Stream Gaging and Flood Forecasting

A Partnership of the 

U.S. Geological Survey and the National Weather Service

Flash Flood Watch

National Weather Service, Washingtonr D.C 

 4:30 am EDT Tuesday, June 27,1995

The National VJeather Service in Washington, D;C has issued a flash flood watch..f Valtd 

until 10 pm. EDT this evening,.,for a large part of western Virgmia...A southward moving 

cold front will act to focus heavy rain producing showers and thunderstorms today, and 

tonight in and close to the watch area, ^combination of a very moist air massmplace 

over the region and the added focus of the cold front; wul allow for widespread rain to 

develop. Saturated soil from recent rains will add to the, flooding problem,

\ Persons in flood prone areas should monitor rainfall today and have a plan to move to high 

^ground should persistent heavy rains occur.

Floods are among the most frequent 

and costly natural disasters in terms of 

human hardship and economic loss. As 

much as 90 percent of the damage related 

to natural disasters (excluding droughts) 

is caused by floods and associated mud 

and debris flows. Over the last 10 years 

(1985-94), floods have cost the Nation, 

on average, $3.1 billion annually in damages. The long-term (1925-88) annual 

average of lives lost is 95, mostly as a 

result of flash floods. One has only to 

recall the flash flooding of the Big 

Thompson River in Colorado in 1976, 

which killed 139 people as it swept 

through campgrounds and vacation 

homes nestled in a narrow canyon, to realize how unexpected and costly, in human 

life alone, such phenomena can be.

Important elements in the Nation's program to reduce flood damages include 

flood warnings and river forecasts. 

Timely warnings and forecasts save lives 

and aid disaster preparedness, which 

decreases property damage by an estimated $1 billion annually. Although the 

issuance of flood forecasts is now 

accepted as common and routine, their 

preparation is no minor feat. This technical achievement is made possible by the 

joint efforts of several Federal, State, and 

local agencies and many dedicated people 

across the Nation.

A Partnership

The National Weather Service (NWS), 

which is part of the National Oceanic and 

Atmospheric Administration, is widely

known as the Federal agency in charge of 

weather forecasting and warning for the 

Nation. Many people, however, are not 

aware that the NWS also is charged by 

law with the responsibility for issuing 

river forecasts and flood warnings. The 

National Weather Bureau Organic Act of 

1890 (U.S. Code title 15, section 311) 

mandates that the National Weather Service is the responsible agent for "***the 

forecasting of weather, the issue of storm 

warnings, the display of weather and 

flood signals for the benefit of agriculture***." The NWS uses many sources of 

data when developing its flood forecasts. 

The U.S. Geological Survey (USGS) is 

the principal source of data on river depth 

and flow.

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 information is the 

USGS streamflow-gaging station network.

The USGS operates and maintains 

more than 85 percent of the Nation's 

stream-gaging stations, which includes 

98 percent of those that are used for realtime river forecasting. Currently, this network comprises 7,292 stations dispersed 

throughout the Nation, 4,200 of which are 

equipped with earth satellite radios that 

provide real-time communications. The 

NWS uses data from 3,971 of these stations to forecast river depth and flow conditions at 4,017 forecast-service locations 

on major rivers and small streams in 

urban areas (fig. 1).

Figure 1. Locations of U.S. Geological Survey stream-gaging stations that are used by the 

National Weather Service to develop river forecasts.
Stream Gaging

The two most fundamental items of 

hydrologic information about a river are 

stage, which is water depth above some 

arbitrary datum, commonly measured in 

feet, and flow or discharge, which is the 

total volume of water that flows past a 

point on the river for some period of time, 

usually measured in cubic feet per second 

or gallons per minute. These two key 

factors are measured at a location on the 

river called a stream-gaging station 

(fig- 2).

By using automated equipment in the 

gaging station, river stage can be continuously monitored and reported to an accuracy of 1/8 of an inch. Linking batterypowered stage recorders with satellite 

radios enables transmission of stage data 

to computers in USGS and NWS facilities 

even when extreme high waters and 

strong winds disrupt normal telephone 

and power services. In this way, USGS 

and NWS hydrologists know the river 

stage at remote sites and how fast the 

water is rising or falling.

