U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3080

December 2007

Recent Improvements to the U.S. Geological Survey 

Streamgaging Program . . . from the National Streamflow Information Program

This Fact Sheet is one in a series that highlights information or recent research findings from the U.S. Geological Survey National 

Streamflow Information Program (NSIP). The investigations and scientific results reported in this series require a nationally consistent streamgaging network with stable long-term monitoring sites and a rigorous program of data, quality assurance, management, archiving, and synthesis. NSIP produces multipurpose, unbiased surface-water information that is readily accessible to all.

Introduction

The U.S. Geological Survey 

(USGS) established its first streamgage 

in 1889 on the Rio Grande River at 

Embudo, N.M. As the need for streamflow information increased, the USGS 

streamgaging network expanded to its 

current (2007) size of approximately 

7,400 streamgages nationwide.

The USGS streamgaging network, 

for most of its history, required mechanical measuring and recording devices to 

collect station data. Time-consuming and 

labor-intensive site visits were required to 

gather the recorded data for processing in 

the office. Eventually the data were published in paper reports. The USGS has 

progressively improved the streamgaging 

program by incorporating new technologies and techniques that streamline data 

collection, data delivery, and records 

processing while increasing the number 

and quality of product types that can be 

derived from the data. Improvements in 

recent decades that have expanded and 

broadened the streamgaging program are 

highlighted below.

Streamflow Data 

Collection and Processing

Streamgaging generally involves 

(1) obtaining a continuous record of stage 

(height of water), (2) obtaining periodic 

measurements of discharge, (3) defining the 

relation between the stage and the discharge, 

(4) using the stage-discharge relation to 

convert the continuous record of stage into 

a record of discharge, and (5) disseminating 

the streamflow information to a multitude 

of users, including water managers, scientists, engineers, and the general public.

River stage was traditionally 

measured with a float and tape system 

from inside a stilling well (fig. 1) and 

recorded on a paper graph or tape. This 

method of stage measurement served the 

USGS well over many years but with 

some drawbacks. Stilling-well systems 

are expensive to install, tend to fill with 

sediment, can be damaged when the 

pipes in the river move or are destroyed 

by floods, and cause human safety 

problems because of the confined space. 

The mechanical-paper recording devices 

resulted in poor timekeeping, lost data, 

and labor-intensive processing.

Float

Floor

Shelf

Recorder

Satellite

radio antenna

Water surface Water surface

Intakes

Figure 1. Stage measurement using a 

stilling well, float, and recorder.

Gas

bubbles

Bubble system

pressure sensor

Gage house

Battery

Bubble

orifice

Plastic

gas line

Monitored stream

Figure 2. Measurement of stage using a 

pressure-sensing device.

To overcome most of the drawbacks 

with the stilling-well system, the USGS 

has been using various pressure-sensing 

devices to measure stage (fig. 2), and 

electronic data loggers to record data.

Pressure systems transmit gas through 

a small-diameter tube into the river. The 

pressure generated is directly proportional 

to the height of the water, and thus the 

river stage can be determined. Many of 

the challenges associated with stilling 

wells are avoided with pressure systems, 

which have lower installation and maintenance costs and reduced safety concerns.

A major development in stagemeasuring technology beginning to be 

deployed by the USGS for special conditions is the non-contact radar stage sensor 

(fig. 3). This device measures the height 

of the river without touching the water. 

The radar stage sensor is simple and cost 

effective to install; is located above the 

river so it is much less susceptible to being 

moved or destroyed by debris or floods; 

eliminates the confined space safety 

issue; and, as a result of all of the above, 

is economical to operate and maintain.

Figure 3. Stage can be measured using a noncontact radar stage sensor mounted on a bridge.
Measurement of Discharge

Historically, the USGS measured river 

discharge through physical measurements 

of the water velocity and cross-sectional 

area of the river using a mechanical current 

meter placed directly into the water (fig. 4).

The mechanical instruments used to 

measure discharge worked well and have 

a proven track record; however, they are 

labor intensive to use, hinder measurements in difficult locations and under 

extreme flow conditions, and can place 

hydrographers in dangerous situations. 

For these reasons, the USGS has pursued 

the use of new and emerging technologies 

to improve discharge measurements.

