velocities and compute discharge using an ADCP.

3. Measure surface velocities by timing floating drift, or by 

using an optical or other approved noncontact flowmeter.

4. Compute the measurement in the normal manner, using 

the surface velocities as though they were the mean 

velocities in the vertical, and using the depths from step 1.

5. Apply the appropriate velocity coefficient to the discharge 

computed in step 3. Use a coefficient of 0.86 for a natural 

channel and 0.90 for an artificial channel.

The optical current-meter and ADCP measurement procedures are described in previous sections of this chapter and 

by Mueller and Wagner (2009). The optical current meter is 

portable, battery operated, and requires no great skill for quick 

and accurate readings of the surface rate of flow. The meter 

is not immersed, so it does not disturb the flow, and it is in no 

danger of damage from floating debris or ice. In many cases, 

the ADCP has become the most efficient alternative to the 

mechanical current meter where velocities are too great; however, the ADCP also has limitations. See the previous sections 

of this chapter for information on velocity, depth, turbidity, 

and other site-condition limitations of the ADCP.

Keep in mind that just after the crest, the amount of 

floating drift or ice is usually greatly reduced, and it may be 

possible to obtain velocity observations with a current meter. 

These observations can help define the velocity coefficient 

mentioned in step 5 above.
Recording Field Notes 37

Recording Field Notes

Field notes for a discharge measurement may be recorded 

on standard paper note sheets (for example, USGS Forms 

9-275-F, 9-275-I, and other special field forms). With the ADCP 

discharge measurement, the software attached to each instrument contains digital forms for the recording of some of the 

field data. The USGS has developed a paper form for recording 

field data observed during an ADCP discharge measurement 

(fig. 2D). With a current-meter discharge measurement, field 

forms can be recorded using an electronic notebook, such as the 

Aquacalc or a Personal Digital Assistant (PDA). With an ADV 

measurement, there are special field forms to accommodate its 

specifications and details. These methods are described in more 

detail in subsequent paragraphs in this section. The SWAMI 

program with a PDA (commonly used by the USGS) can be 

used to record discharge measurements, inspections, differential 

level surveys, and other field measurements. SWAMI has an 

interface with the National Water Information System (NWIS), 

so measurements are easily uploaded to NWIS (fig. 2C).

Standard Paper Note Keeping for a Mechanical 

Current-Meter Discharge Measurement

Paper note sheets, as shown in figure 2A, are the traditional way to record the field observations for a mechanical 

current meter, ADV, or ADCP discharge measurement. Generally, for each discharge measurement, the hydrographer should 

record the following information, at a minimum, on the front 

sheet of the measurement notes (the information may vary, 

depending on the meter and method being used):

• Measurement number, who computed, and who 

checked the measurement;

• Downstream station identification number and station 

name (station name includes stream name and location, 

to correctly identify an established gaging station). For 

a miscellaneous measurement, record the stream name 

and exact location of site;

• Date of measurement and members of measurement 

party (initials and last name);

• Measured channel width, area, average velocity (computed as a ratio of the measured discharge/measured 

area), average gage height, and discharge;

• Vertical velocity method(s) of measurement, number of 

sections, and change in gage height during the discharge measurement;

• Measurement method coefficient, horizontal-angle 

coefficient, type of meter suspension (for example, rod, 

100#C, and so forth) and whether tags were checked;

• Type of meter (for example, AA or pygmy), the current 

meter’s serial number; and the elevation of the meter 

above the channel bottom;

• Meter rating used (for example, Standard Rating No. 

2) and the most recent spin test results;

• Measurement percentage (after computed) from the 

existing stage-discharge rating, and the indicated shift 

in feet from that rating;

• GAGE READINGS: Do not erase inside this block on 

the front sheet. If an error is made, cross through the 

error and write the correct reading.

◦ Start time measurement using 24-hour clock time, 

and record the time zone (that is, EST, CST, EDT, 

and so forth).

◦ Record inside and outside gage, and also readings 

from recording devices (for example, data logger, 

graphic, and so forth).

◦ Compute weighted mean gage height either by averaging readings, or if sufficient change in gage height 

occurred, by using methods for weighting gage 

height discussed in this chapter.

◦ Compute gage-height correction caused by difference in true gage height (reference gage) and 

recorder or other gage that is reading incorrectly.

◦ Record the correct mean gage height.

• Samples collected: Indicate type of water-quality 

measurements and samples [that is, water-quality, sediment, and (or) biological], and indicate if the measurements are documented on separate sheets (that is, water 

quality, aux./base gage, other);

• Indicate whether the rain gage (if applicable) was 

serviced/calibrated;

• Briefly describe the weather (for example, sunny, 

cloudy, rainy, cold, or other);

• Record the air temperature in degrees Celsius and the 

time of the reading;

• Record the water temperature in degrees Celsius and 

the time of the reading;

• Record the check bar reading (if a wire weight is present), time of the reading, and any adjustments in elevation made to the check bar.

• Indicate the type of measurement (wading, cable, ice 

boat, and so forth) and location of measurement relative to the gage (upstream, downstream, and so forth).

• Rate the measurement based on the hydrologic/hydraulic conditions in which the measurement was made 

[that is, excellent (2 percent), good (5 percent), fair (8 

percent), or poor (more than 8 percent)].

• Flow: Document the hydraulic condition of the flow 

(steady, unsteady, where the flow was within the cross 

section, and so forth).

• Cross section: Geomorphologically describe the cross 

section (that is, sand, clay, cobble, and so forth), shape, 

presence of vegetation, and any other roughness affecting flow.
38 Discharge Measurements at Gaging Stations

• Document if the gage is operating and whether the 

record was removed during this visit;

• Note the battery voltage and the cleaning of the orifice 

or intakes;

• If appropriate, indicate pressure readings of the 

nonsubmersible pressure transducer (that is tank, line, 

bubble-rate (bubbles/minute);

• Indicate if appropriate, readings of extreme indicators 

of high flow.

• Document the condition of the crest stage gage (CSG) 

and record the high water mark (HWM) if a reading is 

available on the CSG and the reference elevation.

• Record any other HWM obtained at the gage, if appropriate.

• Control conditions: Describe what and where the 

control of flow is for the gage pool (that is, gravel riffle 

about 80 ft downstream of the gage, and so forth).

• REMARKS: Use this space to document any unusual 

conditions in the gage reach that might affect the 

measurement, record, or other pertinent information 

regarding the accuracy of the discharge measurement 

and conditions that might affect the stage-discharge 

relation and document any observer reading or results 

of discussions with an observer.

• Measure the gage height of zero flow as many times 

a year as possible; record to the nearest 0.1 ft. Do this 

by recording the gage height at the gage at the time the 

gage height at zero flow is measured. Subtract the gage 

height from the depth of flow at the point the gage 

height at zero flow is measured and rate this measurement as good, fair, or poor.

• Fill in all items on the front sheet or mark with a dash 

after the measurement is completed and computed.

Mechanical Current-Meter Inside Notes

In the inside notes of a mechanical current-meter discharge measurement, identify the measurement starting point by 

either left edge of water or right edge of water (LEW or REW, 

respectively), when facing downstream, and record the time 

you started the measurement. If a significant change in stage is 

expected during the measurement, periodically record the time 

for intermediate verticals during the course of the measurement. 

If possible, synchronize this time with the recording interval of 

the digital recorder or data logger. Intermediate times are important because if there is any appreciable change in stage during 

the measurement, these recorded times are used to determine 

intermediate gage heights, which are then used to compute a 

weighted mean gage height for the measurement, as described 

in a subsequent section of this chapter. When the measurement 

is completed, record the time and the bank of the stream (LEW 

or REW) where the section ends.

Begin the measurement by recording the distance from 

the initial point to the edge of the water. Measure and record 

the depth, and velocity (if any), at the edge of water. Compute 

the width using the midsection method described in a previous 

section of this chapter. Proceed across the measurement section by measuring and recording the distance of each vertical 

from the initial point; the depth at the vertical; the observation 

depths as 0.6, 0.2, 0.8, and so forth; the revolutions and time 

for each velocity observation; and the horizontal angle coefficient if different than 1.00.

Complete all computations required for the inside notes 

to determine the total width, area, and discharge. Transfer 

these values to the front sheet and complete other items on 

the measurement front sheet. The measurement computations 

should be made, and the note sheets completed, before the 

hydrographer leaves the gaging station.

Erasures of original field data are not allowed. This 

includes items such as gage readings, distances, depths, meter 

revolutions, times, horizontal-angle coefficients, and other 

field measurements that cannot be repeated. If a variable 

is remeasured, and it is necessary to change the originally 

recorded value of that variable, cross it out and record the new 

measurement above or adjacent to the original. The original 

measurement should remain legible, even though it is crossed 

out. On the other hand, it is permissible to erase computed 

values, such as velocities, areas, widths, and discharges.

Standard Paper Note Keeping for an ADV 

Discharge Measurement

Paper note sheets, as shown in figure 2B, are the traditional method to record the field observations for an ADV 

discharge measurement. If using paper note keeping with an 

ADV for each measurement, the hydrographer should record 

the information, at a minimum, as if it were a mechanical 

velocity-meter discharge measurement, with a few variations. 

Indicate the filename of the infield diagnostic test performed 

on the ADV during the discharge measurement. Fill in all 

items on the front sheet or mark with a dash after the measurement is completed and computed.

ADV Inside Notes

An electronic summation of an ADV discharge measurement is produced by the ADV software. This information contains much of the information on the front sheet 

of the standard discharge measurement form, plus depths, 

widths, velocities, angles, area, and discharge. Typically ADV 

software does all the computations for an ADV discharge 

measurement. Print this output and attach it to the discharge 

measurement form for archival. Recent programming with 

personal digital assistants (PDAs) has further facilitated the 

collection, processing, and entry of discharge measurements 

into digital databases. See the sections entitled Electronic 

counters” and “Other electronic counters, electronic field notebooks, and personal digital assistants” for further discussion of 

the use of PDAs for field measurements.
Mean Gage Height of Discharge Measurements 39

Mean Gage Height of Discharge 

Measurements

The mean gage height of a discharge measurement 

represents the mean height of the stream during the period 

the measurement was made and is referenced to the datum of 

the gaging station. Just as an accurate determination of the 

discharge is important, so is an accurate determination of the 

mean gage height because it is one of the coordinates used 

in plotting the discharge measurement to establish the stagedischarge relation. The computation of the mean gage height 

presents no problem when the change in stage is small (0.1 ft 

or less). At gage-height changes of less than 0.1 ft, the mean 

gage height for the discharge measurement can be obtained 

by averaging the gage heights at the beginning and end of 

the measurement, without significant error. Measurements, 

however, must sometimes be made during floods or regulation 

when the stage significantly changes more than 0.1 ft.

To compute an accurate mean gage height for a discharge 

measurement, read the gage at the beginning and end of the 

discharge measurement, and several times during the measurement if there are significant changes. If the station is equipped 

with an electronic data logger or DCP that automatically 

records at intervals of 15 minutes or less, you can take the 

intermediate gage-height readings from those instruments after 

the measurement is completed. The hydrographer should accurately synchronize watch time and recorder time, and should 

record watch time for selected verticals at intervals during the 

discharge measurement. If the recording interval is greater 

than 15 minutes (that is, 30 minutes or 1 hour), intermediate 

gage- height readings should be obtained by reading the gage 

once or twice during the discharge measurement.

If the change in stage during the measurement is greater 

than about 0.1 ft (Rantz, 1982, suggests a change of 0.15 ft), 

the mean gage height should be computed by weighting the 

gage-height readings. In the past, the mean gage height was 

computed by weighting the gage readings with partial discharges from the discharge measurement. Later studies show, 

however, that this method tends to overestimate the mean 

gage height. Time weighting has also been used to compute a 

weighted mean gage height, but this method tends to underestimate the mean gage height. Therefore, it is recommended 

that both methods of weighting be used for discharge measurements having stage changes of 0.10 or more, and that an average of the two results be used for the mean gage height.

Plot the gage-height readings so that intermediate readings can be interpolated where necessary. Pay particular attention to breaks in the slope of the gage-height graph. Figure 25 

illustrates a plot of gage heights for a discharge measurement. 

Gage heights for this measurement were determined from the 

stage recorder at 15-minute intervals.

In the discharge-weighting procedure, the partial discharges measured between recorded watch times are used with 

the mean gage height for that same time period. The equation 

used to compute the weighted mean gage height is:

, (11)

where H weighted mean gage height, in feet,

Q total discharge measured, in cubic feet 

per second = q1 + q2 + q3 + ... + qn,

q1, q2, q3,...qn amount of discharge measured during 

time interval 1, 2, 3...n, in cubic feet 

per second,

h1, h2, h3,...hn average gage height during time 

interval 1, 2, 3...n, in feet.

Figure 25 shows the computation of a discharge-weighted 

mean gage height. The graph at the bottom is a reproduction 

of the gage-height graph during the discharge measurement. 

To help explain the method, the discharges are taken from the 

current-meter measurement shown in figure 2A. The upper 

computation of the mean gage height in figure 25 shows the 

computation using the given formula. The lower computation was calculated using a shortcut method to eliminate the 

multiplication of large numbers. In this method, after the average gage height for each time interval has been computed, a 

base gage height, which is usually equal to the lowest average 

gage height, is chosen. Then, the difference between the base 

gage height and the average gage heights is used to weight the 

discharges. When the mean difference has been computed, the 

base gage height is added to it.

In the time-weighting procedure, the arithmetic mean 

gage height for time intervals between breaks in the slope of 

the gage-height graph is used with the duration of those time 

periods. The equation used to compute mean gage height is

, (12)

where H weighted mean gage height, in feet,

T total time for the measurement, in 

minutes (t

1

 + t2 + t3 + ...+ tn),

t

1

, t2, t3, ..., tn duration of time intervals between 

breaks in the slope of the gage height 

graph, in minutes, and

h1, h2, h3, ..., hn average gage height, in feet, during 

time interval 1, 2, 3, ..., n.

Using the data from figure 25, the computation of the 

time-weighted mean gage height is as follows:

Average gage height (h) in ft Time interval (t) in minutes h x t

1.92 15 28.80

1.70 15 25.50

1.67 15 25.05

1.88 15 28.20

Totals 60 107.55

The mean gage height is computed as H = 107.55/60 = 

1.79 ft.

In this example, there is little difference between the 

discharge-weighted mean gage height (1.77 ft) and the timeweighted mean gage height (1.79 ft). The average of the 
40 Discharge Measurements at Gaging Stations

two values, 1.78 ft, is the preferred mean gage height for the 

discharge measurement.

When extremely rapid changes in stage occur during 

a measurement, the weighted mean gage height is not truly 

applicable to the discharge measured. To reduce the range in 

stage during the measurement, try to reduce the time required 

for making the discharge measurement; however, keep in mind 

that shortcuts in the measurement procedure usually reduce the 

accuracy of the measured discharge. Therefore, measurement 

procedures during rapidly changing stage must be optimized 

to produce a minimal combined error in measured discharge 

and computed mean gage height. The following section of this 

chapter describes procedures for making discharge measurements during periods of rapidly changing stage.

Figure 25. Graph of readings used to compute a weighted mean gage height.
Discharge Measurements During Rapidly Changing Stage 41

Discharge Measurements During 

Rapidly Changing Stage

Discharge measurements during periods of rapidly changing stage are more difficult to obtain, and accuracy is often 

not as good as for measurements made when the stage is fairly 

constant. The computation of total discharge and the corresponding stage are both subject to more uncertainty when the 

stage significantly changes during the period of the measurement. Two procedures are suggested for shortening the time 

required for a discharge measurement. The first procedure is 

applicable more for large streams where the stage changes are 

usually not as great as for small streams. The second procedure is designed more for the flash-flood conditions experienced with small streams, where peaks can be of momentary 

duration, and where the rising and falling stage is rapid. Keep 

in mind that the moving-boat ADCP discharge measurement 

can be used during these conditions. For a more detailed discussion of procedures for using an ADCP moving-boat during 

rapidly changing stage, see Mueller and Wagoner (2009).

Measurements of Large Streams With Rapidly 

Changing Stage

During periods of rapidly changing stage in a large stream, 

make measurements as quickly as possible to keep the change in 

stage to a minimum. This will minimize discharge errors caused 

by the shifting of flow patterns and other variables as the stage 

changes, and will provide a more accurate stage computation 

for the measurement. To reduce the time required for making 

a discharge measurement, make fewer than the usual number 

of observations and shorten the time to make them. This is 

sometimes referred to as a shortcut method. Following is a list 

of some of the things that can be done to reduce the time:

1. Use the 0.6-depth method rather than the 0.2 and 0.8 method. 

If the 0.6 method cannot be used because the flow is too 

swift, or if debris makes it too hazardous, then use the 0.2-

depth method or the subsurface method. If an optical meter is 

available, use surface-velocity measurements.

2. Reduce the velocity observation time to about 20 to 

30 seconds. This is referred to as half-counts.

3. Only measure about 15 to 18 sections. For some conditions, even use less than 15 sections may be used.

4. Observe and record the watch time at about every third 

vertical. If possible, observe and record the stage once or 

twice during the measurement.

By incorporating the above practices, a measurement 

can usually be made in 15 to 20 minutes. The surface or 

subsurface method for observing velocities is used, then some 

vertical-velocity curves will be needed later to establish coefficients to convert observed velocity to mean velocity. Compute 

a weighted mean gage height for the discharge measurement, 

as described in a previous section of this chapter.

Use the determination of error in discharge measurements, as described by Sauer and Meyer (1992), to illustrate 

the difference in errors between the standard measurement 

procedure and the shortcut procedure. The results shown 

in table 9 are based on a firm and smooth streambed. The 

additional uncertainty caused by using the shortcut method 

is generally less than the error that can be expected from the 

shifting of flow patterns and other variables that may occur 

during periods of rapidly changing stage.

Measurements of Small Streams With Rapidly 

Changing Stage

A series of “instantaneous” discharge measurements can 

be made during flash flood conditions on small streams by rating individual subsections, or verticals. This method requires 

repeated observations of gage height, depth, and velocity at 

selected verticals during the rise and fall of the flood wave. 