It is much more difficult to measure 

river discharge accurately and continuously. As a matter of practicality, discharge is usually estimated from preestablished stage/discharge relations, or 

rating curves. The rating curves are constructed by USGS field personnel who 

periodically visit the gaging station to 

measure river discharge (fig.3). For more 

information about measurement of river 

discharge see Wahl and others (1995).

Changes in river cross sections that 

result from the scour or deposition of sediment or changes in streambed and bank

-Satellite 

radio antenna

National Weather Service 

State College, Pa

Per your letter request dated June 5, enclosed are updated rtitirigtables for the following 

USGS streamflow gaging stations in Virginia: James River at Lick Run, Jackson River 

below Gathrighi Dam, Jackson River at Covmgtori, Maury River near Buena Vista, Rappahannock River near Fredericksburg, dndCraing Creek at Parr. Ratings at the other six 

sites are unchanged.,.,The low-end portion of the Gtithright Dam rating is currently 

Undergoing reassessment but the high flow portion of the rating is not expected to change

U.S. Geological Survey 

Richmond, Virginia

roughness alter the stage/discharge relation. Such changes are particularly prevalent during floods. Occasionally, changes 

are so severe as to require development of 

a new stage/discharge rating; this 

occurred at the North River at Stokesville, 

Va., as a result of a major flood in 1985 

(fig. 4). Thus, even after a stage/discharge 

rating is well established, additional discharge measurements are required periodically to detect and track changes and to 

update the rating. Updated rating curves 

are provided to the NWS. Because documentation of flood discharges is so important, USGS field personnel are routinely 

deployed to stream-gaging stations during 

periods of high flow to measure river discharge during inclement weather, day or 

night.

By using an up-to-date stage/discharge 

rating and a river-stage reading, an accurate estimate of the river discharge can be 

produced. An important characteristic of 

a stage/discharge rating is that the process 

also works in reverse; given a discharge 

estimate, the corresponding river stage 

can be determined. This functionality 

enables the NWS to transform an obscure 

river parameter, its discharge, into an eas10 

Hi

LU 

LL

? 5 Stage/discharge rating 

after flood of 1985,

Stage/discharge rating 

before flood of 1985

0.1 1 10 100 1,000 10,000 

RIVER DISCHARGE, IN CUBIC FEET PER SECOND

Figure 4. Stage/discharge relations at North 

River near Stokesville, Va., before and after 

the flood of 1985.

ily visualized and well-understood measure of public risk, the flood stage.

Flood Forecasting

River-flood forecasts are prepared by 

13 NWS river-forecast centers and disseminated by NWS offices to the public. 

During periods of flooding, the NWS 

river-forecast centers issue forecasts for 

the height of the flood crest, the date and 

time when the river is expected to overflow its banks, and the date and time 

when the flow in the river is expected to 

recede to within its banks. These forecasts are updated as new information is 

acquired.

Figure 2. Schematic of a stilling well and 

shelter at a stream-gaging station.

Figure 3. Crane, current meter, and weight 

used for measuring the discharge of a river 

from a bridge.

River Flood Warning 

National Weather Service). 

Washington, D.C. , 

4:15 pm EDT Tuesday, June T7\ 1995 !

Heavy rain across the Rappahannock River 

basin in northern Virginia win cause significant flooding. At 4:10 pm the Rappa-: 

hannock River at Remington was 12,4 feet 

and rising sharply: The river should reach 

its 15 foot flood stage tonight and crest 

between 1$ and 20 feet early Wednesday 

morning.
To develop flood forecasts, the NWS 

develops and calibrates complex mathematical models of how the Nation's rivers 

and streams respond to rainfall and snowmelt. These models are developed for 

preselected forecast service points, which 

are usually located along major rivers or 

on small streams near urban areas that 

have a history of flooding. In every case, 

records of river discharge must be available so the NWS can develop a river 

model. An important hydraulic input to 

these models is the USGS stage/discharge 

rating. The resulting model is rarely 

exact, but it provides estimates of river 

response to rainfall. Thereafter, when 

heavy rainfall is forecast for the river 

basin, those amounts are entered into the 

model, and the model estimates the river 

stage and discharge that will result. As 

new river and rainfall data are collected 

during a storm, the new data are entered 

into the computer, and new river forecasts 

are produced (fig. 5).

