Frequently Asked Questions about the USGS and Floods

What is the USGS?

Chartered in 1879 by Congress to classify the public lands and to examine the geologic structure, mineral resources, and products of the national domain, the USGS is the Nation's leading earth science information agency. As part of its mission, the USGS provides practical information about the Nation's rivers and streams that is useful for mitigation of hazards associated with floods and droughts and defines the hydrologic and hydraulic characteristics needed for the design and operation of engineering projects such as dams and levees. The primary source of this water-resource information is the USGS streamflow-gaging-station network.

What is the role of the USGS during floods?

A streamflow-gaging station provides the two most fundamental items of hydrologic information about a river: stage (also called gage height) and streamflow (also called discharge).
Stage is the distance above a selected point and commonly is measured in feet.
Streamflow is the total volume of water that flows past a point on the river for a given period of time; it usually is measured in cubic feet per second or gallons per minute.
Automated equipment in the gaging station can measure stage to an accuracy of 1/8 inch and record stage on a set interval, typically 15 minutes or one hour.
Streamflow is more difficult to measure accurately and continuously. As a matter of practicality, streamflow usually is computed from pre-established stage/streamflow relations, or rating curves. The rating curves are plotted by USGS field personnel who periodically visit the gaging station to measure streamflow.
Changes in the streambed and banks can alter the stage/streamflow relation. Such changes particularly are prevalent during floods. Additional streamflow measurements are required periodically to detect and track changes and to update the rating. Because documentation of flood streamflow is important, USGS field personnel visit streamflow-gaging stations during periods of high water to measure streamflow. The high-water measurements help USGS personnel calibrate the stage/streamflow rating, allowing the USGS to provide dependable flood information to those who need it.

The above information consists of excerpts from: Stream Gaging and Flood Forecasting, A Partnership of the U.S. Geological Survey and the National Weather Service by R.R. Mason, Jr., and B.A. Weiger, USGS Fact Sheet 209-95, http://water.usgs.gov/wid/FS_209-95/mason-weiger.html

Does the USGS forecast flood crests or issue flood warnings?

No. The National Weather Service (NWS), which is part of the National Oceanic and Atmospheric Administration, is charged by law with the responsibility of issuing river forecasts and flood warnings. The NWS uses many sources of data when developing its flood forecasts. The USGS is the principal source of data on river depth and streamflow (Stream Gaging and Flood Forecasting, A Partnership of the U.S. Geological Survey and the National Weather Service by R.R. Mason, Jr., and B.A. Weiger, USGS Fact Sheet 209-95, http://water.usgs.gov/wid/FS_209-95/mason-weiger.html.

NWS river forecasts for stations across the Nation can be obtained at: http://weather.gov/rivers_tab.php

How many streamflow-gaging stations are there?

There are more than 7,000 gages in the national USGS streamflow-gaging-station network.

To see a map of the current active stations, go to: http://water.usgs.gov/nsip/nsipmaps/currentgages.html.

In Indiana, there are more than 160 stations.

To see a map of stations in Indiana, go to: http://water.usgs.gov/nsip/nsipmaps/in_base.html.

How does a streamflow-gaging station work?

Instruments housed in a weather-proof shelter near streams or rivers record stage (also referred to as gage height). These stage data are transmitted to USGS computers at established times in near real time by satellite or telephone telemetry. For more information about how streamflow-gaging stations work, see:

Does “stage” or “gage height” mean how deep a river is?

No. Stage, or gage height, is the water depth above a selected point at a measuring site. The stage plus elevation of the selected point equal the elevation of the water surface. The actual depth of the river at a given gage height can vary significantly upstream or downstream from that measuring point. Because riverbeds can change, the depth at the measuring site could change over time for the same gage height.

How do you measure streamflow?

The USGS usually measures streamflow (also called “discharge”) by taking measurements of the water width, depth, and speed. Streamflow is computed by multiplying width and depth to compute area; then area is multiplied by average water speed to yield discharge. Streamflow usually is computed for a number of sections across the steam; the individual section streamflows then are added up to compute total streamflow.
For many years, the USGS used mechanical current meters to measure streamflow. Now the USGS often uses state-of-the-art Doppler sonar flowmeters to measure streamflow.