Two procedures are described below, with the primary difference being the method of determining depth at each vertical. 

In the first procedure, the streambed elevation referenced to 

gage datum is predetermined for each selected vertical. Depth 

is then determined as the difference between the gage height 

and the streambed elevation. In the second procedure, depth 

is measured at the selected verticals by sounding each time 

velocity is observed. The first procedure is faster; however, it 

may not be suitable if the streambed is unstable.

The method of computing the discharge measurements is 

also slightly different for the two procedures. For both procedures, a rating of gage height versus mean velocity is required 

Table 9. Comparison of discharge measurement error for standard and shortcut methods.

[No., number; s, seconds; ft/s, feet per second; ft, feet]

Measurement Meter 

type

No. of 

verticals

Points in 

vertical Suspension Observation time, s

Mean 

velocity, 

ft/s

Mean 

depth, ft

Uncertainty

percent

Measurement 

rated

Standard AA 30 2 cable 50 2.50 10.0 2.3 Excellent/good.

Shortcut AA 15 1 cable 25 2.50 10.0 5.1 Good/fair.
42 Discharge Measurements at Gaging Stations

for each subsection, or vertical. For the second procedure, a 

rating of gage height versus depth is required for each vertical. 

The two procedures are described below.

Procedure 1—Depth Is Computed From 

Predetermined Streambed Elevations at Each 

Vertical

1. Select about 10 verticals, or subsections. For very small 

streams, you may use fewer verticals. Mark the selected 

verticals in some way so that repeated observations can be 

made at the same vertical each time.

2. Determine the streambed elevation referenced to gage 

datum for each selected vertical prior to making the series 

of discharge measurements. After the flood recedes, determine the streambed elevations at each vertical again to see 

if changes occurred during the flood. If the streambed is 

not stable, it will be necessary to interpolate the changes 

based on time and the best judgment of the hydrographer. 

Depth is determined at each vertical as the difference 

between this elevation and the gage height.

3. Take velocity observations at each vertical using the 

0.6-depth method. Full counts of 40 seconds or more are 

recommended, but half-counts may be used if the stream 

is rising or falling extremely fast. If the 0.6 method cannot 

be used, then take velocity observations at the 0.2 depth 

or the subsurface depth. If an optical or other approve 

noncontact flowmeter is available, use it to take surfacevelocity readings. For surface- and subsurface-velocity 

readings, it will be necessary to determine the coefficient 

required for converting the readings to a mean velocity. 

Meter positions should be based on the depth, as computed in item 2 above.

4. Make observations of other factors that would affect the 

computation of discharge, such as horizontal-angle coefficients.

5. Repeat the velocity and other observations at each of the 

selected verticals several times over the duration of the 

flood wave.

6. Record the watch time of each vertical measurement, and 

make corresponding gage- height observations frequently 

during the period of the flood wave.

7. Develop a rating of gage height versus mean velocity for 

each of the selected verticals. If surface- or subsurfacevelocity observations were made, apply adjustments so 

that the rating represents mean velocity in the vertical. 

In some cases, it may be necessary to develop more than 

one rating for each vertical. For instance, a rating for the 

rising side of the flood wave, and a separate rating for the 

falling side of the flood wave, may be necessary.

8. Select a gage height for which a discharge measurement 

is to be computed. Use a standard discharge measurement 

note sheet for computing the discharge measurement. 

Enter the stationing for the edge of water and for each of 

the selected verticals. Enter the depths at each vertical, 

computed on the basis of the selected gage height minus 

the streambed elevation. Enter the mean velocity at each 

vertical on the basis of the gage height versus mean velocity ratings. Enter other adjustments, such as horizontalangle coefficients, as observed during the observation of 

velocities. Compute the discharge measurement similar to 

a regular discharge measurement.

9. Repeat the process described in item 8 above for other 

selected gage heights. If the ratings of gage height versus 

mean velocity change, such as for rising and falling stage, 

then compute separate discharge measurements for the 

rising and falling limbs of the flood wave.

Procedure 2—Depth Is Measured by Sounding 

at Each Vertical

1. Select about 10 verticals, or subsections, as described in 

the first procedure above. For very small streams, fewer 

verticals may be used. Mark the selected verticals in some 

way so that repeated observations can be made at the 

same vertical each time.

2. Determine the depth for each selected vertical by sounding the streambed each time you measure a vertical. Use 

this method when it is possible to easily make soundings, 

and when there is a likelihood of streambed elevation 

changes caused by scour or fill during the course of the 

measurement.

3. Take velocity observations at each vertical using the 

0.6-depth method. Full counts of 40 seconds or more are 

recommended, but half-counts may be used if the stream 

is rising or falling extremely fast. It is not likely that you 

will need to make surface or subsurface observations 

because depth soundings are possible with this procedure. 

Meter positions should be based on the sounded depth.

4. Make observations of other factors that would affect the 

computation of discharge, such as horizontal-angle coefficients.

5. Repeat the observations of depth, velocity, and other variables at each of the selected verticals several times over 

the duration of the flood wave.

6. Record the watch time of each vertical measurement, and 

make frequent, corresponding gage-height observations 

during the period of the flood wave.

7. Develop a rating of gage height versus mean velocity for 

each of the selected verticals. As described in the first procedure, it may be necessary to develop more than one rating, 

such as for the rising and falling sides of the flood wave.

8. Develop a rating of gage height versus depth for each of 

the selected verticals. If streambed changes occur during 

the measurements, it will be necessary to take these into 

account by making appropriate corrections.
Correction of Discharge for Storage During Measurement 43

9. Select a gage height for which a discharge measurement is 

to be computed. Using a standard discharge-measurement 

note sheet, enter the stationing for the edge of water and 

for each of the selected verticals. Enter the depths at each 

vertical, based on the selected gage height and the ratings 

of gage height versus depth. Enter the mean velocity at 

each vertical on the basis of the gage height versus mean 

velocity ratings. Enter other adjustments, such as horizontal-angle coefficients, as observed during the observation 

of depths and velocities. Compute the discharge measurement similar to a regular discharge measurement.

10. Repeat the process described in item 9 above for other 

selected gage heights. If the depth and (or) mean velocity ratings change, such as for rising and falling stage, 

or for streambed scour or fill, then compute separate 

discharge measurements for conditions before and after 

the changes.

Correction of Discharge for Storage 

During Measurement

Most discharge measurements are made at or near the 

gaging station and the gage control. However, at some gages, 

it may be necessary to make discharge measurements at a 

substantial distance away from the gage and (or) control. For 

instance, during a flood, the only place to measure may be at a 

bridge located some distance from the gage. Or for some sites, 

the low-water section control may be located at a substantial 

distance downstream from the gage. If a discharge measurement is made at a substantial distance from the gage control 

during a change in stage, the discharge passing the control 

during the measurement will not be the same as the discharge 

at the measurement section. In these situations, an adjustment must be applied to the measured discharge to account 

for the change in channel storage that occurs between the 

measurement section and the control during the period of the 

measurement. The adjustment for channel storage is computed 

by multiplying the channel surface area by the average rate of 

change in stage in the reach between the measurement section 

and the control. The equation is

, (13)

where QG discharge going over the control, in cubic feet 

per second,

Qm measured discharge, in cubic feet per second,

W average width of stream between measuring 

section and control, in feet,

L length of reach between measuring section and 

control, in feet,

Δh average change in stage in the reach L during the 

measurement, in feet, and

Δt elapsed time during measurement, in seconds.

Determine the change in stage at each end of the reach 

(that is, at the control and at the measuring section) and use 

an average of these two values. Generally, the gage height at 

the gage is used at one end of the reach, and a reference point 

(RP) or a temporary gage is set at the other end of the reach. 

The water-surface elevation at each end of the reach is determined before and after the measurement to compute Δh. If the 

measurement is made upstream from the control, the adjustment will be plus for falling stages and minus for rising stages; 

if it is made downstream from the control, it will be minus for 

falling stages and plus for rising stages.

An example computation for a flood measurement that 

was made 0.6 mile upstream from the gage (and control) during a period of changing stage is shown below:

• Measurement made 0.6 mile upstream, L = 3,170 feet.

Average width between measuring section and control, 

W = 150 ft.

• Gage height at beginning of measurement, at the gage 

(and control) = 5.84 ft.

Gage height at end of measurement, at the gage (and 

control) = 6.74 ft.

Change in stage at gage (and control), 6.74 – 5.84 = 

+0.90 ft.

• Gage height at beginning of measurement, at measuring section, = 12.72 ft.

Gage height at end of measurement, at measuring section = 13.74 ft.

Change in stage at measuring section, 13.74 – 12.72 = 

+1.02 ft.

Readings taken at measuring section from a reference 

point before and after measurement.

• Average change in stage in the reach, Dh = 

(0.90+1.02)/2 = 0.96 ft.

Elapsed time during measurement, Dt = l.25 hours = 

4,500 seconds.

Measured discharge, Qm = 8,494 ft3/s.

,(14)

This discharge should then be rounded to 8,390 ft3/s, which 

represents the discharge at the gage, or control.

The adjustment of the measured discharge for storage 

between the gage (or control) and measuring site, as described 

above, is a separate and distinct problem from that of making 

adjustments owing to variable water-surface slopes caused by 

changing discharge. Those adjustments are related to stagedischarge rating analysis, and are described by Kennedy 

(1984), and by Rantz (1982). The storage adjustment they 

described should be made immediately following the completion of the discharge measurement, and the resulting adjusted 

discharge is later used for rating analysis.
44 Discharge Measurements at Gaging Stations

Instruments and Equipment

Point-velocity current-meter and profiler measurements 

are usually classified in terms of the method used to cross the 

stream during the measurement (that is, wading, bridge, cableway, or ice), the method used to compute the discharge, such 

as midsection, ADCP, flume, or volumetric, and the method 

used to compute the velocity (if applicable to the discharge 

method), such as Price AA meter, ADCP, or ADV. Instruments 

and equipment used in making current-meter measurements 

will vary, depending upon which of these measurement types 

are being used. Current meters, timers, and electronic and 

other counting equipment are generally common to all types 

of current-meter and profiler measurements. This section 

describes equipment currently used by USGS field offices.

Current Meters

A point-velocity current meter, in the context of this 

report, is a precision instrument calibrated to measure the 

velocity of flowing water in a single point or fixed volume. 

Several types of current meters are available for use, including 

rotating-element mechanical meters, electromagnetic meters, 

acoustic Doppler velocimeters (ADVs or FlowTrackers), 

acoustic digital current meters (ADCs), and optical meters.

The principle of operation for a mechanical meter is 

based on the proportionality between the velocity of the 

water and the resulting angular velocity of the meter rotor. By 

placing a mechanical current meter at a point in a stream and 

counting the number of revolutions of the rotor during a measured interval of time, the velocity of water at that point can be 

determined from the meter rating.

An electromagnetic current meter is based on the principle that a conductor (water) moving through a magnetic field 

will produce an electrical current. By measuring this current 

and the resultant distortion in the magnetic field, the instrument can be calibrated to determine point velocities of flowing 

water.

The acoustic meter or profiler (for example, ADV or 

ADCP) uses the Doppler principle to determine velocities of 

flowing water. Acoustic meters and profilers have been developed to measure point velocities in the vertical profile of an 

open-channel flow, as well as multicell vertical-velocity profiles. 

The ADCP has been adapted for use with the moving-boat 

method of measuring discharge, as described by Mueller and 

Wagner (2009). The ADV has been developed and adapted 

to mount on the standard USGS wading rod to measure point 

velocities in a manner similar to the method for measuring 

velocity with a vertical-axis rotating-element mechanical meter. 

ADVs will be described in a later section of this chapter.

The following sections describe the various types of 

current meters and profilers in more detail, give advantages 

and disadvantages of each, and provide guidance on care and 

maintenance.

Current Meters—Mechanical and Vertical Axis

Historically, the most commonly used meter by the USGS 

to measure open-channel velocities in rivers and streams has 

been the vertical-axis, mechanical current meter. The original 

prototype for this kind of current meter was designed and built 

in 1882 by W.G. Price, while he was working with the Mississippi River Commission. The Price current meter has evolved 

through a number of different models and refinements since 

1882, but the basic theory and concepts remain the same. The 

Price AA meter is the most commonly used mechanical current 

meter for discharge measurements made by the USGS; however 

there are other variations of this meter, such as the Price AA 

slow velocity, the Price pygmy, and the Price AA winter meter. 

The following sections describe the various Price meters in 

more detail, and table 10 summarizes the various configurations 

and recommendations for the Price current meter, Price AA low 

velocity, the Price pygmy, the Price AA winter yoke with polymer and metal cups, and the SonTek FlowTracker ADV.

Price AA Meter

Historically, most current-meter measurements made by 

the USGS have been made with the vertical-axis Price AA and 

the Price pygmy current meters, as shown in figures 26 and 27. 

The basic components of the Price AA meter include the shaft 

and rotor (bucket wheel) assembly, the contact chamber, the 

yoke, and the tailpiece. The rotor, or bucket wheel, is 5 in. in 

diameter and 2 in. high with six cone-shaped cups mounted on 

a stainless-steel shaft. A vertical pivot supports the vertical shaft 

of the rotor, hence the name vertical-axis current meter. The 

contact chamber houses the upper part of the shaft and provides 

a method of counting the number of revolutions the rotor makes. 

A reduction gear (commonly referred to as the penta gear) on 

the lower part of the shaft allows counting every fifth revolution of the rotor when it is activated. The penta gear is used 

in discharge measurements with very high velocities. Contact 

chambers that can be used on the Price AA meter are described 

in a later section of this chapter. The yoke is the framework that 

holds the other components of the meter. A tailpiece is used for 

balance and keeps the meter pointing into the current.

When placed in flowing water, the rotors of the Price current 

meters turn at a speed proportional to the speed of the water. For 

practical purposes, these current meters are considered nondirectional because they register the maximum velocity of the water, 

even though they may be placed at an angle to the direction of 

flow. Advantages of the vertical-axis current meter are:

1. They operate in lower velocities than do horizontal-axis 

meters.

2. Bearings are well protected from silt-laden water.

3. The rotor is easily repairable in the field without adversely 

affecting the rating.

4. USGS standard ratings apply to the Price AA and Price 

pygmy meters.

5. A single rotor serves for the entire range of velocities.
Instruments and Equipment 45

Table 10. Price current meter and SonTek ADV configurations, usages, and recommended ranges of depth (without boundary 

interference) and velocity.

Meter Contact chamber Counting method Rating

Velocity 

range, feet 

per second1

Depth range,

feet Remarks

Price AA

Standard, 

cat’swhisker 

and penta gear

Headphones, 

CMD2

, or 

EFN3

Standard 0.2 to 12

1.25 or greater

The Price AA meter can be 

used as a low-velocity 

meter if equipped with an 

optic contact chamber.

Individual 0.1 to 12

Magnetic 

CMD2

 or EFN3

Standard 0.2 to 12

Individual 0.1 to 12

Optic Standard or individual 0.1 to 12

Price AA, low 

velocity

Cat’s-whisker 

with double 

contact lobe on 

shaft. No penta 

gear. 

Headphones, 

CMD2

, or 

EFN3

Individual 0.1 to 12 1.25 or greater

This is the traditional Price 

AA low velocity meter. An individual rating 

is recommended; however, 

a standard rating can be 

used if less accuracy is 

acceptable.

Price pygmy Cat’s whisker

Headphones, 

CMD2

, or 

EFN3

Standard or individual 0.2 to 4.0 0.3 to 1.5 --

Price, winter 

WSC4 yoke, 

polymer cups

Cat’s whisker

Headphones, 

CMD2

, or 

EFN3

Individual, with 

suspension 

device

0.1 to 12 1.25 or greater

This meter is recommended 

for conditions where slush 

Magnetic or ice is present.

Optic CMD2

 or EFN3

Price, winter 

WSC4 yoke, 

metal cups

Cat’s whisker

Headphones, 

CMD2

, or 

EFN3

Individual, with 

suspension 

device

0.1 to 12 1.25 or greater

This meter is recommended 

for conditions where slush 

Magnetic or ice is not present.

Optic CMD2

 or EFN3

SonTek 

FlowTracker 

ADV

N/A N/A Individual 0.003 to 135 0.25 or greater

The FlowTracker has been 

documented by the USGS 

to provide velocities 

comparable to mechanical vertical axis current 

meters.

1

Low- and high-velocity limits shown in the table are based on a small-to-moderate extrapolation of the lower and upper meter calibration limits. It is not 

recommended that the meters be used for velocities less than the lower limit. The velocity rating for the Price meter may allow additional extrapolation in the 

upper range to about 20 feet per second. The upper range of the Price pygmy meter rating may be extrapolated to about 5 feet per second. Standard errors within 

the meter calibration limits are less than ±5% in all cases. Standard errors in the extrapolated range of velocities are unknown, but are probably within ± 5%. 

2

Current-meter digitizer. Observe cautions for low velocities. See text.

3

Electronic field notebook, such as Aquacalc or DMX. Observe cautions for low velocities. See text.

4

Water Survey of Canada.

5

Manufacturer’s specification.
46 Discharge Measurements at Gaging Stations

Price AA Meter (Slow Velocity)

In addition to the Price AA meter described above, there 

is a Price AA meter modified slightly for use in measuring 

low velocities. To reduce friction, the penta gear has been 

removed from this meter, and the shaft has two eccentrics 

making two contacts with the cat’s whisker per revolution. The 

low-velocity meter normally is rated from 0.2 to 2.5 ft/s and is 

recommended when the mean velocity at a cross section is less 

than 1 ft/s.

Price Pygmy Meter

A miniature version of the Price AA meter is the Price 

pygmy meter, as shown in figures 26 and 27, which is used 

for measuring velocities in shallow depths. The Price pygmy 

meter is scaled two-fifths the size of the standard meter and 

has neither a tailpiece nor a penta gear. The contact chamber 

is an integral part of the yoke of the meter. The Price pygmy 

meter makes one contact for each revolution and is used only 

for rod suspension.

