U.S. Department of the Interior

U.S. Geological Survey

Techniques and Methods 3–A7

Stage Measurement at Gaging Stations

Chapter 7 of Book 3, Section A
Front cover: Looking upstream into Glacier National Park from Swiftcurrent Creek at Many Glacier, 

Mont. (U.S. Geological Survey station no. 05014500).

Back cover: On the top, hydrographer inspecting the streamgage at Prairie Dog Town Fork Red River 

near Canyon, Tex. (USGS station no. 07297500) in August 1925 (site is now discontinued). On the bottom, 

Streamgage construction crew at the Suwannee River at Ellaville, Fla. (USGS station no. 02319500), 

February 20, 1933.
Stage Measurement at Gaging Stations

By Vernon B. Sauer and D. Phil Turnipseed

Techniques and Methods 3–A7

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

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Sauer, V.B., and Turnipseed, D.P., 2010, Stage measurement at gaging stations: U.S. Geological Survey Techniques and 

Methods book 3, chap. A7, 45 p. (Also available at http://pubs.usgs.gov/tm/tm3-a7/.)

ISBN 978–1–4113–2989–8
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 A7 of book 3 (TM 3–A7) deals with stage measurement at 

gaging stations. The original version of this chapter was published in 1968 as U.S. Geological 

Survey (USGS) Techniques for Water-Resources Investigations, chapter A7 of book 3. New and 

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

edition of “Stage measurement at gaging stations.”

This edition supersedes USGS Techniques of Water-Resources Investigations 3A–7, 

1968, “Stage measurement at gaging stations,” by T.J. Buchanan and W.P. Somers, available at 

http://pubs.usgs.gov/twri/twri3a7/, 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 “Stage measurement at gaging stations” is published online 

at http://pubs.usgs.gov/tm/tm3-a7/ 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

Introduction and Purpose .............................................................................................................................1

Basic Requirements for Collecting Stage Data ........................................................................................2

Gage-Component Definitions ..............................................................................................................2

Gage Datum ...........................................................................................................................................3

Stage-Accuracy Requirements ..........................................................................................................3

Sources of Stage-Measurement Errors ...................................................................................4

Datum Errors ........................................................................................................................4

Gage-Reading Errors ..........................................................................................................4

Stage-Sensor Errors ...........................................................................................................4

Water Surface-to-Sensor-to-Recorder Errors ...............................................................4

Hydraulically Induced Errors ............................................................................................5

Recorder Errors ...................................................................................................................5

Retrieval Errors ...................................................................................................................5

Verification Errors ...............................................................................................................5

Gage Structures .............................................................................................................................................6

Stilling Wells ..........................................................................................................................................6

Instrument Shelters ............................................................................................................................11

Lightning Protection ...........................................................................................................................12

Instrumentation ............................................................................................................................................12

Nonrecording Gages ..........................................................................................................................13

Staff Gages..................................................................................................................................13

Electric-Tape Gages ..................................................................................................................14

Wire-Weight Gages ...................................................................................................................14

Cantilevered Wire-Weight Gages ...........................................................................................15

Float-Tape Gages .......................................................................................................................16

Float-Tape Maximum- and Minimum-Stage Indicators .......................................................17

Crest-Stage Gages ....................................................................................................................17

Water-Level Sensors ..........................................................................................................................19

Float-Driven Sensors .................................................................................................................19

Basic Float System ...........................................................................................................19

Float and Shaft Encoder ..................................................................................................19

Float and Potentiometer ..................................................................................................20

Bubble Gages .............................................................................................................................20

Gas-Purge Systems ..........................................................................................................21

Bubble-Gage Orifices .......................................................................................................22

Nonsubmersible Pressure Transducers .......................................................................24

Submersible Pressure Transducers .......................................................................................25

Noncontact Water-Level Sensors...........................................................................................26

Acoustic..............................................................................................................................26

Radar ...................................................................................................................................28
vi

Figures

1. Reinforced concrete stilling well and shelter..........................................................................6

2. Corrugated-galvanized-steel stilling well and shelter............................................................6

3. Concrete pipe stilling well and shelter......................................................................................7

4. Concrete block stilling well and shelter....................................................................................7

5. Steel pipe stilling well and shelter attached to bridge abutment.........................................7

6. Corrugated-steel pipe stilling well and shelter attached to bridge pier .............................7

7. Schematic of typical flushing system for intakes....................................................................8

Rapid Deployment Gages ................................................................................................30

Optical (Laser) ...................................................................................................................30

Water-Level Recorders......................................................................................................................30

Paper Chart Recorders .............................................................................................................30

Paper Punch-Tape Recorders .................................................................................................31

Electronic Data Loggers ...........................................................................................................32

Data Collection Platforms.........................................................................................................33

Telemetry Systems..............................................................................................................................33

Timers....................................................................................................................................................35

Power Supplies ...................................................................................................................................35

Typical Gaging-Station Instrumentation Configurations.......................................................................36

Stilling Well, Float Sensor, Shaft Encoder, and Data-Collection Platform.................................36

Instrument Shelter, Bubble Gage, Nonsubmersible Pressure Transducer, and 

Electronic Data Logger/Data Collection Platform............................................................37

Instrument Shelter, Submersible Pressure Transducer or Noncontact Radar-Stage 

Sensor and Electronic Data Logger/Data-Collection Platform......................................38

Data Retrieval and Conversion..................................................................................................................40

New Stage-Station Design.........................................................................................................................40

Site Selection.......................................................................................................................................41

Sensor Selection.................................................................................................................................41

Recorder Selection.............................................................................................................................41

Power Requirements..........................................................................................................................41

Operation of Stage-Measurement Station ..............................................................................................42

Clock, Timer, and Battery Check.......................................................................................................42

Gage Readings ....................................................................................................................................42

Record Retrieval..................................................................................................................................42

Float-Sensor, Gage-Well, and Intake Inspection...........................................................................42

Bubble-Gage, Gas-System, and Orifice Inspection ......................................................................43

Submersible Pressure Transducers ................................................................................................43

Noncontact Radar-Stage Sensors...................................................................................................43

Maximum- and Minimum-Stage Determinations ..........................................................................43

Final Recheck ......................................................................................................................................43

General Considerations .....................................................................................................................43

Safety.............................................................................................................................................................44

References Cited..........................................................................................................................................44
vii

8. In-bank stilling well and silt trap................................................................................................9

9. Schematic of typical in-bank silt trap........................................................................................9

10. Static tube for intake pipe.........................................................................................................10

11. Instrument shelter located on a stream bank........................................................................11

12. Instrument shelter located on a bridge abutment ................................................................11

13. Instrument shelter located on a dam ......................................................................................11

14. Look-in type of instrument shelter...........................................................................................12

15. Outside, vertical-staff gage, attached to 2-by-6-foot wood backing.................................13

16. Inclined-staff gage .....................................................................................................................13

17. Electric-tape gage and cylindrical weight .............................................................................14

18. Type A wire-weight gage ..........................................................................................................15

19. Cantilevered wire-weight gage................................................................................................15

20. Float-tape gage with analog or SDI–12 shaft encoder ........................................................16

21. Details of a crest-stage gage ...................................................................................................18

22. Vaisala Model 436A and Sutron 5600–530 shaft encoders..................................................19

23. Design Analysis Associates WaterLOG Model H–510 shaft encoder and 

memory card................................................................................................................................20

24. Stage potentiometer...................................................................................................................20

25. Conoflow gas-purge system.....................................................................................................21

26. Hydrological Services Model HS–55 self-contained bubbler system ...............................21

27. Design Analysis Associates Model H–355 self-contained gas-purge system.................22

28. Sutron Accububbler Model 5600–0131–1 self-contained gas-purge system...................22

29. Details of a bubble-orifice assembly.......................................................................................23

30. Bubble-orifice assembly............................................................................................................23

31. Details of a standard orifice static tube..................................................................................23

32. Paroscientific Model PS–2 nonsubmersible pressure transducer....................................24

33. Sutron Accubar Model 5600–0125–3 (Accubar-3) nonsubmersible pressure 

transducer....................................................................................................................................24

34. Design Analysis Associates Model H–350XL nonsubmersible pressure 

transducer....................................................................................................................................24

35. Design Analysis Associates Model H–312 submersible pressure tranducer..................25

36. KPSI Model level and pressure transducer...........................................................................25

37. YSI Model 600XL vented water-level submersible transducer...........................................25

38. Onset HOBO water-level data-logger models U20–001–01 .................................................26

39. In-Situ Aqua Troll 200 .................................................................................................................26

40. Aquatrak Absolute Liquid Level Sensor..................................................................................27

41. Design Analysis Associates H–3611 Radar Level Sensor....................................................29

43. Ohmart Vega VegaPuls62 Radar Gauge..................................................................................29

42. Ott RLS Radar Level Sensor......................................................................................................29

44. Typical USGS rapid deployment streamgage with a DCP and Ott RLS 

radar stage sensor .....................................................................................................................30

45. Stevens A–35 strip-chart recorder..........................................................................................31

46. Stevens analog-digital recorder (ADR)...................................................................................31

47. Typical installation of float-tape gage, electronic shaft encoder, and 

electronic data logger (EDL).....................................................................................................32

48. Schematic showing flow of hydrological data for a DCP transmission from 
viii

a gaging station to the Internet................................................................................................34

49. Design Analysis Associates H–424–MS SDI–12 Radio Link................................................35

50–53. Schematics of—

50. a stilling well, float sensor, incremental shaft encoder, and DCP .............................36

51. bubble gage, nonsubmersible pressure transducer, EDL, and (or) DCP..................37

52. a submersible pressure transducer used to measure stage with EDL 

and (or) DCP........................................................................................................................38

53. a radar-level sensor used to measure stage with EDL, and (or) DCP.......................39

Tables

1. Driving force and torque developed when a float of the indicated size 

is displaced by 0.01 ft.................................................................................................................20

2. Sensor specifications for the Design Analysis H–3611 radar level sensor, 

the Ohmart Vega VegaPuls61 and VegaPuls62 radar gages, and 

the Ott RLS radar level sensor..................................................................................................28

3. Summary of currently available data-collection platforms (DCPs) and 

electronic data loggers (EDLs).................................................................................................32

4. Equipment requirements for a site with a stage-recording gage ......................................40

Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Pressure

pound per square inch (lb/in2

) 6.895 kilopascal (kPa)

ounce force per square inch (ozf/in2

) 110.316 kilopascal (kPa)

pound force per square foot (lb/ft2

) 0.0479 kilopascal (kPa) 

Density

pound per cubic foot (lb/ft3

) 16.02 kilogram per cubic meter (kg/m3)

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 “National Geodetic Vertical Datum of 1929 

(NGVD 29)” or the “North American Vertical Datum of 1988 (NAVD 88).”
ix

Abbreviations and Acronyms

AC alternating current power

ADAPS U.S. Geological Survey National Water Information System Automatic Data 

Processing System

ADR analog-digital recorder

ADVM acoustic Doppler velocity meters

Accubar-3 Sutron Accubar Model 5600–0125–3

bps baud rate expressed in bits per second

Bubbler gas-purge Streamgaging system

CMP corrugated metal pipe

CSG crest stage gage

C type Columbus type

DCP data collection platform

DECODES device conversion and delivery system

EDL electronic data logger

FCC Federal Communications Commission

GOES Geostationary Operational Environmental Satellite

GPS global positioning system

HDR high data rate radio/transmitter

HIF U.S. Geological Survey Hydrologic Instrumentation Facility

laser light amplification by stimulated emission of radiation

LDR low data rate radio/transmitter

NAVD88 North American Vertical Datum of 1988

NGVD29 National Geodetic Vertical Datum of 1929

NIST National Institute of Standards and Technology

NOAA-NESDIS National Oceanic and Atmospheric Administration’s National Environmental 

Satellite, Data, and Information Service

NOAA-NOS National Oceanic and Atmospheric Administration’s National Ocean Service

NWS National Oceanic and Atmospheric Administration’s National Weather Service

NSIP U.S. Geological Survey National Streamflow Information Program

NWIS U.S. Geological Survey National Water Information System

NWISWeb U.S. Geological Survey National Water Information System Web site

OFR U.S. Geological Survey Open-File Report

OSW U.S. Geological Survey Office of Surface Water

PDA portable digital assistant

PVC polyvinyl chloride

radar radio detection and ranging

TM U.S. Geological Survey Techniques and Methods

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

UPS uninterruptable power supply

USGS U.S. Geological Survey

WRD Water Resources Division/Discipline

WWVB National Institute for Standards and Technology time signal radio station near 

Fort Collins, Colo., that radio-controlled clocks throughout North America use 

to synchronize themselves
x

Definitions of Symbols

Aw area of stilling well (in units of distance squared)

a

p combined cross-sectional area of intake pipes (in units of distance squared)

d average diameter of intake pipes (in units of distance)

rate of change of stage (in units of distance per unit time)

Δh time lag of a stilling well’s inside gage height (in units of distance)

g acceleration of gravity (in units of distance per unit time squared)

K general loss coefficient to account for various intake pipe fittings

Kv 3-way stem-cock valve loss coefficient, equal to 1.8 (do not use this coefficient if the 

intake is flush mounted or if a gate valve is used) 

Ksot side-outlet tee loss coefficient, equal to 1.1

Kst static tube loss coefficient, equal to 1.6

L average length of intake pipes (in units of distance)
Stage Measurement at Gaging Stations

By Vernon B. Sauer and D. Phil Turnipseed

The water-surface elevation for most rivers and streams is 

measured above an arbitrary or predetermined gage datum and 

is called the gage height of the river or stream. Gage height is 

often used interchangeably with the more general term stage, 

although the term gage height is more appropriate when used 

with a specific reading on a gage. Stage or gage height is usually expressed in feet and hundredths of a foot, or in meters 

and hundredths or thousandths of a meter.

A record of stream stage is useful in itself for designing 

bridges, embankments, levees, and other structures affected 

by stream elevations, or in planning for the use of flood plains. 

In streamgaging, gage heights are used as the independent 

variable in a stage-discharge relation to compute discharges. 

Reliability of the discharge record is therefore dependent on 

the reliability of the gage-height record, as well as the stagedischarge relation. Elevation records of lakes and reservoirs 

provide an index of lake-surface area and volume, as well as 

the elevation of the lake or reservoir.

Gage-height records may be obtained by systematic 

observation of a nonrecording gage, or with automatic waterlevel sensors and recorders. Various types of transmitting 

systems are frequently used to automatically relay gage-height 

information from remote gaging stations to office computers.

New technology, especially in the field of electronics, has 

led to a number of innovations in sensing, recording, and transmitting gage-height data. Whereas in the past, the majority of 

gaging stations used floats in stilling wells as the primary method 

of sensing gage height, the current trend is the use of submersible or nonsubmersible pressure transducers or radio detection 

and ranging (radar) sensors, which do not require a stilling well, 

are generally safer, and require less maintenance to operate. 

Unique to radar and optical [light amplification by stimulated 

emission of radiation (laser)] sensors, these instruments are 

usually operated out of the water, thus presenting a new set of 

uncertainties (for example, wind and waves, debris floating on 

the water surface, and air gaps), which are not encountered when 

the sensor is actually submerged or on the water surface.

Electronic data recorders and satellite transmission 

systems are now being used extensively. These new methods 

and equipment generally require only an instrument shelter 

with a simple bubble-orifice device in the stream, connected 

by tubing to a nonsubmersible transducer in the instrument 

shelter.

Abstract

Stream and reservoir stage are critical parameters in the 

computation of stream discharge and reservoir volume, respectively. In addition, a record of stream stage is useful in the 

design of structures that may be affected by stream elevation, 

as well as for the planning for various uses of flood plains. This 

report describes equipment and methodology for the observation, sensing, and recording of stage in streams and reservoirs. 

Although the U.S. Geological Survey (USGS) still uses the 

traditional, basic stilling-well float system as a predominant 

gaging station, modern electronic stage sensors and water-level 

recorders are now commonly used. Bubble gages coupled with 

nonsubmersible pressure transducers eliminate the need for 

stilling wells. Submersible pressure transducers have become 

common in use for the measurement of stage in both rivers and 

lakes. Furthermore, noncontact methods, such as radar, acoustic, 

and laser methods of sensing water levels, are being developed 

and tested, and in the case of radar, are commonly used for the 

measurement of stage. This report describes commonly used 

gaging-station structures, as well as the design and operation of 

gaging stations. Almost all of the equipment and instruments 

described in this report will meet the accuracy standard set by 

the USGS Office of Surface Water (OSW) for the measurement 

of stage for most applications, which is ±0.01 foot (ft) or 0.2 

percent of the effective stage. Several telemetry systems are 

used to transmit stage data from the gaging station to the office, 

although satellite telemetry has become the standard. These 

telemetry systems provide near real-time stage data, as well as 

other information that alerts the hydrographer to extreme or 

abnormal events, and instrument malfunctions.