C.

B.

A.

Figure 4. Current-meter method 

equipment. A, Price AA current meter. 

B, Current meter attached to a wading rod. 

C, Current meter suspended from a weightcable system (Olson and Norris, 2007; 

photograph by Michael Nolan, USGS).

For the last 10 to 15 years the USGS 

gradually has been replacing mechanical 

meters with hydroacoustic meters for 

discharge measurements. Hydroacoustic 

meters currently (2007) are used to make 

about 30 percent of USGS discharge 

measurements. Hydroacoustic meters 

use the Doppler shift to measure water 

velocity. Pulses of sound originate from a 

transducer, travel through the water, and 

are reflected back to the transducer when 

the pulse bounces off of solid particles in 

the water. The shift in the frequency of 

the sound is proportional to the velocity 

of the solid particles moving in the 

water. The velocity of the solid particles 

is assumed to be the same as the water 

velocity. Hydroacoustic meters, such as 

the acoustic Doppler velocimeter (ADV), 

can be mounted on a wading rod (fig. 5), 

allowing discharge measurements to be 

taken while wading in a stream. ADVs 

provide a more accurate flow measurement during low-flow and drought conditions than does a mechanical meter.

Wading

 rod

ADV

Figure 5. An 

acoustic Doppler 

velocimeter (ADV) 

mounted on a 

wading rod (Olson 

and Norris, 2007).

More complex acoustic meter systems, such as the acoustic Doppler current profiler (ADCP; fig. 6), are used to 

measure water velocity, water depth, and 

boat velocity. Onboard computers then 

calculate the discharge. The ADCP can be 

mounted on a variety of platforms 

(fig. 7), such as manned boats, tethered 

boats connected to an operator by a rope or 

cable and used from a bridge or cableway, 

and remotely controlled boats operated 

from shore. The USGS continues to work 

with hydroacoustic instrument manufacturers to develop instruments that meet the 

needs and requirements of the USGS.

Hydroacoustic instrumentation 

greatly improves the ability of the USGS 

Figure 7. Acoustic Doppler current 

profilers can be mounted on a variety 

of platforms. A, Manned (photograph 

by Paul Baker, USGS). B, Tethered. 

C, Remotely controlled. 

A. B.

C.

to make discharge measurements under 

extreme flow conditions and in difficult 

measuring locations. The ADCP has 

improved the quality of the measurements 

by providing more detailed information 

about the river flow. ADCPs are the most 

efficient method for measuring discharge, 

taking minutes rather than the hours associated with other methods (table 1).

Hydroacoustic meters allow the 

USGS to downsize the vehicles needed 

for field work. Instead of large trucks to 

transport heavy cranes, reels, and weights 

for use with mechanical meters, the 

USGS increasingly uses more economical vehicles to carry the smaller, lighter 

hydroacoustic equipment. 

The ideal method for measuring discharge would be to measure 

it continuously without contacting the 

water. Recent research conducted by the 

USGS has shown that it is possible to 

use several types of radar instruments 

to measure both the surface velocity of 

Finish

Start Path of boat

on river surface

Path of boat

on river bottom

Figure 6. River discharge measured using 

an acoustic Doppler current profiler mounted 

on a boat.
the water (fig. 8) and the cross-sectional 

area of the river so that discharge can be 

monitored continuously without contacting the water.

Table 1. Acoustic Doppler current profilers 

(ADCPs)—used in Indiana to measure 

discharge during a July 2003 flood—greatly 

increased the efficiency of measurement in 

terms of time and staff compared to mechanical 

current meters used in a January 1991 flood.

Flood month 

and year

Discharge 

measurements, in 

10 days

Average

time per 

discharge

measurement, in 

minutes

Hydrographers

January 1991

(Current 

meters)

52 96 11

July 2003 

(ADCPs)

62 18 6

Figure 8. Ultra-high frequency (UHF) and 

microwave radar system can measure the 

surface velocity of a river.