Figure 26. Assembly drawing of the A, Price AA current meter and B, Price pygmy current meter.

Tailfin screw

Price Pygmy Meter 

Round-head set screw

Binding post

assembly

Contact chamber cap 

 Pivot nut with set screw

Slotted set screw

Shipping plug

Hub assembly with 

bucket-wheel nut

Hub nut

Bucket wheel

Slotted set screw

Binding post bushing

Thumb nut

Price pygmy shaft

Upper-bushing

bearing

Yoke alignment tool

 Pivot with nut and set screw

Single-count

binding post

Thumb nut Cap

Binding post bushing

 Shaft

Hanger screw

Tailfin screw

Penta-count

binding post

Price AA Current Meter

Nut with screw

Screw

Pivot with nut

and screw

Contact chamber

pivot screw 

Raising nut

Bucket wheel

hub with nuts

Hub nut

Shaft with 

magnet

 Spring

Reed switch 

with spring

Yoke alignment 

tool

Bucket wheel

Tailpiece assembly

 Balance weight

Magnetic head contact shamber

Penta Gear

assembly

 Binding post

Contact 

chamber

A

B
Instruments and Equipment 47

Price AA Winter Meter

In streams where slush ice is present, use a modified 

Price AA meter, as shown in figure 28. This meter is built 

with a WSCan winter-style yoke, and uses a polymer rotor 

(bucket wheel) in place of the standard metal-cup rotor. The 

WSCan winter-style yoke meters require an individual rating 

due to increased tolerances in manufacturing. The solid polymer rotor has the advantage that it does not fill with slush 

ice during a measurement, and the slush ice does not easily 

adhere to it. If slush ice is not present, alternative methods 

can be used, including replacing the polymer rotors with 

metal cups or using a pygmy meter. Regular Price AA meters 

with metal-cup rotors are also acceptable for slush-free 

conditions, if there are no problems with cutting the required 

larger holes through the ice. In recent years, the USGS has 

increasingly used ADCPs and ADVs as alternatives to the 

Price AA for making discharge measurements in streams 

where ice is present.

Current Meters—Mechanical and Horizontal 

Axis

A number of mechanical current meters are available that 

have a propeller-, or vane-, type of rotor mounted on a horizontal shaft. These meters are used extensively in Europe and 

some Asian countries, but very little in the United States, and 

are generally not recommended by the USGS because they are 

not as durable as the Price current meter. Horizontal-axis current 

meters include the Ott (Germany), Neyrpic (France), Haskell 

(U.S.), Hoff (U.S.), Braystoke (United Kingdom), and Valeport. 

Various models of each of these are also available. As a group, 

horizontal-axis current meters have the following advantages:

1. The rotor, or propeller, disturbs flow less than verticalaxis rotors because of axial symmetry with flow direction.

2. The rotor is less likely to be entangled by debris than 

vertical-axis rotors.

3. Bearing friction is less than for vertical-axis rotors 

because bending moments on the rotor are eliminated.

4. In oblique currents, some of these meters (for example, 

the Ott meter) measure the velocity normal to the measuring section when the meter is held normal to the measuring section.

5. Rotors with propellers of different pitches are available 

for some of the meters, allowing measurement of a considerable range of velocity.

See figures 29, 30, and 31, respectively, for examples of 

the Ott, Hoff, and Valeport current meters.

Figure 27. A, Price AA and B, Price pygmy current meters.

A

B

A

B

Figure 28. Price AA meter 

with winter-style yoke and 

polymer rotor.
48 Discharge Measurements at Gaging Stations

Current-Meter Contact Heads (Vertical Axis)

The Price current meter is normally fitted with a contact 

chamber that has a cat’s-whisker type of circuitry used for 

counting the number of revolutions of the rotor. Two other 

types of contact chambers—the magnetic switch type and the 

optical type—can be fitted to the Price AA meter.

Cat’s Whisker

When placed in flowing water, the rotor of the current 

meter turns at a speed proportional to the speed of the water. 

The number of revolutions of the rotor is obtained by counting 

electrical impulses generated in the contact chamber. An eccentric 

contact on the upper end of the rotor shaft wipes a slender alloy 

wire (cat’s whisker) attached to the binding post, which closes an 

electrical circuit. This electrical impulse produces an audible click 

in a headphone or registers a unit on a counting device. Contact 

points in the chamber are designed to complete the electrical 

circuit at selected frequencies of revolution, such as twice per 

revolution, once per revolution, or once per five revolutions (by 

use of the penta gear). The penta gear, wire, and binding post provide a contact each time the rotor makes five revolutions. Figure 

32 shows the contact chamber and shaft for the cat’s-whisker-type 

chamber, with a single-count binding post.

Two types of cat’s-whisker wires have been used: one is 

the simple bronze wire, and the other is the old type of wire 

with a small solder bead on the end of it. Adjust the cat’s 

whisker for the penta gear so that it always touches the penta 

eccentric, even when the penta counter is not in use. Otherwise, the meter rating may be affected.

Magnetic Switch

A contact chamber housing a magnetic-type switch, as 

shown in figure 32, is available to replace the cat’s-whiskercontact chamber. The magnetic switch is composed of glass that 

is enclosed in a hydrogen atmosphere and hermetically sealed. 

The switch assembly is rigidly fixed in the top of the meter head 

just above the tip of the shaft. The switch is operated by a small 

permanent magnet rigidly fastened to the shaft. Two types of 

magnets used: (1) a bar magnet and (2) a circular magnet. If 

the contact chamber uses the bar magnet, it should be identified 

with an “A” stamped on the top surface of the chamber to indicate it has been modified. Older, unmodified contact chambers 

with the bar magnet were found to under-register for velocities 

greater than about 2 ft/s. The chambers that utilize the circular 

magnets fit the standard rating throughout its range.

The magnetic switch quickly closes when the magnet is 

aligned with it, and then promptly opens when the magnet moves 

away. The magnet is properly balanced on the shaft. Any type 

of AA meter can have a magnetic switch added by replacing the 

shaft and the contact chamber. The magnetic switch is placed in 

the contact chamber through the tapped hole for the binding post. 

The rating of the meter is not altered by the change.

Figure 29. Ott current meter.

Figure 30. Hoff current meter.

Figure 31. Valeport current meter.
Instruments and Equipment 49

An automatic counter, as described in a later section of 

this chapter, is used with the magnetic-switch contact chamber. Do not use a headphone with the magnetic head because 

arcing can weld the contacts.

Optical Head

A contact head utilizing fiber-optics technology is available 

for reading the pulse rate of the Price AA current meter. A special 

rotor containing two fiber-optic bundles is attached to the upper 

end of the bucket-wheel shaft. The rotation of these fiber-optic 

bundles gates infrared light from a photo diode to a photo transistor, creating a pulse rate that is proportional to the rotor’s revolutions. The pulses are counted, stored, and then compared with a 

quartz crystal oscillator. This information is processed to display 

stream velocity on a liquid crystal readout. The display has three 

averaging periods selected by a rotary switch. The averaging 

periods range from a minimum of about 5 seconds to a maximum 

of about 90 seconds. The unit is powered by a 9-volt battery.

Output of pulses from the optical sensing unit can be 

counted by the current-meter digitizer and the electronic field 

notebooks described in subsequent sections of this chapter. A 

standard rating table based on tow-tank calibration tests at the 

USGS Hydrologic Instrumentation Facility (HIF) is used to 

convert pulse rate to stream velocity.

A special tail-fin assembly is required for the optical 

meter so it will balance properly when submerged. The vertical section of this tail fin is marked with the letters OAA, and 

the horizontal section is marked PAA.

Vertical-Axis Current-Meter Timers and Counters

The determination of velocity using a mechanical current 

meter requires that the number of revolutions of the rotor be 

counted during a specified time interval, usually 40 to 70 seconds. Several methods are available for timing and counting 

the revolutions, as described in the following paragraphs.

Stopwatch and Headset

For current meters having a cat’s-whisker-type contact 

chamber, an electrical circuit is closed each time the contact wire 

touches the single or penta eccentric of the current meter. A battery and headphone, as shown in figure 33, are parts of the electrical circuit, and an audible click can be heard in the headphone 

at each electrical closure. Some hydrographers have adapted 

compact, comfortable hearing-aid-type phones to replace headphones. Beepers that can be heard without the headset are also 

sometimes used. Do not use a headset, or similar device, with the 

magnetic contact chamber because arcing can weld the contacts.

Measure the time interval to the nearest second with a 

stopwatch. Figure 33 shows the standard analog stopwatch; 

however, a digital wrist watch can also be used.

A B

C

Figure 32. A, Price AA magnetic head current meter contact 

chamber; B, single count binding post; and C, shaft with magnet 

for magnetic heads.

Figure 33. A, Analog stopwatch and B, current-meter headset.

A B
50 Discharge Measurements at Gaging Stations

Electronic Counters

Current-Meter Digitizer (CMD)

A current-meter digitizer (CMD), or automatic electronic 

counter, as shown in figure 34, was developed for use with 

the cat’s-whisker-, optic-, and magnetic-contact chambers. It 

can be used with any of the mechanical, vertical-axis current 

meters, but be careful to avoid false counts when using it for 

low velocities when the cat’s-whisker-contact chamber is used. 

The CMD automatically counts and displays the number of 

revolutions of the current-meter rotor and the elapsed time. 

A buzzer produces an audible signal at each contact closure, 

and the total counts and elapsed time are shown in the display 

at the completion of the velocity measurement. A coded chip 

can be installed in the CMD that will compute and display 

the velocity from the standard rating table for the particular 

meter being used. The CMD is powered with five rechargeable 

batteries, and has an adapter that can be used to attach it to the 

top of a wading rod.

Other Electronic Counters, Electronic Field Notebooks, 

and Personal Digital Assistants

Electronic counters and timers for mechanical current 

meters are also available in some commercially available devices. 

Electronic field notebooks (EFNs), such as the JBS Instruments 

Aquacalc Pro Discharge Measurement Computer, are designed 

for electronic recording of discharge-measurement data. These 

will be described in more detail in a later section of this chapter. 

Both the Aquacalc and the Hydrological Services Current Meter 

Counter signal processor (CMCsp) contain built-in digitizers 

that count and time the current-meter rotor revolutions. Built-in 

ratings convert the revolutions and elapsed time to velocity. Just 

as with the CMD described above, the EFNs should be used with 

caution to avoid false counts when measuring low velocities with 

meters equipped with a cat’s-whisker-contact chamber.

The USGS is increasing development and use of personal 

digital assistants (PDAs) for primary data collection and processing of discharge measurements and surface-water field data. 

Software programs such as the Surface WAter Measurement 

and Inspection application (SWAMI) continue to streamline 

field activities by reducing the need for paper forms in the field 

through the development of electronic field forms written for the 

PDA. SWAMI, as described in previous sections of this chapter, 

is used in many USGS Water Science Centers, and other PDA 

applications and programs are continually being developed.

Care of the Vertical-Axis Mechanical Current 

Meter

There are a number of documents describing the care and 

maintenance of the vertical-axis current meters. Among these, 

the most important originated from the USGS and are by 

Smoot and Novak (1968) and by Rantz (1982), and Office of 

Surface Water Technical Memorandum No. 89.07 (1989) and 

Office of Surface Water Technical Memorandum No. 99.06 

(1999). These instructions represent a long history of experience based on field use of the meters, as well as from individuals in the Office of Surface Water Hydraulic Laboratory who 

have repaired and adjusted current meters to calibrate them 

within close tolerances. A brief description of the recommended procedure for checking the condition of a current 

meter, and for its care and cleaning during daily field use, is 

presented in the next few paragraphs. For complete details, 

consult the above-mentioned documents.

Figure 34. Current-meter digitizer.
Instruments and Equipment 51

Recommended Procedure Before, During, and 

After Each Discharge Measurement

1. Before each discharge measurement, make a visual 

examination of the meter cups or vanes, pivot, bearing, 

and shaft for damage, wear, or faulty alignment. Inspect 

the bearing surface for water. This will usually appear as a 

milky emulsification of oil and water on the lower bearing 

and pivot, and in the contact chamber. If water is found, 

dry the meter parts and re-oil because the presence of 

water will affect the performance of the meter. The lower 

bearing is probably the most susceptible to the entrance of 

water.

2. Spin the rotor to make certain it operates freely, and allow 

it to slowly return to a resting position. If the rotor does 

not turn smoothly, or if it stops abruptly, then it is a sign 

of some problem and it should be corrected before using 

the meter. Check the balance and alignment of the meter 

on the hanger or wading rod. Be sure that the conductor 

wire does not interfere with meter balance and rotor spin.

3. During measurements, check the meter periodically when 

it is out of the water to be sure that the rotor spins freely, 

and that there is no debris or other substance obstructing 

it.

4. After a measurement is completed, make another visual 

inspection as described above to ensure that nothing was 

damaged or caused the meter to malfunction during the 

measurement. If there is a problem, you may have to 

make another discharge measurement.

5. Timed spin tests (described later in this chapter) are not 

required for each discharge measurement. The visual 

inspection described above is preferred over timed spin 

tests made in the field.

Recommended Procedure After a Day of Use in 

the Field

1. Examine the pivot and bearing surfaces for wear and 

damage, especially the pivot point. The pivot should 

feel sharp, not rounded or dull. It should not have a burr 

detectable visually or with the fingernail. A magnifying 

glass is helpful in making this examination. If the pivot is 

dull or burred, replace it with a new one.

2. Clean and lightly oil the pivot, bearing, and upper shaft 

with current-meter oil. Do not use regular machine oil, 

such as “3-in-1,” because it tends to become gummy 

when exposed to water.

3. Check and carefully adjust cat’s-whisker contacts, if necessary. Cat’s whiskers should be made of simple bronze 

wire, not beaded wire.

4. After replacing the contact-chamber cap, spin the meter to 

see if it is operating correctly, as previously described. A 

timed spin test is not required.

Recommended Procedure After Each Field Trip

After each field trip, completely disassemble, inspect, and 

clean current meters. Make any necessary repairs. Detailed 

instructions for the disassembly, inspection, and adjustment 

of Price AA (both standard and magnetic head) and pygmy 

current meters are contained in the attachments to Office of 

Surface Water Technical Memorandum No. 99.06 (1999). A 

timed spin test may also be performed after each field trip, and 

after meter repairs.

Inactive Current Meters

Disassemble, inspect, and clean current meters as 

described above, prior to storing them. If the period of storage is less than 1 year, the meter may be used without further 

maintenance if an inspection and a spin test indicate it is 

operating properly. If the meter has been in storage longer than 

1 year, or an indeterminate period, complete inspection, cleaning, and adjustment before using the meter.

Spin Tests

A timed spin test, made in the field before and after each 

discharge measurement, is no longer a requirement as it was 

in the past. The visual tests as described above are adequate 

for checking the meter in the field. Note “OK” or “free” in the 

spaces on the front sheet of the discharge measurement for 

spin test information to indicate that the visual check of the 

meter was acceptable.

Perform the full-timed spin test under controlled conditions between field trips, when the meter is suspect, and before 

and after repairs. Place the meter on a stable, level surface 

to perform the spin test. There should be no wind currents 

or drafts that can affect the rotor spin. Sharply spin the rotor 

while starting a stopwatch. Stop the stopwatch when the rotor 

comes to a complete stop. The minimum, acceptable spin 

times are as follows:

All types of Price pygmy meters 0:45 seconds

All types of Price AA meters 2:00 minutes

These are considered to be absolute minimum spin times. 

Meters in good condition will perform substantially better.

Recordkeeping

Maintain a current-meter log to record the results of the 

timed spin tests for each current meter. In addition, the log 

should contain information that identifies the meter and rotor, 

a history of repairs to the meter, as well as the name of the 

person who checked the meter, and dates of occurrences. Figure 35 shows a recommended format for the current meter log. 

The current-meter log should become a permanent record and 

archived with other water-resources data.
52 Discharge Measurements at Gaging Stations

Figure 35. Example of a current-meter log (suggested format).
Instruments and Equipment 53

Rating of Mechanical Current Meters

In order to determine the velocity of the water from the 

revolutions of the rotor of a mechanical current meter, a relation must be established between the angular velocity of the 

rotor and the velocity of the water turning it. This relation is 

referred to as the current-meter rating, and is expressed in an 

equation or in tabular format.

The current-meter rating facility is operated by the 

USGS, and is located at the Hydraulics Laboratory of the HIF 

at Stennis Space Center, Miss. The rating facility consists of 

a sheltered, reinforced concrete basin 400 ft long, 6 ft wide, 

and 6 ft deep, commonly called the tow tank. An electrically 

driven car rides on rails alongside and extending the length 

of the water-filled basin, and carries the current meter at a 

constant rate through the still water. Although the rate of travel 

can be accurately adjusted by means of a hydraulic regulating gear, the average velocity of the moving car is determined 

for each run by making an independent measurement of the 

distance it travels during the time that the revolutions of the 

rotor are electrically counted. A scale graduated in feet and 

tenths of a foot is used for this purpose. Eight pairs of runs are 

usually made for each current meter. A pair of runs consists of 

two traverses of the basin, one in each direction, at approximately the same speed. Practical considerations usually limit 

the ratings to velocities ranging from 0.1 ft/s to about 15 ft/s, 

although the rating car can be operated at lower speeds. Unless 

a special request is made for a more extensive rating, the lowest velocity used in the rating is about 0.2 ft/s, and the highest 

is about 8.0 ft/s.

Because there is rigid control in the manufacture of 

the Price meter, virtually identical meters are produced and, 

for practical purposes, their rating equations are identical. 

Therefore, there is no need to calibrate the meters individually, 

which is a major advantage and time saver. Instead, a standard 

rating is established by calibrating a group of meters that have 

been constructed according to USGS specifications. This standard rating is essentially an average rating for the calibration 

group, and it is then supplied with all meters manufactured 

according to USGS specifications. Identicalness of meters is 

ensured by supplying the dies and fixtures for the construction 

of Price current meters to the manufacturer who makes the 

successful bid. Another advantage of the standard rating is that 

field repairs can be made to a meter without requiring that it 

be recalibrated. On the other hand, there are somewhat larger 

errors associated with the standard ratings, as opposed to the 

individual meter ratings. For additional details see Office of 

Surface Water Technical Memoranda Nos. 91.01 (1991) and 

99.05 (1999).

