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Flood Climatology Glossary

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Action Stage

The stage which, when reached by a rising stream, represents the level where the NWS or a partner/user needs to take some type of mitigation action in preparation for possible significant hydrologic activity. The appropriate action is usually defined in a weather forecast office (WFO) hydrologic services manual. Action stage can be the same as forecast issuance stage (see / forecast issuance stage/).

Alert Stage

The stage which, when reached by a rising stream, represents the level where appropriate officials (e.g., county sheriff, civil defense officials, or bypass gate operators) are notified of the threat of possible flooding. (Used if different from action stage, and at the discretion of the WFO or river forecast center [RFC].) The term "alert stage" is to be used instead of warning stage. Monitor stage or caution stage may be used instead of alert stage in some parts of the country.

Annual Flood

The maximum discharge peak during a given water year (October 1 - September 30).


In hydrologic terms, permeable layers of underground rock, or sand that hold or transmit groundwater below the water table that will yield water to a well in sufficient quantities to produce water for beneficial use.

Artificial Control

In hydrologic terms, a weir or other man-made structure which serves as the control for a stream-gaging station.


Backwater Effect

The effect which a dam or other obstruction has in raising the surface of the water upstream from it.

Backwater Flooding

Hydrologic terms, upstream flooding caused by downstream conditions such as channel restriction and/or high flow in a downstream confluence stream.


The margins of a channel. Banks are called right or left as viewed facing in the direction of the flow.


The water level, or stage, at which a stream, river or lake is at the top of its banks and any further rise would result in water moving into the flood plain.

Bankfull Stage

An established gage height at a given location along a river or stream, above which a rise in water surface will cause the river or stream to overflow the lowest natural stream bank somewhere in the corresponding reach. The term "lowest bank" is however, not intended to apply to an unusually low place or a break in the natural bank through which the water inundates a small area. Bankfull stage is not necessarily the same as flood stage.


Streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as ground water flow, or dry-weather flow.


An area having a common outlet for its surface runoff. Also called a "Drainage Basin."

Basin Boundary

The topographic dividing line around the perimeter of a basin, beyond which overland flow (i.e.; runoff) drains away into another basin.

Basin Lag

The time it takes from the centroid of rainfall for the hydrograph to peak.

Basin Recharge

Rainfall that adds to the residual moisture of the basin in order to help recharge the water deficit. i.e; water absorbed into the soil that does not take the form of direct runoff.


Caution Stage

The stage which, when reached by a rising stream, represents the level where appropriate officials (e.g., county sheriff, civil defense officials, or bypass gate operators) are notified of the threat of possible flooding. Alert stage or caution stage are used instead of caution stage in some parts of the country.

Cubic Feet per Second (CFS)

The flow rate or discharge equal to one cubic foot (of water, usually) per second. This rate is equivalent to approximately 7.48 gallons per second. This is also referred to as a second-foot.


The volume of water discharged in twenty four hours, with a flow of one cubic foot per second is widely used; 1 cfs-day is 24 x 60 x 60 = 86,000 cubic feet, 1.983471 acre-feet, or 646,317 gallons. The average flow in cubic feet per second for any time period is the volume of flow in cfs-days.


Also known as Watercourse; an open conduit either naturally or artificially created which periodically, or continuously contains moving water, or forms a connecting link between two bodies of water. River, creek, run, branch, anabranch, and tributary are some of the terms used to describe natural channels. Natural channels may be single or braided. Canal and floodway are some of the terms used to describe artificial channels.


A small stream of water which serves as the natural drainage course for a drainage basin of nominal, or small size. The term is a relative one as to size, some creeks in the humid section would be called rivers if they occurred in the arid portion.


In hydrologic terms, (1) The highest stage or level of a flood wave as it passes a point. (2) The top of a dam, dike, spillway, or weir, to which water must rise before passing over the structure.

Crest Gage

A gage used to obtain a record of flood crests at sites where recording gages are installed.


A horizontal movement of water. Currents may be classified as tidal and nontidal. Tidal currents are caused by gravitational interactions between the sun, moon, and earth and are a part of the same general movement of the sea that is manifested in the vertical rise and fall, called TIDE. Tidal currents are periodic with a net velocity of zero over the tidal cycle. Nontidal currents include the permanent currents in the general circulatory systems of the sea as well as temporary currents arising from more pronounced meteorological variability. The SET of a current is the direction toward which it flows; the DRIFT is its speed.


The total number of flood events in the year up to the given month.


Deformed Ice

A general term for ice which has been squeezed together and forced upwards and downwards in places. Subdivisions are rated ice, ridge ice, hummocked ice, and other similar deformations.


The rate at which water passes a given point. Discharge is expressed in a volume per time with units of L3/T. Discharge is often used interchangeably with streamflow.

Drainage Area

An area having a common outlet for its surface runoff (also see Watershed and Catchment Area).

Drainage Basin

A part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.


Flood Categories

Terms defined for each forecast point which describe the severity of flood impacts in the corresponding river/stream reach. The severity of flooding at a given stage is not necessarily the same at all locations along a river reach due to varying channel/bank characteristics or presence of levees on portions of the reach. Therefore, the upper and lower stages for a given flood category are usually associated with water levels corresponding to the most significant flood impacts somewhere in the reach. The flood categories used in the National Weather Service are minor, moderate and major.