Stage-Discharge Relation

Periodic discharge measurements, 

typically at intervals of every 6 to 

8 weeks, paired with concurrent measurement of the river stage are used to create 

and maintain a rating curve (see figure 9 

for a graph showing the relation of stage 

to discharge). Over time, however, river 

channels can change and shift due to 

erosion of the bed or bank, deposition of 

sediment, movement of dunes or bars, 

and changes in aquatic vegetation. It is 

often necessary to apply a temporary 

shift to the primary rating curve (fig. 9) to 

account for the changed stage-discharge 

relation. Periodic discharge measurements are made to maintain the relation. 

USGS hydrographers spend considerable time developing and maintaining 

these rating curves and determining appropriate shifts to compute the discharge 

record. Until recently, the USGS manually 

developed and maintained all rating curves 

and shifts. In 2006, the USGS completed 

work with a software vendor on the development of a Graphical Ratings and Shift 

Application Tool (GRSAT). The GRSAT 

allows the user, through a computer graphical user interface, to build rating curves 

and apply shifts (fig. 10). Use of GRSAT 

over the past year has shown that about 

8 hours of labor are saved on average 

each year per streamgage—a significant 

cost savings when applied to the national 

network of approximately 7,400 gages. 

10

5

AGE, IN FEET

Stage equals 3.3 feet

/s

ST

Discharge3 equals 40 ft

1

1 10 100 1,000

DISCHARGE, IN CUBIC FEET PER SECOND

Shift

Figure 9. Example of a stage-discharge 

rating curve showing increasing stage 

upward on the y-axis and increasing 

discharge outward on the x-axis. A shift to 

the rating curve is indicated by the red line.

Index-Velocity Streamgage

For some rivers, the typical stagedischarge relation does not apply because 

water height cannot be uniquely related 

to discharge. These rivers are typically 

affected by tides, have little slope, or 

experience density currents. Sites on 

these rivers have been very difficult or 

even impossible to measure. However, 

with the development of hydroacoustic 

and ultrasonic velocity meters, continuous records of stream discharge for these 

types of rivers can be determined. To do 

this, a hydroacoustic or ultrasonic velocity meter is placed in the river (fig. 11) to 

continuously monitor the water velocity 

for a section of the river; this is called the 

index velocity. Periodic discharge measurements are paired with the index velocity to 

develop a relation between the index velocity and the mean velocity. This index velocity versus mean velocity relation is used 

along with a continuous record of the stage 

and a stage versus cross-sectional area relation to determine the continuous record of 

discharge at these sites. These meters and 

index-velocity-type gaging stations allow 

the USGS to routinely measure streamflow 

along the coastal parts of the Nation.

AQUARIUS GRSAT - Rating: STGQ-10

File View Edit

Rating: STGQ-10

Site Number: 05014500 - Site Name: Swiftcurrent Creek at Many Glacier MT - DD: 2 - Rating Number: 10 Shift: 10/13/2004 12:30

Time Series View

Ready DD: 2 TCODE Aging Code: A Rating Number: 10

Setup Overlay Rating Window Help

Site Number: 05014500 - Site Name: Swiftcurrent Creek at Many Glacier MT - DD: 2 - Rating Number: 10

Shift: 10/13/2004 12:30

Rating Zoom 1 Rating Zoom 2

Site Number: 05014500 X:

t

6.86

eef n4.86 i ,PCD M2.86 ORF thg1.86 ie

1.66

h e1.46 gaG 1.26

10 100 1,000

Discharge, in cfs

tee7.86 f ni 6.86 P,CD5.86 mor4.86 f thg3.86 ieh 2.86 ega1.86

G

500 1,000 1,500 2,000 2,500 3,000 3,500

Discharge, in cfs

Y:

Figure 10. View of a Graphical Rating and Shift Application Tool (GRSAT) screen. GRSAT has 

been developed to build and maintain rating curves.

Sideward

Piling looking

ADVM

Figure 11. Sideward looking acoustic Doppler 

velocity meter (ADVM) used to continuously 

measure a river index velocity (Ruhl and 

Simpson, 2005).
Flood Hardening of Streamgages

Because of their primary purpose 

and close proximity to water, USGS 

streamgages are often vulnerable to flooding and other potentially destructive hazards. As part of continuous improvements 

to its streamgaging program, the USGS 

is beginning to strengthen or “harden” its 

gages. Hardening a streamgage generally means modifying the structure to 

be able to withstand a flood with an 

estimated 200-year recurrence interval. 

