FAQ's about Rivers

The term "100-year flood," is used to describe the recurrence interval of floods. As the table below shows, the "100-year recurrence interval" means that a flood of that magnitude has a one percent chance of occurring in any given year. In other words, the chances that a river will flow as high as the 100-year flood stage this year is 1 in 100. Statistically, each year begins with the same 1-percent chance that a 100-year event will occur.

But, just because a 100-year flood happened last year doesn't mean that it won't happen this year, too. In other words, future rainfall and floods don't depend on the rainfall and floods that happened in the past. The past records are mainly used to show what kind of river flows can be expected. So, when you hear about a 100-year flood, at least you have a general idea that it does mean a BIG flood, and if you hear of a 200-year flood you know that it means one even BIGGER! As an example, in July of 1994, some places in south Georgia received more than 20 inches of rainfall in a few days -- the floods they produced were tremendous... way over the 100-year flood. At Senoia, Ga., the maximum amount of water flowing by the Line Creek gage was 2.4 times greater than the 100-year flood level.

Several types of data can be collected to assist hydrologists predict when and where floods might occur. The first is monitoring the amount of rainfall occurring on a realtime basis. Second, monitoring the rate of change in river stage on a realtime basis can help indicate the severity and immediacy of the threat. Third, knowledge about the type of storm producing the moisture, such as duration, intensity and areal extent, is valuable for determining possible severity of the flooding. And fourth, knowledge about the characteristics of a river's drainage basin, such as soil-moisture conditions, ground temperature, snowpack, topography, vegetation cover and impermeable land area, can help to predict how extensive and damaging a flood might become.

The National Weather Service collects and interprets rainfall data throughout the United States and issues flood watches and warnings as appropriate. The National Weather Service uses statistical models and flood histories to try to predict the results of expected storms. The USGS maintains a network of streamflow-gaging stations throughout the country for which the discharge and stage are monitored. Flood estimation maps are generally produced by estimating a flood with a certain recurrence interval or probability and simulating the inundation levels based on flood plain and channel characteristics. More information on floods is available from the USGS Hydrologic Information Center at http://www.nws.noaa.gov/oh/hic and from the USGS national home page at http://water.usgs.gov. For more information on real-time flood monitoring, please see USGS Fact Sheet FS-209-95, which is available on-line at http://water.usgs.gov/public/wid/FS_209-95/mason-weiger.html.

To accomplish these tasks, the USGS has over 150 field offices where personnel are involved in the following activities:

How high and how fast a river will rise during a storm depends on many things. Most important, of course, is how much rain is falling. But also we have to look at other things, such as the stage of the river when the storm begins, at what the soil is like in the drainage basin where it is raining (is the soil already saturated with water from a previous storm?), and at how hard and in what parts of the basin the rain is falling. The USGS has studied these things at many places across the country for many years, and thus is often able to make predictions about if and where a flood will occur and how bad that flood will be.

With the advent of modern computer and satellite technology, the USGS can monitor the stage of many streams almost instantly. Since some streams, especially those in the normally arid Western U.S., can rise dramatically in a matter of minutes during a major storm, it is important to be able to remotely monitor how fast water is rising "in real time" in order to warn people that might be affected by a dangerous flood. Recreational users of streams, such as kayakers, also use "real-time" stream-stage data to tell them if certain streams are at the right height for kayaking. The USGS can now gather data on stream stage and even produce graphs showing stage as the rain is falling. In fact, some of these real-time data and graphics are being made available for you to use via the World Wide Web. You can access current stream conditions for your state right now.

NOAA is the main source of bathymetric data for the world, and here is the site you can search for their data: http://www.ngdc.noaa.gov/mgg/bathymetry/relief.html

Real-time streamflow data are available from the U.S. Geological Survey for over 4200 stations throughout the United States. These data are available only through the World-Wide Web.

The American Whitewater Affiliation provides a compilation of web pages and telephone numbers where real-time streamflow and reservoir information can be obtained across the United States.

Note that direct telephone access to U.S. Geological Survey stream-gaging stations is not authorized except for official use, including those stations where National Weather Service equipment is co-located.

Access to these stations must be restricted to official use so that data are available during emergencies.