Introduction and Purpose

The stage of a stream or lake is the height or elevation of 

the water surface above an established datum plane. For rivers, 

lakes, reservoirs, coastal streams, estuaries, and surface-water 

bodies, the height of the water surface is usually referred to 

as an elevation measured above the North American Vertical 

Datum of 1988 (NAVD88), the National Geodetic Vertical 

Datum of 1929 (NGVD29), or some other citable datum.
2 Stage Measurement at Gaging Stations

The purpose of this report is to describe the instrumentation and methods currently being used for the acquisition 

of gage-height data. This includes the traditional float/stilling-well method, and the newer methods utilizing pressure 

transducers, gas-purge systems, and radar sensors. The report 

first describes basic requirements necessary to all gages, such 

as gage-datum and stage-accuracy standards. Gage structures 

and the various types of instrumentation are described, such 

as reference and auxiliary gages, crest-stage gages, float 

gages, shaft encoders, bubble gages, recorders, telemetry 

systems, power supplies, submersible and nonsubmersible 

transducers, radar sensors, and other new instruments that are 

still in the development and testing phase. This report also 

describes various instrumentation configurations, methods of 

data retrieval, site selection, and gaging-station design and 

operation criteria.

Basic Requirements for Collecting 

Stage Data

The collection of stage data, either manually or automatically, requires various instrumentation, or components, 

established at a gaging site. For stage data to be useful for 

their intended purposes, requirements for maintaining a permanent gage datum and meeting specified accuracy limits are 

important. This section of the report provides definitions of 

the components, as well as the basic accuracy requirements. 

Descriptions of specific instrumentation are given in subsequent parts of this report.

Gage-Component Definitions

The reference gage for an automatic recording gaging station is a nonrecording gage used to set recorders, data loggers, 

or transmitters from which the primary gage-height record is 

obtained. The reference gage designated for this purpose is 

sometimes called the base gage, but this report uses the term 

“reference gage,” as defined by Rantz (1982). For gaging stations utilizing a stilling well, the reference gage is usually a 

staff gage, float-tape gage, or electric-tape gage located inside 

the stilling well. These gages are generally referred to as inside 

gages. For gages without stilling wells, such as a bubble gage, 

the reference gage is usually mounted directly in or over the 

stream, designated specifically to be the reference gage, and 

generally referred to as an outside gage. This may be a staff 

gage, wire-weight gage, chain gage, or other type of gage. For 

a nonrecording station, the reference gage is usually an outside 

gage, such as a staff gage, wire-weight gage, chain gage, or 

other type of gage read by an observer.

An auxiliary gage is any nonrecording gage other than 

the reference gage, and is used primarily for comparison and 

checking of the reference gage. For instance, where the inside 

gage is the reference gage, an outside gage is considered an 

auxiliary gage.

A stage sensor is a device that automatically determines 

(senses) the vertical position of the water surface. This may be 

a float riding on the water surface inside a stilling well. It may 

be a nonsubmersible pressure transducer coupled with a gaspurge bubbler orifice. It may be a submerged pressure transducer coupled with an electronic cable to transmit the vertical 

position of the water surface, and a venting tube to vent the 

submerged transducer to atmospheric pressure. Or it may be 

an acoustic, radar, laser, or optical pulse that reflects from the 

water surface to other instruments designed and calibrated for 

measuring or recording the gage height.

A stilling well and intake system consists of a closed well 

connected to the stream by intake pipes. It is designed so that 

an accurate water level of the stream can be determined inside 

the stilling well, even during rapidly rising or falling conditions, but without any appreciable surge or wave action. Stilling wells are generally used in conjunction with a float sensor, 

but can be used with gas-purge bubbler orifices.

A gas-purge bubbler orifice is a device placed in the 

stream, or sometimes near the bottom of a stilling well, to emit 

bubbles at a set rate. This device, when connected by a gas-fed 

tube to a gas-purge system and pressure transducer, becomes 

part of a bubble-gage stage-sensing system.

A pressure (or static) head, for the purposes of this report, 

is defined as the ratio between the pressure (pounds per square 

foot) and the density [pounds per cubic foot (lb/ft3

)] of a liquid, which for this report is typically the density of water (62.4 

lb/ft3). This is also known as the depth of the water at a given 

point.

Effective stage, for purposes of this report, is defined 

as the height of the water surface above an orifice, intake, or 

other point of exposure of the sensor to the water body.

A stage recorder is a graphical, digital, or electronic 

device that automatically records and stores gage-height 

readings sensed by a stage sensor. Graphical (analog) recorders produce a continuous chart of gage height. Digital and 

electronic recorders generally store gage heights at predetermined time intervals, such as every 5 minutes, 15 minutes, or 

1 hour. Sometimes other uniform time intervals are used, as 

well as nonuniform time intervals based on preprogrammed 

conditions.

A gage-height retrieval is the means by which gageheight data are extracted from the recorder. This may be 

simply by manually removing a chart or paper-punch tape, by 

downloading the data from the recorder to a personal digital 

assistant (PDA) or field computer, or by removing an electronic memory device from the recorder.

A telemetry system, for purposes of this report, is the 

means by which gage-height and other environmental data are 

automatically transmitted from the field recorder to another 

location. Telephone, radio, or satellite communication may 

perform this function.
Basic Requirements for Collecting Stage Data 3

Gage Datum

The datum of the gage may be either a recognized datum, 

such as the North American Vertical Datum of 1988 (NAVD 

88), the National Geodetic Vertical Datum of 1929 (NGVD 

29), or an arbitrary datum chosen for convenience. NGVD 29 

was the predominant datum used to establish lake and reservoir 

gages, and streamflow gages, including those located in tidal 

zones or coastal areas; however, with its inception, the NAVD 

88 is currently the datum the USGS recommends as the vertical 

datum for the USGS streamgaging network. Where NAVD 88 

exists, all gages referenced to other datums should be resurveyed or converted to NAVD 88. An arbitrary datum plane is 

usually used for streamgaging sites where it is desirable for all 

recorded gage heights to be relatively low numbers.

Select the arbitrary datum plane for a streamgaging site 

to avoid negative values of gage height. This requires the 

arbitrary datum plane to be below the lowest expected gage 

height, which will be at, or below, the elevation of zero flow 

on the control for all conditions.

Maintain a permanent gage datum, if at all possible, so that 

only one datum for the gage-height record is used for the life of 

the gaging station. For each gaging station, maintain a permanent datum that has at least three permanent reference marks 

that are independent of the gage structure. For gaging stations 

located at bridges, use at least one reference mark that is located 

away from the bridge structure, preferably out of the right-ofway easement. To make sure that the reference gage and the 

auxiliary gages have not changed relative to the established 

datum, and to determine the magnitude of any changes, run 

levels periodically to all gages and reference marks. Procedures 

for running levels at gaging stations are historically described 

by Thomas and Jackson (1981) and Kennedy (1990). With the 

publication of Kenney (2010), this is the standard for differential level surveys at USGS streamgaging stations.

The gage datum may need to be changed if there is excessive channel scour or a manmade channel change. Make such a 

change in increments of whole feet (or meters) so that the new 

datum can easily relate to the old datum. In some instances, the 

gage itself may need to be relocated to another site. The relation 

between the datum for the new gage site and the datum for the 

old gage site should be defined by leveling; however, it is not 

usually necessary to use the same datum at both sites. Keep a 

permanent record, or history, of all datum changes.

If you use an arbitrary datum plane for a gaging station, 

establish its relation to NAVD 88 by levels in order to maintain a national datum for the gage-height record. This can be 

done by using a Global Positioning System (GPS) survey, if it 

is not practical to do a level survey from a known established 

bench mark. This allows for recovery of the gage datum if the 

gage and local reference marks are destroyed.

Stage-Accuracy Requirements

Stage and elevation data are used primarily as an index 

for computing stream discharge and reservoir contents. The 

established methods require that stage data be measured and 

stored as instantaneous values rather than averaged values. 

Subsequent data processing and analysis will provide the 

means for any required averaging. The following paragraphs 

on accuracy requirements and stage-measurement error pertain 

to instantaneous stage values.

A number of factors enter into the specification of 

stage-accuracy requirements. For instance, the specific use 

for which the stage data are collected is an important factor. 

Stage data used to compute streamflow records must be 

significantly more accurate than stage data used for some 

design applications, or for certain flood-plain management 

applications. The primary use of stage data by the USGS is 

for computation of streamflow records; consequently, stageaccuracy requirements are stringent. In accordance with 

this primary use, and because the use of stage data cannot 

be predicted, the overall accuracy of stage data established 

for USGS gaging stations is either 0.01 foot or 0.2 percent 

of the effective stage, whichever is greater. For example, 

the required accuracy would be 0.06 ft at an effective stage 

of 30 ft, 0.02 ft at 10 ft, and 0.01 ft at all effective stages 

less than 7.5 ft. Effective stage is defined as the height of 

the water surface above the orifice, intake, or other point of 

exposure of the sensor to the water body. The instrument 

should be installed in the field with the orifice or intake 

only slightly below the zero-flow stage, or other defined 

low point of use.

The accuracy criteria stated above applies to the complete 

streamgaging station configuration, and is a composite of 

errors, or total error, from all of the components necessary for 

sensing, recording, and retrieving the data. See USGS Office 

of Surface Water (OSW) Technical Memorandum No. 93.07 

(1992) and OSW Technical Memorandum 96.05 (1996a). The 

individual sources of stage-measurement errors are described 

in the next section of this report.

The same accuracy requirements apply at reservoirs, 

lakes, and estuaries as those for stream sites. Vertical accuracy 

is needed for the computation of storage changes in reservoirs, 

for computation of discharge using slope ratings and (or) 

unsteady-flow models.

When field conditions, such as high velocities, wave 

action, or channel instability, make it impossible to collect 

accurate stage data or to define an accurate stage-discharge 

relation, stage data should be collected with the greatest accuracy feasible. Select appropriate instruments and methods to fit 

the field conditions.
4 Stage Measurement at Gaging Stations

Sources of Stage-Measurement Errors

The measured stage of a stream or other water body at 

any given point in time is subject to numerous sources of 

incremental errors. The combined effect of these errors should 

be within the accuracy requirements stated in the preceding 

section. The accuracy requirement for any single component 

of a stage-measuring system will generally be more stringent 

than the requirement for the system as a whole; however, it 

is not always possible to isolate, or pinpoint, an error and 

attribute it to one specific component. This part of the report 

describes the various sources of error, in general. For additional descriptions, see Rantz (1982).

Datum Errors

The gage datum is described in a previous section of 

this report. Movement of a gage caused by uplift or settlement of the supporting structure can cause datum errors 

that can only be detected by running levels. Gage datum for 

reference gages should be maintained to an accuracy of 0.01 

ft (Rantz, 1982), which can usually be achieved by running 

levels to established reference marks every 2 or 3 years. 

Where conditions are not stable, levels may be required at 

more frequent intervals. Generally, gages do not need to be 

adjusted unless datum discrepancies exceed 0.02 ft. Procedures for surveying differential levels at gaging stations are 

described by Kenney (2010).

Gage-Reading Errors

Errors can result from inaccurate gage readings, where 

it may be difficult to detect the water line against a staff gage 

because of poor lighting or very clear water. In other instances, 

accurate gage readings may be difficult to make because of 

water surge. These errors can be reduced or eliminated by 

careful observation, and in the case of surge, by averaging 

several observations. In almost all cases, read gages to the 

nearest 0.01 ft.

Stage-Sensor Errors

Stage sensors, such as floats, pressure transducers, and 

other stage-sensing devices, may introduce gage-height 

errors. The float in a stilling well may sometimes leak, the 

float-tape clamp may have slipped, or small animals or 

snakes may rest on the float. In most instances, problems 

with the float will cause it to float lower than originally set, 

causing gage readings to be too low. Stilling-well intake 

pipes may also become partly clogged, where the stage 

inside the stilling well lags in time behind the actual stage of 

the stream. Lagging intakes are discussed in depth in subsequent sections of this report.

Pressure transducers may have, or may develop, calibration errors. These errors can result in plus or minus deviations 

from the true gage height. The standard for acceptable errors 

in nonsubmersible pressure transducers is 0.01 ft or 0.10 

percent of the effective stage, whichever is greater (see OSW 

Technical Memorandum No. 96.05, 1996). In other words, 

the acceptable error is 0.01 ft for an effective stage of 10 ft or 

less, 0.03 ft for 30 ft, and 0.05 ft for 50 ft. These are acceptable errors for the pressure transducer only, which is only one 

component of the overall stage-measuring system. Various 

makes and models of pressure transducers have been tested by 

the USGS Hydrologic Instrumentation Facility1

 (HIF) to determine if they meet these standards. The HIF also tests pressure 

transducers after they have been used in the field. [See Water 

Resources Division (WRD) Instrument News issue 90 (U.S. 

Geological Survey, 1998a).]

Noncontact radar sensors are available and are being 

used in many locations in the USGS national streamgaging 

network. These sensors have been tested at the HIF and some 

of them are within 0.01 ft of accuracy. But noncontact radar 

sensors come with a new set of uncertainties, such as wind, 

air gap between the sensor and the water surface, waves, and 

debris floating on the water surface. The HIF continues to 

strive to stay abreast of testing to determine if this genre of 

stage sensors meets vertical standards.

Other noncontact stage-measuring devices are available 

that use water-surface sensing methods, such as acoustic 

wave transmission and optical (laser) transmission systems. 

These methods have and continue to undergo testing and 

have shown potential. Under controlled conditions, some 

of these devices are accurate to within 0.01 ft. Sources of 

error include temperature variations, density variations, and 

compositional variations in the transmission column. In some 

cases, obstacles such as snow, rain, or dust can affect accuracy. The electronics of these systems can be very sensitive 

and affect accuracy.

Water Surface-to-Sensor-to-Recorder Errors

The communication link between the stream-water 

surface, the stage sensor, and the data recorder can sometimes 

develop problems, or have inherent problems that result in 

gage-height errors. For instance, for a stilling-well-and-float 

system, the intakes may become clogged, or excessive sediment may settle in the stilling well, or the float tape may hang. 

These are major problems that usually result in a complete 

loss of data. More subtle problems can also occur that are not 

so obvious, but may result in small gage-height errors. For 

instance, as the stage rises in a stilling well, the float tape that 

1

Contact information for the HIF is available on their Web site home page at 

http://wwwhif.er.usgs.gov/.
Basic Requirements for Collecting Stage Data 5

connects the float to the data recorder via the float pulley is 

gradually transferred from one side of the float pulley to the 

other. This shift in weight can cause the float to ride slightly 

higher in the water, causing small positive errors in the 

recorded gage height. Rantz (1982) describes and quantifies 

this and other sources of error, such as float lag and submergence of the float-tape counterweight.

Likewise, for gas-bubbler systems, errors can result 

from gas friction in the line between the bubbler orifice 

and the nonsubmerged pressure transducer, the variation in 

weight of the gas column with stage, and the gas-bubble rate. 

Rantz (1982), Smith (1991), and Kirby (1991) discuss these 

error sources in detail, and quantify their magnitudes. Submersible pressure transducers show variation in measured 

water pressure because of water temperature, water density, 

and other factors.

In the case of electronic stage sensors, errors may occur 

as a result of converting an electrical signal to a numerical 

value of stage suitable for recording. In other instances, errors 

can happen when mechanical movement is converted to a 

numerical value of stage.

Noncontact gages have other sources of error. For 

instance, radar gages are often affected by waves on the water 

surface. Laser gages, although seldom used, are affected by 

the clarity of the water.

Hydraulically Induced Errors

High velocity in the stream near the outside end of the 

intake pipes can cause drawdown, or sometimes buildup, 

of the water surface inside a stilling well. A similar condition can occur when high velocity occurs near a bubblegage orifice. For example, where a sensor is located on the 

downstream or upstream side of a pier, the drawdown or 

buildup can be very large during large flows, on the order 

of 0.5 ft or more. This condition should be investigated by 

making simultaneous readings of outside and inside auxiliary 

gages, or recorder readings, during periods of high stages 

and (or) high velocity. It can also be checked by determining 

outside and inside high-water elevations. (See a subsequent 

section of this report on “Maximum- and minimum-stage 

determinations”).

Hydraulically induced errors can be reduced or eliminated through the use of an intake-static tube, or in the case 

of a bubble gage, an orifice-static tube. Relocating the intakes 

or orifice to a zone of low velocity may also help. Where 

drawdown or buildup cannot be completely eliminated, it may 

be necessary to develop an inside-outside gage relation to use 

over the effective range in stage for correcting inside gage 

readings to represent the actual outside gage height.