In most cases, hardening involves raising the structure to a higher elevation, 

improving the structural integrity of the 

instrument shelter, and upgrading the 

data transmission capabilities. In fiscal 

year 2005, the USGS received a Congressional supplemental appropriation for the 

purpose of hardening streamgages that 

were impacted by Hurricanes Katrina and 

Rita in the Gulf Coast States. The USGS 

is using these funds to harden about 

130 streamgages within 100 miles of the 

Gulf Coast that also serve as National 

Weather Service flood forecast sites.

Streamflow Information

The goal of the USGS National 

Streamflow Information Program 

(NSIP) is to provide timely streamflow 

information to a multitude of users, 

including water managers, scientists, 

and the general public. For most of the 

last 100 years, the stage and discharge 

data and streamflow statistics have been 

made available annually through printed 

reports. Streamflow data in these reports 

in recent decades were usually available 

about 7 months (at the earliest) after the 

end of the previous water year.

Streamflow Information 

on the Internet

The annual USGS Water Resources 

Data Report, which serves as the citable 

reference for published USGS water data, 

is now available electronically. Approved, 

citable USGS streamflow data can be 

accessed through the Internet by using 

a text-based search (http://web10capp.

er.usgs.gov/adr06_lookup/search.jsp) or 

by using a map interface search (fig. 12) 

(http://web10capp.er.usgs.gov/imf/sites/

adr06/launch2.jsp).

Significant advancements have 

been made in developing a database and 

system to provide streamflow information 

electronically, in real time (updated at 

1- to 4-hour intervals) over the Internet through the USGS National Water 

Infor­mation System (NWIS). To access 

real-time streamflow information from 

the USGS, a user simply goes to the Web 

page http://waterdata.usgs.gov/nwis/rt

(fig. 13) and clicks on the State of their 

choice; after the State map comes up, 

the user clicks on the streamgage of 

interest, and the real-time river stage 

and discharge are displayed. The many 

uses of streamflow data are enhanced by 

making these data available quickly to 

a wide audience. Recreationists can use 

current information to plan their outings. 

During floods, real-time streamflow data 

contribute to saving lives and property.

The USGS also provides historic 

daily values of stage and discharge; 

annual instantaneous peak stage and 

discharge values; discharge measurements; and daily, monthly, and annual 

streamflow statistics through NWIS 

(http://waterdata.usgs.gov/nwis/sw).

Figure 12. View of the Annual Water Data Reports Mapper Launch Page (http://web10capp.

er.usgs.gov/imf/sites/adr06/launch2.jsp).

DISCHARGE, IN CUBIC

FEET PER SECOND 5,000

3,000

2,000

1,000

100

Thursday, October 4, 2007 9:30ET

Sept 27

--- Provisional Data Subject to Revision ---

Sept 28 Sept 29 Sept 30 Oct 1 Oct 2 Oct 3 Oct 4

USGS 05387500 Upper Iowa River at Decorah, IA

Figure 13. Streamflow data area available 

in real time from the USGS National Water 

Information System (http://water.usgs.gov/

waterwatch/). Maps of real-time streamflow 

are compared to historical streamflow for 

the day and year.
Historic Instantaneous 

Streamflow Data

USGS instantaneous streamflow 

values, usually recorded at 15-minute 

intervals, generally have been available 

over the Internet for only the previous 

31 days. A long-term electronically 

available archive of historic instantaneous values of streamflow, however, 

are important to many users, such as 

the National Weather Service, the Army 

Corps of Engineers, and consulting 

engineers for (1) hydraulic and hydrologic modeling for flood-plain mapping, 

flood routing, and issuing flood forecasts and warnings, and (2) estimating 

pollutant transport. In response, the 

USGS developed an Instantaneous Data 

Archive (IDA; http://ida.water.usgs.

gov/ida/index.cfm) that provides historic 

instantaneous data available in electronic 

form through the Internet back to about 

1990. This database has recently been 

made available to the general public, 

although information is not available for 

all streamgaging stations in each State. 

This is an ongoing effort within the 

USGS, as resources are made available.