Water being drawn from a well was once precipitation that fell onto Earth's surface. It seeped into the ground and, over time, occupied the porous space in some subsurface material. Naturally, big particles that can be found in streams, such as leaf chunks, will not be seen in ground water. So, yes, big particles are filtered out. But ground water can contain other items that you can't see. Some are naturally occurring and some are human-made substances. Ground water can contain hydrogen sulfide or other naturally occurring chemicals. Ground water also may contain petroleum, organic compounds, or other chemicals introduced by humans' activities.

Contaminated ground water can occur if the well is located near land that is used for farming where certain kinds of chemicals are applied to crops, or near a gas station that has a leaking storage tank. Leakage from septic tanks and/or waste-disposal sites also can contaminate ground water. A septic tank can introduce bacteria to the water, and pesticides and fertilizers that seep into farmed soil can eventually end up in water drawn from a well. Or, a well might have been placed in land that was once used for something like a garbage or chemical dump site. In any case, it is wise to have your well water tested for contaminates.

Not directly. You cannot say that because a stream rises (doubles) from a 10-foot stage to a 20-foot stage that the amount of water flowing also doubles. Think of a cereal bowl with a rounded bottom. Pour one inch of milk in it. It doesn't take much milk to make it up to the one inch level because the bowl is least wide near the bottom. Now, pour in milk until it is two inches deep -- it takes a lot more milk than it did to fill the first inch because the bowl gets wider as you go up. The same thing happens in a stream -- the stream banks will generally be narrower at the bottom and tend to widen as you go up the bank. So, the amount of water flowing in a stream might double when the stage rises from 1 to 2 feet of stage, but then it might quadruple when it goes from 3 to 4 feet. This graphic helps to illustrate:

To find out how much water is flowing in a stream or river, USGS personnel have to go out and make a "discharge measurement." USGS uses the term "discharge" to refer to how much water is flowing, and discharge is usually expressed in "cubic feet per second" (think of a cube of water one foot on a side, and how many of those move past a point in one second). To do this, we often have to go out and stand in the creek, measure the depth and how fast the water is moving at many places across the creek. By doing this many, many times, and at many stream stages, over the years we can develop a relation between stream stage and discharge. Stream stages are not always cooperative, so its not uncommon for someone to have to go measure a stream at 2:00 in the morning during a storm, sometimes in freezing conditions! Also, the stream can be uncooperative in that it changes -- a big storm may come along and scour out bottom material of a creek, or lodge a big log sideways in the creek, or sometimes do both at the same time. These kind of changes result in changes in the relation between stage and discharge.

A more detailed explanation is available.

The U.S. Geological Survey (USGS) stream-gaging program provides streamflow data for a variety of purposes that range from current needs, such as flood forecasting, to future or long-term needs, such as detection of changes in streamflow due to human activities or global warming. The development of data on the flow of the Nation's rivers mirrors the development of the country. From the establishment of the first stream-gaging station operated by the USGS in 1889, this program has grown to include 7,292 stations in operation as of 1994. Data from the active stations, as well as from discontinued stations, are stored in a computer data base that currently holds mean daily-discharge data for about 18,500 locations and more than 400,000 station-years of record. The stream-discharge data base is an ever-growing resource for water-resources planning and design, hydrologic research, and operation of water-resources projects.

The USGS stream-gaging program provides hydrologic information needed to help define, use, and manage the Nation's water resources. The program provides a continuous, well-documented, well-archived, unbiased, and broad-based source of reliable and consistent water data. Because of the nationally consistent, prescribed standards by which the data are collected and processed, the data from individual stations are commonly used for purposes beyond the original purpose for an individual station. Those possible uses include the following:

• Enhancing the public safety by providing data for forecasting and managing floods
• Characterizing current water-quality conditions
•  Determining input rates of various pollutants into lakes, reservoirs, or estuaries
• Computing the loads of sediment and chemical constituents
• Understanding the biological effects of contamination
• Delineating and managing flood plains
• Setting permit requirements for discharge of treated wastewater
• Designing highway bridges and culverts
• Setting minimum flow requirements for meeting aquatic life goals
• Monitoring compliance with minimum flow requirements
• Developing or operating recreation facilities
• Scheduling power production
• Designing, operating, and maintaining navigation facilities
• Allocating water for municipal, industrial, and irrigation uses
• Administering compacts or resolving conflicts on interstate rivers
• Defining and apportioning the water resources at our international borders
• Evaluating surface- and ground-water interaction
• Undertaking scientific studies of long-term changes in the hydrologic cycle