Recorder Errors

Automatic stage recorders include analog (graphic) 

recorders, digital (punch tape) recorders, data collection platforms (DCPs), and electronic data loggers (EDLs). Analog and 

digital recorders are mechanical recorders that can have play 

within the drive chains, gears, and (or) other linkages. Graphic 

recorders may also experience paper expansion and (or) reversal errors that can lead to data errors. Electronic recorders can 

have play within the mechanical float wheels and chain drives 

connected to them. Some of these errors can be corrected with 

care, good maintenance, and proper verification.

DCPs and EDLs may record incorrect stage readings 

because of errors or inconsistencies in equations and algorithms that convert electrical signals to recorded stages. This 

is usually a programming problem that can be corrected. 

Extreme temperatures can cause recording errors; however, 

most EDLs are rated to record accurately from about -40° to 

+60° Celsius, and some data loggers are rated for even greater 

extremes. Tests indicate that in almost all cases, the data loggers record correctly at extreme temperatures.

Retrieval Errors

Retrieval errors resulting from DCPs and EDLs occur 

because downloading the data from the data logger to a field 

computer is an electronic process that can sometimes result 

in incorrect stage readings and lost data. An even more serious problem would be the complete loss of stage data. Data 

errors or loss can also occur when downloading the data 

from an EDL or DCP to a removable data card or to an office 

computer. (See subsequent sections of this report for a more 

detailed discussion of data retrieval.)

Verification Errors

Stage readings require frequent and consistent verification to ensure that errors are reduced or eliminated. Failure 

to perform proper verification standards can be the source of 

undetected, and possibly significant, stage errors. Verification 

procedures include frequent reading of independent auxiliary gages, comparison of inside- and outside-gage readings, 

observation of high-water marks, redundant recording of peaks 

and troughs by use of maximum/minimum stage-tape indicators (also referred to as Dahman indicators), use of crest-stage 

gages, and regular maintenance of gage datum by differentiallevel surveys. These checks should be augmented as appropriate for unusual field conditions. Hydrographers should notice 

and keep records of instrument performance, including comparisons of recorded stages with the reference gage reading, 

and any applied corrections.
6 Stage Measurement at Gaging Stations

Figure 2. A–C, Corrugated-galvanized-steel stilling well and 

shelter.

A

C

B

C

Gage Structures

Stream and reservoir gages require some type of instrument shelter, and in the case of gages that use float sensors, 

they also require a stilling well. In many cases, these structures provide protection against vandalism, and from natural 

hazards such as rain, floods, wind, and lightning. The following sections describe the various structures used by the 

USGS. Streamgages identified as part of the USGS National 

Streamflow Information System (NSIP) are by mandate to be 

maintained above the 0.5 percent annual exceedance probability (200-year) flood stage. (See more discussion of NSIP 

streamgages in a subsequent section of this report.)

Stilling Wells

The stilling well protects the float and dampens the fluctuations in the stream caused by wind and turbulence, and perhaps could be considered the most accurate measure of stream 

stage. Stilling wells are made of concrete, reinforced concrete, 

concrete block, concrete pipe, corrugated-galvanized-steel 

pipe, aluminum pipe, PVC pipe and occasionally wood. They 

may be placed in the bank of the stream as shown in figures 

1, 2, 3, and 4, but often are placed directly in the stream and 

attached to bridge piers or abutments, as shown in figures 5 

and 6.

The stilling well should be deep enough for its bottom 

to be at least a foot below the minimum stage anticipated and 

its top above the level of the 2-percent annual-exceedance 

probability (50-year) flood [0.5-percent annual-exceedance 

probability (200-year) flood for NSIP streamgages]. The inside 

of the well should be big enough to permit free operation of all 

the equipment to be installed. Normally, a pipe 4 ft in diameter or a well with inside dimensions 4 by 4 ft is of satisfactory size, but pipes as small as 18 inches (in.) in diameter 

have been used for temporary installations where equipment 

Figure 1. Reinforced concrete stilling well and shelter.
Gage Structures 7

Figure 3. Concrete pipe stilling well and shelter.

Figure 4. Concrete block stilling well and shelter.

Figure 5. A, Steel pipe stilling well and B, shelter attached to 

bridge abutment.

Figure 6. A and B, Corrugated-steel pipe stilling well and shelter 

attached to bridge pier.

A

B

A B
8 Stage Measurement at Gaging Stations

requirements are not substantial. The 4-by-4-ft well provides 

ample space for the hydrographer to enter the well from the 

top, via a ladder, to clean it or to repair equipment. The smaller 

metal wells and the deep wells should have doors at various 

elevations to facilitate easy entry for cleaning and repairing. All personnel must follow appropriate safety rules and 

regulations when entering confined space, as defined in several 

official WRD memoranda, the latest being USGS WRD Technical Memorandum 2005.03 (2005). This memorandum also 

directs the hydrographer to other safety-related memoranda 

that specifically deal with safety guidelines for working in and 

around gaging stations. (See a subsequent section of this report 

on “Safety” for additional safety considerations.)

When placed in the bank of the stream, the stilling well 

should have a sealed bottom so that groundwater cannot seep 

into it nor stream water leak out of it. Water from the stream 

enters and leaves the stilling well through one or more intakes 

so that the water in the well is at the same elevation as the 

water in the stream. If the stilling well is in the bank of the 

stream, the intake consists of a length of pipe connecting the 

stilling well and the stream. The intake should be at an elevation at least 0.5 ft lower than the lowest expected stage in 

the stream, and at least 0.5 ft above the bottom of the stilling 

well to prevent silt buildup from plugging the intake. In cold 

climates, the intake should be below the frostline. If the well 

is placed in the stream, holes drilled in the stilling well may 

act as intakes, taking the place of pipe intakes. Some wells 

placed in the stream have a sloping hopper bottom that serves 

as an intake. These are designed to allow silt to slide out of the 

stilling well, thus preventing a buildup of silt that might cause 

a loss of gage-height record.

Two or more pipe intakes are commonly installed at vertical intervals of about 1 ft. During high water, silt may cover 

the stream end of the lower intakes while the higher ones 

continue to operate.

Most stations that have intakes subject to clogging are 

provided with flushing systems, as shown in figure 7, whereby 

water under several feet of head can be applied to the gagewell end of an intake. Ordinarily, a pump raises water from the 

well to an elevated tank. The water is then released through 

the intake by operation of a valve. Intakes without flushing 

systems may be cleaned with a plumber’s snake or rod, or by 

building up a head of water in the well with a portable pump 

to force an obstruction out of the intakes.

A silt trap is constructed at some stations where silt 

is a persistent and recurring problem. The silt trap is a 

Figure 7. Schematic of typical flushing system for intakes.
Gage Structures 9

low-elevation well located between the main stilling well and 

the stream, and through which the intakes pass before reaching the main stilling well. Baffles are sometimes placed in the 

silt trap to facilitate settlement of the silt before reaching the 

main stilling well, thereby leaving the main stilling well clear 

of silt so that it can function without clogging. The silt trap 

usually has a large entrance on top so that it can be accessed 

and cleaned easily. Figure 8 shows an installation with a silt 

trap, and figure 9 is a schematic illustrating a typical silt trap 

installation.

The intakes for stilling wells placed in the bank of the 

stream are usually galvanized-steel pipe. The most common 

size used is 2-in.-diameter pipe, but in some places up to 4-in.-

diameter pipe is used. After the size and location of the well 

have been decided, the size and number of intakes should be 

determined. The intake pipes should be of sufficient number 

and size for the water in the well to follow the rise and fall 

of stage without significant delay. Smith, Hanson, and Cruff 

(1965) related intake lag to rate of change of stage of the 

stream, and to various components, such as valves, tees, and 

static tubes, which are used in the stilling-well intake system. 

Based on their studies, the following equation may be used to 

determine the approximate lag for an intake pipe (or pipes) for 

any given rate of change of stage. Keep in mind that there will 

always be some intake lag during a change in stage, but it can 

be minimized if sufficient intakes are provided. This equation, 

although not exact, will provide a reasonable indication of the 

lag that can be expected for various combinations of intakes 

and pipe fittings. An intake system designed to keep intake lag 

at 0.1 ft or less, for the maximum expected rate of rise or fall, 

is probably adequate.

Figure 8. In-bank stilling well and silt trap.

Figure 9. Schematic of typical in-bank silt trap.
10 Stage Measurement at Gaging Stations

The usual methods of preventing the formation of ice in 

a stilling well are insulating measures, such as subfloors and 

heaters. Subfloors are effective if the station is placed in the 

bank and has plenty of earth fill around it. If the subfloor is 

built in the well below the frostline in the ground, ice will not 

normally form in the well as long as the stage remains below 

the subfloor. Holes are cut in the subfloor for the recorder float 

and weights to pass through, and removable covers are placed 

over the holes. Subfloors prevent air circulation in the well and 

the attendant loss of heat that contributes to the formation of 

ice in the well.

An electric heater or heat lamps with reflectors may be 

used to keep the well free of ice. The cost of operation and the 

availability of electric service at the gaging station are governing factors. Heating cables are often placed in intake pipes to 

prevent ice from forming.

Oil (for example, mineral oil, vegetable oil, or Isopar 

M) is sometimes used in oil cylinders placed in the stilling 

well to prevent freezing; however, because of the danger 

of leakage of the oil from the cylinder to the stream, it is 

highly discouraged, as stated in USGS WRD Memorandum 

No. 77.49 (1977). If it is necessary to use an oil cylinder, the 

gage height reading must be corrected for the displacement 

the oil causes on the water column in the cylinder because 

of the density difference of the oil and water. Never put 

oil directly in the stilling well. Likewise, do not use an oil 

cylinder if there is a real danger that the oil could escape to 

the stream. Wherever possible, use an alternative measure, 

such as an insulated subfloor or heater. Pressure transducers 

and bubble gages, as described in subsequent sections of this 

report, are also alternatives.

Figure 10. Static tube for intake pipe.

, (1)

where Δh = lag, in feet,

g = acceleration of gravity, in feet per second 

per second,

L = average length of intake pipes, in feet,

d = average diameter of intake pipes, in feet,

Aw = area of stilling well, in square feet,

ap = combined cross-sectional area of intake 

pipes, in square feet,

= rate of change of stage, in feet per second, 

and

K = loss coefficient to account for various intake 

pipe fittings (see below).

The loss coefficient, K, is the summation of loss coefficients for various pipe fittings, as defined by Smith, Hanson, 

and Cruff (1965), and as given in the following equation:

, (2)

where Kv = three-way stem-cock valve loss coefficient, 

equal to 1.8 (do not use this coefficient 

if the intake is flush mounted or if a gate 

valve is used);

Ksot = side-outlet tee loss coefficient, equal to 1.1; 

and

Kst = static tube loss coefficient, equal to 1.6.

If there are two or more intakes of different sizes and 

lengths, use the average length and the average diameter of 

the pipes for L and d, respectively; however, use the total 

cross-section area of all of the pipes for ap. If the intakes 

have different fittings, compute K for each pipe and use an 

average value of K for the system. These recommendations 

will not give exact results for intake lag, but will be approximately correct.

Place the intake pipes at right angles to the direction 

of flow where they are level. If the velocity past the ends of 

the intakes is high, drawdown or pileup of the water level in 

the stilling well may occur. To reduce the drawdown effect, 

attach static tubes to the stream end of the intake pipes. A 

static tube is a short length of pipe attached to an elbow or 

tee on the end of the intake pipe, extending horizontally 

downstream, as shown in figure 10. The end of the static tube 

is capped and water enters or leaves through holes drilled in 

the tube.

Use a special “orifice static tube” for a bubble-gage station equipped with a gas-purge system. Orifice static tubes will 

be described in subsequent sections of this report dealing with 

bubble gages.
Gage Structures 11

Instrument Shelters

Instrument shelters are made of almost every building 

material available and in various sizes and shapes, depending 

on local custom and conditions. See figures 1–6 for examples 

of shelters placed on top of stilling wells. Instrument shelters 

are also required for gages, such as bubble gages, that do not 

require stilling wells. These may be placed on a concrete slab 

or other suitable foundation directly on a stream bank, on a 

bridge, or on some other structure located near the stream, 

as shown in figures 11–13. Most instrument shelters have an 

antenna for data transmission, and solar panels for maintaining battery charge. Some type of mast is usually required for 

these sites, but the solar panel may also function adequately 

mounted to the roof or wall of the shelter.

Figure 11. Instrument shelter located on a stream bank.

Figure 12. Instrument shelter located on a bridge abutment. Figure 13. A–C, Instrument shelter located on a dam.

A

B

C
12 Stage Measurement at Gaging Stations

Lightning Protection

A lightning-protection system is needed for gaging 

structures to ensure uninterrupted data collection and to 

minimize expensive repairs to instruments and equipment that 

might otherwise be damaged by lightning. The best and most 

effective lightning protection for instruments, such as satellite 

data-collection platforms (DCPs), EDLs, stage sensors, telephone modems, computers, and other microprocessor-based 

instrument systems, is protection designed for and built into 

the instrument circuitry. Built-in protection can more closely 

match the protection needs of the circuitry than can protection 

that is added to the instrument after it is manufactured. Provide supplemental protection when built-in lightning protection is inadequate or not part of the equipment.

Supplemental protection includes alternating current 

power-line and telephone-surge suppressor devices. When 

telephone lines are used, but alternating current (AC) power 

is not part of a system, use a telephone-line-surge suppressor. Coaxial cables used for antennas should have a transient 

protector device to protect the DCP transmitter, not only from 

lightning, but also from voltage differences in an electrostatic 

discharge. Ground and protect sensor lines from induced lightning transients.

An effective, low-resistance grounding system is also 

required. Common-point grounding is necessary to keep the 

system components within the gage house at the same voltage 

potential relative to one another anytime the system becomes 

part of the lightning discharge circuit. Connect the commonpoint ground to a low-resistance (5-ohm) earth ground. A 

grounding rod buried below the soil frost line provides a yearround, uniform, low-resistance ground. In some cases, the 

stilling well may provide a low-resistance earth ground. An 

earth grounding kit can be obtained from the HIF, as described 

in WRD Instrument News issue 83 (U.S. Geological Survey, 

1998b).

Use as many layers of lightning protection as possible. 

However, even with internal instrument protection and supplemental protection, a direct lightning strike will likely destroy 

the electronic components. An excellent, detailed description 

of providing lightning protection is given in WRD Instrument 

News issue 73 (U.S. Geological Survey, 1996b) and issue 75 

(U.S. Geological Survey, 1996c).

Instrumentation

Many instruments are available for observing, sensing, 

recording, and transmitting stage data. Such instrumentation ranges from the simple nonrecording auxiliary gages to 

sophisticated water-level sensors, noncontact radar sensors, 

electronic data loggers, and telemetry systems, such as satellite data-collection platforms (DCPs). This section describes 

most of the currently available instruments used for stage data 

collection in open channels and reservoirs.

A walk-in shelter is the most convenient type of shelter, 

allowing the hydrographer to enter while standing and to be 

protected from the weather. A shelter with inside dimensions 

4 by 4 ft with ceiling height 7 ft above the floor is about the 

ideal size, either for a stilling well gage or a bubble gage, 

where a stilling well is not required.

Look-in shelters are used at sites where a limited amount 

of equipment is to be installed and if a portable and inexpensive shelter is desired. Look-in shelters provide sufficient 

space for equipment such as bubblers. Figure 14 is an example 

of a look-in type of shelter.

In humid climates, shelters should be well ventilated and 

have a tight floor to prevent entry of water vapor from the 

well. Screening and other barriers should be used over ventilators and other open places in the well and shelter to help 

prevent the entry of insects, rodents, and reptiles.

Instrument shelters not requiring a stilling well, such as 

for a bubble gage, may be installed at any convenient location above the reach of floodwaters. Shelters similar to those 

in figures 11–14 would be adequate. Such a gage may be used 

to take advantage of existing natural or artificial features in a 

stream without costly excavation for well or intake and without need for any external structural support. The bubble orifice 

is placed at least 0.5 ft below the lowest expected stage in the 

stream. The plastic tube connecting the orifice and the instrument is encased in metal pipe or conduit, or buried to protect it 

from the elements, animals, and vandalism. The bubble gage is 

especially well suited for short-term installations because the 

entire station is readily dismantled and relocated with practically no loss of investment.