Satellite Telemetry

The USGS uses satellite telemetry as its primary method to transmit 

streamflow data from the streamgage to 

the NWIS for processing (fig. 14). The 

original radios installed in streamgages 

transmit data to the satellite once every 

4 hours. Once the data are transmitted, 

it takes on average about 7 minutes for 

the data to move from the streamgage, 

through NWIS processing, to posting on 

the Internet. The transmission of data 

once every 4 hours is often not sufficient for smaller streams and locations 

that have rapid increases and decreases 

in streamflow. The satellite radios have 

recently been improved to allow transmissions once every hour and as rapid 

changes in stage occur. The USGS is 

upgrading streamgages by installing new 

high-speed satellite radios so that routine 

transmissions will be made once every 

hour. Currently (2007), about 2,500 of the 

approximately 7,000 streamgages with 

satellite telemetry have had these new 

satellite radios installed. The USGS plans 

to complete the conversion to the new 

satellite radios by the end of 2013.

Domestic

Satellite

(DOMSAT)

Geostationary

Operational

Environmental

Satellite

(GOES)

NWIS Web site

Command and Data

National Water Acquisition (CDA),

Wallops Island, Va. Information System

(NWIS)

Local

Readout

Ground

Station

(LRGS)

Field station with 

Data Collection Platform (DCP)

Figure 14. Satellite telemetry is used to transmit near real-time data from streamgages to the 

USGS National Water Information System for processing (from Nielsen and Norris, 2007).

Satellite Telemetry and 

Ground Station Backup

All of the streamflow data transmitted by satellite telemetry goes through a 

single data acquisition system operated 

by the National Oceanic and Atmospheric Administration (NOAA), at 

Wallops Island, Va. The USGS, NOAA, 

and many other partners and data users 

recognize the potential for interruptions to satellite telemetry capability and 

continuous national data if the Wallops 

Island facility was not able to function for 

some reason, such as a hurricane strike. 

Virtually all satellite telemetry capability 

across the United States would be lost 

until the Wallops Island facility could be 

restored to operation. The USGS, therefore, is partnering with NOAA and other 

Federal agencies to establish a backup 

system at the USGS Earth Resources 

and Observation Science (EROS) facility located near Sioux Falls, S.D. This 

backup system is expected to be operational by the end of 2007 and will help to 

ensure uninterrupted satellite telemetry.

Real-Time Data 

Quality Assurance

The USGS values its reputation 

for credible and accurate data, and 

recognizes the critical need to flag, as 

quickly as possible, potentially erroneous 

real-time information that might result 

from equipment malfunctions or sudden physical changes to a stream. Even 

though such data are identified as provisional and subject to change, it is important that the real-time streamflow data 

be as accurate as possible. The USGS, 

therefore, has developed quality-assurance software to (1) identify potentially 

erroneous data, which usually appears in 

the form of a “data spike,” and (2) to alert 

the appropriate USGS Water Science 

Center to its existence so the data can be 

checked and corrected if necessary.

Alerts are sent daily by e-mail to 

hydrographers responsible for the affected 

streamgage. Since implementation of 

the quality-assurance software in 2002, 

the number of potentially erroneous data 

displayed on the Internet have dropped to 

almost negligible levels (fig. 15).

140

120

NUMBER OF SITES100

80

60

40

20

0

S O N D J F MAM J J A S O N D J F MAM J J A S O N D J F M

2002 2003 2004 2005

EXPLANATION

At least one spike

At least one spike more than 3 days old

Figure 15. The number of data spikes (reflecting 

potential errors) in real-time streamflow data has 

diminished substantially since the installation of 

the quality-assurance software in 2002.
Robust Data-Delivery Network

Users of the USGS real-time 

streamflow information include those 

who use the streams for recreation, as 

well as those who require the data for 

daily operational needs. Many of these 

organizations could not carry out their 

missions without reliable access to 

the real-time streamflow information 

provided by the USGS. For this reason, 

the USGS has invested significant 

resources to develop a robust datadelivery network to ensure that real-time 

data are available when needed. The 

USGS provides real-time streamflow 

data through computer servers located in 

three locations: Menlo Park, Calif.; Sioux 

Falls, S.D.; and Reston, Va. Requests 

for real-time data are directed to the 

server location with the least workload 

so that data requests are met as rapidly 

as possible. The servers also function as 

backups; if one location ceases to function, the other two locations will continue to provide data. Additionally, the 

USGS built a system to provide backup 

real-time data receiving and processing capabilities (fig. 16). Through this 

system, real-time data for the States in 

the eastern half of the United States are 

received by servers at Menlo Park, Calif. 