Standard current-meter ratings are not mandatory for use 

with the Price meters. For some applications, it may be appropriate to use individually rated meters to avoid the additional 

uncertainty of the standard ratings. All winter-style meters 

must be individually rated with the suspension device that will 

be used with it.

Standard current-meter ratings, as of 1999, have been 

defined for the Price AA with the cat’s-whisker- and magneticcontact chambers, and the Price pygmy with the cat’s-whiskercontact chamber. The standard rating for the Price AA with the 

fiber-optic-contact chamber was defined in 1991. These ratings 

are as follows:

• Price AA with cat’s-whisker- and magnetic-contact 

chambers (Standard rating No. 2)

V = 2.2048R + 0.0178 (15)

• Price pygmy with cat’s-whisker-contact chamber 

(Standard rating No. 2)

V = 0.9604R + 0.0312 (16)

• Price AA with fiber-optic-contact chamber

V = 2.194R + 0.014 (R<0.856) (17)

V = 2.162R + 0.041 (R>0.856) (18)

where V velocity, in feet per second (ft/s), and

R the number of rotor revolutions per second.

For convenience in field use, the data from the currentmeter ratings are reproduced in tables, samples of which are 

shown in figures 36 and 37. In figure 36, the velocities corresponding to a range of 3 to 200 revolutions of the rotor within 

a period of 40 to 70 seconds are listed in the tables.

In figure 37, the velocities corresponding to a range of 

3 to 350 revolutions of the rotor within a period of 40 to 70 

seconds are listed in the tables. This range in revolution and 

time has been found to cover general field requirements. To 

provide the necessary information for extending a table for the 

few instances where extensions are required, the equation of 

the rating table is shown in the heading.

Meters that have been rated by means of rod suspension, 

and then by means of cable suspension using Columbus-type 

weights and hangers, have not shown significant differences in 

their ratings. Therefore, no suspension coefficients are needed 

if weights and hangers are properly used.

The preceding discussion relates primarily to the Price 

current meters. Other types of meters, such as the horizontalaxis meters and the electromagnetic meters, can also be 

calibrated in the tow tank in a similar manner as the Price 

meters. The HIF designed and constructed a special tow tank 

for testing and calibrating the acoustic Doppler point-velocity 

FlowTracker velocimeter. In the near future, there are HIF 

plans to build and operate a tow tank designed to test and 

calibrate ADCPs.
54 Discharge Measurements at Gaging Stations

STANDARD RATING TABLE NO. 2 FOR PYGMY CURRENT METER (6/99)

EQUATION: V = 0.9604 R+ 0.0312 (R=revolutions per second)

Seconds

VELOCITY IN FEET PER SECOND

Revolutions

3 5 7 10 15 20 25 30 40 50 60 80 100 150 200

40 0.103 0.151 0.199 0.271 0.391 0.511 0.631 0.752 0.992 1.23 1.47 1.95 2.43 3.63 4.83

41 0.101 0.148 0.195 0.265 0.383 0.500 0.617 0.734 0.968 1.20 1.44 1.91 2.37 3.54 4.72

42 0.100 0.146 0.191 0.260 0.374 0.489 0.603 0.717 0.946 1.17 1.40 1.86 2.32 3.46 4.60

43 0.098 0.143 0.188 0.255 0.366 0.478 0.590 0.701 0.925 1.15 1.37 1.82 2.26 3.38 4.50

44 0.097 0.140 0.184 0.249 0.359 0.468 0.577 0.686 0.904 1.12 1.34 1.78 2.21 3.31 4.40

45 0.095 0.138 0.181 0.245 0.351 0.458 0.565 0.671 0.885 1.10 1.31 1.74 2.17 3.23 4.30

46 0.094 0.136 0.177 0.240 0.344 0.449 0.553 0.658 0.866 1.08 1.28 1.70 2.12 3.16 4.21

47 0.093 0.133 0.174 0.236 0.338 0.440 0.542 0.644 0.849 1.05 1.26 1.67 2.07 3.10 4.12

48 0.091 0.131 0.171 0.231 0.331 0.431 0.531 0.631 0.832 1.03 1.23 1.63 2.03 3.03 4.03

49 0.090 0.129 0.168 0.227 0.325 0.423 0.521 0.619 0.815 1.01 1.21 1.60 1.99 2.97 3.95

50 0.089 0.127 0.166 0.223 0.319 0.415 0.511 0.607 0.800 0.992 1.18 1.57 1.95 2.91 3.87

51 0.088 0.125 0.163 0.220 0.314 0.408 0.502 0.596 0.784 0.973 1.16 1.54 1.91 2.86 3.80

52 0.087 0.124 0.160 0.216 0.308 0.401 0.493 0.585 0.770 0.955 1.14 1.51 1.88 2.80 3.73

53 0.086 0.122 0.158 0.212 0.303 0.394 0.484 0.575 0.756 0.937 1.12 1.48 1.84 2.75 3.66

54 0.085 0.120 0.156 0.209 0.298 0.387 0.476 0.565 0.743 0.920 1.10 1.45 1.81 2.70 3.59

55 0.084 0.119 0.153 0.206 0.293 0.380 0.468 0.555 0.730 0.904 1.08 1.43 1.78 2.65 3.52

56 0.083 0.117 0.151 0.203 0.288 0.374 0.460 0.546 0.717 0.889 1.06 1.40 1.75 2.60 3.46

57 0.082 0.115 0.149 0.200 0.284 0.368 0.452 0.537 0.705 0.874 1.04 1.38 1.72 2.56 3.40

58 0.081 0.114 0.147 0.197 0.280 0.362 0.445 0.528 0.694 0.859 1.02 1.36 1.69 2.51 3.34

59 0.080 0.113 0.145 0.194 0.275 0.357 0.438 0.520 0.682 0.845 1.01 1.33 1.66 2.47 3.29

60 0.079 0.111 0.143 0.191 0.271 0.351 0.431 0.511 0.671 0.832 0.992 1.31 1.63 2.43 3.23

61 0.078 0.110 0.141 0.189 0.267 0.346 0.425 0.504 0.661 0.818 0.976 1.29 1.61 2.39 3.18

62 0.078 0.109 0.140 0.186 0.264 0.341 0.418 0.496 0.651 0.806 0.961 1.27 1.58 2.35 3.13

63 0.077 0.107 0.138 0.184 0.260 0.336 0.412 0.489 0.641 0.793 0.946 1.25 1.56 2.32 3.08

64 0.076 0.106 0.136 0.181 0.256 0.331 0.406 0.481 0.631 0.782 0.932 1.23 1.53 2.28 3.03

65 0.076 0.105 0.135 0.179 0.253 0.327 0.401 0.474 0.622 0.770 0.918 1.21 1.51 2.25 2.99

66 0.075 0.104 0.133 0.177 0.249 0.322 0.395 0.468 0.613 0.759 0.904 1.20 1.49 2.21 2.94

67 0.074 0.103 0.132 0.175 0.246 0.318 0.390 0.461 0.605 0.748 0.891 1.18 1.46 2.18 2.90

68 0.074 0.102 0.130 0.172 0.243 0.314 0.384 0.455 0.596 0.737 0.879 1.16 1.44 2.15 2.86

69 0.073 0.101 0.129 0.170 0.240 0.310 0.379 0.449 0.588 0.727 0.866 1.14 1.42 2.12 2.81

70 0.072 0.100 0.127 0.168 0.237 0.306 0.374 0.443 0.580 0.717 0.854 1.13 1.40 2.09 2.78

3 5 7 10 15 20 25 30 40 50 60 80 100 150 200

Figure 36. Example 

of a standard currentmeter rating table 

No. 2 for Price pygmy 

current meters with 

cat’s-whisker contact 

chamber.
Instruments and Equipment 55

Figure 37. Example of a standard current-meter rating table No. 2 for Price AA current meters with cat’s-whisker and magnetic 

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56 Discharge Measurements at Gaging Stations

Electromagnetic Current Meters

Electromagnetic current meters, with no moving parts, 

are commercially available for measuring point velocities. 

These meters are based on the principle that a conductor (in 

this case, water) moving through a magnetic field will produce 

an electric current. The velocity of the moving water can be 

related to the electric current produced, and the distortion created in the magnetic field. The electromagnetic meters can be 

accurately calibrated in a tow tank, similar to the calibration of 

mechanical meters; however, tests have shown that the electromagnetic meters are less accurate than the Price AA meters, 

especially at low velocities (less than about 0.5 ft/s). The Price 

AA meters also have less variance than the electromagnetic 

meters at all velocities. Advantages of the electromagnetic current meter are as follows: no moving parts; direct readout of 

velocity; and, in oblique flow, the velocity measured is normal 

to the measuring section when the meter is held normal to it.

Marsh-McBirney 2000

An electromagnetic current meter successfully used by the 

USGS for making discharge measurements is the Model 2000, 

produced by Marsh-McBirney. This meter, as shown in figure 

38, is designed to mount on a standard round or top-setting wading rod. The meter is not designed for cable suspension.

A display meter, also shown in figure 38, shows a direct 

readout of the velocity. No conversion equation or table is 

necessary. The meter must be kept clean for accurate readings, 

and it is recommended that the rating be occasionally spot 

checked to verify that it is still accurate. This can be done in 

two ways. First, submerge the meter in a bucket of still water 

to verify the zero point of the rating. Second, place the meter 

in close proximity to a Price AA meter, in flowing water, to 

verify that it gives the same velocity reading. If there are differences, rate the electromagnetic meter again in the tow tank.

Ott Electromagnetic Current Meter

An Ott electromagnetic current meter is available; however, it has not been used extensively in the United States. The 

Ott meter, shown in figure 39, works in a manner similar to the 

Marsh-McBirney meter.

Acoustic Current Meters

Acoustic Doppler current meters, with no moving parts, 

are commercially available for measuring point velocities. These meters are based on the Doppler principle. The 

velocity of the moving water is measured using the transmitted and received signals from sound pulses reflecting off 

particles in the moving water column. These acoustic meters 

can be accurately calibrated in a tow tank, similar to the 

calibration of mechanical meters. Advantages of an acoustic 

Doppler current meter are as follows: no moving parts; direct 

readout of velocity; and ability to sense very low velocities 

less than the rated velocities in standard mechanical current 

meters.

Acoustic Doppler Velocimeter (ADV)

The SonTek/YSI FlowTracker handheld ADV 

(”FlowTracker” and “ADV” are used interchangeably in 

this chapter) is designed as an alternative to the Price AA 

and pygmy meters for wading discharge measurements. The 

FlowTracker operates at an acoustic frequency of 10 MHz 

and measures the phase change caused by the Doppler shift 

in acoustic frequency that occurs when a transmitted acoustic 

signal reflects off particles in the flow. The magnitude of the 

phase change is proportional to the flow velocity. The phase 

difference can be positive or negative, allowing ADVs to measure positive and negative velocities. The FlowTrackermeasures the velocity at a rate of approximately 10 MHz, averages 

the data, and records 1-second velocity-vector data.

The maximum velocity the FlowTracker can measure is 

reduced when measuring flow that is not perpendicular to the 

transmitting transducer. The receiving transducers can measure a velocity range of only ±1.15 m/s (3.77 ft/s). A velocity 

component placed directly toward or away from the receiving 

transducers larger than 1.15 m/s (3.77 ft/s) will result in erroneous velocities. Because of the geometric arrangement of the 

transmitting and receiving transducers, a velocity of 4.5 m/s 

flowing perpendicular to the transmitting transducer face will 

result in the maximum velocity towards a receiving transducer 

of 1.15 m/s (3.77 ft/s).

The FlowTracker probe is mounted to a standard topsetting wading rod with a special offset-mounting bracket 

(fig. 40). This bracket is designed to locate the FlowTracker

probe at the front of the wading rod, with the sampling volume 

Figure 39. Ott electromagnetic current meter.

Figure 38. Marsh-McBirney Model 2000 electromagnetic 

flowmeter and display meter.
Instruments and Equipment 57

about 2 in. (5 cm) to the right of the wading rod. Although the 

probe is inserted into the flow, the sampling volume is about 

4 in. (10 cm) away from all physical parts of the probe, to 

minimize flow disturbance in the sampling volume.

FlowTrackers have several unique data-processing 

requirements because of their method of operation and 

some of the inherent limitations of the acoustic Doppler 

measurement technique. Unlike mechanical meters that use 

the momentum of the water to turn a propeller and directly 

measure the velocity of the water, the FlowTracker does not 

measure the velocity of the water. The FlowTracker measures 

the velocity of particles (sediment, small organisms, and 

bubbles) suspended in the flow, assuming that these particles 

travel at the same velocity as the water. Therefore, the quality 

of the measurement is dependent on the presence of particles 

within the sampling volume that reflect a transmitted signal. 

The FlowTrackerrecords the signal-to-noise ratio (SNR), 

standard error of velocity (based on 1-second data), angle of 

the measured flow (relative to the x-axis of the FlowTracker 

probe), number of filtered velocity spikes, and a boundary 

quality-control flag. These velocity and quality-assurance data 

may be used to evaluate the measurement conditions. Few 

similar quality-assurance data are available for Price currentmeter measurements.

Although a FlowTracker can measure within about 1.2 in. 

(3 cm) of a boundary, the velocity measurement might be 

affected by acoustic interference when the sampling volume 

is close to boundaries or underwater objects, even when the 

sampling volume is not directly on or past the boundary. At the 

start of each velocity measurement, if the probe detects nearby 

acoustic boundaries that could cause interference with the 

velocity measurement, a boundary adjustment is automatically 

made. The boundary adjustment attempts to overcome the 

possible interference by reducing the lag times of the acoustic 

signals transmitted by the FlowTracker, causing a reduction 

of the velocity range that can be measured. Any changes are 

noted in the boundary quality-control flag. Because the sampling volume is located about 4 in. (10 cm) from the transmitting transducer it can be difficult to ascertain the precise 

location of the sampling volume. If the sampling volume is 

on or past a boundary, the velocity data will be erroneous. Be 

careful to avoid boundaries while making measurements in 

depths less than 3.54 in. (9 cm), especially in channels with 

irregular bottoms.

Spikes in velocity data occur with any acoustic Doppler 

velocity sensor such as the FlowTracker. Spikes may have 

a variety of causes (for example, large particles in the flow, 

air bubbles, or acoustic anomalies). Velocity data from each 

FlowTracker measurement are evaluated to look for spikes. 

The FlowTracker spike filter is a variation on a method called 

“Tukey’s Outlier.” In this method, a histogram of each velocity component is calculated. The FlowTracker determines the 

lower quartile (Q1; 25 percent of samples are less than this 

value), the upper quartile (Q3; 75 percent of samples are less 

than this value), and the interquartile range (IQR = Q3-Q1). 

If the IQR is less than 0.015 m/s (0.049 ft/s), IQR is set to 

A

B

Figure 40. A, SonTek/YSI FlowTracker acoustic Doppler 

velocimeter (ADV) mounted on a standard top-setting wading rod 

and B, closer view of transmitting and receiving transducers and 

offset-mounting bracket.
58 Discharge Measurements at Gaging Stations

0.015 m/s (0.049 ft/s). Any value less than (Q1–2*IQR) or 

greater than (Q3+2*IQR) is considered a spike and is not used 

for mean-velocity calculations.

The FlowTracker measures magnitude and direction of 

velocity. The operator must keep the wading rod perpendicular 

to the tag line so that the pulse generated by the transmitter is 

parallel to the tag line, regardless of flow direction. To compute discharge, the FlowTracker uses the component of velocity perpendicular to the transmitting transducer and reports the 

flow angle from the FlowTracker’s x-axis as a quality-control 

value. A flow angle measured by the FlowTracker may be 

the result of flow that is not perpendicular to the tag line, or 

a wading rod that is not being held perpendicular to the tag 

line (operator error). Flow angles of less than 20 degrees with 

small variations between verticals are not unusual. Large 

fluctuations of flow angles between verticals, however, may 

indicate a poorly measured cross section. If there is angular 

flow, and the wading rod is oriented with the flow, the velocity used and resulting discharge would be biased high. If the 

flow is truly perpendicular to the cross section, but the wading 

rod is erroneously held at an angle, the velocity and resulting 

discharge would be biased low. To avoid possible errors in the 

measured velocities, it is important that the operator always 

carefully and accurately aligns the wading rod.

Signal-to-Noise Ratio (SNR)

Adequate signal-to-noise ratio is needed to obtain an 

accurate measurement of the flow velocity. SNR is a measure 

of the strength of the reflected acoustic signal relative to the 

ambient noise level of the instrument. SNR is a function of 

the concentration and size distribution of the particles that 

reflect the acoustic signal. SNR is recorded for each beam with 

each 1-second sample. The manufacturer states that optimal 

SNR is 10 decibels (dB) or above (SonTek/YSI, 2002). USGS 

policy is that FlowTrackers should not be used for measuring 

discharge if the SNR for any single beam is less than 4 dB.

Speed of Sound

The accuracy of hydroacoustics instruments like the 

FlowTracker is dependent on an accurate speed of sound. The 

speed of sound is primarily a function of the temperature and 

salinity of the water. The FlowTracker has a built-in temperature sensor. To verify that the temperature sensor is working 

correctly, take an independent water-temperature measurement prior to each discharge measurement. If the FlowTracker 

has been stored in an environment with a different ambient 

temperature from the water, the probe may need to be placed 

in the water for a period of time, allowing it to equilibrate with 

the water temperature. A 5°F error in temperature will result in 

approximately a 1-percent bias in the measured velocity. The 

speed of sound is also sensitive to salinity. A 5-part-per-thousand error in salinity would result in an approximate velocity bias of 1 percent, when used in saline environments like 

estuaries; therefore, the operator needs to measure the salinity 

and input the value into the FlowTracker.