Figure 14. Look-in type of instrument shelter.
Instrumentation 13

Nonrecording Gages

One method of obtaining a record of stage is by the 

systematic observations of a nonrecording gage. In the early 

days of the USGS, this was the means generally used to obtain 

records of stage, and is still used at a few gaging stations, but 

today water-level sensors and automatic water-stage recorders are the predominant instruments used at practically all 

gaging stations. Nonrecording gages are still in general use as 

auxiliary gages at water-stage recorder installations and serve 

the following purposes:

• As an auxiliary or reference gage to indicate the watersurface elevation in the stream or reservoir. This gage 

is referred to as the outside gage, and in the case where 

a stilling well is not used, this gage is usually the reference gage used for setting the automatic recorder.

• As an auxiliary or reference gage to indicate the watersurface elevation in the stilling well. This gage is 

referred to as an inside gage, and auxiliary gage readings in the stream are compared with the gage readings 

in the stilling well to determine whether outside stream 

stage is accurately transmitted into the stilling well via 

the intake pipes. The inside gage is usually considered 

the reference gage.

• As a temporary substitute for the recorder when the 

intakes are plugged or there is an equipment failure. 

The outside auxiliary gage can be read as needed by a 

local observer to continue the record of stage during 

the malfunction.

The types of nonrecording gages generally used are staff, 

wire weight, chain, float tape, and electric tape. Staff, wireweight, and chain gages are normally used as outside auxiliary 

gages at recording gaging stations. Float- and electric-tape 

gages and the vertical-staff gage are normally used inside stilling wells. Staff gages are read directly, whereas the other four 

types are read by measurement from a fixed point to the water.

Staff Gages

The staff gage is either vertical or inclined. The standard 

USGS vertical-staff gage consists of porcelain-enameled iron 

sections 4 in. wide and 3.4 ft long and graduated every 0.02 

ft. Figure 15 shows a vertical, outside staff gage. The verticalstaff gage is also used in stilling wells as an inside gage. 

Vertical-staff gages are set by leveling directly to the gages.

An inclined staff gage is used for an outside gage and usually consists of a graduated heavy timber securely attached to a 

permanent foundation. Inclined staff gages built flush with the 

streambank are less likely to be damaged by floods, floating ice, 

or drift than are projecting vertical staffs. Inclined-staff gages 

must be individually calibrated by leveling to several points 

along the length of the gage, interpolating intermediate points, 

and marking these points with a relatively permanent marking 

system. An inclined-staff gage is shown in figure 16.

Figure 15. Outside, vertical-staff gage, attached to 2-by-6-foot 

wood backing.

Figure 16. A and B, Inclined-staff gage.

A

B
14 Stage Measurement at Gaging Stations

to the positive terminal of the voltmeter. The negative terminal 

of the voltmeter is connected to the weight, through the frame, 

reel, and tape. One of two power supplies may be used with 

electric-tape gages. Prior to 1989, electric-tape gages were 

manufactured with a 5-volt meter, which requires a 4.5-volt 

battery. After 1989, electric-tape gages were manufactured 

with a 15-volt meter, which requires a 12-volt battery. The old 

5-volt meter can be replaced with a 15-volt meter so that the 

older model electric-tape gage can be connected to a 12-volt 

battery.

Attach the electric-tape gage to the instrument shelf, and 

insulate it if the shelf is metal. Ground the negative terminal of 

the battery to the stilling well if the stilling well is metal, such 

as a corrugated metal pipe (CMP) or culvert-pipe well. If the 

stilling well is nonmetallic, attach the ground wire to a small 

metal plate placed on the floor of the stilling well. Attach 

the positive terminal of the electric-tape gage to the positive 

terminal of the battery.

Run differential-level surveys to determine the gage 

datum elevation of the index marker of the electric-tape gage. 

Lower the weight so that at the index marker, the tape reads an 

elevation somewhat less than the elevation of the index marker 

that was determined by levels. For example, lower the tape to 

read 1.00 ft less than the elevation of the index marker. Hold 

the tape at this position and adjust the position of the weight 

on the tape so that the bottom of the weight is exactly 1.00 

ft below the index marker, as measured with a pocket tape 

or carpenter’s rule. The electric-tape gage is now set to read 

water levels precisely to gage datum.

To determine the gage height of the water level, the 

weight is lowered until it barely contacts the water surface. 

This contact completes the electric circuit and produces a 

signal on the voltmeter. With the weight held in the position 

of first contact, observe the tape reading at the index marker, 

which is the gage height of the water level.

Wire-Weight Gages

The type A wire-weight gage is usually attached to a 

bridge handrail and is generally used as an outside auxiliary 

gage. At some sites it is used as a reference gage. A type A 

wire-weight gage consists of a drum wound with a single 

layer of cable, a bronze weight attached to the end of the 

cable, a graduated disc, and a Veeder counter, all within a 

cast-aluminum box. Figure 18 shows a type A wire-weight 

gage. The disc is graduated in tenths and hundredths of a 

foot and is permanently connected to the counter and to the 

shaft of the drum. The disk is adjustable by loosening the set 

screws and moving the disk to the desired setting. The cable is 

made of 0.045-in.-diameter stainless-steel wire, and is guided 

to its position on the drum by a threading sheave. The reel 

is equipped with a pawl and ratchet for holding the weight 

within about 0.1 ft of any desired elevation. The diameter of 

the drum of the reel is such that each complete turn represents 

l ft of movement of the weight. A horizontal checking bar is 

mounted at the lower edge of the instrument so that when the 

Electric-Tape Gages

The electric-tape gage, as shown in figure 17, consists 

of a steel tape graduated in feet and hundredths, to which is 

fastened a cylindrical weight, a reel in a frame for the tape, 

and a voltmeter. Terminals are provided so that a voltmeter can 

be connected to a battery. The negative terminal of the battery 

is attached to a ground connection, and the positive terminal 

Figure 17. A, C, and D, Electric-tape gages and B, cylindrical 

weight.

A

D

B C
Instrumentation 15

bar is moved to the forward position the bottom of the weight 

will rest on it.

The gage should be set by lowering the weight to a position a few feet above the water surface and where leveling can 

be used to determine the elevation of the bottom of the weight. 

Hold the weight at this position and set the Veeder counter and 

graduated disk to read the same elevation as determined by 

levels. The elevation of the check bar should be determined 

by levels, and also by setting the weight on the check bar and 

reading the elevation from the Veeder counter and dial. These 

two elevations should be identical; however, there will sometimes be a small difference, especially if the vertical distance 

between the gage and low water is large. Both check-bar 

elevations should be recorded, and the one determined from 

the dial reading should be used for future checking to verify 

that the gage adjustments have not changed. Some hydrographers record the correct check-bar elevation and the date it 

was determined inside the wire-weight gage box, so it is readily available anytime the gage is used.

The gage height of the water surface is determined by 

lowering the weight to the water surface until it just touches it. 

The Veeder counter and graduated dial are read to obtain the 

gage height. If there are waves or turbulence, it may be necessary to take several readings at the crest and trough, and use 

the average of these for the water-surface elevation. In very 

still water, it is sometimes difficult to tell when the weight 

touches the water. Various methods are used, such as creating 

a slight pendulum motion that will disturb the water surface 

at the low point of the swing. Another method is to lower the 

weight into the water a few hundredths of a foot, and raise 

it a hundredth at a time, each time making a quick upward 

movement of the weight. If the weight is in the water or just 

at the water surface, the quick vertical movement will create 

a visible disturbance of the water surface. If there is no noted 

disturbance, than the previous elevation is the water-surface 

elevation.

Cantilevered Wire-Weight Gages

A cantilevered wire-weight gage is sometimes used 

where outside staff gages are hard to maintain and where a 

bridge, dock, or other structure over the water is not available 

for the location of a wire-weight gage. A wire-weight gage can 

be mounted on a cantilevered arm that extends out over the 

stream, or which is made in such a way that it can be tilted to 

extend over the stream. Figure 19 shows a cantilevered wireweight gage. To ensure no vertical movement, use a stable 

horizontal structure while using this instrument.

The cantilevered wire-weight gage consists of the cantilevered arm, which is held permanently in place, and a standard 

USGS wire weight, which runs over a pulley on the streamward end of the cantilever.

The wire-weight gage is set usually by leveling to the 

bottom of the weight, similar to the method described in the 

previous section for wire-weight gages. The wire-weight drum 

can be adjusted to read the correct elevation.

Stage is determined by lowering the weight until the 

bottom of the weight just touches the water surface, just as 

described previously for a wire-weight gage. The gage height 

is then read from the wire weight.

Figure 18. A and B, Type A wire-weight gage.

A

B

Figure 19. Cantilevered wire-weight gage.
16 Stage Measurement at Gaging Stations

Float-Tape Gages

Historically, the float-tape gage consisted of a float, a 

graduated steel tape, a lead or stainless-steel counterweight, 

and a pulley. The float pulley usually was a diameter of 6 in., 

was grooved on the circumference to accommodate the tape, 

and was mounted in a standard. An arm extended from the 

standard to a point slightly beyond the tape to carry an adjustable index. The tape was connected to the float by a clamp that 

also could be used for making adjustments to the tape reading, 

in case they were too large to be accommodated by the adjustable index.

Today, a float-tape gage is similar and typically consists 

of a float, a graduated steel tape, a stainless-steel counterweight, and a pulley wheel attached to an analog or SDI–12 

shaft encoder as shown in figure 20. The float pulley is typically a diameter of about 4 in., is grooved on the circumference to accommodate the tape, and is mounted directly to the 

shaft of the encoder. The shaft encoder is connected to a DCP 

or EDL where timed measurements of stage are recorded. 

An arm typically mounted to a gage shelf extends to a point 

slightly beyond the tape to carry an adjustable index. The tape 

is connected to the float by a clamp that also may be used for 

making adjustments to the tape reading, in case the adjustments are too large to be accommodated by the adjustable 

index.

B Figure 20. A and B, Float-tape gage with analog or SDI–12 shaft 

encoder.

A
Instrumentation 17

Floats having diameters ranging from 2 to 12 in. are 

commonly made of plastic, although older floats were made of 

copper. The float-tape gage is used chiefly as an inside reference gage.

The float-tape gage can be an independent assembly, or 

it can be an integral part of an electronic encoder interfaced to 

a DCP or EDL, as shown in figure 20. In all cases, the index 

marker is set by determining the water-surface elevation in the 

stilling well by reading the inside auxiliary gage, and by setting the index marker to read the same on the float tape. Today 

roughly 40 percent of USGS streamgages still use float-tape 

assemblies in a stilling-well for the measurement and recording of river stage.

Float-Tape Maximum- and Minimum-Stage 

Indicators

An advantage of a float-tape gage is that maximum- and 

minimum-stage indicators can be used on the tape so that 

the maximum and minimum stage can be determined for 

the period between station visits. These indicators are wire 

clips (similar to a paper clip and also referred to as Dahman 

indicators), or small magnets that are attached to the float tape 

beneath the instrument shelf; however, magnets will not work 

on stainless-steel tapes. The clips or magnets are designed so 

they slide easily on the float tape, and are large enough that 

they will not pass through the float-tape hole in the instrument shelf. As the stage rises, the clip or magnet will come 

against the instrument shelf and slide along the tape until the 

stage reaches a peak and begins to recede. The clip or magnet will then retain its position on the tape, unless a higher 

stage occurs at a later date. During the inspection visit to the 

gage, the hydrographer can then raise the float until the clip 

or magnet just touches the instrument shelf, and read the tape 

indicator at that point to determine the peak stage. Alternately, 

the float-tape reading at the clip or magnet can be read, and a 

correction subtracted from this reading that will account for 

distance between the float-tape indicator and the bottom of the 

instrument shelf.

A wire clip or magnet, as described above, can also be 

used on the counterweight side of the float tape to determine 

the minimum stage that occurred since the last visit to the 

gaging station. The operation of the minimum-stage indicator 

is similar to the peak-stage indicator as described above. After 

obtaining the readings of both the maximum- and minimumstage indicators, the clips or magnets should be reset on the 

float tape before leaving the station so they are against the 

bottom of the instrument shelf.

Crest-Stage Gages

The crest-stage gage is a simple, economical, reliable, 

and easily installed device for obtaining the elevation of 

the flood crest of streams. Although many different types of 

crest-stage gages have been tested, the most functional one 

is a vertical piece of 2-in. galvanized pipe containing a wood 

or aluminum staff held in a fixed position with relation to 

a datum reference, as shown in figure 21. The bottom cap 

has six intake holes located around its circumference so that 

when aligned correctly with the flow as shown in figure 21, 

drawdown or head buildup inside the pipe is kept to a minimum. Tests have shown that this arrangement of intake holes 

will be effective with velocities up to 10 ft per second, and at 

angles up to 30 degrees with the direction of flow. The top cap 

contains one small vent hole. For additional information, see 

Friday (1965) and Carter and Gamble (1963).

The bottom cap or a perforated tin cup or copper screening in a cup shape attached to the lower end of the staff contains regranulated cork. As the water rises inside the pipe, the 

cork floats on its surface. When the water reaches its peak and 

starts to recede, the cork adheres to the staff inside the pipe, 

thereby retaining the crest stage of the flood. The gage height 

of a peak is obtained by measuring the interval between the 

reference point on the staff and the flood mark. Scaling can be 

simplified by graduating the staff. The datum of the crest-stage 

gage should be checked by levels run from a reference mark to 

the top of the staff, or to the top of the bottom cap. Crest-stage 

gage pipes should be routinely checked for slippage.
18 Stage Measurement at Gaging Stations

Figure 21. Details of a crest-stage gage.

30°

30°

30°

30°

FLOW

1⁄4-inch intake

holes

Section A–A'

Note: Set 8-penny nail at 

top to measuring stick for

flush fit with cap.

3

/

16-inch vent hole

2-inch pipe

3⁄4-inch-by11⁄2-inch 

measuring

stick

A A'

Perforated tin 

cup for 

regranulated 

cork
Instrumentation 19

Water-Level Sensors

A water-level sensor, or stage sensor, was defined in 

a previous section of this report as a device that automatically determines, or senses, the vertical position of the water 

surface. In reality, stage sensors are usually not a single 

device, but a combination of two or more components that 

work together to sense the water-level position. The two most 

commonly used stage sensors include float-driven systems and 

gas-purge (bubbler) systems. Other less commonly used stage 

sensors are submersible pressure transducers, and noncontact 

methods used are acoustic, radar, and optical (laser) systems. 

The following sections describe the various instruments used 

for automatically determining water levels.

Float-Driven Sensors

Basic Float System

The basic float sensor consists of a float resting on the 

water surface in a stilling well. The float is attached to a tape 

or cable passing over a pulley, with a counter weight attached 

to the other end of the tape or cable. This is identical to the 

float-tape reference gage described in a previous section of 

this report, and is shown in figure 20. The float follows the rise 

and fall of the water level, and if the system uses a graduated 

tape, the water stage can be read visually by using an index 

marker. For automatically recording the water stage, the pulley 

is typically connected to a digital shaft encoder and DCP or 

EDL.

Float and Shaft Encoder

A shaft encoder is a float-driven device that is connected 

with a shaft to the pulley of a basic float system. The shaft 

encoder interprets the rotational position and the number of 

revolutions of the shaft to determine the water stage. Shaft 

encoders may or may not have visual readouts to indicate the 

water stage. In addition, shaft encoders are programmable to 

transmit the encoded stage to an EDL or to a DCP based on 

user-specified instructions. Some shaft encoders have their own 

internal recording system, such as a computer memory card.

Several commercial manufacturers, such as Vaisala, Inc., 

Schmitz Engineering Liaison, and Design Analysis Associates, 

Inc., and others, make various models of shaft encoders that meet 

USGS accuracy standards. Figure 22 shows a Vaisala model 

436A shaft encoder and a Sutron 5600–530 shaft encoder, both of 

which are designed to transmit water stages to an EDL or DCP. 

Figure 23 shows a WaterLOG model H–510 shaft encoder by 

Design Analysis Associates, Inc., which is designed to record 

water-stage data on an internal-computer memory card.

The float system that drives a shaft encoder must produce enough initial force to overcome the starting torque of 

the encoder. For example, when the float is displaced 0.01 

ft, the driving force must be sufficient to produce movement 

of the encoder shaft. The driving force will vary, depending 

on the diameter of the float and on the diameter of the pulley 

attached to the encoder shaft. Table 1 shows the driving force 

and torque developed for 0.01-ft movement of various sizes of 

floats, and a 12-in.-diameter pulley. In order to allow for other 

sources of error, OSW Technical Memorandum 96.12 (1996d) 

Figure 22. A, Vaisala Model 436A shaft encoder and B, Sutron 5600–530 shaft encoder.

A B
20 Stage Measurement at Gaging Stations

Table 1. Driving force and torque developed when a float of the 

indicated size is displaced by 0.01 ft.