As forecasts are prepared, water that 

flows into large rivers from upstream 

points and tributary streams must be considered; in fact, gaging important tributary streams is often needed even at 

locations where forecast services are not 

provided. These points are used in the 

forecast models as model control points. 

Because none of the models can predict 

exactly what will happen on a river, the 

use of river stages and the associated rating curve to reassess continuously how 

much water is in every stream is a vital 

part of the forecast process.

River Flood Statement 

National Weather Service; 

" Washington, D.C

i EDT Tuesday June 27, 1995

ZW 5

1995

Figure 5. Observed and computer model estimates of river stage and selected model 

updates for the Rappahannock River at Remington, Va., during the flood of June 1995.

At W:OQ pm the Rappahannock River at 

Remington waslSJQZfeet and still rising. 

The river has reached its 15 foot flood stage 

and will crest between 16 and 18 feet overnight.

 These rises will continue to move down-

 stream toward Fredericksburg. At 10:00 

:pm the Fredericksburg gage was reading

 6.4i$. A sharp rise and crest of around 20 

feel is expected Wednesday afternoon.

Even a well-calibrated model is an 

ephemeral commodity. Once a river 

model is developed, changes in watershed 

characteristics, such as increasing urbanization, drainage improvements, and construction of dams and levees, can make 

the model obsolete. A continuing cycle of 

model calibration, collection of river-discharge and rainfall data, and model recalibration is required to provide a current, 

useful, and accurate flood-forecasting 

tool.

Working Together

During a flood, the USGS and the 

NWS work together to collect and use the 

most up-to-date hydrologic data. The 

USGS furnishes continuous information 

on river stage and discharge and provides 

rating revisions to the NWS as they 

become available. The NWS uses its river 

models and hydrometeorological data 

(and forecasts) to predict the discharge at 

each forecast service point and the most 

up-to-date stage/discharge rating to forecast how deep the water will get.

The 1993 Midwest flood presented 

both agencies with an unprecedented 

challenge. During the floods, USGS 

hydrographers made more than 2,000 visits to stream-gaging stations in the floodaffected areas to verify that instruments 

were working and communicating properly, to make repairs as needed, and to 

measure river discharge. The NWS issued 

more than 135 river flood warnings and 

2,562 river flood statements from June 1 

to August 15, 1993. Both agencies supplied the U.S. Army Corps of Engineers 

(COE), the Federal Emergency Management Agency (FEMA), and many State 

and local agencies with a continuous 

stream of water-related information for 

their use in flood management and disaster mitigation.

Although the economic damages 

caused by the 1993 Midwest flood were a 

financial disaster for the Nation, the loss 

of human lives was relatively small; a 

smaller flood in 1903 claimed 100 rives. 

Most of the savings in human lives can be 

attributed directly to the early and accurate river forecasts that were made possible by recent advances in remote streamgaging telemetry and data-intensive riverflow modeling, as well as to flood-control 

dams, locks, and levees.

River Flood Statement 

National V^eather Service, 

Washington, D.C , 

3:00 pm EDT Wednesday, June 2flt 1995

"Atl :00 am the Rappahannock River at 

-, Remington river stage was 17.6 feet and { 

l rising. 1

; * 

1 Stages are at hazardous levels throughout

the Rappahannock basin. 

The Future

In addition to their role in flood forecasting, USGS stream-gaging stations 

provide information that is useful for the 

design and operation of dams, levees, 

bridges, water- and wastewater-treatment 

facilities, and for other engineering 

works. The data also are used in the preparation of forecasts of public water supplies, monitoring of water quality, and 

assessment of environmental regulation. 

In addition to their use by the USGS and 

the NWS, USGS real-time streamflow 

data are used by water management agencies, such as the COE, the Bureau of Reclamation, the Tennessee Valley 

Authority, and the Natural Resources 

Conservation Service, emergency management officials, such as FEMA, and 

other Federal, State, and local agencies.

Of all USGS stream-gaging stations, 90 

percent are operated by the USGS in 

cooperation with other Federal, State, and 

local agencies. About 50 percent of the 

stations are funded through cost-sharing
vides one-half of the funds for the stations 

and the cooperating agencies provide 

the other half. Another 40 percent of the 

stations are funded entirely by the cooperating agency. However, the resulting 

streamflow data are available to all 

potential users through USGS data bases, 

on the Internet, and through USGS 

publications.

