U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007–3043

June 2007

From the River to You: 

USGS Real-Time Streamflow Information

...from the National Streamflow Information Program

Printed on recycled paper

This Fact Sheet is one in a series that highlights information or recent research findings from the USGS 

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.

Figure 1. Stream stage is the height of the water surface above 

a reference level or datum. Stage can be measured by a variety of 

methods, including a float, as shown in this illustration.

Introduction

As part of the National Streamflow Information Program, the U.S. 

Geological Survey (USGS) operates 

more than 7,400 streamgages nationwide to provide streamflow information for a wide variety of uses. These 

uses include prediction of floods, 

management and allocation of water 

resources, design and operation of 

engineering structures, scientific 

research, operation of locks and 

dams, and for recreational safety and 

enjoyment. These streamgages are 

operated by the USGS in partnerships 

with more than 800 other Federal, 

State, Tribal, and local cooperating 

agencies. Today (2007), nearly 

90 percent of streamgages record and 

transmit streamflow information electronically so that streamflow information is available on the World Wide 

Web in real time (http://waterdata.

usgs.gov). Most streamgages transmit 

information via satellite, but a few use 

telephone and radio telemetry.

Collecting and delivering streamflow information from the streamgage 

to the World Wide Web in real time 

by satellite involves numerous steps 

and the simultaneous activities of 

a large array of computer hardware 

and software. These processes work 

together to ensure the prompt delivery 

of streamflow information to many 

diverse users.

Collecting and Transmitting 

Streamflow Information

The streamflow information 

collected at most streamgages 

is stream stage (also called gage 

height). This is the height of the 

water surface above a reference level 

or datum. Stream stage is measured 

by a variety of methods including 

floats, pressure transducers, and 

acoustic or optical sensors (fig. 1).

Stage data are measured at the 

time interval necessary to monitor the 

rise and fall of a given river, typically 

every 15 minutes (fig. 2). These 

data are stored in a data logger in 

the gagehouse. 

On a preset schedule, typically 

every 1 to 4 hours, the streamgage 

transmits all the stage information 

recorded since the last transmission to 

a Geostationary Operational Environmental Satellite (GOES). Many 

streamgages have predetermined 

stage thresholds. When these thresholds are exceeded, the time between 

transmissions to the satellite will 

decrease from 1 to 4 hours to every 

15 minutes to provide more timely 

data during flooding or other emergency situations.

Intake

Float

Recorder

Satellite Radio

Stilling

Well
Transmissions from the 

streamgage are sent from the GOES 

satellite to the National Oceanic 

and Atmospheric Administration’s 

(NOAA) Command and Data 

Acquisition (CDA) facility at Wallops Island, Virginia. The received 

information is then immediately 

rebroadcast at much higher power to 

a domestic communications satellite 

(DOMSAT). This allows the information to be received at smaller Local 

Readout Ground Stations (LRGS) 

throughout the United States. The 

USGS maintains 21 LRGS systems to 

provide redundancy, in the event one 

or more systems are not operating.

Upon receipt at the LRGS, stage 

information is transferred almost 

immediately into the USGS National 

Water Information System (NWIS). 

NWIS consists of a network of more 

than 50 computers that collectively 

process all USGS water-resources 

data and store more than 100 years 

of streamflow, water-quality, and 

ground-water data from hundreds of 

thousands of sites across the country. 

Processing Streamflow 

Information

Stream stage is an important 

parameter for some applications, 

but most users of stream information 

are interested in streamflow (or discharge). Because streamflow cannot 

currently be measured directly at the 

streamgage, it must be estimated from 

stage data. This is done using a stagedischarge rating (fig. 3). A rating is 

the relation between stream stage and 

streamflow; a rating is developed by 

making manual discharge measurements that cover the range of stream 

stage at each streamgage. Because the 

configuration of the stream channel 

at each streamgage is different, an 

individual rating must be developed 

for each streamgage. In addition, 

changing channel conditions require 

that ratings be constantly checked by 

making additional measurements 

and adjusted as appropriate (fig. 4). 