Float diameter, inches (in.) 12.00 10.00 2.50 1.50

Driving force, ounces (oz) 7.84 5.45 0.34 0.123

Driving torque, inch-ounce (in-oz) 

(12-in. pulley)

15.00 10.40 0.65 0.230

One-third of driving torque, inchounce (in-oz)5.00 3.47 0.217 0.077

Figure 23. Design Analysis Associates WaterLOG Model H–510 

shaft encoder and memory card.

recommends that the starting torque of shaft encoders be no 

more than one-third of the values shown in table 1. Also, it is 

recommended that shaft encoders should have a starting torque 

that meets the criterion for a 1.5-in. float [less than 0.08 inchounces (in-oz)]. This would allow interchange of shaft encoders 

among gaging sites where smaller floats might be used.

Figure 24. Stage potentiometer.

Bubble Gages

The term bubble gage is a general term that refers to 

various configurations of instrumentation designed to measure, 

or sense, the water level and stage on the basis of pressure 

differentials. A gas, such as nitrogen or air, is forced (bubbled) 

through a fixed orifice mounted in the stream, and the water 

pressure at the orifice is transmitted through the gas tube to a 

pressure sensor located in the gage house where it is converted 

to a measurement of stream stage. Original bubble gages, first 

used in the early 1960s, used a mercury manometer to measure 

pressure differences. However, because of the hazardous 

nature of mercury, these manometers were banned in the late 

1980s, and installation of new mercury manometers was prohibited. All existing mercury manometers in use by the USGS 

were removed over a period of a few years in the 1990s.

Modern day bubble gages use some type of nonsubmersible pressure transducer as the method of measuring pressure 

differentials. A number of different pressure transducers are 

available that meet USGS accuracy requirements; some are 

described in the following sections of this report.

Two essential components of a bubble-gage system, in 

addition to the pressure sensors, are (1) a gas-purge system, 

and (2) a bubble-gage orifice. These are described in the following sections.

Float and Potentiometer

The stage potentiometer, as shown in figure 24, is designed 

as a stage sensor when used in combination with a basic float 

system. The stage potentiometer does not meet the accuracy 

requirements for primary stage records, and errors of ±0.05 ft 

are common. However, it is useful for noncritical applications 

with moderate accuracy requirements, and where low cost is an 

important factor. Stage potentiometers should not be used where 

continuous float motion (painting) occurs. Continuous movement of the water surface can cause wear of the internal contacts 

of the potentiometer, resulting in unreliable data.
Instrumentation 21

Gas-Purge Systems

The gas-purge system is a critical component of a bubblegas system. It is designed to feed a gas, usually nitrogen, 

through a system of valves, regulators, and tubing to an orifice 

located at a fixed elevation in the stream. The continuous 

formation of bubbles at the orifice transmits the pressure head 

(depth of water over the orifice) caused by the stream stage, by 

way of the connecting tubing, to the pressure sensor located in 

the gage house.

Several gas-purge systems are available for use with 

bubble gages. The standard USGS Conoflow bubbler system is 

a commonly used gas-purge system. Other gas-purge systems 

include the Hydrological Services Model HS–55 self-contained bubbler system, the Fluid Data Systems Safe Purge II 

unit, the Design Analysis Associates Model H–355, and the 

Sutron Accububbler.

The Conoflow bubbler system is shown in figure 25. The 

system consists of the Conoflow differential regulator, the 

sight-feed and needle-valve assembly, and various valves and 

tubing. The sight-feed and needle-valve assembly includes an 

oil reservoir where the bubble rate can be visually adjusted. 

One tube leads to the stream and orifice, and another tube 

leads to the pressure sensor. The Conoflow bubbler system is a 

proven method that meets USGS accuracy standards.

The Hydrological Services Model HS–55 self-contained 

bubbler system is a gas-purge system that converts water-level 

head to pressure in conjunction with a water-level pressuresensing instrument. This system is designed to supply a 

constant bubble rate by way of a regulator. Bubble rate is set 

by rotating a micrometer dial, thus eliminating the need for a 

sight glass and oil reservoir, as used in the Conoflow system. 

The unit has a gas-supply inlet for use with dry nitrogen, an 

instrument outlet for connection to a pressure-sensing device, 

and an orifice outlet for head-pressure measurement and 

system-purging operations. Tests have shown that it compares 

favorably with the Conoflow system. Figure 26 shows an 

HS–55 self-contained bubbler system.

The Design Analysis Associates Model H–355 and the 

Sutron Accububbler model 5600–0131–1 are self-contained 

gas-purge systems with pressure sensors and controller units. 

They do not produce constant bubble rates; they purge the 

orifice line before each scheduled reading. These systems can 

be automatic and (or) manually controlled to purge the orifice 

line. They wait for the bubble turbulence to subside before 

taking a stage reading. These types of systems are considered 

an advantage in streams where sediment infiltration may 

cause clogging of the orifice lines. Tests have shown that they 

compare favorably to the Conoflow system. Figures 27 and 28 

show the Design Analysis Associates Model H–355 selfcontained gas-purging system and the Sutron Accububbler 

model 5600–0131–1 gas-purge units, respectively.

Figure 26. Hydrological 

Services Model HS–55 selfcontained bubbler system.

Figure 25. Conoflow gas-purge system.
22 Stage Measurement at Gaging Stations

Figure 28. Sutron Accububbler Model 5600–0131–1 selfcontained gas-purge system.

Bubble-Gage Orifices

The gas-purge system bubbles a gas into the stream 

through an orifice. The standard USGS orifice is mounted 

in a 2-in. pipe cap so that it can be attached to a 2-in. pipe. 

Figure 29 is a detailed drawing of an orifice assembly, and figure 30 shows an inside and outside view of an actual standard 

USGS orifice assembly.

The orifice assembly is installed so that the orifice 

remains at a fixed elevation in the stream, and the proper 

placement in the stream is essential for obtaining an accurate record of stage. If possible, the orifice assembly should 

be installed where stream currents are not high, and where 

sediment accumulations are not likely to cover the orifice. If 

high velocities are expected to occur near the orifice, install 

it inside a static tube—a vertical mount perpendicular to the 

direction of flow.

Details of a standard orifice static tube are shown in 

figure 31. A more detailed description for the installation of a 

bubble-gage orifice is given by Craig (1983).

Figure 27. Design Analysis Associates Model H–355 self-contained gas-purge system.
Instrumentation 23

Figure 31. Details of a standard orifice static tube.

Pressure hole

Open end of union

Vent

hole

Figure 30. A, Inside and B, outside view of a bubble-orifice assembly.

Figure 29. Details of a bubble-orifice assembly.

A B
24 Stage Measurement at Gaging Stations

Nonsubmersible Pressure Transducers

Nonsubmersible pressure transducers are generally used as 

the pressure sensor for bubble-gage systems. These transducers 

are connected to the gas-purge unit to receive the pressure input 

from the stream. The transducers are internally programmed to 

convert the gas pressure to units of water head (feet of water 

over the orifice), and to then transmit the data to an EDL or 

DCP by way of a serial digital interface, such as the SDI–12. 

Pressure transducers generally have internal compensation for 

temperature changes and various other adjustment provisions 

to compensate for water-density variations, purge-gas weight, 

local-gravity variations, and gage-datum adjustments.

Three nonsubmersible pressure transducers tested by the 

HIF, and found to meet USGS accuracy requirements, are (1) 

the Paroscientific Model PS–2, (2) the Sutron Accubar Model 

5600–0125–3 (Accubar-3), and (3) the Design Analysis Associates Model H–350XL. These models are shown in figures 

32, 33, and 34, respectively.

Figure 32. Paroscientific Model PS–2 nonsubmersible pressure 

transducer.

Figure 33. Sutron Accubar Model 5600–0125–3 (Accubar-3) 

nonsubmersible pressure transducer.

Figure 34. A and B, Design Analysis Associates Model H–350XL 

nonsubmersible pressure transducer.
Instrumentation 25

Submersible Pressure Transducers

A number of submersible pressure transducers are commercially available for use, primarily in groundwater wells, 

although in a growing number of applications, they have been 

used for streams and reservoirs. Submersible pressure transducers are usually self-contained units having a transducer element and electronic circuitry. Some also contain a data logger 

and battery. These units can be mounted directly in the stream 

or reservoir. Temperature variations will affect the accuracy 

of submersible units; historically, these sensors generally 

have not met USGS accuracy requirements for primary stage 

records, although in recent years the HIF has tested several 

models that do meet OSW accuracy standards. Submersible 

units will be damaged or destroyed by freezing temperatures. 

Use of submersible pressure transducers may be acceptable in 

environments where temperature changes are small and where 

freezing is not a factor, such as in deep-reservoir installations. Tropical, subtropical, and temperate streams may also 

be acceptable locations, but without protection from freezing temperatures, these instruments are of limited use in the 

northern climates.

Submersible pressure transducers receive the pressure 

input from the water pressure of the stream above them and 

are internally programmed to convert pressure to units of 

water head (feet of water over the orifice). Units of water head 

then transmit the data to an EDL or DCP by way of a serial 

digital interface, such as the SDI–12.

Examples of submersible pressure transducers tested by 

the HIF, and found to meet USGS accuracy requirements, 

are: Design Analysis Associates Model H–312 submersible 

pressure transducer (0–22 psi; figure 35), the KPSI Model 

level and pressure transducer (0–15 psi; figure 36), and the 

YSI Model 600XL vented water-level submersible transducer 

(0–15 psi; fig. 37).

Submersible pressure transducers tested by the HIF, 

which did not meet USGS accuracy requirements, but may 

be applicable in certain environments, are: the Onset HOBO 

water-level data-logger models U20–001–01 and U20–001–

02, and the In-Situ Aqua Troll 200. Two of these models are 

shown in figures 38 and 39.

Since September 2005, the OSW has actively pursued 

using submersible transducers, such as those shown in figures 

35 through 39, for deployment in the track of hurricanes that 

come on shore in the United States. The data obtained from 

these transducers have helped modelers, decisionmakers, and 

emergency managers in the monitoring of storm surge caused 

by large hurricanes.

Figure 35. Design Analysis Associates Model H–312 submersible 

pressure tranducer. Figure 36. KPSI Model level and pressure transducer (0–15 psi).

Figure 37. YSI Model 600XL vented water-level submersible transducer (0–15 psi).
26 Stage Measurement at Gaging Stations

Storm tide levels, magnitude, and duration are critical 

parameters in the calibration of coastal models that depict 

and predict storm tides during hurricanes. This information 

is then used by emergency managers for the preparation, 

warning, and response to hurricanes. Since 2005, the OSW, 

in concert with USGS Water Science Centers, has planned 

and designed methods to collect a reliable time-series that 

can be used to construct a storm-surge dome model (that 

is, a three-dimensional image of the penetration, height, 

and abatement of the surge) for hurricanes that strike the 

U.S. coast. Such data yield valuable information for proactive design of coastal infrastructure, as well as emergency 

preparedness information for state departments of transportation and the National Weather Service. Data collected along 

beaches can help identify and describe wave dynamics and 

beach instability.

Based on the NOAA National Hurricane Center’s 

48-hour forecast of hurricane landfalls, USGS hydrographers 

rapidly deploy water-level sensors shown in figures 35 through 

39. These sensors are installed specifically at the coastline or 

near shore estuaries to capture detailed tidal-stage wave-height 

data for use by emergency managers and hurricane modelers. 

Improving emergency preparedness and coastal protection 

is the USGS goal by optimizing the collection of time-series 

storm-surge data that can improve the predictive capability of 

hurricane-inundation models, help in the design of manmade 

infrastructure, and help protect coastal ecosystems. These 

sensors collect not only storm-tide levels but also can be programmed to collect wave data.

Figure 39. In-Situ Aqua Troll 200.

Figure 38. Onset HOBO 

water-level data-logger models 

U20–001–01 (0–30 ft).

Noncontact Water-Level Sensors

Almost all of today’s instrumentation for measuring water 

levels requires that some part of the stage-sensing element be 

in contact with the water. Recent developments using acoustic, radar, and optical methods are beginning to emerge with 

instrumentation that will eventually provide accuracy and 

convenience of measuring water surfaces without direct contact. 

Most of these types of instruments are still in the development 

stage and, consequently, are relatively expensive, and some are 

not yet able to achieve USGS accuracy standards. The following 

sections provide a brief description of noncontact water-level 

sensors. For a more detailed description, see issue 80 of WRD 

Instrument News (U.S. Geological Survey, 1998a).

Acoustic

The acoustic noncontact methodology for measuring 

stage generally uses a high-frequency acoustic transducer 

that propagates a sound wave through the air to the water 

surface. The reflected acoustic wave is received at the transducer, converted to an electrical signal, and processed into 

a water-surface stage. The transmitted and reflected signals 

are affected by air temperature and air density, which in turn 

affects accuracy, especially when the vertical distance between 

the transducer and the water surface is large (that is, greater 

than about 10 ft). Without compensating for air temperature 

and density, this acoustic method has limitations in meeting 

USGS accuracy standards for primary stage records.

One notable exception in the use of acoustic noncontact methodology for measuring stage is the acoustic sensor 

instrumentation developed in cooperation with Aquatrak 

Corporation by the National Oceanic and Atmospheric 

Administration’s National Ocean Service (NOAA–NOS). 

The Aquatrak Absolute Liquid Level Sensor is an instrument 

based on sonic-pressure pulse comparison that is measured 

inside a 6-in. polyvinyl chloride (PVC) tube. The Aquatrak 

uses a fixed reference in its calibration tube approximately 

4 ft below the sensor to compensate each reading for propagation speed changes due to variations in air temperature 

and density. This instrument in preliminary tests by the HIF 

shows promise as a water-level sensor. Initial tests by the 

HIF indicate the Aquatrak Absolute Liquid Level Sensor 

passes USGS standards for vertical accuracy (that is, ±0.01 ft 

or 0.2 percent of the effective stage). The Aquatrak Absolute 

Liquid Level Sensor is shown in figure 40 without the PVC 

housing and installed at an NOS station.

The Aquatrak sensor is used across the United States by 

NOAA–NOS as the standard for the collection of water-level 

data. NOAA–NOS operates over 200 permanent near-coast 

tidal and Great Lakes gages in the United States.

In-stream acoustic beams are used for the measurement of depth, typically on acoustic Doppler velocity meters 

(ADVMs) at index velocity streamgages. The USGS in recent 

years has built hundreds of index-velocity gages with an 

ADVM or other acoustic-velocity meter for the measurement 
Instrumentation 27

and a submersed pressure transducer for the measurement of 

depth. These measurements can readily be converted to stage 

with an offset. Currently (2010), the HIF has done only limited 

testing to determine the accuracy of these instruments for 

stage measurement, but the technology seems promising as 

long as temperature and density constraints within the vertical 

column of water being measured remain constant. Typically 

in the USGS, ADVM stage measurements are used as an auxiliary or backup stage for the primary stage sensor mentioned 

previously.

of streamflow in reaches where unsteady (varied, nonuniform) 

streamflow is prevalent that prevents the development of a 

conventional stage-discharge rating. The ADVM measures 

point velocity and typically these gages include a dedicated 

stage sensor (for example, stilling well, nonsubmersible or 

submersible pressure transducer, or radar-level sensor) for the 

recording and computation of stage and discharge. However, 

ADVMs also now readily have available a separate in-stream 

acoustic beam or submersed pressure sensor for the measurement of depth. Some ADVMs have both an acoustic beam 

Figure 40. A, Aquatrak Absolute Liquid Level Sensor; B, sensor without polyvinyl chloride (PVC) 

housing; and C , installed at a National Oceanic and Atmospheric Administration National Ocean 

Service monitoring station with PVC housing.

A

C B
28 Stage Measurement at Gaging Stations

Radar

Radar, or radio-frequency transmission, is a distancemeasuring method that has been used since before World 

War II. A radio frequency is the propagation of an electromagnetic field, and therefore, is performed at the speed of 

light. The advantages of radar are that the signal is generally immune to weather conditions, such as snow and rain, 

and the radio wave used for this application is harmless to 

humans and wildlife. The usable sensor-to-water sensing 

range is minimally from near zero to about 66 ft, and can 

be greater, depending on the radar instrument. The technology for using radar for measurement of water levels is still 

new, although several commercially developed instruments 

are available. Some of these units are being tested by the 

HIF. Also, as in all radio-frequency transmissions, the radar 

frequency must meet approval from the USGS Office of 

Information Services and ultimately the U.S. Federal Communications Commission (FCC).

Several radar sensors are commercially available for 

use in streams and reservoirs. Radar sensors are usually 

self-contained units having typically a horn-like transmitting 

device and electronic circuitry. Radar sensors are internally 

programmed to convert radar-frequency reflections from units 

of distance to the water surface to stage.