Similarly, the real-time data in the western half of the United States are received 

by the servers in Reston, Va. This backup 

system ensures that real-time data can 

be processed and posted to the Internet 

should any one of the USGS State Water 

Science Centers not be able to do so.

CA

VA

MN

HI

WA

MT ND

OR ID

SD

WY

NV

NE

UT

CA

CO KS

AZ OK NM

ME VT

NH

WI NY MA

MI CT

IA PA

OH NJ IL IN

WV

MO VA KY

NC TN

AR SC

MS AL GA

LA TX

FL

PR

MD

AK

EXPLANATION

NWIS replicated to Virginia

NWIS replicated to California

Figure 16. Backup system for receiving and processing real-time streamflow data.

Streamflow Synthesis Products

The approximately 7,400 streamgages currently operated by the USGS 

nationwide, and the record of streamflow 

data extending back more than 100 years, 

serve as valuable records to which current 

streamflow conditions can be compared. 

The USGS developed a real-time data 

synthesis and analysis tool called WaterWatch (http://water.usgs.gov/waterwatch/) 

that puts the current streamflow conditions in perspective both spatially and 

temporally. The colors on the WaterWatch 

map (fig. 13) represent real-time streamflow compared to percentiles of historical 

daily streamflow for the day of the year 

being shown on the map. Only stations 

having at least 30 years of record are used 

to generate the WaterWatch map. WaterWatch also provides descriptions and 

analyses of hydrologic conditions (such 

as droughts and floods) by using several 

types of maps, graphs, and tables.

Continued Assessment of 

the National Streamflow 

Information Program

USGS streamgaging activities are 

carried out under the National Streamflow Information Program (NSIP). The 

plan for NSIP was developed to provide a 

national perspective, guidance, planning, 

and leadership for streamgaging activities 

of the USGS. The NSIP plan provides for 

a unified, stable network of streamgages 

to meet long-term national needs for 

streamflow information while ensuring 

that local needs continue to be met. More 

information about NSIP is available at 

http://water.usgs.gov/nsip/.

The USGS continues to evaluate 

the activities carried out under NSIP to 

ensure that the program design remains 

valid and that activities are conducted in 

an efficient and cost-effective manner. 

Several evaluations of NSIP have been 

conducted, including by the (1) Advisory Committee on Water Information 

Streamgaging Task Force (http://water.

usgs.gov/nsip/taskforce/task_force.html), 

(2) National Research Council (NRC) 

(http://water.usgs.gov/nsip/nasreport/

es/NRC_Report.html), (3) U.S. Government Accounting Office (http://www.

gao.gov/new.items/d05376.pdf), and 

(4) National Hydrologic Warning 

Council (http://water.usgs.gov/nsip/pubs/

nhwc_report.pdf). The NRC concluded 

“...that the National Streamflow Information Program is a sound, well-conceived 

program that meets the nation’s needs for 

streamflow measurement, interpretation, 

and information delivery.”

The USGS strives to operate a 

national streamgaging program that is 

efficient, relevant, cost effective, and 

robust, and is dedicated to its 

continuous improvement.

References Cited

Nielsen, J.P., and Norris, J.M., 2007, From the 

river to you­—USGS real-time streamflow 

information . . . from the National Streamflow Information Program: U.S. Geological 

Survey Fact Sheet 2007–3043, 4 p.

Olson, S.A., and Norris, J.M., 2007, 

U.S. Geological Survey streamgaging . . . 

from the National Streamflow Information 

Program: U.S. Geological Survey 

Fact Sheet 2005–3131, 4 p.

Ruhl, C.A., and Simpson, M.R., 2005, 

Computation of discharge using the indexvelocity method in tidally affected areas: 

U.S. Geological Survey Scientific Investigations Report 2005–5004, 31 p.

By Stephen F. Blanchard

For additional information visit 

http://water.usgs.gov/nsip/ 

or contact:

J. Michael Norris

U.S. Geological Survey

603-226-7847; mnorris@usgs.gov