River Flood Statement 

"- National Weather Service, 

Washington, D.C. 

4:26:am EDT TJmrsday, June 29, 1995

, River stage? are falling across the Rapidan 

and Rappahannock Rivers. At 3:45 amtlie 

level on the Rappahannock River at Remington was 13,46 feet, well below its 15

i foot flood stage. The Rappahannock River 

at Fredericksburg crested at around 25.1 

feetat 2:30 am this morning.

I This will be the last statement of this flood 

event.

The NWS has developed extensive 

river-forecasting services that are based 

on access to USGS data. When cooperating agencies have obtained the information that they need from a stream-gaging 

station, they usually discontinue funding 

for that station. When either party (USGS 

or its cooperators) discontinues funding 

for a gage as a result of budget reductions 

or for other reasons, the operation of the 

station must be discontinued. This 

arrangement has an unintended consequence for the NWS and the communities 

that depend on NWS river-forecast services; gaging stations that are critical to 

the forecast service may be discontinued 

owing to circumstances beyond the control of the NWS or of its customers. Since 

1983, 57 river-forecast service points 

have been affected by closure of one or 

more USGS stream-gaging stations and 

the trend accelerated during the early 

1990's.

Demand for NWS river-forecast services continues to grow owing to an 

expanding population, urbanization, and 

economic growth NWS now provides 

forecast services at about 4,000 locations. 

Although new radar technologies and 

computer visualization techniques hold 

significant promise for improving the 

timeliness and accuracy of river forecasts 

and flood warnings, ground-based verification will still be needed even after such 

technologies are in place. The need for 

real-time verification of river discharge 

and subsequent model adjustment is more 

than a scientific quest for accuracy; it is 

critically important to maintain model 

accuracy to minimize economic damage 

and human suffering. The detail and timeliness of the required data can be furnished only by on-site stream-gaging 

stations

Robert R. Mason, Jr.,

U.S. Geological Survey, and 

Benjamin A. Weiger, 

National Weather Service

References

Gilbert, B.K., 1995, Water data program: 

U.S. Geological Survey Fact Sheet 

FS--065-95,4 p.

Stallings, Eugene, 1991, Institutional and 

management aspects, flood forecasting 

and drought prediction by the National 

Weather Service, in Paulson, R.W., 

Chase, E.B., Roberts, R., and Moody, 

D.W., comps., 1991, National water summary 1988-89-Hydrologic events and 

floods and droughts: U.S. Geological Survey Water-Supply Paper 2375, p. 117- 

122.

National Weather Service, 1994, The 

great flood of 1993: National Weather 

Service, Natural Disaster Survey Report, 

Washington, D.C., 281 p.

U.S. Geological Survey, 1995, FederalState cooperative water-resources program: U.S. Geological Survey Fact Sheet 

FS-052-95,4 p.

van der Leeden, Frits, Troise, F.L., Todd, 

D.K., 1990, The water encyclopedia (2d 

ed.): Chelsea, Michigan, Lewis Publishers, Inc., 808 p.

Wahl, K.L., Thomas, W.O., Jr., and 

Hirsch, R.M., 1995, The stream-gaging 

program of the U.S. Geological Survey: 

U.S. Geological Survey Circular 1123, 

22 p.

 1995, An overview of the streamgaging program: U.S. Geological Survey 

Fact Sheet FS-066-95, 4 p.

For more information, contact 

any of the following:

U.S. Geological Survey 

Office of Surface Water 

415 National Center 

Reston, Virginia 22092 

(703) 648-5977

National Weather Service 

Office of Hydrology 

1325 East-West Highway 

Silver Spring, Maryland 20910 

(301) 713-0006

Additional earth science information can 

be obtained by accessing the USGS 

"Home page" on the World Wide Web at

"http://www.usgs.gov" 

or the NWS "Home page" at

"http://www.nws.noaa.gov."

For more information on all USGS 

reports and products (including maps, 

images, and computerized data), call 

1-800-USA-MAPS.

August 1995 Fact Sheet FS 209-95