To use these sensors, the hydrographer will usually have 

to provide a separate data logger and battery. These units are 

mounted directly above the stream or reservoir using a bridge 

handrail or other stable structure. In recent years, the HIF has 

tested several of these sensors that do meet OSW accuracy 

standards.

Typically, radar sensors used to measure stream stage 

have a beam angle of 8 degrees (table 2). This means at 25 ft 

of air gap, the footprint measured has a diameter of 7 ft on 

the water surface. If using one of these instruments for the 

measurement of stage, make routine checks to ensure the 

instrument footprint does not have obstructions that will affect 

the measurement of stage, such as debris, boulders, and other 

material in the stream.

Examples of radar sensors tested by the HIF, which 

meet USGS accuracy requirements, are: the Design Analysis 

Associates H–3611, the Ohmart Vega VegaPuls 61, the Ohmart 

Vega VegaPuls 62, and the Ott RLS radar level sensors. These 

sensors show promise in measuring stream stage although 

more research is needed to refine the uncertainties associated 

with noncontact radar sensors. Some of the manufacturers’ 

specifications for three radar sensors tested by the HIF are 

presented in table 2. Three of the four sensors are shown in 

figures 41, 42, and 43.

Table 2. Sensor specifications for the Design Analysis H–3611 radar level sensor, the Ohmart Vega VegaPuls61 and VegaPuls62 radar 

gages, and the Ott RLS radar level sensor.

[TOF, time of flight; GHz, gigahertz; FCC, Federal Communications Commission; ft, feet; °, angular degrees; lb, pound; °C, degrees Celsius; mA, milliampere; 

V, volt; DC, direct current; mW, milliwatt]

Specification Design Analysis Ohmart Vega Ohmart Vega Ott

Model H3611 Puls 61 Puls 62 RLS

Technology Pulse TOF Pulse TOF Pulse TOF Pulse TOF

Frequency 26 GHz 26 GHz 26 GHz 24 GHz

License required FCC approved FCC approved FCC approved FCC approved

Range 0.75 to 72 ft 0.75 to 72 ft 0.75 to 72 ft 2.62 to 35 ft

Accuracy 0.01 0.01 0.01 ~0.02

Beam angle 8°

10°

8° 8° 8°

Measurement footprint diameter at 25 feet or 

height

8° = 7 ft

10° = 9 ft

7ft 8° = 7 ft

10° = 9 ft

5 ft

Weight 8 lb ~5 lb 8 lb 4.6 lb

Communications interface SDI–12 4-20 mA, Hart, SDI–12 4-20 mA, Hart, SDI–12 SDI–12, RS485

Supply voltage 10 to 16 V DC Nominal 24 V DC Nominal 24 V DC 12 to 24 V DC

Current draw 1 mW 1 mW 1 mW <5 mW

Operating temperature range -40 to 80°C -40 to 80°C -40 to 80°C -10 to 40°C

Antenna Open Sealed Open Sealed
Instrumentation 29

Figure 43. Ohmart Vega VegaPuls62 Radar Gauge.

Figure 41. Design Analysis Associates H–3611 Radar Level 

Sensor.

Figure 42. Ott RLS Radar Level Sensor.
30 Stage Measurement at Gaging Stations

These gages could also be used for wildfires, major 

chemical spills, and other natural and anthropogenic disasters. The need for rapidly deployable streamgages has grown 

sequentially over time.

Optical (Laser)

Optical methods of measuring distance are confined 

mainly to the use of laser, a technology that deserves further 

investigation for use in measuring water-level stage. Eye safety 

is a consideration with lasers because direct exposure of the 

eyes to a laser beam could be dangerous and cause permanent 

vision loss or other damage to the eyes. Generally, for waterlevel measurements, this will not be a problem because the 

power levels of the beam are well within safe standards. As 

with radar, a few commercially developed laser instruments are 

now available and are being considered for testing by the HIF. 

In general, existing lasers in industrial use are recommended 

for opaque and unclear liquids. There is currently a need to do 

further research in the use of laser technology before it can be 

evaluated for use in the measurement of stream stage.

Water-Level Recorders

A water-level recorder is an instrument that automatically 

records a continuous, or quasicontinuous, record of the watersurface stage with respect to time. Water-level recorders may 

be paper chart recorders (analog), paper punch-tape analogdigital recorders (ADRs), EDLs, or DCPs. The basic requirements for a recorder are to systematically and accurately keep 

a record of the gage height with respect to time so that a stage 

hydrograph of the fluctuations of the water surface can be produced for later archiving and analysis. Accuracy requirements 

are described in a previous section of this report.

Paper Chart Recorders

A paper chart recorder, also referred to as an analog or 

graphical chart recorder, provides a continuous graphical 

trace of water stage with respect to time. It was the primary type of stage recorder used by the USGS from about 

1900 until the mid-1960s, when the ADR paper-punch tape 

recorder was introduced. The paper chart recorder is still 

used at some gaging stations, but in most instances it is 

used only as a backup recorder for verifying peak stages of 

hydrologic events. The Stevens A–35 strip-chart recorder, as 

shown in figure 45, is the most common analog recorder, but 

there are variations such as the gold strip-chart recorder, and 

the horizontal-drum recorder.

The Stevens A–35 recorder uses a roll of chart paper called 

a strip chart, which is driven by a clock at a uniform speed to 

provide the timing element of the stage hydrograph. Clocks 

are either spring driven or weight driven and are reasonably 

accurate if properly adjusted. Timing errors of 1 to 2 hours are 

normal over a period of several weeks; however, larger errors 

can sometimes occur. An ink pen and (or) pencil that rides on 

Rapid Deployment Gages

In recent years, the USGS has implemented a program whereby the HIF constructs and stores portable look-in 

shelters with stage sensors and preconfigured DCPs, batteries, 

solar panels, and other equipment to use in a rapid deployment 

during floods. Currently, the stage sensor for most of these 

gages is a noncontact radar-stage sensor. These portable gages 

are constructed and stored at the HIF, then quickly shipped to 

regions experiencing a flood or other environmental disaster, 

and when installed, provide near real-time access to stage and 

other hydraulic and hydrologic parameters to decisionmakers 

during emergency disasters (fig. 44). The equipment in these 

gages is typically a noncontact radar-stage sensor, a DCP, a Yagi 

antenna (top-of-pole-mount type), a 30-watt solar panel with 

custom bracket, and a GPS antenna. All cables are included and 

prewired, with two standard USGS padlocks for security.

Rapid deployment gages are useful where there is not an 

existing gage but there is a need to obtain information quickly, 

where a gage was destroyed by a flood, fire, or other cause, and 

perhaps for supplementing other gaging information at a site.

A

B

Figure 44. A and B, Typical USGS rapid deployment streamgage 

with a DCP and Ott RLS radar stage sensor.
Instrumentation 31

the strip chart is connected through a series of gear-and-chain 

linkages to the float wheel. As the stage rises and falls, the pen 

and (or) pencil draws a trace on the 10-in.-wide chart at a fixed 

scale, usually 1 ft of stage per 2 in. of chart scale, although 

other scales can be used. A reversal mechanism allows the pen 

and (or) pencil to reverse direction at the edge of the chart, thus 

providing a stage scale limited only by the vertical travel range 

of the float. Most strip-chart records will operate for several 

months without servicing, whereas the horizontal-drum recorder 

must be serviced at weekly intervals. Attachments are available for the strip-chart recorder to record water temperature or 

rainfall on the same chart with stage.

The Stevens A–35 recorder is a time-proven instrument 

that served as the primary method of recording stream stage 

for many years. A major advantage of this recorder is that it 

produces a graphical record of stage that gives a visual picture 

of the rise and fall of the stream. The graphical record also 

provides an easy means of spotting malfunctions, such as 

clogged intakes at stilling well streamgages. A disadvantage 

is that the graphical stage record cannot be easily scanned for 

electronic archiving and analysis.

Paper Punch-Tape Recorders

Paper punch-tape recorders, commonly referred to as 

ADRs, were introduced in the early 1960s as a replacement of 

the paper strip-chart recorders and used through the 1990s. The 

most common type of ADR used by the USGS is the Stevens 

digital recorder shown in figure 46. Recorders of this type have 

been replaced by EDLs and DCPs. A description of ADRs in 

this report is presented because of the many decades of service 

they provided and the billions of instantaneous stages that are 

still retained in hardcopy archives with the USGS.

The ADR (Carter, 1963) is a battery-operated, slow-speed, 

paper-tape recorder that punches a 4-digit number on a 16-channel paper tape at preselected time intervals. Stage is recorded in 

increments of a hundredth of a foot from zero to 99.99 ft and is 

transmitted to the instrument by rotation of the input shaft, which 

was generally driven by a basic float system, although these 

instruments were also geared to manometers. Shaft rotation is 

converted by the instrument into a coded punch-tape record that 

can be read visually from the tape, but is designed to be read by 

digital-tape readers for input to an electronic computing system.

The code consists of four groups of four punches each. In 

each group, the first punch represents “1,” the second “2,” the 

third “4,” and the fourth “8.” Thus, a combination of up to three 

punches in a group represents digits from one to nine, with a 

blank space for zero, and the four groups of punches represent 

all numbers from 1 to 9,999. Coding is done by means of two 

discs containing raised ridges in accordance with the punch 

code outlined above. One disc is mounted directly on the input 

shaft. The second code disc is connected to the first by a 100:1 

worm gear so that 100 revolutions of the input shaft rotate the 

second, or high-order disc, one complete revolution. A paper 

tape is moved upward through a punch block that is mounted 

on a movable arm hinged at the base of the recorder. The punch 

Figure 45. A and B, Stevens A–35 strip-chart recorder.

A

B

Figure 46. Stevens analog-digital recorder (ADR).
32 Stage Measurement at Gaging Stations

block contains a single row of 18 pins—16 pins for the information punches and 2 pins for punching feed holes.

Throughout decades of use, a major advantage of ADRs was 

that the paper-punch tapes could be automatically read into an 

electronic computing system for archiving and analysis; however, 

it was a slow and tedious process to read tapes by visual inspection. This made field inspection of the stage record difficult for 

spotting malfunctions of the gage. ADRs could also miss the 

absolute peak stage, especially on flashy streams. The mechanically punched tape of an ADR was very practical for field use 

under widely varying conditions of temperature and moisture.

Electronic Data Loggers

Electronic data loggers (EDLs) are devices that can 

be programmed to electronically record stage data (or other 

variables) on a specific, regular time interval, or on a userdefined schedule that may vary according to stage or other 

variable. There are a number of manufacturers and vendors of 

EDLs, which include Sutron, Coastal Environmental Systems, 

Campbell Scientific, Synergetics, Vaisala, Design Analysis 

Associates, Stevens, Unidata, and others. New models of 

current EDLs and new EDLs from new manufacturers are 

frequently offered. For this reason, it is not practical to provide 

a detailed description of specific manufacturers or models 

of EDLs in this report, as such listings and descriptions are 

subject to change in a short time. Descriptions and test results 

can be obtained from the HIF. EDLs are commonly combined 

with DCPs, which are described in the next section of this 

report. Table 3 contains a selected listing of currently available 

models of EDLs and DCPs that have been tested by the HIF.

EDLs are usually powered by an external battery, which 

may be rechargeable. In many cases, the batteries are charged 

through the use of a solar panel. Where reliable AC power is 

available, power may be supplied from the AC source. EDLs 

also usually have internal batteries to maintain programming 

and data in the event of a power failure.

Electronic clocks used in EDLs can generally be relied upon 

to keep accurate time. See a subsequent section of this report on 

“Timers” about the method of setting the timers in EDLs.

The stage of a stream or reservoir is sensed by either a 

float-driven shaft encoder, by a bubble gage and nonsubmersible pressure transducer arrangement, by a submersible pressure 

transducer, or by a radar sensor. The encoded stage data are then 

relayed to the EDL through a hard-wired or wireless connection 

for storage and (or) transmittal to a remote site. See figure 47 for 

a typical installation of a float-driven shaft encoder and EDL.

EDLs store stage data either internally on a memory 

module, or the EDL may store the data on a removable 

Table 3. Summary of currently available data-collection 

platforms (DCPs) and electronic data loggers (EDLs).

Manufacturer Model number Primary function

Sutron 8200

8200A

8210

8400

9210

7210

EDL and DCP

EDL and DCP

EDL and DCP

EDL

EDL and DCP

EDL and DCP

Vaisala 555A EDL and DCP

Campbell Scientific CR101

CR10X1

CR 295

CR 850

CR 1000

EDL

EDL

EDL and DCP

EDL

EDL

Design Analysis H-3501

H-522jr

H-522+

H-350XL1

H-500XL

H-510

DH-21

EDL2

EDL/DCP

EDL/DCP

EDL2

EDL

EDL

EDL2

Stevens GS-93A1

GS-93B1

EDL

EDL

1

Not currently supported by the HIF.

2

EDL and nonsubmersible pressure transducer (combination).

Figure 47. A, Typical installation of float-tape gage and B, 

electronic shaft encoder and electronic data logger (EDL).

A

B
Instrumentation 33

memory card. Most EDLs allow for both methods of data storage. Data are retrieved either by downloading directly from 

the EDL to a PDA or other field computer, or by removing the 

memory card and transferring the data from the card to a PDA 

or other field computer. The portable memory card can then 

be carried to the field office for downloading. Data can also be 

retrieved by transmitting via telephone, radio, or satellite.

EDLs can store relatively large amounts of data without 

frequent servicing, and they can be programmed to collect data 

according to specific needs. They can be easily connected to 

transmitting devices, such as land-based radios, telephones, or 

satellite radio systems, so that data can be retrieved in near realtime mode. The internal electronic clocks of EDLs are generally 

accurate. A disadvantage is that usually the complete stage record 

cannot be easily viewed in the field by graphical methods without 

downloading the data to a PDA or portable field computer that 

has plotting software designed for stage-hydrograph viewing.

Data Collection Platforms

Data Collection Platforms (DCPs) are referred to in the 

commercial industry as a unit designed to acquire data and 

transfer the data to another location. Data may be transferred by 

a telephone modem (land-line based or cellular), a land based 

radio, or satellite. The USGS uses the term DCP in reference 

to devices that collect data and transfer it over the NOAA, 

National Environmental Satellite, Data, and Information Service 

(NOAA–NESDIS) Geostationary Operational Environmental 

Satellite (GOES). The GOES transmitter is considered an integral part of a DCP. High Data Rate (HDR) transmitters are now 

available and transmit at baud rates of from 300 to 1200 bits per 

second (bps). NOAA–NESDIS has mandated the replacement 

of all 100 bps radios with 300 or 1200 bps radios by 2013.

DCPs store relatively large amounts of data; some units 

may provide for storage modules or cards that can also store 

a relatively large amount of data. In some cases, the GOES 

transmitter may be an integral part of an electronic data logger, or may be connected to an EDL in such a way that large 

amounts of data can be stored as well as transmitted. Like 

EDLs, there are a number of makes and models of DCPs by 

some of the same companies listed in the previous section on 

EDLs. Descriptions and manufacturers’ specifications of specific DCPs are not given in this report because currently available DCPs are subject to change, and new makes and models 

are frequently introduced. Descriptions and test results can be 

obtained from the HIF, and a summary of currently available 

units is given in table 3.

The power source for a DCP can be a rechargeable battery or a reliable AC electrical source. Solar panels are typically used to keep batteries charged, and the same battery may 

provide power to other instruments in the gage house.

Electronic timers are used for DCPs, and are generally 

very accurate. See a subsequent section of this report on “Timers” for a description of a device that can be used for setting 

the electronic clock of a DCP. HDR transmitters include 

internal atomic clocks.

DCPs receive stage (or other variable) data from a shaft 

encoder, submersible or nonsubmersible pressure transducer, 

radar water-level sensor, water quality sondes or other sensors, 

just as described previously for EDLs. The data are transmitted on a predefined schedule, such as a 1-, 3-, or 4-hour interval 

(after 2013, current plans are for 3- and 4-hour transmission to 

be phased out), thus providing near real-time access to the data. 

Overlapping periods of data are transmitted each time a transmission is made, providing a certain amount of redundancy; and this 

allows for the recovery of data that may sometimes be lost during 

a transmission. Redundant transmissions have proved a reliable 

means of plugging short gaps from data loss either through a 

missed transmission or brief malfunction of equipment.

DCPs typically are equipped with two channels for transmission. One channel is designated for normal transmissions 

(for example, at 1, 3, or 4 hours), and one for emergency transmissions (for example, typically transmitted at the recording 

interval of the DCP, such as at 5, 15, or 60 minutes). Emergency transmissions are random and usually triggered either 

by the stage reaching a predefined trigger, such as a NOAA 

National Weather Service flood stage, or a rate of change of 

stage, usually at flashy streams.