Maintenance and Care

Although the built-in QCTest is reliable for detecting 

issues, a BeamCheck stores more system performance data 

and still may be needed to evaluate the unit in more detail 

when there is a potential issue.

QCTests and BeamChecks

• Perform a QCTest and store it with each measurement. 

When a QCTest is completed as part of a measurement, 

it will print out on the measurement summary.

• Complete a QCTest in flowing water with the sample 

volume away from any boundaries.

• Perform a BeamCheck if you notice any anomalies in the 

QCTest. Any failures in a QCTest require a BeamCheck.

• Perform a BeamCheck after any possible physical damage (drop, and so forth), firmware upgrade, or repair.

As stated previously, the FlowTracker is an acoustic 

Doppler velocimeter (ADV) that has been adapted to fit on 

a typical USGS streamgaging wading rod, developed by the 

USGS in cooperation with the SonTek/YSI Inc., and is widely 

used by the USGS. The FlowTracker has undergone extensive 

testing to evaluate differences between the FlowTracker performance and vertical-axis current meters (that is, Price AA, 

pygmy, and so forth).

The USGS Office of Surface Water, through the HIF, has 

put into place a process that will check and recalibrate each 

FlowTracker approximately every 3 years to ensure the quality 

assurance/quality control of this instrument in the measurement 

of the Nation’s surface-water resources. For additional details, 

see Office of Surface Water Memorandum 2010.02 (2010).
Instruments and Equipment 59

Acoustic Digital Current Meter (ADC)

Another development that is a potential alternative to the 

Price AA and pygmy meters for wading discharge measurements 

is the Ott acoustic digital current meter (ADC) (fig. 41). The Ott 

ADC operates with two transducers at an acoustic frequency of 

6 MHz, and measures the phase change caused by the Doppler 

shift in acoustic frequency that occurs when a transmitted acoustic signal reflects off particles in the flow. The phase measurement is restricted to ±180 degrees and uses a pulse scheme with 

two different time delays to resolve the phase ambiguity.

The velocity (V) is computed using the following formula:

, (19)

where V velocity (in distance per unit time),

c speed of sound in water (in distance per unit 

time),

ΔΦ computed phase difference, and

τ time lag between pulses.

The Ott ADCuses a pulse-coherent technique. Transmitted pulses have a known lag time (τ). Backscatter echoes are 

amplified in the sensor head and then sent to the handheld 

display where they are digitized. A stable quartz oscillator 

controls the measurement sequence.

The Ott ADC has several unique data-processing requirements because of its method of operation and some of the inherent limitations of the acoustic Doppler measurement technique. 

Unlike mechanical meters that use the momentum of the water 

to turn a propeller and directly measure the velocity of the water, 

the Ott ADC, as with other acoustic Doppler current meters in 

use in the USGS and elsewhere, does not measure the velocity 

of the water. The Ott ADC measures the velocity of particles 

(sediment, small organisms, and bubbles) suspended in the flow, 

assuming that these particles travel at the same velocity as the 

A

B C D

Figure 41. A, Ott acoustic digital current meter (ADC) mounted on a standard top-setting wading rod; B, closer view of transmitting and 

receiving transducers; C, ADC datalogger; and D, schematic of the sampling volume and transducers.
60 Discharge Measurements at Gaging Stations

Commonly, the transducer assembly consists of three to nine 

transducers that operate at a fixed, ultrasonic frequency, typically 300 to 3,000 kilohertz (kHz). The transducers are horizontally spaced around the transducer assembly; all transducers have the same fixed angle from the vertical, referred to as 

a “beam angle,” that is typically between 20 and 30 degrees. 

The transducer assembly may have a convex or concave 

configuration or, in the case of the phased array, an essentially 

flat surface. The pressure case is attached to the transducer 

assembly. Examples of several different ADCPs used in the 

USGS are shown in figure 43.

When an ADCP is deployed from a moving boat, it is 

connected by cable to a power source and by cable or radio 

modem to a portable microcomputer. The computer is used 

to program the instrument, monitor its operation, and collect 

and store the data. For a detailed description of how an ADCP 

measures velocity and computes discharge, and detailed 

instruction in the use of ADCP technology with reference to 

moving boats, see Mueller and Wagner (2009).

Midsection Method With an ADCP

When an ADCP is deployed to perform a midsection 

method discharge measurement, most best practices used 

for mechanical-meter discharge measurements still apply. 

These practices, including site selection, are well documented in Rantz and others (1982). The midsection method 

with an ADCP is similar to the midsection method used for 

mechanical-meter discharge measurements and involves 

measuring the channel area and water velocities of a stream 

at a cross section; however, instead of point measurements of 

velocity, with this method, the average velocity is obtained 

by profiling the velocity in the water column at each section. 

The channel is divided into a number of vertical subsections. 

Most natural channels must be divided into 20 or 30 subsections to adequately characterize their irregular geometry. The 

depth and average velocity are measured at each subsection 

and are applied to a subarea whose width extends halfway to 

the preceding and following observation points. The area of 

each subsection is determined by directly measuring width 

and depth. The average water velocity in each subsection is 

estimated using the measured velocity at elected locations in 

the vertical. The total discharge within the stream is the sum of 

the individual subsection discharges.

Additional details and in-depth discussion of ADCP 

technology, methodology, and quality assurance and quality 

control can be found in USGS publications by Simpson and 

Oltmann (1993), in Lipscomb (1995), Morlock (1996), Oberg 

and others (2005), and Mueller and Wagner (2009). The ADCP 

method is a relatively new and evolving technology, and as a 

result, there are ongoing changes to the hardware, software, 

and firmware.

water. Therefore, the quality of the measurement is dependent on 

the presence of particles within the sampling volume that reflect 

a transmitted signal. In addition, the Ott ADC measures depth 

by using an absolute piezoresistive pressure cell with a range of 

0 to about 16.4 ft (0–5 m). The pressure cell is located inside the 

probe and protected by the wading rod adapter, which is securely 

fastened with two screws. The hydrographer can estimate a depth 

without reading the top-setting rod.

Preliminary testing of the Ott ADC at the HIF indicates 

this technology has a lot of potential for use as an alternative 

to mechanical current meters.

Acoustic Doppler Current Profiler (ADCP)

The ADCP measures velocity magnitude and direction 

using the Doppler shift of acoustic energy reflected by material suspended in the water column, providing essentially a 

complete vertical profile of velocity.

Broadband, narrowband, and other pulse-to-pulse coherent ADCPs transmit pairs of acoustic pulses along a narrow 

beam from each of multiple transducers. As the pulses travel 

through the water column, they strike suspended sediment and 

organic particles (referred to as “scatterers”) that reflect some 

of the acoustic energy back to the ADCP. The ADCP receives 

and records the reflected pulses. The reflected pulses are separated by time differences (range gating) into successive volumes called “depth cells.” The frequency shift (known as the 

Doppler effect) is proportional to the velocity of the scatterers 

relative to the ADCP. The ADCP computes a velocity component along each beam, because the beams are positioned at a 

known angle from the vertical (usually 20 or 30 degrees) and 

in known horizontal orientations so that trigonometric relations can be used to compute three-dimensional water-velocity 

vectors for each depth cell. Therefore, the ADCP produces 

vertical profiles of velocity composed of water speeds and 

directions at regularly spaced intervals, vertical profiles of 

velocity, discharge profiles, and a wealth of information for a 

discharge measurement (fig. 42).

The Broadband ADCPs (such as the Teledyne RD Instruments Rio Grande ADCPs) use phase-coded pulses, such that 

many independent measurements of velocity can be made 

by a single Broadband pulse of the same length as a narrowband pulse. These independent measurements are averaged 

to produce a velocity with a lower uncertainty than would be 

possible with a single measurement. Narrow-band systems 

(such as the SonTek/YSI RiverSurveyor S5 and M9) typically 

compensate for this characteristic by pinging faster (sending 

more pulses per second, up to 20 Hz) and reporting a velocity 

based on the average of many pulses with a typical velocity 

output of 1 Hz.

Currently, the most common main external components 

of an ADCP are a transducer assembly and a pressure case. 
Instruments and Equipment 61 Figure 42. Screen capture of an ADCP measurement of discharge for the Pearl River near Columbia, MS (02489000).
62 Discharge Measurements at Gaging Stations

Figure 43. Examples of acoustic Doppler current profilers (ADCPs) used to measure discharge at gaging stations; A, Teledyne RD Instruments 600 kHz Rio Grande ADCP; B, 

Teledyne RD Instruments 600 kHz RiverRay phased array ADCP, and C, closeup view of phased array transducer; D, SonTek/YSI RiverSurveyor S5; E, SonTek/YSI RiverSurveyor 

M9; and F, Teledyne RD Instruments StreamPro ADCP.
Instruments and Equipment 63

Optical Current Meters

An optical current meter, as shown in figure 44, is a 

stroboscopic device designed to measure surface velocities in 

open channels without immersing equipment in the stream. 

The optical current meter is used principally in measurements 

of surface velocity during floods when it is impossible to use 

streamgaging equipment that requires placement in the water, 

because of extremely high velocities and high-debris content 

in the stream. Use of this technology and other technologies, 

such as radar, deserves further investigation in the measurement of discharge using surface velocities.

Sounding Equipment

Sounding (determination of depth) is commonly done 

mechanically; the equipment used depends upon the type of 

measurement being made. Measure depth and position in the 

vertical with a rigid rod or by a sounding weight suspended 

from a cable. The cable is controlled either by a reel or by a 

handline. A sonic sounder is also available, but it is usually 

used in conjunction with a reel and a sounding weight. The 

various equipment that can be used for sounding is described 

in the following paragraphs. In addition, ADCPs can sound 

depths using sophisticated algorithms that may have global 

positioning features and capabilities. Mueller and Wagner 

(2009) discuss ADCP sounding in more detail.

Top-Setting Wading Rods

The two types of wading rods commonly used are the 

top-setting rod and the round rod. The top-setting rod is preferred because of the convenience in setting the meter at the 

proper depth and because the hydrographer can keep his hands 

dry.

The top-setting wading rod, as shown in figure 45, has 

a 1⁄2-in. hexagonal main rod for measuring depth and a 3⁄8-in. 

diameter round rod for setting the position of the current 

meter.

The rod is placed in the stream so the base plate rests on 

the streambed, and the depth of water is read on the graduated 

main rod. When the setting rod is adjusted to read the depth of 

water, the meter is positioned automatically for the 0.6-depth 

method, as shown in figure 46. The 0.6-depth setting is the 

setting measured down from the water surface. This setting 

is the same as the 0.4-depth position when measured up from 

the streambed. When the depth of water is divided by 2, and 

this value is set on the setting rod, the meter will be at the 

0.8-depth position from the water surface. When the depth of 

water is multiplied by 2, and this value is set, the meter will be 

at the 0.2-depth position from the water surface.

Figure 44. Hydrographer using an optical current meter to 

measure surface velocity.

Figure 45. A, Top-setting wading rod and B, schematic of a topsetting wading rod with Price AA current meter and current-meter 

digitizer (CMD) attached.

A

B
64 Discharge Measurements at Gaging Stations

Round Wading Rods

The round wading rod, as shown in figure 47, consists 

of a base plate, lower section, sliding support, three or four 

intermediate sections, and a rod end (not essential). The parts 

are assembled as shown in figure 48. The meter is mounted on 

the sliding support and is set at the desired position on the rod 

by sliding the support. The round rod can be assembled into 

various lengths using the 1-ft sections, and it is easy to store 

and transport when disassembled.

Figure 46. Closeup view of setting scale on handle of top-setting 

wading rod.

Figure 48. Round wading rod with Price AA current meter 

attached.

Figure 47. Parts for the round wading rod.
Instruments and Equipment 65

Hanger Bars

The weight hanger is attached to the end of the sounding 

line by a connector. The current meter is attached to the hanger 

bar beneath the connector, and the sounding weight is attached 

to the lower end of the hanger bar.

There are three types of weight hanger bars, as shown in 

figure 51:

1. The Columbus or C type, 1⁄8 × 3⁄4 × 12 in. (for weights up 

to 150 pounds);

2. Heavy weight, 1⁄8 × 3⁄4 × 18 in. (for 200- and 300-pound 

weights);

3. Heavy weight, 1⁄8 × l 1⁄2 ×18 in. (for 200- and 300-pound 

sounding weights that have the slots properly extended to 

accommodate a l1⁄2-in. wide hanger bar).

Winter-Style Suspension Wading Rods

Measurements made under ice cover should use the 

WSCan winter sounding rods, either in the 1⁄2- or 1-in.-

diameter versions. These rods are available in sections so that 

the desired length can be assembled. A special foot fits the 

lower section, and the rods will accommodate the winter-style 

current-meter yoke, as shown in figure 49.

Sounding Weights

If a stream is too deep or too swift to wade, the current 

meter is suspended in the water from a boat, bridge, or cableway. A sounding weight is suspended below the current meter to 

keep it stationary in the water. The weight also prevents damage 

to the meter when the assembly is lowered to the streambed.

The sounding weights currently used are the Columbus weights, commonly called the C type, and are shown in 

figure 50. The weights are streamlined to offer minimum resistance to flowing water. The weights are available in 15-, 30-, 

50-, 75-, 100-, 150-, 200-, and 300-pound sizes. Each weight 

has a vertical slot and a drilled horizontal hole to accommodate a weight hanger and securing pin.

Figure 49. Water Survey Canada winter-style round 1-inch 

suspension rod and meter: A, with Price AA meter in a winterstyle yoke attached and B, closer view of round 1-inch diameter 

suspension rod.

A B

Figure 50. Columbus C-type sounding weights (15 through 300 

pounds).
66 Discharge Measurements at Gaging Stations

The Columbus hanger bar contains three holes in order to 

properly position the meter. The hanger screw of the currentmeter yoke is placed through the bottom hole to support the 

meter when a 30-pound sounding weight is used. The center of the meter cups is then 0.5 ft above the bottom of the 

weight. This arrangement is designated as 30 C .5, which 

means that a 30-pound Columbus weight is being used and 

the center of the meter cups is 0.5 ft above the bottom of the 

weight. The hanger screw goes through the middle hole when 

15- or 50-pound weights are used. The designations for these 

arrangements are 15 C .5 and 50 C .55. The hanger screw goes 

through the upper hole when 50-, 75-, 100-, and 150-pound 

weights are used. The designations for these arrangements are 

50 C .9, 75 C 1.0, 100 C 1.0, and 150 C 1.0. Each of the two 

heavy-weight hangers has only one hole for the hanger screw 

of the meter. The designations for these arrangements are 200 

C 1.5 and 300 C 1.5.

Weight-hanger pins of various lengths, as shown in figure 51, are available for attaching the sounding weight to the 

hanger bar. The stainless steel pins are threaded on one end to 

screw into the hanger bar and slotted on the other.

Sounding Reels

Several different types of sounding reels are available for 

use with the Columbus C-type weights. In general, a sounding 

reel has a drum for winding the sounding cable, a crank-andratchet assembly for raising and lowering the weight or holding it in any desired position, and a depth indicator. Table 11 

contains detailed information on each of the five most commonly used reels.

The A-pack reel, as shown in figure 52, is light, compact, 

and ideal for use at cableway sites a considerable distance 

from the highway. It can also be used on cranes, bridge boards, 

and boat booms.

Figure 51. Sounding weight hanger bars and hanger pins.

Table 11. Sounding reel data.

Reel Sounding cable

Cable

diameter,

in inches

Drum

circumference, 

in feet

Cable 

capacity, in 

feet

Maximum 

weight, in 

pounds

Depth indicator Brake Type of operation

A-pack Ellsworth 0.084 1 45 50 Counter No Hand.

Canfield Single conductor1 .0625 1 45 50 Counter No Hand.

A-55 Ellsworth .084

.10 1 9580

50

100 Self computing No Hand.

B-56 Ellsworth .10

.125 1.5 144115

150

200 Self computing Yes Hand or power.

E-53 Ellsworth .10

.125 2 206165

150

300 Self computing Yes Power.

1

Some Canfield reels have been converted to double-conductor cable but most of them are still used as single-conductor reels.

Figure 52. A-pack reel.
Instruments and Equipment 67

The B–56 reel (a major modification of the B–50 reel) 

can handle all but the heaviest sounding weights and has the 

advantage that it can be used with a hand crank or power 

equipment, as shown in figure 54.

The E–53 reel is the largest reel commonly used for 

current-meter measurements. This reel will handle the heaviest 

sounding weights and is designed exclusively for use with 

power equipment. It has a hand crank for emergency use, as 

shown in figure 55.

Sounding Cable

Ellsworth reverse-lay two-conductor stranded cable is 

normally used on all sounding reels, except the single-conductor Canfield reel, which uses galvanized steel aircraft cord. 

Ellsworth cables are normally available in 0.084-, 0.100-, and 

0.125-in. diameters. It is important to use the appropriate size 

cable-laying sheave on the reels.

For safety purposes, when measuring floods, it is important to connect the sounding cable to the sounding reel in such 

a way that the cable will break in the event that heavy debris is 

caught and cannot be released. The cable will usually unwind 

(pay out) from the sounding reel until it reaches its end, at 

which point there is danger to the equipment and the hydrographer unless the cable is cut or breaks. Precut some of the 

cable strands when installing the cable on the reel so that the 

remaining strands will break when the load reaches a specified limit. Table 8 provides information about cable strength 

and number of strands to cut to provide the necessary safety 

margin. Complete instructions for making the cable installation are given in a HIF technical information sheet dated April 

1999.

The Canfield reel, as shown in figure 53, is also compact 

with uses similar to that of the A-pack reel. The Canfield reel 

is not available from the HIF, and must be obtained from 

Leupold and Stevens Instruments, Inc.

The A–55 reel is for general purpose use with the lighter 

sounding weights, as shown in figure 54.

Figure 53. Canfield reel.

Figure 54. A, A–55 reel and B, B–56 reel.