A description of USGS methods of measuring streamflow can be found at: http://water.usgs.gov/pubs/circ/circ1123/.
A description of how Doppler sonar flowmeters are used to measure streamflow can be found at: http://in.water.usgs.gov/hydroacoustics/adcpuses.shtml.

How can I get USGS streamflow-gaging station data?

Real-time data from USGS streamflow-gaging stations in Indiana can be accessed at: http://waterdata.usgs.gov/in/nwis/rt.
Water-resources data collected at streamflow-gaging stations across 50 states, the District of Columbia, and Puerto Rico are available online at http://waterdata.usgs.gov/nwis/.
Real-time data are available from many of these stations at http://waterdata.usgs.gov/nwis/rt.

Is there a way I can quickly see how many stations in Indiana are above flood stage?

Yes.

A map of Indiana showing which stations are above flood stage is available online at: http://water.usgs.gov/cgi-bin/waterwatch?state=in&map_type=flood&web_type=map.
A map of the United States showing this information is available at: http://water.usgs.gov/cgi-bin/dailyMainW?state=us&map_type=flood&web_type=map

What is a 100-year flood? Will I only see one every 100 years?

The term "100-year flood" is a statistical designation and is a way to categorize the size of a flood. There is a 1-in-100 chance (or a 1-percent chance) that a 100-year flood will happen during any year. A better term might be the "1-in-100 chance flood."
Scientists collect data and study past floods to get a minimum of 10 years of information about a river; a longer record provides a better estimate of the "1-in-100 chance flood." Scientists use statistics and observe how frequently different sizes of floods occur, and the average number of years between them, to determine the probability that a flood of any given size will be equaled or exceeded during any year.
The actual number of years between floods of any given size varies. Floods designated as 100-year floods happen irregularly because the climate naturally varies over the years. As more data are collected, or when a river basin is altered in a way that affects the flow of water in the river, scientists re-evaluate the frequency of flooding. Dams and urban development are examples of some manmade changes in a basin that affect floods.

The above information consists of excerpts from: USGS Fact Sheet 229-96, The "100-Year Flood,” by Karen Dinicola, http://water.usgs.gov/pubs/fs/FS-229-96/

I live near a stream that floods frequently, but there is no gaging station on that stream. Why isn’t there a station near me?

State, local, and other federal agencies that work with the USGS select station sites based upon the agencies' needs. Stations are funded jointly by State and local agencies and the USGS through the USGS Cooperative Program or by other federal agencies. This means that the State, local government agencies, or other federal agencies would need to provide funding for any new USGS Indiana stations.

For more information on the USGS Cooperative Program see: Cooperative Water Program—A Partnership in the Nation's Water-Resources Program by Bruce E. Taggart, USGS Fact Sheet 2004-3068, http://water.usgs.gov/pubs/fs/2004/3068/

Isn’t measuring streamflow in floods dangerous?

The safety of USGS personnel is always the top priority in any USGS effort. Like many jobs that involve working outside in adverse conditions, there are risks. The USGS does take steps to manage and reduce these risks. The USGS has stringent safety policies, and each USGS office has a Safety Officer to make sure those policies are enforced. Some examples of actions the USGS takes to manage risks during floods include

Unmanned boats are used to make streamflow measurements where possible, eliminating risks of personnel being in a boat during floods.
All personnel are required to wear personal flotation devices when working in, near, or above water.
USGS boat operators must be certified by a boat-operator class and must keep their certification current by taking refresher training.
During floods, USGS personnel are teamed up so that no one works alone.
All field personnel carry cell phones and are required to check in with the office periodically.
Personnel who are in boats during cold weather wear survival suits that provide full flotation and are designed to keep a person warm in cold water.
USGS boats are safety equipped and capable of operating in floods. The primary boat of the USGS Indiana District has numerous safety features and a powerful jet outboard motor to handle swift water. Photos of the jet boat can be seen in the photos section of this web page.

Are you catching anything?

This is perhaps the most frequently asked question of our crews in the field! USGS hydrographers frequently can be seen on bridges, holding a rope that is attached to a tethered Doppler sonar flowmeter boat—making it look like they are fishing for a large catch. Actually, hydrographers are catching something: they are catching streamflow data that can help many people and agencies during times of floods.

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