Telemetry Systems

The primary telemetry method used by the USGS is the 

GOES satellite transmission system, which is an integral part 

of DCPs, as described in the previous section of this report. 

Radio and telephone-transmitting systems are also used, 

primarily by agencies other than the USGS, but the USGS 

does have several flood-warning and real-time systems that 

are transmitted through radio and telephone systems. Through 

2013, the GOES satellite transmission will allow transmission 

through low data rate (LDR) satellite transmitters. LDRs transmit typically at a baud rate of 100 bps. After 2013, agencies 

using the GOES are currently mandated to upgrade all LDR 

radios to high data rate (HDR) radios. HDR radios transmit 

at 300 or 1200 bps. A schematic of the flow of data from the 

streamgage to the Internet is provided in figure 48.

Other telemetry systems for transmitting data from a 

gaging station to a remote location include electronic modems, 

cellular telephone modems, land based radio systems, and 

new satellite technology, such as Meteor Communications 

Corporation systems, which uses ionized meteor trails as a 

means of radio signal propagation. These meteor trails exist in 

the 50- to 75-mile region above the Earth’s surface. Some of 

these systems are relatively new, and still in the development 

stage. Wireless data-transmitting systems are in a rapid state of 

development with new systems on the horizon.

A short-range radio transmitter can be used to send data 

from a stage sensor, such as a float-tape gage or bubble gage, 

to a data logger. Two units are required—one for sending and 

one for receiving. An example of a radio system of this type 

is the Design Analysis Associates H–424–MS SDI–12 Radio 

Link, as shown in figure 49. This unit has a range of 1⁄8 to 1⁄4 
34 Stage Measurement at Gaging Stations

Figure 48. Schematic showing flow of hydrological data for a DCP transmission from a gaging station to the Internet.

Geostationary Operational

Environmental Satellite

Domestic

Satellite

Local Readout

Ground Station

NWIS Web site

Command and Data

Acquisition,

Wallops Island, Va.

National Water

Information System

(NWIS)

Streamgage
Instrumentation 35

Timers

Timers for data recorders and data loggers range from 

mechanical clocks to highly sophisticated electronic clocks to 

GPS signals. These have been referenced in previous sections of this report that describe data recorders and DCPs. 

The accuracy of the time associated with each recorded stage 

is dependent on the accuracy of the timer, and the advent of 

electronic timers in recent years has resulted in highly accurate 

time records.

Portable, electronic clocks are still available that can be 

used for setting the time on EDLs, DCPs, and other instruments requiring an accurate time setting; however, most EDLs 

and DCPs come with GPS-corrected timers that are highly 

accurate. These clocks receive a radio signal directly from the 

Atomic Clock (WWVB) in Boulder, Colo. The units typically 

use an omnidirectional antenna, and reception of the signal 

automatically sets and displays the time, in hours, minutes, 

and seconds, on a liquid crystal display. They are typically 

accurate to 0.0001 of a second. In addition to the time, a.m. 

and p.m., the month, date, day of the week, time zone, and 

signal strength are also displayed. The clocks automatically 

adjust for daylight savings time, leap year, and Earth-rotation 

corrections. Future instrumentation will incorporate automatic 

calibration of internal timers, and manual setting of timers will 

not be needed.

With the advent of HDR transmissions, it is critical to be 

accurate within a second of true atomic time. As stated previously, HDR transmitters include internal atomic clocks.

Power Supplies

Most gaging stations require some type of electrical 

power, either AC or DC, to operate the various instruments. 

Dry cell and wet cell batteries are the usual DC power sources; 

however, if a reliable source of AC current is available, it can 

be used, usually with an AC-to-DC converter. Very few, if 

any, instruments operate directly from AC current. If a system 

is operating from AC power, it should be equipped with an 

appropriate uninterruptible power supply (UPS) and surge protector. Automatic battery chargers that operate from AC power 

can be used to keep rechargeable batteries fully charged. 

Where AC power is not available, solar panels are frequently 

used as the primary method of recharging batteries.

Figure 49. Design Analysis Associates H–424–MS 

SDI–12 Radio Link.

mile; however, this range can be increased by substituting a 

directional antenna for the omnidirectional antenna that comes 

with the unit. It is basically a line-of-sight unit, but can tolerate minor obstructions. Other short-range radio transmitters 

are available from other manufacturers.
36 Stage Measurement at Gaging Stations

Typical Gaging-Station Instrumentation 

Configurations

The diversity of instrumentation available for sensing, 

recording, and transmitting stage-data results in several configurations of instrument setups provides stage data according 

to USGS standards for input to the National Water Information 

System (NWIS). In the following sections, four typical configurations are described. There are other possible and likely 

variations of these configurations.

Stilling Well, Float Sensor, Shaft Encoder, and 

Data-Collection Platform

The traditional, basic stage-recording system, used since 

the early 1900s, consisted of a stilling well located in or near 

the stream with a float used as the stage sensor. To this day, 

this method remains the most consistent and least uncertain 

method for the measurement of stream stage. The inside 

auxiliary gage is typically used as the reference gage for 

setting the recorder. This has proven to be a reliable system 

and is still used at about 4 of 10 USGS streamgaging stations 

today. Pictures of stilling wells are shown in figures 1–6 and 8. 

Nonelectronic recorders are shown in figures 45 and 46. This 

configuration with a shaft encoder and DCP is shown schematically in figure 50.

From the early 1900s through the 1990s, stage was 

recorded by a graphical or digital (nonelectronic) recorder. 

Since the 1990s, most of the instrumentation has been electronic 

for the logging, recording, and transmitting of stage data.

Figure 50 shows a stilling well and float sensor, an 

incremental shaft encoder (see figs. 22 and 23) and a DCP. 

The encoded stage, as determined by the float-actuated shaft 

encoder, is passed to the DCP by a hard-wired connection. The 

inside reference gage is used for setting the shaft encoder. The 

DCP transmits data through the GOES to the NWIS for 

reception, decoding, processing, analysis, and near virtual 

issuance of preliminary stage data to the NWIS World Wide 

Web (NWISWeb).

Figure 50. Schematic of stilling well, float sensor, incremental shaft encoder, and DCP.
Typical Gaging-Station Instrumentation Configurations 37

Instrument Shelter, Bubble Gage, 

Nonsubmersible Pressure Transducer, and 

Electronic Data Logger/Data Collection Platform

At gage sites where a stilling well is not used or is not 

practical, stage may be sensed with a pressure transducer 

coupled to a compressed-gas-purge system (bubble gage). 

Use nitrogen gas or a self-contained air-compression apparatus as the bubbler and also to purge the orifice. Connect the 

gas-purge system with an orifice line (tubing) to a bubblegage orifice that is located in the stream, and to the pressure 

transducer located in the instrument shelter. The stage, as 

sensed by the pressure transducer, is passed to the DCP by a 

hard-wired connection. The encoded stage is stored and later 

transmitted by the GOES transmitter. All instruments are 

housed in an instrument shelter. An outside reference gage is 

used for setting the pressure transducer. This configuration is 

shown in figure 51.

Figure 51. Schematic of bubble gage, nonsubmersible pressure transducer, EDL, and (or) DCP.
38 Stage Measurement at Gaging Stations

Instrument Shelter, Submersible Pressure 

Transducer or Noncontact Radar-Stage Sensor 

and Electronic Data Logger/Data-Collection 

Platform

At gage sites where neither a stilling well nor a nonsubmersible pressure transducer is used or is practical, stage may 

be sensed with a submersible pressure transducer or a noncontact radar-level sensor. Submersible pressure transducers can 

be mounted directly in the stream or reservoir. Temperature 

variations will affect the accuracy of submersible units, but 

several have passed the USGS standard for vertical accuracy 

(figs. 35–37). The ideal conditions for this sensor are typically 

a deep stream where temperature changes are minimal throughout the year. The stage, as sensed by the submersible pressure 

transducer, is passed to the DCP by a hard-wired connection, 

typically SDI–12. The encoded stage is stored and later transmitted by the GOES transmitter. All instruments, except the 

transducer, are housed in an instrument shelter. An outside reference gage is used for setting the pressure transducer.

Submersible pressure transducers are usually self-contained units having a transducer element and electronic circuitry. 

Some also contain a data logger and battery. These units can 

be mounted directly in the stream or reservoir, and with some 

planning in most locations, can be mounted so that maintenance 

and replacement can be accomplished without the hydrographer 

entering the water. As stated previously, temperature variations 

will affect the accuracy of submersible sensors, even those that 

meet USGS accuracy standards. Submersible units can be damaged or destroyed by freezing temperatures. Submersible pressure transducers receive the pressure input from the water pressure of the stream above them and are internally programmed 

to convert pressure to units of water head (feet of water over the 

orifice). Units of water head then transmit the data to an EDL 

or DCP by way of a serial digital interface such as the SDI–12. 

Use an outside reference gage for setting and maintaining stage 

measured with the pressure transducer.

The configuration for a submersible pressure transducer 

sensor is shown in figure 52.

Figure 52. Schematic of a submersible pressure transducer used to measure stage with EDL and (or) DCP.
Typical Gaging-Station Instrumentation Configurations 39

The noncontact radar-level sensor is mounted on a bridge 

handrail or other stable structure above the water surface. 

Radar-level sensors are generally immune to weather conditions, such as snow and rain, and the radio wave used for this 

application is harmless to humans and wildlife. The usable 

sensor-to-water sensing range is minimally from near zero 

to about 66 ft, and can be greater, depending on the radar 

instrument. Radar sensors are usually self-contained units 

having typically a horn-like transmitting device and electronic 

circuitry. Radar sensors are internally programmed to convert 

radar-frequency reflections from units of distance to the water 

surface to stage. The stage, as sensed by the radar, is transmitted to the DCP by a hard-wired connection, typically SDI–12. 

The encoded stage is stored and typically later transmitted by 

the GOES transmitter. All instruments, except the transducer, 

are housed in an instrument shelter. An outside reference gage 

is used for setting and maintaining stage in the noncontact 

radar-level sensor.

The configuration for a radar-level sensor is shown in 

figure 53.

Figure 53. Schematic of a radar-level sensor used to measure stage with EDL, and (or) DCP.
40 Stage Measurement at Gaging Stations

Data Retrieval and Conversion

The retrieval of stage data varies according to the type of 

recorder used, and to the urgency of acquiring the data. The 

overwhelming majority of USGS stage data is transmitted, 

received, decoded, and entered into NWIS through the GOES 

system, as shown in figure 48. EDLs may require a visit to 

the gaging station to retrieve the data; however, if the EDL is 

part of a DCP configuration, the data are retrieved by radio 

transmission via the GOES. Data may also be retrieved from 

an EDL and (or) DCP by downloading to a PDA or portable 

field computer, or by extracting a data card from the EDL and 

downloading data from the card to a computer. Data may also 

be retrieved by transmission via telephone, radio, or one of the 

other telemetry methods described in previous sections of this 

report. Some systems may be programmed to provide an automatic alert at critical stages, such as National Weather Service 

(NWS) flood stage or rate of change of stage. In the USGS, 

data recorders that produce a paper chart or paper-punch tape 

are no longer used as primary record, although paper chart 

recorders still exist as backup recorders at some stations. In 

particular, verification of instantaneous peak stages is sometimes done using A–35 chart recorders.

All data retrieved by way of charts, electronic data cards, 

and electronic downloads are considered original stage data 

and should be protected against loss or damage. Data that 

are transmitted via DCP/GOES satellite, or other telemetry 

method, are considered original and must be preserved as 

such. Original data for automated data-collection sites are 

defined as unaltered (no changes in magnitude) data acquired 

from the primary sensor (and backup sensor, as needed) and 

converted to conventional (engineering) units and a standard 

format. USGS policy defining original data is described by 

Hubbard (1992).

The various DCP and EDL record data in various formats 

must be deciphered and converted into a standard format that 

can be easily used by data-processing systems, such as the 

USGS NWIS Automatic Data Processing System (ADAPS). 

The Device Conversion and Delivery System (DECODES) 

performs this task. To convert data from a variety of devices at 

many sites, DECODES also manages a database that contains configuration information about these devices and sites. 

DECODES can convert data recorded at either fixed or variable time intervals from most EDLs, as long as the recorder 

can produce appropriate data files and identify any format 

changes unambiguously. For example, if the data format 

changes, some code must be present to identify for DECODES 

that the format has changed. DECODES can convert data from 

all the DCPs in use, whether recorded in ASCII, BINARY, or 

the LABARGE (pseudo ASCII) format. DECODES has been 

designed to operate under the Unix and MS–DOS operating 

systems. In the USGS, DCP data are automatically processed 

through DECODES for input to NWIS.

New Stage-Station Design

A new gaging station is usually established for the purpose of obtaining a record of streamflow, reservoir contents, 

tidal fluctuation, or other purpose. Acquisition of stage data 

may be only one of several factors that must be considered. 

For purposes of this report, however, only the requirements 

for obtaining an accurate and representative stage record are 

described.

Stage data can be obtained through periodic readings of 

a reference gage by an observer, but this method is usually 

used only at sites where a short record is needed and where 

occasional gage readings will suffice. Although such a record 

may be inexpensive to obtain, its accuracy and completeness 

will not be as good as a stage record obtained with an automatic stage sensor and DCP or EDL. The following sections 

pertain primarily to sites where an automatic recording gage 

will be used. Table 4 summarizes some of the equipment 

requirements.

Table 4. Equipment requirements for a site with a stage-recording gage.

Sensor Recorder Stilling-well 

required

DC power 

required

Meets USGS stage

accuracy requirements

Float with shaft encoder EDL and (or) DCP Yes Yes Yes

Float with potentiometer EDL and (or) DCP Yes Yes No

Bubble gage with nonsubmersible pressure transducer EDL and (or) DCP No Yes Yes

Submersible pressure transducer EDL and (or) DCP No Yes Yes1

Noncontact sensor EDL and (or) DCP No Yes Yes1

1

Should meet USGS stage accuracy requirements.
New Stage-Station Design 41

Site Selection

Choose the gage site for stage measurement based on the 

following criteria:

• If the stage record is to be used for computing streamflow, consider the requirements for controls, rating 

curves, backwater, and other streamflow variables in 

selecting the site, as well as the acquisition of stage 

data (see Rantz and others, 1982).

• Select the site so the intakes or orifice are in a pool, if 

possible, where stream velocity is low and not subject 

to significant turbulence. If this is not possible, place 

the intakes or orifice in a slack-water zone, where they 

are protected from high velocity.

• The gage stilling well (if used) and the instrument 

shelter may be located on a stream bank, bridge, dam, 

or other suitable structure, provided the other site 

selection criteria are met as closely as possible. Do not 

place the gage structure where it might sustain damage 

during floods.

• If the gage is located at or near a bridge, make sure it 

is on the downstream side. If this is not possible, and 

it must be located upstream of the bridge, then place it 

far enough upstream to be out of the zone of drawdown 

caused by the bridge during medium and high water.

• Select the site where either a stilling well with intakes can 

be easily installed, or where an instrument shelter can be 

installed for housing a bubble gage. If a bubble gage is to 

be used, the site must provide suitable conditions to install 

the necessary bubble tubing and orifice static tube. For 

bank installations, place the tubing underground between 

the gage shelter and the stream. For bridge installations, 

attach the tubing to the bridge members and pier or piling. 

Firmly anchor the orifice static tube in the stream, preferably in a zone of low velocity.

• Place the gage intakes or orifice low enough to record 

the lowest expected stage. In cold climates, place them 

below the frost line to protect them from freezing, if 

possible.

• The instrument shelter should be high enough to be 

above the 0.5 percent exceedance- (200-year) flood 

level, if possible.

• Minimize the distance between the stream and the stilling well and (or) instrument shelter.

• The site should have a suitable location for one or 

more outside auxiliary gages. These could be staff 

gages, wire-weight gages, or tape-down reference 

points. Make sure the auxiliary gages are easily accessible and located in a position so that accurate gage 

readings can be easily made. They should be in the 

same pool as the gage intakes, or orifice, and should 

provide readings that are indicative of the readings 

obtained through the intakes or orifice.

• If the gage site is for the purpose of measuring stage 

in a lake or reservoir and it is near the outlet structure, 

then make sure the gage intakes, or orifice, are located 

upstream of the zone of drawdown of the outlet structure.

• Make sure site conditions are such that an accurate 

datum can be maintained. Appropriate reference marks 

and reference points should be located both on and off 

the gaging structure to maintain accurate and timely 

level surveys of the gage. See Kenney (2010).