B

A

Figure 55. E–53 reel.
68 Discharge Measurements at Gaging Stations

Handlines

A handline, shown in figure 58, is a device used for 

making discharge measurements from bridges using a 15- 

or 30-pound sounding weight. The advantages of using the 

handline are that it is easy to set up, it eliminates the use of a 

sounding reel and supporting equipment, and it reduces the 

difficulty in making measurements from bridges, which have 

interfering members, such as trusses. The disadvantages of 

using the handline are that there is a greater possibility of 

making errors in determining depth because of slippage of the 

handline, measuring scale, or tape, and it requires great physical exertion, especially in deep streams. Handlines can be used 

from cable cars, but this is not recommended because of the 

disadvantages mentioned above.

Ellsworth cable is recommended for handlines because of its 

flexibility and durability. Two-conductor electrical service cord is 

used between the headset connector and the handline reel.

The pressed-sleeve connector or the C1 connector is used 

on handlines because they are lighter in weight than the type 

B connector, yet strong enough for the sounding weights used 

with handlines.

Connectors

A connector is used to join the end of the sounding cable 

to the sounding-weight hanger. The three types of connectors 

generally used are types B, C1, and pressed sleeve, as shown 

in figure 56. The type B connector is used with A–55, B–56, 

and E–53 reels. The C1 connector is used with the A-pack and 

Canfield reels, although the pressed-sleeve connector can be 

used on these reels. The pressed-sleeve connector is also used 

on handlines.

Depth Indicators

A computing depth indicator, as shown in figure 57, is 

used on the A–55, B–56, and E–53 reels. The stainless-steel 

indicator is less than 3 in. in diameter and has nylon bushings 

that do not require oil. The main dial is graduated in feet and 

tenths of a foot from 0 to 10 ft. The depth is indicated by a 

pointer. Tens of feet are read on a numbered inner dial through 

an aperture near the top of the main dial.

The main dial has a graduated spiral to indicate directly 

the 0.8-depth position for depths up to 30 ft.

The A-pack and Canfield reels, shown in figures 52 and 

53, are equipped with counters for indicating depths. Figure 58. Handline.

Figure 57. Computing depth indicator.

Figure 56. Connectors for attaching sounding cable to soundingweight hanger; A, Type B; B, Type C1; and C, pressed sleeve.

A

B

C
Instruments and Equipment 69

Sonic Sounder

A commercial, compact, portable sonic sounder has been 

adapted to measure stream depth. The sounder is powered by 

either a 6- or 12-volt storage battery and will operate continuously for 10 hours on a single battery charge. Three recording speeds are available—36, 90, and 180 in. per hour. Four 

operating ranges—0 to 60 ft, 60 to 120 ft, 120 to 180 ft, and 

180 to 240 ft—allow intervals of 60 ft of depth. The sounder 

is portable, weighing only 46 pounds. The depth recorded is 

from the water surface to the streambed. The transducer has 

a narrow beam angle of 6 degrees, which minimizes errors 

on inclined streambeds and allows the hydrographer to work 

close to piers or other obstructions.

Measurements can be made with this equipment without 

lowering the meter and weight to the streambed. As soon as the 

weight is in the water, the depth will be recorded. The meter 

can then be set at the 0.2 depth, or just below the water surface where a velocity reading is obtained. Then a coefficient is 

applied to convert measured velocity to the mean in the vertical.

Temperature change affects the sound propagation velocity, but this error is limited to about plus or minus 2 percent in 

freshwater. This error can be eliminated completely by adjusting the sounder to read correctly at a particular average depth 

determined by other means.

Cableway Equipment

The USGS cableway provides a track for the operation of 

a manned cable car from which the hydrographer makes a current-meter measurement. Most cableways have a clear span of 

1,000 ft or less, although a few structures have been built with 

clear spans approaching 2,000 ft. The design and construction 

of cableways are described in detail by Wagner (1995).

Cable cars provide a movable platform from which the 

hydrographer, sounding reel, and other necessary equipment 

are supported. The newer versions of these cable cars are 

fabricated from aluminum, and have a standard follower brake 

and integral-reel mounts, which will accept all standard sounding reels, including battery-powered reels. Cable cars can also 

be equipped with the Sandpoint type cable-car brake, which 

allows the cable car to be slowed or stopped. Both sitdown 

and standup types of manually propelled cable cars are used in 

streamgaging, as shown in figures 60 and 61, and have space 

for two people to work. Some older cable cars still in use are 

fabricated partially from wood, may or may not have permanent reel mounts, and may have space for only one person.

Manned cable cars are moved from one point to another 

on the cableway by means of cable-car pullers, as shown in 

figure 62. The standard car puller is an aluminum-cast handle 

with a snub attached. The snub, usually four-ply belting, is 

placed between one of the car sheaves and the cable to prevent 

movement of the car along the cable. A second type of puller, 

also shown in figure 62, is used when a car is equipped with 

a follower brake. A third type, the Colorado River cable-car 

Power Unit

Power units, as shown in figure 59, are available for the 

B–56 and E–53 reels to raise and lower the sounding weight 

and meter. The power unit can be used with 6-, 12-, 18-, or 

24-volt batteries.

Figure 59. Power units for sounding reels; A, J & H Single Speed 

Power Drive and B, USGS Variable Speed Reel Drive System.

A

B
70 Discharge Measurements at Gaging Stations

Figure 61. Stand-up cable car.

Figure 60. Sit-down cable car.
Instruments and Equipment 71

puller, is the same in principle as the puller used on cars 

equipped with a follower brake.

Power-operated cable cars, such as the battery-powered 

car shown in figure 63, are available for extremely long spans 

or other special situations where extensive streamgaging and 

monitoring is required. The power assist on these cable cars is 

also utilized to operate a type E sounding reel.

Unmanned, remotely operated cable carriages, such as 

the Hydrological Services Hornet, are used for discharge 

measurements as well as for sediment sampling. The cable 

carriage and sounding equipment can be remotely operated 

from the stream bank, as shown in figure 64. They are used 

in areas where it is impossible to wade, where no bridges are 

available, and where it is not practical to build or maintain a 

manned cableway.

Unmanned cableways are used more in Europe than 

in the United States, but are becoming more common in 

the United States. These bank-operated cableways have 

obvious benefits in safety and convenience (fig. 65). Both 

permanent and portable bank-operated cableways are 

becoming more useful in the measurement of discharge, 

especially for more narrow streams, generally 300 ft or less 

in top-bank width.

Figure 62. Cable-car pullers: A, with belt and B, with follower 

brake.

A

B

Figure 63. Battery-powered cable car.
72 Discharge Measurements at Gaging Stations

Figure 65. Bank-operated 

cableway.

Figure 64. Remotely operated 

Hydrological Services Hornet cable 

carriage with ADCP and trimaran 

used by the USGS.
Instruments and Equipment 73

Bridge Equipment

Streamflow measurements are frequently made from a 

bridge. The meter and sounding weight can be supported by 

a handline, a bridge board, or by a sounding reel mounted on 

a crane. An ADCP mounted on a tethered craft can also be 

deployed from a bridge. Tethered ADCP craft are rapidly becoming the prevalent means of measuring discharge from a bridge.

Handlines and Bridge Boards

A handline, as described in a previous section of this 

chapter, is the simplest form of bridge-measuring equipment. 

Used extensively in the 20th century, it does not require any 

separate reels or handling equipment; however, it can only 

be used with light sounding weights, such as the 15- and 

30-pound size. It also requires that depth be measured with 

tags, and a tape or measuring stick.

A bridge board is a portable platform made from wood or 

metal upon which a small reel can be mounted. Bridge boards 

may be used with an A-pack, A–55, or B–56 sounding reel and 

weights up to 75 pounds. A bridge board is usually about 6 to 

8 ft long, with a sheave at one end over which the meter cable 

passes, and a reel seat near the other end. The board is placed 

on the bridge rail so that the force exerted by the sounding 

weight suspended from the reel cable is counterbalanced by 

the weight of the sounding reel. The bridge board may be 

hinged near the middle to allow one end to be placed on the 

sidewalk or roadway. Figure 66 shows a bridge board in use.

Figure 67. Type A crane mounted on a three-wheel base.

Figure 66. Measuring from a bridge with a bridge board.

Figure 68. Type E crane mounted on a four-wheel base.

Portable Cranes

Two types of hand-operated portable cranes are the type A 

for weights up to 100 pounds, and the type E for weights greater 

than 100 pounds. The type A crane mounts on a three-wheel or 

four-wheel base or truck, and the type E crane mounts on a fourwheel base or truck. Cranes can be easily moved by hand along 

the sidewalk or floor of the bridge. Figure 67 shows a type A 

crane mounted on a three-wheel base, and figure 68 shows a 

type E crane mounted on a four-wheel base.

Any of the reels described in table 11 may be used on 

either of the portable cranes; however, the power-driven reels 

(B–56 and E–53) are used only with the Type E crane. Various 

combinations of cranes, bases (trucks), and reels are possible.
74 Discharge Measurements at Gaging Stations

radio-modem telemetry of acoustic Doppler current profiler 

(ADCP) data, potential platform-hull sources have been investigated, and many hull configurations have been tested and 

evaluated.

Platforms, which included a variety of hull configurations, were tested for drag and stability at the USGS Hydraulic 

Laboratory tow tank and at a field site below a reservoir. The 

testing indicated that trimaran designs provided the best allaround performance under a range of conditions. The trimaran 

designs house the ADCP in the center hull. Waterproof radio 

modems that operate at 900 MHz are used to communicate 

wirelessly with instruments at high-baud rates.

A tethered-platform design with a trimaran hull, and 900-

MHz radio modems, are commercially available from several 

vendors. Continued field use has resulted in USGS procedures for making tethered-platform discharge measurements, 

including methods for tethered-boat deployment, moving-bed 

tests, and measurement of edge distances. Figure 71 shows a 

tethered craft ADCP in wide usage in the USGS (Mueller and 

Wagner, 2009; Rehmel and others, 2003).

All cranes are designed so that the crane can be tilted forward over the bridge rail far enough for the meter and weight 

to clear most rails and be lowered to the water. Where bridge 

members obstruct passage of the crane along the bridge, the 

weight and meter can be raised and the crane can be tilted 

back to pass by the obstruction.

Use cast-iron counterweights weighing 60 pounds with 

four-wheel-base cranes. The number of such weights needed 

depends upon the size of sounding weight being supported, 

the depth and velocity of the stream, and the amount of debris 

being carried by the stream.

Use a protractor on the outer end of cranes to measure 

the angle the sounding line makes with the vertical when the 

weight and meter are dragged downstream by high-velocity 

water. The protractor is a graduated circle clamped to an 

aluminum plate. A plastic tube, partly filled with colored 

antifreeze (ethylene glycol), is the protractor index. This tube 

is fitted in a groove between the graduated circle and the aluminum plate. A stainless-steel rod is attached to the lower end 

of the plate to ride against the downstream side of the sounding cable. The protractor will measure vertical angles from 

-25 degrees to +90 degrees. Figure 69 is a close-up view of a 

protractor mounted at the outer end of the boom.

Power-Driven Cranes

Many special arrangements for measuring from bridges 

have been devised to suit a particular purpose. Vehicle-mounted 

cranes are often used for measuring from bridges over larger 

rivers, as shown in figure 70. Monorail streamgaging cars have 

also been developed for large rivers. The car is suspended from 

the substructure of bridges by means of I-beams.

Tethered Craft

The USGS, in cooperation with manufacturers, continues to test and refine tethered-platform designs for measuring 

streamflow. Platform specifications have been developed for 

Figure 70. Vehicle-mounted, power-driven crane.

Figure 69. Protractor used for measuring vertical angles.
Instruments and Equipment 75

Boat Equipment

There are five basic types of boat measurements: the 

manual stationary boat, the manual moving boat, the automatic moving boat, the ADCP moving boat, and the remotecontrolled ADCP moving boat. Equipment requirements for 

each of these boat measurement types are described in the 

following paragraphs.

Manual Stationary Boat

The manual stationary boat method uses a boat as a 

platform for the hydrographer and the sounding equipment. 

The boat is attached to a tag line or cable to stabilize it at each 

vertical where soundings are made. The heavy-duty tag lines 

required for boat measurements are described in a previous 

section of this chapter.

Special equipment assemblies, as shown in figure 72, are 

necessary to suspend the meter from the boat if the depths do 

not allow using rod suspension. A crosspiece spanning the boat 

is clamped to its sides, and a boom attached to the center of the 

crosspiece extends out over the bow. The crosspiece is equipped 

with a guide sheave and clamp arrangement at each end to 

attach the boat to the tag line, and makes it possible to slide the 

boat along the tag line from one station to the next. A small rope 

can be attached to these clamps so that in an emergency a tug on 

the rope will release the boat from the tag line. The crosspiece 

also has a clamp that prevents lateral movement of the boat 

along the tag line during readings. The boom consists of two 

structural aluminum channels, one telescoped within the other 

to permit adjustments in length. The boom is equipped with a 

reel plate on one end and a sheave over which the meter cable 

passes. The sheave end of the boom is designed so that by adding a cable clip to the sounding cable a short distance above the 

connector, the sheave end of the boom can be retracted when 

the meter is raised out of the water. The raised meter is easy to 

clean and is in a convenient position when not being operated. 

All sounding reels fit the boat boom, except the A-pack and the 

Canfield, which can be made to fit by drilling additional holes in 

the reel plate on the boom.

In addition to the equipment already mentioned, the following items are needed for making boat measurements:

1. A stable boat big enough to support the hydrographers and 

equipment;

2. A motor that can easily move the boat against the maximum current in the stream;

3. A pair of oars for standby use;

4. A personal floatation device (PFD) for each hydrographer; 

and

5. A bailing device.

Manual Moving Boat

Equipment requirements for the manual moving boat 

method of making a discharge measurement are described in 

detail by Smoot and Novak (1969); they are not described in 

this chapter. In summary, the manual moving-boat method 

requires a sonic sounder, a vane with indicator, a special 

current meter, and an easily maneuverable small boat. The 

manual moving-boat method is seldom used since the advent 

of the ADCP moving-boat method, which is described below.

Automatic Moving Boat

The automatic moving-boat method is similar to the manual 

moving-boat method, except that all readings of depth, velocity, angles, and distance are recorded automatically by onboard 

computer equipment. Equipment requirements are more complex 

in order to enable the automatic sensing and recording.

Figure 72. Manual stationary-boat equipment assembly.

Figure 71. Measuring with a tethered ADCP, DGPS, and trimaran.
76 Discharge Measurements at Gaging Stations

Like the manual moving-boat method, the automatic 

moving-boat method is seldom used since the advent of the 

ADCP moving-boat method.

ADCP Moving Boat

Currently, the ADCP moving-boat method is the most 

common moving-boat method in the USGS. All readings of 

depth, velocity, angles, and distance are recorded to a laptop or 

PDA and the discharge measurement is computed using ADCP 

software and input from the hydrographer.

As shown in figure 73, described in other sections of this 

chapter, and in depth by Mueller and Wagoner (2009), ADCP 

moving-boat measurement techniques have almost entirely 

superseded the automatic moving method.

Remote-Controlled ADCP Moving Boat

There are commercially available unmanned, remotecontrolled craft with ADCPs used by the USGS and others 

for the measurement of discharge where a manned boat or 

tethered boat may not be feasible. Similar to but smaller 

than the ADCP moving-boat, these remote-controlled craft 

typically come with self-contained motors and a remotecontrolled system for driving the boat across a stream or 

river. See Mueller and Wagner (2009) for an in depth discussion of remote-controlled ADCP moving boat discharge 

measurements.

Figure 73. ADCP equipment 

mounted and operated on A, a 

moving boat and B, a tethered 

platform.

A

B
Instruments and Equipment 77

Electronic Field Notebooks and Personal Digital 

Assistants (PDAs)

Recent developments in electronics have produced commercially available electronic field notebooks (EFNs) and 

personal digital assistants (PDAs), designed specifically for the 

purpose of recording field notes while making a discharge measurement (fig. 74) . The PDAs are also designed for other field 

procedures, including discharge-measurement data collection 

and processing, station inspection, differential-level survey notes, 

and water-quality data collection and equipment calibration. The 

USGS commonly uses the Surface Water Measurements and 

Inspections (SWAMI) program with a PDA. The program can be 

used to record discharge measurements, inspections, differentiallevel surveys, and other field measurements. This software provides an efficient means of collecting field data (fig. 2C) and has 

been specifically designed to interface with the USGS NWIS.

The recording process is semiautomatic. Basic information and data still must be entered manually for measurements 

of stream depth, stationing, horizontal angle of flow, equipment, and other basic data.

EFNs automatically count meter revolutions and elapsed 

times, and make the conversion to stream velocity. They also 

assist the hydrographer with certain tasks, such as locating each 

subsection so that no more than 10 percent of the total flow will 

be included in each subsection. The notebook makes all of the 

measurement calculations to obtain the final discharge, and to 

summarize all pertinent items of the measurement.

The EFNs and PDAs can store many discharge-measurement reports. The EFN and (or) PDA reports for a discharge 

measurement are similar to the paper note sheets used for 

manual note keeping. A header, similar to the paper “front 

sheet,” contains site information, equipment information, and 

a summary of measurement data. The report also contains 

Figure 74. Personal digital assistants (PDAs) and electronic field notebooks (EFNs), clockwise from the top: A, the Trimble Juno SB 

PDA with GPS, Bluetooth, and Parani Serial Adapter; B, the Trimble Nomad 800 GL PDA with GPS and Bluetooth; C, the Trimble Recon 

PDA without GPS; D, the JBS Instruments Aquacalc Pro Discharge Measurement Computer; and E, the Hydrological Services Current 

Meter Counter Signal Processor with Bluetooth (CMCsp).
78 Discharge Measurements at Gaging Stations

complete measurement data, similar to the paper “inside 

notes,” for all of the individual subsections. In addition, the 

report contains various warning flags and quality-control 

information. Complete reports for each discharge measurement can be downloaded to a computer for viewing, printing, 

and other analysis.