Data for one or more of these purposes are needed at some point in time on virtually every stream in the country, and a data-collection system must be in place to provide the required information. The general objective of the stream-gaging program is to provide information on or to develop estimates of flow characteristics at any point on any stream. Streamflow data are needed for immediate decision making and future planning and project design. Data, such as that needed to issue and update flood forecasts, are referred to as "data for current needs." Other data, such as that needed for the design of a future, but currently unplanned, bridge or reservoir or development of basinwide pollution control plans, are referred to as "data for future or long-term needs." Some data, of course, fit into both classifications; for example, a station that supplies data for flood forecasting and also provides data to define long-term trends. Reference: Wahl, K.L., Thomas, W.O., Jr., and Hirsch, R.M., 1995The stream-gaging program of the U.S. Geological Survey: U.S. Geological Survey Circular 1123, 22 p.

For more information on the National Streamflow Information Network (NSIP) go to: http://water.usgs.gov/nsip/

No. In ground-water work the USGS puts a lot of effort in measuring the water levels in observation wells. Since water levels in aquifers can change (for a variety of reasons) we need to keep accurate records of these changes, and what factors affect them. Water levels in wells can definitely be affected by water withdrawals nearby -- and sometimes far away.

One way to keep a record of the water levels in a well is to place a float at the end of a wire and lower it into a well. The float will go up and down as the water in the well goes up and down. The other end of the wire is attached to a machine that has a pen-like instrument attached, and the pen point moves up and down according to the action of the float. A roll of paper slowly rolls past the pen, so a record of water level is plotted continuously on the paper. Sometimes we don't use paper -- we just log the changes straight into computer memory.

The depth to the water table can change (rise or fall) depending on the time of year. During the late winter and spring when accumulated snow starts to melt and spring rainfall is plentiful, water on the surface of the earth infiltrates into the ground and the water table rises. When water-loving plants start to grow again in the spring and precipitation gives way to hot, dry summers, the water table will fall because of evapotranspiration. The most reliable method of obtaining the depth to the water table at any given time is to measure the water level in a shallow well with a tape. If no wells are available, surface geophysical methods can sometimes be used, depending on surface accessibility for placing electric or acoustic probes. Data bases containing depth-to-water measurements made in the past are maintained by the USGS. Your state government probably maintains a data base of drillers' logs that have water-levels recorded when a well was drilled, and hydrologic consultants often have reports that contain water-level data from shallow boreholes. Consulting any or all of these sources is a good first step in finding out the depth to the water table.

All the gaging stations that appear on the "Current Conditions" section of our website have satellite telemetry that basically works like this--An electronic data logger, using a 12-volt battery supply, monitors and records gage heights at selected intervals (usually 15 minutes). The data are periodically transmitted to a satellite in geo-stationary orbit over the equator. The transmitter is called a GOES radio transmitter, and USGS stations typically transmit data every 4 hours. The data are relayed via the satellite to a groundstation in Maryland, and then from Maryland via satellite to a USGS groundstation in Carson City, Nevada. From Nevada, the data are transmitted via landline to our computer system. USGS software decodes the data, which often (but not always) arrives in binary format, and puts it in a format that our hydrologic-data processing software (ADAPS) can recognize. The gage-height data are stored and manipulated to provide streamflow in cubic feet per second. USGS website software continuously accesses the various data files (site information, gage height, and discharge) and portrays the information graphically. Most of the data-logging systems use 12-volt power from a wet-cell battery with a solar panel recharging system. Data can also be transmitted via cellular and FM frequencies, but both require direct line of sight to a repeater.

USGS has used consistent procedures for over 100 years to collect streamflow data, and currently operates over 7000 stream-gaging stations across the United States. All regular stream-gaging stations record stage, or water depth, at a fixed time interval, usually every 15, 30, or 60 minutes. Stage is most often measured using either a float and pulley device or a pressure transducer.

A relationship is developed by USGS hydrographers between stage (usually expressed as feet) and discharge (usually expressed as cubic feet per second). This relationship is developed by making frequent direct discharge measurements at stream-gaging stations. Discharge is the primary streamflow data product of USGS, and operations have been specifically designed to provide accurate determinations of average daily streamflow, flood peaks, minimum flows, and flows associated with water-quality samples. In most cases, USGS operations have not been designed or funded to provide accurate instantaneous streamflow data, although customer demand for these data is increasing. In any case, the accuracy of instantaneous streamflow data reported by USGS is acceptable for developing the principal data products and for most other applications. The accuracy of data may not be sufficient for applications requiring high resolution.