Sensor Selection

There are four basic categories of stage sensors, as 

described in previous sections of this report. They are (1) 

float-driven sensors, (2) bubble gage with nonsubmersible 

pressure transducer, (3) submersible pressure transducers, 

and (4) noncontact water-level sensors. The first two—floatdriven sensors and bubble gages—can be configured with 

other instrumentation to provide stage data that meet the 

accuracy requirements for a primary stage record. The third 

and fourth—submersible pressure transducers and noncontact sensors—meet USGS accuracy requirements, but some 

manufacturers’ sensors do not, so refer to HIF testing results 

to make sure they do. If somewhat less-accurate stage data are 

acceptable, then these may be suitable sensors to use.

In some situations, it may be impossible to maintain a 

stilling well or bubble-gage orifice in the stream because of 

very high-velocity conditions or significant debris that can 

damage or wash out the gage. The noncontact sensor may be 

the best alternative for such a site, even though it does not 

provide the accuracy of a float gage.

Float-driven sensors require a stilling well, whereas the 

other sensor types do not. All, however, require some type 

of instrument shelter. See previous sections of this report on 

“Water-level sensors” for detailed descriptions.

Recorder Selection

Generally, two types of recorders are currently available 

for recording stage data, as described in the previous section 

“Water-level recorders.” These are EDLs and DCPs. Analog 

paper charts still exist in some parts of the country, but are 

typically used only for backup and verification of instantaneous 

peak stages during flooding events. Selection of an EDL or DCP 

may be dependent to some degree on the type of sensor used 

and the other related instrumentation as shown in table 4.

Power Requirements

Almost all of the currently used instrumentation requires 

12-volt DC electrical power, as described in previous sections 

of this report. This may be supplied by dry or wet cell batteries, 

or AC power converted to DC power. Battery chargers or solar 

panels may be used to keep rechargeable batteries fully charged.
42 Stage Measurement at Gaging Stations

Operation of Stage-Measurement 

Station

There are numerous and varied types of stage-measuring 

equipment, as well as different configurations of this equipment, as described in previous sections of this report. Today’s 

equipment options are considerably greater than in previous 

years when only a graphic or digital punch-tape recorder was 

used. It is not practical to provide detailed instructions for 

servicing every type of recorder, data logger, DCP, gas-purge 

system, and other stage-measuring device. The operation of 

each stage station will vary according to the type of equipment 

used; become familiar with each device that is serviced. Each 

hydrographer will generally have their own specific routine, 

which may be different from that of another hydrographer. 

Therefore, the following operational steps do not necessarily 

apply to all sites, nor are they in a specific order that must be 

followed. As closely as possible, base the operation of a stage 

measurement station on the steps listed below.

Clock, Timer, and Battery Check

One of the first things the hydrographer should do when 

arriving at a gaging station is to check the clock in the DCP 

or EDL. HDR DCPs come with an atomic clock, but they 

should also be verified as necessary. Some smart sensors also 

have timing associated with them. Check these to make sure 

they are correct. For paper strip-chart recorders, check the pen 

and chart to see if the recorder is running slow or fast, or if 

the clock may have stopped. Make sure there is a mark at the 

point where the pen is resting on the chart, and that the date 

and watch time are recorded on the chart. Adjust the clock if 

there is a significant time error. In some cases, it may be necessary to replace the clock.

DCPs and EDLs have internal electronic clocks that are 

usually very accurate. Even so, these should be compared 

to watch time and reset as necessary. A portable electronic 

clock and antenna, as described in a previous section of this 

report, can be used to set the time very accurately on a DCP or 

EDL. The more modern instrumentation, such as HDR DCPs, 

have internal time checks and automatic time adjustments, as 

described in previous sections of this report.

Most gaging stations use 12-volt batteries for clocks, timers, EDLs, and DCPs. Battery voltage should be checked each 

time the station is visited, and replaced if voltage is below the 

specified limit. If an AC electrical source or a solar-charging 

panel is used, it should be inspected to be sure it is operating correctly. Solar panels should be cleaned periodically to 

ensure optimal performance.

Gage Readings

All auxiliary gages should be read before the gage-height 

record is retrieved from the recorder. Record these readings, 

along with the time, on the appropriate form (for example, 

USGS inspection sheet, 9–275F, 9–275I, electronic form, 

and so forth). EDLs and DCPs usually have no way to input 

auxiliary-gage readings; therefore, the readings recorded on 

the gage inspection forms are used for comparison and verification that the EDL and (or) DCP are operating correctly.

Take the reference gage reading at the same time as the 

recorder reading and compare both to determine if there are 

any discrepancies. If a discrepancy is found, then determine 

and correct the cause, if possible.

Record Retrieval

If necessary, the gage record should be retrieved from 

the DCP or EDL, after the time and gage readings have been 

determined and recorded. Electronically recorded data may 

be retrieved from EDLs and (or) DCPs by removing the data 

card and (or) downloading the data from the card to a portable 

computer or PDA or by hardwiring the PDA or computer to 

the EDL or DCP. In all cases, it is possible to make a direct 

download of the data into a PDA or portable computer. The 

electronically recorded data should be inspected for malfunctions and unusual events, such as floods, through the use of 

data files and (or) graphics on the portable computer.

Float-Sensor, Gage-Well, and Intake Inspection

Float-sensor gages require that the gage well and intakes 

be free and clear of sediment and other obstructions that would 

impair their performance. Following the initial readings of 

time and gage readings, and the removal of the gage record, 

if necessary, carefully inspect the float, gage well, and intakes 

to be sure everything is in good working order. Inspecting the 

gage record in the office (if the site is transmitting in near real 

time) or on site, as mentioned in the preceding paragraphs, 

may reveal sediment and (or) intake problems that cannot 

be detected otherwise. Probe the bottom of the gage well for 

sediment deposits, and clean, if necessary. Likewise, if there 

is any indication of intake problems, flush the intakes. Inspect 

the outside end of the intakes, if possible, and clean sediment 

or other debris from the intakes to allow free inflow. In some 

cases, a problem may develop from the float itself. Inspect 

the float for leaks, slippage of the float-tape-clamp screw, or 

debris that may be lodged on top of the float.

Inspection of the gage record in the office (before visiting 

the gage, if possible), or on a PDA or field computer by graphing the downloaded data, can help indicate problems with the 

recorded stage data. When these types of problems occur, the 

hydrographer should try to find the cause and make appropriate corrections. For instance, it may be that the lowest intake is 

not low enough for the lowest stage experienced. In this case, 

a new (and lower) intake may need to be installed. Or it may 

be that the float hangs on an obstruction in the gage well at a 

medium or high stage. Move or remove the obstruction so that 

the float can freely pass.
Operation of Stage-Measurement Station 43

Bubble-Gage, Gas-System, and Orifice 

Inspection

If the station has a bubble-gage sensor, inspect the bubble 

orifice to make sure it has not been buried by sediment. If the 

station has a nitrogen tank for pressure to the bubble gage 

sensor, keep a log of gas-feed rate, gas consumption, and gascylinder replacement to ensure a continuous supply of gas and 

to help spot leakage in the system. There is no serious leak 

in the gas-purge system if (1) the nonsubmersible pressure 

transducer operates to indicate stage correctly and (2) the gas 

consumption based on the average bubble rate over a period of 

time corresponds with the gas consumption computed from the 

decrease in cylinder pressure. If a leak is evident, its location 

can be determined by isolating various parts of the gas-purge 

system, sequentially closing of valves in the system. Gas leaks 

should be fixed to ensure continuous operation of the bubble 

gage.

Submersible Pressure Transducers

If the station has a submersible pressure transducer, query 

the sensor to make sure it is recording properly, and the slope 

and offsets are set correctly. If possible, inspect the orifice or 

pipe end containing the sensor to ensure it has not been buried 

in sediment or has another obstruction that could cause drawdown or buildup of the streamflow around the sensor, thereby 

causing the sensor to read low or high, respectively.

Noncontact Radar-Stage Sensors

If the station has a noncontact radar-stage sensor, query 

the sensor to make sure it is correctly programmed and properly recording. If the sensor has an opening to the atmosphere 

at the horn through which the radar is emitted, make sure it is 

clear of spider webs, mud daubers (dirt dobbers), wasps, and 

other insects and vermin. Inspect the water-surface area to 

make sure no obstructions, such as logs or other debris, have 

become lodged in the footprint of the sensor. These could create drawdown or buildup of the streamflow around the sensor 

footprint, causing the sensor to read low or high, respectively. 

As a precaution, make sure the radar signal is not impinging off a pier or other obstruction that would affect the stage 

measurement.

Maximum- and Minimum-Stage Determinations

If a high discharge has occurred since the previous visit 

to a stilling-well station, look for high-water marks both in the 

stilling well and outside the well. After obtaining high-water 

marks inside a stilling well, the marks should be cleaned and 

marked to prevent confusion with high-water marks that will 

be left by subsequent peak stages. In the case of a bubblegage station, submersible transducer, or radar sensor, look for 

high-water marks along the stream bank in the vicinity of the 

gas orifice, location of the submersible transducer, or measurement footprint of the radar sensor. Use a crest-stage gage, as 

described in a previous section of this report, as the source of 

corroboration of high-water marks at gages that do not have 

a stilling well. When a low peak follows a high peak, it may 

be possible to find high-water marks for both peaks. Regardless of the source, high-water marks (Benson and Dalrymple, 

1967) are used as an independent check on the maximum stage 

shown by the stage recorder, and determination of these marks 

should be considered a very important part of the gaging-station operation. Float-tape maximum and minimum indicators, 

as described in a previous section, are part of another method 

for determining maximum and minimum stages.

Make sure crest-stage gages are serviced on a regular 

basis to ensure there is always fresh cork in the cup, and to be 

sure the intake holes and vent hole are open. Mark and date 

cork lines defining high-water marks, then erase the cork lines 

after determining their elevations to avoid confusion with 

high-water marks left at a later time. Never remove the staff 

when water stands high in the pipe because reinserting it will 

result in a surge of the water in the pipe, thereby resulting in 

an artificial “high-water mark” on the staff.

Final Recheck

After other work is done at the gaging station, such as 

making a discharge measurement, painting, differential-level 

survey, or other maintenance, before leaving that station, 

return to the gage and make a final check of the DCP, EDL, 

and other instruments. Make another complete set of gage 

readings, recording all, including the watch time, on inspection sheets or other forms, as appropriate. Do not leave the 

station without being sure that all instruments are functioning 

as they should.

General Considerations

There are a few guidelines to consider concerning the 

maintenance of continuously recording streamgaging stations 

to increase the accuracy and to improve the continuity of the 

stage record. When needed, periodically clean and oil equipment. Keep all electronic equipment clean, free of dust, and 

properly ventilated. When needed, check and replace desiccant, which is kept inside of the gage house, shelter, or instrument. Intakes, stilling wells, and sediment traps should be 

thoroughly cleaned, at least once a year, but more often when 

sediment is a recurring problem. Carefully check gage-purge 

systems of bubble gages for leaks during each gage visit. 

Reduce excessive humidity and temperatures in the gage shelter to a minimum by providing proper ventilation, and in some 

cases, modify extreme cold temperatures using heating units 

and (or) insulation. Gage datum should be maintained within 

specified limits by periodically running a differential-level 

survey (Kenney, 2010); to check all gages, and by correcting 
44 Stage Measurement at Gaging Stations

gages as necessary. At new gage sites, and at sites where the 

datum is not stable, levels should be run at least once a year. 

After it is confirmed that the datum is fairly stable, levels can 

be run every 2 or 3 years, and in some cases, an even longer 

time between levels may be acceptable. Experience has shown 

that a program of careful inspection and maintenance will 

result in a complete gage-height record about 98 to 99 percent 

of the time.

Safety

Safety should be a primary consideration when working 

at gaging stations. Many times hydrographers work alone at 

a gaging station in a remote location, and, therefore, should 

always consider possible hazards in doing the work, and the 

fact that help may not be available if an accident occurs. Even 

when working with other people, one’s personal safety and the 

safety of others should be of paramount importance.

The USGS is particularly concerned about the development of safe working habits and the observance of all safety 

requirements. Details of safety requirements are given in several official USGS WRD memoranda, the latest being WRD 

memorandum No. 99.32 (U.S. Geological Survey, 1999). This 

memorandum also directs the hydrographer to other safetyrelated memoranda that specifically deal with safety guidelines 

for working in and around gaging stations. Each field hydrographer should obtain a copy of these memoranda, and carefully 

study them to become familiar with all safety requirements. 

USGS Water Science Centers should provide training in safety 

to each employee. Documents such as job hazards analyses 

and traffic closure plans should be an integral part of station 

folders in the office, the field vehicle, and the gage shelter. 

Keep these documents current by reviewing and updating 

them as necessary and follow the documented procedures 

outlined in these safety guides.

References Cited

Benson, M.A., and Dalrymple, T., 1967, General field 

and office procedures for indirect discharge measurements: U.S. Geological Survey Techniques of WaterResources Investigations, book 3, chap. A1, 30 p. (Also 

available at http://pubs.usgs.gov/twri/twri3-a1/html/pdf/

twri_3-A1_a.pdf).

Carter, J.R., and Gamble, C.R., 1963, Tests of crest-stage gage 

intakes: U.S. Geological Survey Open-File Report 63–147, 

10 p.

Carter, R.W., Anderson, W.L., Isherwood, W.L., Rolfe, K.W., 

Showen, C.R., and Smith, W., 1963, Automation of streamflow records: U.S. Geological Survey Circular 474, 18 p. 

(Also available at http://pubs.usgs.gov/circ/circ474/).

Craig, J.D., 1983, Installation and service manual for U. S. 

Geological Survey manometers: U.S. Geological Survey 

Techniques of Water-Resources Investigations, book 8, 

chap. A2, 57 p. (Also available at http://pubs.usgs.gov/twri/

twri8a2/pdf/TWRI_8-A2.pdf).

Friday, John, 1965, Tests of crest-stage intake systems: U.S. 

Geological Survey Open-File Report 65–183, 28 p.

Hubbard, E.F., 1992, Policy recommendations for management 

and retention of hydrologic data of the U.S. Geological Survey: U.S. Geological Survey Open-File Report 92–56, 32 p. 

(Also available at http://pubs.usgs.gov/of/1992/ofr92-56/).

Kennedy, E.J., 1990, Levels at streamflow gaging stations: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 3, chap. A19, 27 p. (Also available at 

http://pubs.usgs.gov/twri/twri3-A19/pdf/twri_3-A19_a.pdf).

Kenney, T.A., 2010, Levels at gaging stations: U.S. Geological 

Survey Techniques and Methods, book 3, chap. A19, 60 p. 

(Also available at http://pubs.usgs.gov/tm/tm3A19/).

Kirby, W.H., 1991, Temperature sensitivity of mercurymanometer bubble gages: U.S. Geologcial Survey WaterResources Investigations Report 91–4038, 19 p. (Also available at http://pubs.usgs.gov/wri/wri91-4038/).

Rantz, S.E., and others, 1982, Measurement and computation of 

streamflow, v. 2: U.S. Geological Survey Water-Supply Paper 

2175, v. 2, 631 p. (Also available at http://pubs.usgs.gov/wsp/

wsp2175/pdf/WSP2175_vol2a.pdf).

Smith, Winchell, 1991, Bubble gage registration errors caused 

by gas column density, in Subitsky, Seymour (ed.), Selected 

papers in the hydrologic sciences: U.S. Geological Survey 

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http://pubs.usgs.gov/wsp/wsp2340/).

Smith, Winchell, Hanson, R.L., and Cruff, R. W., 1965, Study 

of intake lag in conventional stream-gaging stilling wells: 

U.S. Geological Survey Open-File Report 65–150, 39 p.

Thomas, N.O., and Jackson, N.M., 1981, Manual for leveling at gaging stations in North Carolina: U.S. Geological 

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U.S. Geological Survey, 1977, Programs and plans—Use 

of oil cylinders to prevent freezing in streamgaging 

wells: U.S. Geological Survey Water Resources Division Memorandum No. 77.49, accessed March 22, 

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accuracy: U.S. Geological Survey Office of Surface Water 

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SW/sw93.07.html).
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U.S. Geological Survey, 1996a, Policy concerning accuracy of 

stage data: U.S. Geological Survey Office of Surface Water 

Technical Memorandum No. 96.05, unpaged, accessed 

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sw05.03.html.

Manuscript approved for publication September 7, 2010.

Prepared by the Reston Publishing Service Center.

Edited by Marilyn A. Billone.

Graphics by Annette L. Goode and Anna N. Glover.

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
Sauer and Turnipseed—Stage Measurement at Gaging Stations—Techniques and Methods 3–A7