One EFN widely used within the USGS is the Aquacalc 

Pro Discharge Measurement Computer (Aquacalc) by JBS 

Instruments. The Current Meter Counter Signal Processor 

(CMCsp) by Hydrological Services is used in concert with a 

PDA to measure discharge with a mechanical current meter, as 

shown in figure 74. The Aquacalc and the CMCsp will work 

with the cat’s-whisker-, magnetic-, and optic-contact chambers 

on the Price current meters. As with all EFNs, care should 

be taken to avoid false counting of meter revolutions when 

measuring low velocities with mechanical meters that have a 

cat’s-whisker-contact chamber.

PDAs are manufactured by many computer companies in 

the United States and abroad. Several are considered reliable 

and have been successfully field tested. At this writing, the 

PDA can interface with most of the ADCPs and electronic data 

loggers used by the USGS.

Miscellaneous Equipment and Personal Items

Other personal equipment and items will be needed 

while making discharge measurements, or when working 

in and around rivers, creeks, and streams. Waders or boots 

should be worn while making wading measurements. Waders 

should be loose fitting, even when allowing for heavy winter 

clothing.

Ice chains, as shown in figure 75, should be strapped onto 

the soles of boots or waders for use on steep or icy stream 

banks, and on rocky or smooth and slippery streambeds.

A properly fitted personal-floatation device (PFD) 

must be worn when working near, in, or over water. This 

includes while wading streams, and working on a cableway, 

bridge, or water retention or control structure, on ice, or in 

a boat.

Figure 75. A, Ice chains for boots and waders and B, Sure Grip 

Ice Treads for boots and waders.

A

B
Accuracy of Current-Meter Discharge Measurements 79

Accuracy of Current-Meter Discharge 

Measurements

The accuracy of a discharge measurement is dependent 

on many factors, including the equipment used, the location 

and characteristics of the measuring section, the number and 

spacing of verticals, the rate of change in stage, the measurement of depth and velocity, presence of ice and (or) debris in 

the measuring section, wind, experience of the hydrographer, 

carefulness (or carelessness) of the hydrographer, and various conditions that can occur during the process of making the 

measurement. The evaluation of the accuracy of a measurement 

has long been a qualitative assessment that takes some or all of 

these factors into account. A quantitative measure of the accuracy for some discharge measurements can also be made. The 

following two sections of this chapter describe these methods.

Qualitative Evaluation

Every discharge measurement should be evaluated for 

accuracy using the qualitative method. Historically, this has 

been the preferred method, and the hydrographer should make 

this evaluation immediately after making the measurement. 

The evaluation should be based on the hydrographer’s opinion 

of the accuracy of the measurement—not on how well, or how 

poorly, the measurement plots on the stage-discharge relation. 

It is difficult to provide written guidelines for making a qualitative evaluation of accuracy. A good qualitative evaluation 

depends mostly on the experience and training of the hydrographer. Several of the factors that should be considered by the 

hydrographer are as follows:

Measuring section.—Consider factors such as the uniformity of depths, the smoothness of the streambed, the streambed 

material (that is, smooth sand; small, firm gravel; large rocks; 

soft muck; and so forth), the ability to accurately measure the 

depth, the approach conditions, presence of bridge piers, and 

other conditions that would affect measurement accuracy.

Velocity conditions.—Consider smoothness of velocity, 

uniformity of velocity, very slow velocity, very high velocity, 

turbulence, obstructions that may affect the vertical velocity 

distribution, use of one-point or two-point method, length of 

counting (40 or more seconds versus half-counts), and other 

factors that affect accuracy of velocity measurements.

Equipment.—Consider the type of current meter used 

(Price AA, Price pygmy, acoustic, or electromagnetic), the 

type of depth-sounding equipment, and the condition of the 

equipment.

Spacing of observation verticals.—Use about 25 to 30 

verticals for a discharge measurement, spaced so that no more 

than 5 percent of the total discharge is contained in each subsection. Although this is frequently difficult to attain, except in 

unusual cases, no more than 10 percent of the total discharge 

should be in a subsection. Otherwise, the accuracy will be 

negatively affected.

Rapidly changing stage.—Although discussed in previous 

sections of this chapter, this condition should also be considered when assessing the accuracy of the measurement. Using 

the shortcut methods previously described will result in less 

accurate measurements of discharge.

Ice measurements.—Making discharge measurements 

under ice cover is usually more difficult, and sometimes less 

accurate, than making open-water discharge measurements. 

Presence of slush ice, layered ice, and anchor ice will have 

adverse affects on accurate measurement of depth and velocity. Velocity distribution will be affected if the water surface is 

in contact with the ice. Freezing of water in the meter cups and 

pivot chamber may affect performance of the equipment.

Wind.—Wind can affect the accuracy of a discharge 

measurement by obscuring the angle of the current, or by 

creating waves that make it difficult to sense the water surface 

prior to making depth soundings. Wind can also affect the 

vertical-velocity distribution, particularly near the surface, and 

can cause vertical and (or) horizontal movement of the current 

meter while making a boat measurement, introducing possible 

error in velocity measurements.

The qualitative method of assessing the accuracy of a discharge measurement requires that the hydrographer consider 

all of the above items and their cumulative effect on the measurement accuracy. The front page of the discharge measurement note sheet (see figure 2) has space for describing (1) the 

cross section, (2) the flow, (3) the weather, and (4) any other 

flow conditions that relate to the accuracy. These descriptions, 

along with the type of equipment, number of verticals, velocity measurement method, and other measurement conditions, 

should provide the basis for rating the measurement as excellent (2 percent), good (5 percent), fair (8 percent), or poor 

(more than 8 percent).

For instance, a measurement might be rated as excellent (2 percent) if (1) the cross section is smooth, firm, and 

uniform; (2) the velocity is smooth and evenly distributed; (3) 

the equipment is in good condition; (4) the two-point velocity 

measurement method was used; and (5) weather conditions are 

good (no wind or ice). On the other hand, if several of these 

factors make it difficult to accurately measure depth and (or) 

velocity, the measurement might be rated fair (8 percent), or 

even poor (more than 8 percent).

As stated previously, it is not possible to provide absolute 

guidelines for making the qualitative evaluation of accuracy. 

As a general rule, the accuracy of most discharge measurements will be about 5 percent, or qualitatively a “good” 

measurement. This is sometimes used as the base-line accuracy, with accuracy upgraded to “excellent” when measuring 

conditions are substantially better than average, and accuracy 

downgraded to “fair” or “poor” when conditions are substantially worse than average. The qualitative-accuracy evaluation 

is based on the hydrographer’s judgment. For more detailed 

qualitative-evaluation information on discharge measurements 

using ADCPs, see Oberg and others (2005) and Mueller and 

Wagner (2009).
80 Discharge Measurements at Gaging Stations

Quantitative Evaluation

A quantitative-accuracy evaluation can be made for some 

current-meter discharge measurements by using the procedure 

described by Sauer and Meyer (1992), Herschy (1994), and 

the International Organization for Standardization (1997). 

These procedures compute the uncertainty, or standard error, 

using a root-mean-square error analysis of individual component errors. The component errors include errors in the 

measurement of width, depth, and velocity, and in computation procedures. These procedures can be used to compute the 

standard error for most discharge measurements made with 

the vertical-axis, cup-type current meter. These procedures do 

not apply to measurements made with other types of current 

meters, or other methods of making discharge measurements. 

Likewise, they do not apply to discharge measurements where 

wind, ice, boundary effects, flow obstructions, improper 

equipment, incorrect measuring procedures, and hydrographer 

carelessness are factors in the measurement.

The details of the Sauer and Meyer (1992) method are 

described in USGS Open-File Report 92–144, and therefore 

are not included in this chapter. A computer program is available to compute the standard error for individual discharge 

measurements, and it is recommended that this quantitative 

evaluation be made for each discharge measurement for which 

it applies. Computations using this method show that the 

standard error of individual discharge measurements can range 

from about 2 percent for ideal conditions, to about 20 percent 

for very poor measuring conditions. Standard errors range 

from about 3 percent to 6 percent for measurements having 

generally normal measuring conditions. The standard errors 

computed by this method are in close agreement with qualitative evaluations.

ADCP Discharge-Measurement Accuracy

There are many sources of error in an ADCP discharge 

measurement. A complete measurement is composed of the 

ADCP-measured channel subsection, extrapolated top subsection, extrapolated bottom subsection, and edge-estimated 

subsections.

The largest and most substantial subsection is the ADCPmeasured channel subsection. Most errors can be greatly 

reduced if factors, such as moving bed, water temperature, 

salinity, cross-section choice, instrument configuration, and 

boat speed, are carefully considered and accounted for, as 

described in previous sections of this chapter. Software is usually provided by the manufacturer that can be used to compute 

the ADCP instrument error for the measured subsection.

Errors for the extrapolated top, bottom, and edge subsections will vary, depending upon the extrapolation methods and 

relative proportion of the total discharge represented in these 

subsections. Again, these errors can be kept to a minimum 

through proper choice of cross section and careful measurement of variables, such as ADCP transducer depth and distances from each shore to the nearest ADCP section.

Studies by Morlock (1996) and Oberg and Mueller (2007) 

concluded that ADCP discharge measurements can be used 

successfully for streamflow data collection under a variety 

of field conditions. In Morlock (1996), 31 ADCP discharge 

measurements were compared to discharge ratings defined by 

conventional methods for the period over which the ADCP 

measurements were made. These comparisons showed that 25 

ADCP measurements were within 5 percent of the conventional 

measurements. Six of the ADCP measurements differed by 

more than 5 percent, the maximum departure being 7.6 percent.

The study by Morlock (1996) stated that ADCP discharge 

measurement error was indicated by the standard deviations 

of the ADCP discharge measurements. The standard deviations ranged from about 1 to 7 percent of the measurement 

discharges. The estimated error of each ADCP discharge 

measurement also was computed from formulas derived by 

the manufacturer of ADCPs. The computations of estimated 

measurement error assume that ADCP instrument- and 

unmeasured-subsection extrapolation errors are the main 

source of measurement error. The standard deviations for most 

ADCP discharge measurements were higher than the estimated 

measurement errors, indicating that significant components of 

measurement error were not related to the instruments; errors 

of this nature include temporal variations of flow. It was concluded that measurement precision can be positively affected 

by selection of a measurement location with minimal flow 

variations, and negatively affected by instrument- and boatoperation factors.

Uncertainties in Discharge Measurements

All discharge measurements, no matter how carefully 

made, are subject to uncertainty. The measurement uncertainty 

can be thought of as a quantitative measure of the dispersion of 

the measured discharge about the true discharge. This uncertainty arises because each measurement is subject to errors of 

unknown magnitude. The total uncertainty in a discharge measurement may arise from several sources, including:

• uncertainty in the measurement of the cross-sectional 

area, which in turn arises from the following:

◦ uncertainty in measurements of width; and

◦ uncertainty in measurements of depth;

• uncertainty in the measurement of the water-velocity 

profile, which in turn arises from the following:

◦ instrument uncertainty;

◦ pulsation and turbulence in open-channel flow;

◦ deviation from our assumptions about the verticalvelocity distribution; and

◦ uncertainty due to oblique angles in the flow velocity;

• uncertainty due to deviation from assumptions used in 

the computation procedures; and

• other random or systematic errors.
Quality Assurance and Quality Control 81

These component uncertainties can be combined to estimate the total uncertainty of a single discharge measurement. 

Where feasible, values for these component uncertainties 

should be estimated independently for each site.

The uncertainty is often expressed as a standard deviation. If we assume that measurement errors are normally 

distributed, then this uncertainty can be used to construct 

confidence intervals for the measured discharge value. For 

example, the true discharge can be expected to be within one 

standard deviation of the measured value at the 68-percent 

confidence level. At the 95-percent confidence level, the true 

discharge can be expected to be within two standard deviations of a single measured value.

Quality Assurance and Quality Control

It should be the goal of each hydrographer to make discharge measurements of the highest quality and with as little 

error as possible. As explained in other sections of this chapter 

there are many actions that must be performed before, during, 

and after the actual measuring process. In the many implicit 

decisions that must be made during the course of a discharge 

measurement, the hydrographer, through training and experience, must develop a keen sense of what is correct and incorrect 

through hydrologic/engineering judgment, and strive to continually take the correct course of action in making a discharge measurement. This is commonly known as quality assurance and 

quality control, sometimes referred to as QA/QC. Some of the 

QA/QC functions are implicit; that is, they are generally understood, performed automatically, and are not specifically defined 

in the measurement notes and sometimes must be accomplished 

through hydrologic/engineering judgement. Careful regard for 

safety, good hydrologic/engineering judgment, and observance 

of proper procedure are implicit functions that cannot be overstressed in making a precise and accurate discharge measurement. On the other hand, some actions are explicit, such as 

performing regular spin tests of current meters, or making check 

measurements when the first measurement may be suspect. Following are some of the QA/QC actions that should be observed 

for making high-quality discharge measurements. These are 

not all inclusive, and each hydrographer should always include 

and document any other actions that relate to the quality of the 

measurement. Additional QA/QC requirements are given in the 

QA/QC plan for each USGS Water Science Center.

• Care of current meters, current profilers, and sounding 

equipment.—Previous sections of this chapter describe 

the proper care of current meters, current profilers, and 

sounding equipment. Current meters are especially 

susceptible to damage and misalignment while in use, as 

well as in transit, if they are not properly protected. The 

hydrographer should follow all established guidelines 

to ensure that the streamgaging equipment, especially 

the current meter and (or) profiler, are in good working condition. While making a discharge measurement, 

the current meter should be periodically observed and 

checked to be sure it is operating smoothly and has not 

become fouled by debris, ice, or other obstructions.

• Spin tests of current meters.—One of the requirements 

for maintaining and checking current meters is a periodic, timed spin test under controlled conditions. The 

procedure for making a timed spin test is described in 

a previous section of this chapter. In addition, before, 

during, and after a discharge measurement, check that 

the rotor is turning smoothly and does not come to an 

abrupt stop.

• Carefulness, good judgment, and proper procedure.—It 

is the hydrographer’s responsibility to apply proper 

procedures with care and good judgment while making 

streamflow measurements. These implicit functions of 

QA/QC should be observed at all times.

• Computing and plotting the measurement on site.—

Compute a discharge measurement as soon as possible 

after it is completed. Do this at the site before leaving. If 

the measurement does not plot within 5 percent (or other 

specified percentage) of the rating curve in use, or if it 

is not in line with the previous trend of measurements, 

try to find an explanation. For instance, there may be 

an obvious change of the control that would explain the 

deviation. All such explanations should be documented 

in the measurement notes. If a satisfactory explanation 

cannot be found, then make a check measurement.

• Making check measurements.—If possible, while 

making a check measurement, select a different cross 

section from the original section and use a different 

current meter. Make the check measurement as close 

in time and gage height to the original measurement as 

possible.

• Checking discharge measurements.—In general, handcomputed discharge measurements are not checked for 

mathematical errors. Nevertheless, check measurements 

that do not plot within an acceptable percentage of the 

rating curve, or within the previous trend of measurements. Likewise, check measurements that define a 

significant extrapolation of the low end or high end of 

a rating curve. Discharge measurements recorded in an 

electronic notebook, such as the Aquacalc, are automatically computed and do not require checking.

• Documentation of QA/QC.—Document in the measurement notes, if possible, all measures taken to ensure that 

discharge measurements are accurate and of high quality. 

Some QA/QC measures require specific documentation 

independent of the measurement notes. For example, 

current-meter spin tests have specific forms that document the spin-test results and all repairs to the meter.
82 Discharge Measurements at Gaging Stations

Safety Requirements

Practicing personal and overall safety is of utmost importance when working near, in, and above water. It is not the 

purpose of this chapter to describe all of the safety requirements; however, each hydrographer should be familiar with, 

and should observe, those requirements. Other documents 

provide the details of specific safety requirements for making wading measurements, and for working on ice, bridges, 

cableways, and boats. For instance, each USGS Water Science 

Center flood plan addresses such things as one-person versus multiple-person field parties, use of an approved PFD, 

and traffic control while making discharge measurements 

from bridges, cableway safety, and boat safety. USGS WRD 

Memorandum No. 99.32 (1999) provides safety guidance as 

it is related to discharge measurements, sampling, and other 

related streamgaging activities. Each Water Science Center has 

a safety officer and a safety plan; both should be consulted for 

specific safety issues.

Portable Weir-Plate Measurements

Current-meter measurements made in shallow depths 

and low velocities are usually inaccurate, if not impossible, to 

obtain. Under these conditions, a portable weir plate is a useful 

device for measuring the discharge.

A 90-degree V-notch weir is suitable because of its favorable accuracy at low flows. A weir made of 10- to 16-gage 

galvanized sheet iron will produce a free-flowing nappe, having the effect of a sharp-crested weir, and will give satisfactory 

performance. The thickness of the plate should vary with the 

size of the weir. Refer to figure 76 for recommended proportions. Decreasing the plate thickness on larger weirs will help 

maintain portability. The notch is cut, without sharpening, 

leaving a flat, even edge. Framing, in the form of small-angle 

irons, is required for medium and large sizes. Canvas attached 

on the downstream or upstream side prevents leakage under 

or around the weir. Eyebolts, properly placed, will secure rods 

driven in earth channels to stabilize the plate.

A

T

90 °

h

B

Staff

gage

Z

L

Weir Z h B L A T Weight (lb)

Large 1.75 1.00 0.75 4.0 1.0 16 ga. 24

Medium 1.25 0.80 0.45 3.0 0.7 14 ga. 17

Small 0.75 0.47 0.28 2.0 0.53 10 ga. 8

Corners may be

trimmed if desired

Figure 76. Portable weir plate.
Portable Parshall-Flume Measurements 83

Attach a staff gage to the upstream face of the weir plate, 

with the zero point at the same elevation as the bottom of the 

weir notch. The staff should be far enough from the notch 

to be outside of the zone of drawdown, which is a distance 

greater than twice the head on the notch. The staff gage is used 

to obtain head on the weir.