These checks and corrections are 

particularly critical during floods 

and droughts or other emergency 

situations. 

Figure 2. Fifteen-minute stage (gage height) data showing the rise and fall of the Potomac 

River during a 1-week period in 2007.

Figure 3. A rating curve relates stream stage to streamflow (discharge) through 

the use of manual discharge measurements. At this streamgage, a stage of 

3.3 feet gives a streamflow of 40 cubic feet per second.

Figure 4. Because all stream channels change with time, additional manual measurements 

must be made to check and update the rating for each streamgage. This is particularly 

important during floods and droughts.

10

5

1

1 10 100 1,000

DISCHARGE, IN CUBIC FEET PER SECOND

STAGE, IN FEET

Stage equals 3.3 feet

Discharge

equals 40 ft3/s

Same stage

Change in cross-section area
When NWIS receives stage 

data from an LRGS, it uses the 

most recent rating developed for the 

streamgage to convert the stream 

stage to streamflow. USGS hydrographers work to ensure that the stored 

ratings for each streamgage match the 

latest discharge measurements at all 

times so that the computed streamflow remains as accurate as possible. 

Redundant NWIS systems exist 

across the country so that data processing can continue in the event of 

the failure of a local NWIS computer.

Delivering Streamflow 

Information

After stream stage is converted to 

streamflow in NWIS, the information 

is sent to the USGS Web site delivery 

system NWISWeb (http://waterdata.

usgs.gov/nwis). On the NWISWeb 

system, current and historical stream 

stage and streamflow data are available in tables and graphs (fig. 5). 

In addition, the WaterWatch 

Web site (fig. 6) provides maps and 

graphs showing real-time streamflow 

conditions in the United States. 

The “WaterWatch” Web site is at 

http://water.usgs.gov/waterwatch/. 

Maps show current streamflow as 

color-coded dots for about 3,000 

streamgages nationwide. The colors 

represent percentiles of streamflow 

that are calculated from historical 

observations for the current day of 

the year. Only streamgages with 

at least 30 years of continuous 

record are used; this ensures that 

the calculated percentiles represent 

historical conditions. 

Figure 5. The streamflow (discharge) of the Potomac River in 2007 as displayed on the USGS 

Web site delivery system NWISWeb (http://waterdata.usgs.gov).

Figure 6. The USGS WaterWatch Web site 

allows users to see the status of the surfacewater resources of the Nation at a glance 

(http://water.usgs.gov/waterwatch/).
Figure 7. Streamflow information from the river to the user passes through numerous satellite and computer systems 

during a 10-minute period.

Need Further Information?

Water-resources programs in each State: http://water.usgs.gov

National Streamflow Information Program: http://water.usgs.gov/nsip

or contact:

J. Michael Norris

U.S. Geological Survey

361 Commerce Way

Pembroke, NH 03275

mnorris@usgs.gov

(603) 226-7847

All systems involved in 

processing USGS streamflow data 

have redundant capabilities to 

minimize the potential for loss of data 

access. For some systems (LRGS, 

NWIS, NWISWeb), this redundant 

capability is automatic or can be 

brought online within a matter of 

minutes if there is a failure of the 

primary system. For other systems 

GOES DOMSAT

LRGS

NWIS Web site

Command and Data

Acquisition (CDA),

Wallops Island, VA

National Water

Information System

(NWIS)

Streamgage

(GOES, NOAA CDA, DOMSAT), 

several hours may be needed to 

switch to the backup system (fig. 7). 

In addition, Federal agencies with 

flood warning and response missions, 

such as the U.S. Army Corps of 

Engineers and the National Weather 

Service, maintain separate receiving 

capabilities to further ensure access 

to USGS streamflow information. 

Although most streamgages have 

only one satellite transmitter, many 

critical flood-warning streamgages 

have redundant telemetry systems 

that couple satellite telemetry with 

radio or telephone transmissions.

By Joseph P. Nielsen and 

J. Michael Norris