All USGS streamflow data, including real-time data, are PROVISIONAL and subject to revision until published in the annual Water-Data Report.

There may be occasional equipment or database problems where erroneous data are reported for short periods of time until corrections can be made. This is why it is important to look at a record of streamflow such as the 7-day hydrograph plots rather than a single point in time. However, most of the time USGS has a high level of confidence in its real-time stage data.

During low streamflow conditions, aquatic grasses may produce increases in stream water level near gages. On smaller streams, debris or rocks on flow control structures may also produce increases in water level. Stage values reported on these pages are believed to be reasonably accurate, but the higher stage readings may produce estimates of discharge that are higher than actual. These higher stage readings at stream gages may be localized and may not be good indicators of stream stage at other locations on the river.

During extreme cold weather, ice can affect stage and discharge determinations at some stream-gaging stations. Data values reported by USGS may be significantly higher or lower than actual streamflow. Adjustment of data for ice effects can only be done after detailed analysis.

Wetlands are transitional areas, sandwiched between permanently flooded deepwater environments and well-drained uplands. They include mangroves, marshes (salt, brackish, intermediate, and fresh), swamps, forested wetlands, bogs, wet prairies, prairie potholes, and vernal pools. They often contain more plants and animals and produce more organic material than either the adjacent water or land areas. Aquatic habitats include permanently flooded parts of estuaries and nearshore environments like seagrass beds, rivers, ponds, and lakes. Aquatic habitats are also critical to fish and wildlife as well as economically and recreationally valuable to humans.

USGS tries to correct an equipment or station problem within several days of its first occurence, and is generally successful in meeting this goal. Occasionally, replacement parts or equipment may not be readily available, or a station may be inaccessible due to weather conditions.

Most USGS stream-gaging stations are operated in cooperation with other agencies. At some stations, the stage transmitting equipment is owned and maintained by other agencies to support their particular public missions and they may be limited in personnel, parts, or funds to maintain the equipment all of the time.

An effort was undertaken to identify and assemble USGS records of daily mean discharge that were judged to be relatively free of anthropogenic effects. The resulting collection of stations is called the Hydroclimatic Data Network or HCDN. The HCDN consists of 1,659 sites throughout the United States and its territories, totaling 73,231 water years of daily mean discharge values.

For more information about this network, visit http://pubs.usgs.gov/of/1992/ofr92-129.

Data from many real-time streamflow gages are relayed to USGS offices through the Geostationary Operational Environmental Satellite (GOES) data-collection system or by telephone. Data are transmitted from each station at intervals of 4 hours and are loaded onto the computer system. This 4-hour interval is based on a transmission time window available for the satellite and is not controlled by USGS.

Data from other stations is obtained through an automatic telephone dial-up, and the computer is set to call on a 4-hour interval. USGS does control this system, but it is limited by the actual time that it takes to dial and download data.

USGS is investigating alternative systems to provide data on a more frequent basis during floods. However, because most existing real-time streamflow gages are located on larger rivers and the hydrologic response time of these rivers is generally much greater than 4 hours, the current system is adequate for most applications.

Real-time streamflow data available on USGS pages are PROVISIONAL data that have not been reviewed or edited. These data may be subject to significant change and are not citable until reviewed and approved by the U.S. Geological Survey. Real-time streamflow data may be changed after review because the stage-discharge relationship may have been affected by:

• backwater from ice or debris such as log jams
• algae and aquatic growth in the stream
• sediment movement
• malfunction of recording equipment

Data are reviewed periodically to ensure accuracy. Each station record is considered PROVISIONAL until the data are published. The data are usually published within 6 months of the end of the water year.

Data users are cautioned to consider carefully the provisional nature of the information before using it for decisions that concern personal or public safety or the conduct of business that involves substantial monetary or operational consequences.

Existing real-time stream-gaging stations have been established to meet the mission requirements of the U.S. Geological Survey and its cooperating agencies, including water resources management, flood warning, and water resources investigations. Most existing stations were established at the specific request of cooperating agencies with funding support from those agencies, but USGS also has a long-term strategic goal of having real-time access to nearly all of its 7000 stream-gaging stations across the United States.

The schedule for adding real-time access to the remainder of the stations is a function of funding for satellite or telephone connections, and will probably take many years to complete. In a few cases, the cost may be relatively inexpensive where phone lines are readily available, but in other cases it may cost $5,000-10,000 to add an individual station.