The general equation for flow over a sharp-edged triangular weir with a 90-degree notch is

, (20)

where Q discharge, in ft3/sec,

h static head, in ft, and

C the coefficient of discharge.

The weir should be rated by determining the flow volumetrically for various values of head, or by having it rated in 

the HIF laboratory. In the absence of a rating, a value of C of 

2.47 may be used.

Flows from 0.02 to 2.0 ft3/s are measured with the large 

weir of figure 76. Discharges can be measured within 3-percent accuracy if the weir is not submerged. A weir is not 

submerged when there is free circulation of air on all sides of 

the nappe.

To place the plate in a sand or silt channel, only a carpenter’s level and a shovel are needed. Push the weir into the 

streambed, and drive the rods through the eyebolts on each end 

to stabilize the weir. Use the level to make the top of the plate 

horizontal and the plate plumb. Another way to level the plate is 

by fastening a staff gage or level bubble to each end of the weir, 

where the staff gages are set at the same elevation. The plate is 

leveled by making the staff-gage readings identical or by using 

the level bubbles. Pack soil and streambed material around the 

ends and bottom of the weir to prevent leakage. Place canvas 

immediately downstream from the weir to prevent the falling jet 

from undercutting the streambed. Let the flow stabilize before 

making a measurement. Read the gage height at half-minute 

intervals for a period of about 3 minutes, and use a mean value 

in the above equation to compute the discharge. Ordinarily, one 

person can measure with a weir of this type. Remove the weir 

after you have completed the measurement.

Portable Parshall-Flume 

Measurements

A portable Parshall measuring flume is useful for measuring 

discharge when the depths are shallow and the velocities are low. 

The standard Parshall flume has a converging section, a throat, 

and a diverging section. The floor of the converging (or upstream) 

section is level both longitudinally and transversely when in 

place. The floor of the throat section slopes downward and the 

floor of the diverging or downstream section slopes upward. The 

standard Parshall flume can be used to measure discharge under 

free-flow conditions, as well as submerged conditions.

The flume used by the USGS is a modified version of the 

standard Parshall flume. The modification consists primarily 

of the removal of the downstream section, which reduces the 

weight of the flume and makes it easier to install. Because it has 

no downstream section, however, it can only be used to measure 

free-flow conditions (that is, where the submergence ratio is 0.6 

or less). This can usually be accomplished by building up the 

streambed by a couple of inches under the level, converging 

floor of the flume when the flume is installed.

Free flow occurs when the ratio of the lower head to the 

upper head is less than 0.6. The discharge under this condition 

depends only on the length of crest (width of throat section) 

and depth of water at the upper gage. A flume that is properly 

constructed has an accuracy of 2 to 3 percent under free-flow 

conditions.

Install the flume by placing it in the channel; fill in with 

available channel bed or bank material around it to prevent 

any water from bypassing it. Use a carpenter’s level to set 

the floor of the converging section level. Some flumes are 

equipped with levels attached to the braces on the flume. After 

the flume is in place, the streamflow is allowed to stabilize 

before reading the gage. After the flow stabilizes, take gage 

readings at about half-minute intervals for about 3 minutes. 

Use an average of the gage readings with the flume rating to 

determine the discharge. Remove the flume after the measurement is complete.

A modified 3-in. Parshall flume is shown in figure 77. 

This modified version is virtually the same as the standard 

Parshall flume except that it does not have a diverging section. 

The gage height, or upstream head on the throat, is read in the 

small stilling well that is hydraulically connected to the flow 

by a 3⁄8-in. hole.

The basic rating equation for a flume is

, (21)

where Q discharge, in cubic feet per second (ft3/s),

C a dimensionless coefficient of discharge that can 

vary with head and other factors,

b width of the throat section, in feet, and

h head, or gage height, in the converging section, 

in feet.
84 Discharge Measurements at Gaging Stations

The rating for the 3-in. modified Parshall flume described 

in this section is given in table 12, and was taken from 

Buchanan and Somers (1969). An identical table is published 

by Kilpatrick and Schneider (1983), and also by Rantz (1982). 

The original source of this rating is unknown, but was probably based on laboratory tests. The rating in table 12 plots as a 

straight line on logarithmic plotting paper, and the equation for 

this rating was computed by regression analysis as follows:

. (22)

This equation is close to one that can be derived for the 3-in. 

modified Parshall flume, based on the procedures given by 

Kilpatrick and Schneider (1983). The above equation will reproduce values of discharge precisely as shown in table 12, except 

for a few instances where the computed discharge deviates by 

0.001 to 0.005 ft3/s. This equation should not be used for values 

of gage height less than 0.01 ft or greater than 0.59 ft.

Figure 77. Modified 3-inch Parshall flume.

Table 12. Rating table for 3-inch modified Parshall flume.

[ft, feet; ft3/s, cubic foot per second]

Gage height (ft) Discharge (ft3/s) Gage height (ft) Discharge (ft3/s) Gage height (ft) Discharge (ft3/s)

0.01 0.0008 0.21 0.097 0.41 0.280

0.02 0.0024 0.22 0.104 0.42 0.290

0.03 0.0045 0.23 0.111 0.43 0.301

0.04 0.0070 0.24 0.119 0.44 0.312

0.05 0.010 0.25 0.127 0.45 0.323

0.06 0.013 0.26 0.135 0.46 0.334

0.07 0.017 0.27 0.144 0.47 0.345

0.08 0.021 0.28 0.153 0.48 0.357

0.09 0.025 0.29 0.162 0.49 0.368

0.10 0.030 0.30 0.170 0.50 0.380

0.11 0.035 0.31 0.179 0.51 0.392

0.12 0.040 0.32 0.188 0.52 0.404

0.13 0.045 0.33 0.198 0.53 0.417

0.14 0.051 0.34 0.208 0.54 0.430

0.15 0.057 0.35 0.218 0.55 0.443

0.16 0.063 0.36 0.228 0.56 0.456

0.17 0.069 0.37 0.238 0.57 0.470

0.18 0.076 0.38 0.248 0.58 0.483

0.19 0.083 0.39 0.259 0.59 0.497

0.20 0.090 0.40 0.269
Float Measurements 85

Volumetric Measurements

The most accurate way of measuring small discharges is 

the volumetric method. This method is performed by observing the time it takes to fill a container of known capacity, 

or the time required to partly fill a calibrated container to a 

known volume. The basic equipment needed for this method is 

a calibrated container and a stopwatch.

Two methods can be used to calibrate the container. The 

first method is to add known volumes of water by increments 

and note the depth of water in the container. The second 

method is to first weigh the empty container and then add 

varying amounts of water to it, each time weighing the container with water, and noting the depth of water in the container. The following equation can then be used to compute the 

volume of water corresponding to the depth that was read:

, (23)

where V volume of water in container, in cubic feet,

W2 weight of container with water, in pounds,

W1 weight of empty container, in pounds,

w unit weight of water, 62.4 lb/ft3.

Volumetric measurements of discharge are made with two 

types of conditions:

• When the flow is or can be concentrated so that all of it 

may be diverted into a calibrated container.

• When the depth of water flowing over broad-crested 

weirs and dams is small and volumetric-increment 

samples can be obtained.

Under the first condition, measurements are made at 

V-notch weirs at artificial controls where all the flow is in a 

notch or catenary, and at places where a small earth dam can 

be built and all the water diverted through a pipe of small 

diameter. Sometimes it is necessary to place a trough against 

the artificial control to carry the water from the control to the 

calibrated container. If a small dam is built, the stage behind 

the dam must be allowed to stabilize before the measurement 

is begun. The measurement is made three or four times to 

ensure error-free and consistent results.

Volumetric measurements are made under the second 

condition by catching a segment of the streamflow with a 

container having a known width of opening. Samples are 

taken at a number of locations across the dam or weir similar 

to procedures used for current-meter measurements. The flow 

rate of each sample is increased by the ratio of the subsection 

width to the sampled width to obtain a discharge rate for each 

subsection. The total discharge of the stream is the summation 

of the discharge rates of each subsection.

Float Measurements

Floats have limited use in streamgaging, but they can be 

used where the velocity is too low to obtain reliable measurements with the current meter, or where flood measurements 

are needed and the measuring structure has been destroyed 

or it is impossible to use a meter. Both surface floats and rod 

floats can be used. Surface floats may be almost anything that 

floats, such as wooden disks, partly filled bottles, oranges, or 

pumpkins. Floating debris or ice cakes may serve as natural 

floats. Rod floats are usually made of wood and weighted on 

one end so they will float upright in the stream. Rod floats are 

sometimes made in sections so their length can be adjusted 

to fit the stream depth; however, they should not touch the 

streambed.

Two cross sections are selected along a reach of straight 

channel for a float measurement. The cross sections should be 

far enough apart so that the time the float takes to pass from one 

cross section to the other can be measured accurately. A travel 

time of at least 20 seconds is recommended, but a shorter time 

can be used on small streams with high velocities, where it is 

impossible to select an adequate length of straight channel. The 

edge of water for both cross sections should be referenced to 

stakes (or other marker) on each bank. Those points will be used 

at a later date, when conditions permit, to survey cross sections 

of the measurement reach, and to obtain the distance between 

cross sections. The surveyed cross sections will be used to 

determine the average cross section for the reach.

Float measurements may sometimes be made through a 

reach extending from the upstream to the downstream side of 

a bridge. This kind of reach may be useful where velocity is 

very slow and velocity observations by current meter are not 

reliable.

The procedure for a float measurement is to distribute 

a number of floats uniformly over the stream width, noting 

the position of each with respect to the bank. They should be 

placed far enough upstream from the first cross section so they 

attain the velocity of the stream before they reach the first 

cross section. Use a stopwatch to time their travel between the 

two cross sections. As each float passes the second cross section, note its distance.

The velocity of the float is equal to the distance between 

the cross sections divided by the time of travel. The mean 

velocity of flow in the vertical is equal to the float velocity multiplied by a coefficient that is based on the shape of the verticalvelocity profile and relative depth of immersion of the float. A 

coefficient of about 0.85 to 0.88 is commonly used to convert 

surface velocity to mean velocity. The coefficient for rod floats 

varies from 0.85 to 1.00, depending upon the shape of the cross 

section, the length of the rod, and the velocity distribution.

The procedure for computing discharge is similar to that 

for a mechanical current-meter measurement. The discharge 

in each partial section is computed by multiplying the average 

area of the partial section by the mean velocity in the vertical 

for that partial section. The total discharge is equal to the sum 

of the discharges for all the partial sections.
86 Discharge Measurements at Gaging Stations

Discharge measurements made with floats under favorable 

conditions may be accurate to within ±10 percent. Wind may 

adversely affect the accuracy of the computed discharge by its 

effect on the velocity of the floats, especially if velocity is very 

slow. If a poor reach is selected and not enough float runs are 

made, the results can be as much as 25 percent in error.

Indirect Discharge Measurements

During floods, it is frequently impossible or impractical 

to measure peak discharges when they occur. Roads may be 

impassable; structures from which current-meter measurements might have been made may be nonexistent, not suitably 

located, or destroyed; knowledge of the flood rise may not be 

available enough in advance to allow reaching the site near 

the time of the peak; the peak may be so sharp that a satisfactory current-meter measurement could not be made, even 

with a hydrographer present at the time; the flow of debris or 

ice can prevent use of a current meter; or personnel limitations might make it impossible to obtain direct measurements 

of high-stage discharge at numerous locations during a short 

flood period. Consequently, many peak discharges must be 

determined after the passage of the flood by indirect methods, such as slope-area, contracted-opening, flow-over-dam, 

or flow-through-culvert, rather than by direct current-meter 

measurement. Detailed descriptions of the procedures used in 

collecting field data and in computing the discharge are given 

by Benson and Dalrymple (1967), Dalrymple and Benson 

(1967), Bodhaine (1968), Matthai (1967), and Hulsing (1967), 

which are in book 3, chapters Al–A5, of the USGS Techniques 

and Methods series. Various computer programs are available 

for computing the discharge for indirect measurements.

Tracer Discharge Measurements

Measurement of discharge by this method depends on 

determination of the degree of dilution of an added tracer solution by the flowing water. A solution of a stable tracer, such 

as a fluorescent dye or a radioactive chemical, is injected into 

the stream at either a constant rate or all at once. The solution 

becomes diluted by the discharge of the stream. Measurement 

of the rate of injection, the concentration of the tracer in the 

injected solution, and the concentration of the tracer at a cross 

section downstream from the injection point permits the computation of stream discharge. The accuracy of the method critically depends upon complete mixing of the injected solution 

through the stream cross section before the sampling station is 

reached and upon no adsorption of the tracer on stream-bottom 

materials. The method is recommended only for those sites 

where conventional methods cannot be employed owing to 

shallow depths, extremely high velocities, or excessive turbulence. A detailed description of the procedures and equipment 

used in measuring discharge by a dye-dilution method is given 

by Kilpatrick and Cobb (1985).

References Cited

Benson, M.A., and Dalrymple, Tate, 1967, General field 

and office procedures for indirect measurements: U.S. 

Geological Survey Techniques of Water-Resources 

Investigations, book 3, chap. Al, 30 p. (Also available at 

http://pubs.usgs.gov/twri/twri3-a1/.)

Bodhaine, G.L., 1968, Measurement of peak discharge at 

culverts by indirect methods: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. A3, 60 p. (Also available at http://pubs.usgs.gov/

twri/twri3-a3/.)

Buchanan, T.J., and Somers, W.P., 1969, Discharge measurements at gaging stations: U.S. Geological Survey Techniques of Water-Resources Investigations, book 3, chap A8, 

65 p. (Also available at http://pubs.usgs.gov/twri/twri3a8/.)

Dalrymple, Tate, and Benson, M.A., 1967, Measurement of 

peak discharge by the slope-area method: U.S. Geological 

Survey Techniques of Water-Resources Investigations, book 

3, chap. A2, 12 p. (Also available at http://pubs.usgs.gov/

twri/twri3-a2/.)

Fulford, J.M., and Sauer, V.B., 1986: Comparison of velocity interpolation methods for computing open-channel 

discharge, in Subitsky, S.Y. , ed., Selected papers in the 

hydrologic sciences: U.S. Geological Survey WaterSupply Paper 2290, p. 139–44. (Also available at 

http://pubs.usgs.gov/wsp/wsp2290/.)

Herschy, Reg, 1994, The analysis of uncertainty in the stagedischarge relation: Flow Measurement and instrumentation, 

v. 5, no. 3, p. 188–190.

Hulsing, Harry, 1967, Measurement of peak discharge at dams 

by indirect methods: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 3, chap. A5, 29 p. 

(Also available at http://pubs.usgs.gov/twri/twri3-a5/.)

International Organization for Standardization, 1997, Measurement of liquid flow in open channels—Velocity-area 

methods, ISO 748:1997E, 41 p.

International Organization for Standardization, 2003, Hydrometry—Echo sounders for water depth measurements: International Organization for Standardization ISO–CD_4366_

(E revised), 15 p.

Kennedy, E.J., 1984, Discharge ratings at gaging stations: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 3, chap. A10, 59 p. (Also available at 

http://pubs.usgs.gov/twri/twri3-a10/.)

Kilpatrick, F.A., and Cobb, E.D., 1985, Measurement of 

discharge using tracers: U.S. Geological Survey Techniques 

of Water-Resources Investigations, book 3, chap. A16, 52 p. 

(Also available at http://pubs.usgs.gov/twri/twri3-a16/.)

Kilpatrick, F.A., and Schneider, V.R., 1983, Use of flumes in 

measuring discharge: U.S. Geological Survey Techniques 
References Cited 87

of Water-Resources Investigations, book 3, chap. A14, 46 p. 

(Also available at http://pubs.usgs.gov/twri/twri3-a14/.)

Lipscomb, S.W., 1995, Quality assurance plan for discharge 

measurements using broadband acoustic Doppler current 

profilers: U.S. Geological Survey Open-File Report 95–701, 

7 p. (Also available at http://pubs.usgs.gov/of/1995/

ofr95-701/.)

Matthai, H.F., 1967, Measurement of peak discharge at width 

contractions by indirect methods: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 3, 

chap. A4, 44p. (Also available at http://pubs.usgs.gov/twri/

twri3-a4/.)

Morlock, Scott, E., 1996, Evaluation of acoustic Doppler 

current profiler measurements of river discharge: U.S. 

Geological Survey Water-Resources Investigations Report 

95–4218, 37 p. (Also available at http://pubs.usgs.gov/wri/

wri95-4218/.)

Mueller, D.S., and Wagner, C.R., 2009, Measuring discharge 

with acoustic Doppler current profilers from a moving boat: 

U.S. Geological Survey Techniques and Methods 3–A22, 72 

p. (Also available at http://pubs.water.usgs.gov/tm3a22.)

Mueller, D.S., and Wagner, C.R., 2006, Application of the 

Loop method for correcting acoustic Doppler current profiler discharge measurements biased by sediment transport: 

U.S. Geological Survey Scientific Investigations Report 

2006–5079, 18 p. (Also available at http://pubs.usgs.gov/

sir/2006/5079/.)

Oberg, K.A., Morlock S.E., and Caldwell, W.S., 2005, Quality 

assurance plan for discharge measurements using acoustic 

Doppler current profilers: U.S. Geological Survey Scientific 

Investigations report 2005–5183, 35 p. (Also available at 

http://pubs.usgs.gov/sir/2005/5183/.)

Oberg, K.A., and Mueller, D.S., 2007, Validation of streamflow measurements made with acoustic Doppler current 

profilers: Journal of Hydraulic Engineers, v. 133, no. 12, 

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Manuscript approved for publication June 1, 2010.

Prepared by the Reston Publishing Service Center.

Edited by Marilyn A. Billone.

Graphics by Annette L. Goode.

Design and typography by Anna N. Glover.

For more information, please contact:

D. Phil Turnipseed

U.S. Geological Survey

415 National Center

Reston, VA 20192

E-mail: pturnip@usgs.gov
Turnipseed and Sauer—Discahrge Measurements at Gaging Stations—Techniques and Methods 3–A8

Printed on recycled paper