Flood Crest

Maximum height of a flood wave as it passes a certain location.

Flood Event

A period of time where one or more river forecast points crest at or above flood stage. Normally two flood events do not overlap in time, since all forecasts points have fallen before flood stage before a second event begins. There are rare cases where a slowly receding forecast point has remained above flood stage before a second event begins. Flood events in this study are usually based on the first location to crest and the last location to crest. See Flood Event Dates for more information.

Flood Event Dates

Flood event dates are usually based on the first location to crest and the last location to crest. Therefore it is possible that flooding actually began earlier and ended later. However in a few cases we do have information on the first and last forecast point exceeding and falling below flood stage.

Flood of Record

The highest observed river stage or discharge at a given location during the period of record keeping. (Not necessarily the highest known stage.)

Flood Plain

The portion of a river valley that has been inundated by the river during historic floods.

Flood Stage

An established gage height for a given location above which a rise in water surface level begins to create a hazard to lives, property, or commerce. The issuance of flood (or in some cases flash flood) warnings is linked to flood stage. Not necessarily the same as bankfull stage.

Forecast Point

A site or location along a river or stream for which hydrologic forecast and warning services are provided by a National Weather Service (NWS) weather forecast office (WFO). The observed forecast stage or discharge for a given forecast point can be assumed to represent conditions in a given reach.



(1) A device for indicating the magnitude or position of a thing in specific units, when such magnitude or position undergoes change, for example: The elevation of a water surface, the velocity of flowing water, the pressure of water, the amount or intensity of precipitation, the depth of snowfall, etc. (2) The act or operation of registering or measuring the magnitude or position of a thing when these characteristics are undergoing change. (3) The operation, including both field and office work, of measuring the discharge of a stream of water in a waterway.

Gage Datum

A horizontal surface used as a zero point for measurement of stage or gage height. This surface usually is located slightly below the lowest point of the stream bottom such that the gage height is usually slightly greater than the maximum depth of water. Because the gage datum is not an actual physical object, the datum is usually defined by specifying the elevations of permanent reference marks such as bridge abutments and survey monuments, and the gage is set to agree with the reference marks. Gage datum is a local datum that is maintained independently of any national geodetic datum. However, if the elevation of the gage datum relative to the national datum (North American Vertical Datum of 1988 or National Geodetic Vertical Datum of 1929) has been determined, then the gage readings can be converted to elevations above the national datum by adding the elevation of the gage datum to the gage reading.


Headwater Basin

A basin at the headwaters of a river. All discharge of the river at this point is developed within the basin.

HSA (Hydrologic Service Area)

A geographical area assigned to Weather Service Forecast Office's/Weather Forecast Office's that embraces one or more rivers.


A graph showing the water level (stage), discharge, or other property of a river volume with respect to time.


Ice terminology

(A) Anchor ice - frazil ice that collects, usually temporarily, on rocks and debris on the streambed, causing a minor increase in stage (from Linsley, Kohler, and Paulhus).

(B) Border ice - sheet ice in the form of a long border attached to the shore.

(C) Candled ice - decaying sheet ice. Solar radiation and warm air temperatures combine to weaken the sheet ice. As the ice decays, it becomes darker in color and fails in thin vertical pieces (candles).

(D) Frazil ice - predominant form of stream ice. Only forms in areas of rapidly moving open water during periods of air temperature below about 20 degrees F. Small, spherical crystals can form at various depths and be transported downstream, where they can accumulate into a stationary ice cover. These crystals are sticky and can form different ice structures. Frazil ice formation ceases where a river freezes over since production requires that open, moving water be in contact with very cold air.

(E) Frazil pans - floating masses of frazil ice that has stuck together. Lateral dimension may range from several feet to several hundred feet.

(F) Frazil slush - loosely packed frazil ice floating on the surface of a stream.

(G) Ice effects - indicates that river ice accumulation in the vicinity of a river gage is altering the channel cross-section (in addition to increasing friction) which produces higher stages than would be expected in the absence of ice. The relationship (rating) between river flow and gage height is no longer valid. A stretch of river does not have to be ice covered to exhibit ice effects. Ice can form along the surface in slow-moving sections, along the banks and sides of the channel, and also on the channel bottom (anchor ice). Abrupt fluctuations in stage of several feet are common and should be expected during extended cold spells.

(H) Ice jam - a stationary accumulation of ice that restricts streamflow. For operational purposes, an ice jam is a mass of ice that produces significant stage fluctuations or backwater upstream of the jam. There are two types of ice jams:

Ice Jam Types

(1) Freezeup jam - generally forms in early to mid winter. Very cold temperatures in combination with steady river flows can lead to the formation of large quantities of river ice, either of the sheet or frazil type, resulting in restricted flow and possible backwater problems. The surface of a freezeup jam is smooth to moderately rough (vertical variations less than two feet). Freezeup jams rarely cause major flooding.

(2) Breakup jam - generally occurs in mid to late winter when runoff from rain and/or snowmelt produces sufficient increase in streamflow to dislodge, breakup, and move the ice cover along a reach of a stream. The dislodged ice is transported downstream and can become jammed where an island, bridge piers, stationary ice, dam, bend in the river, or a confluence serves as an obstacle to the broken ice. The surface of a breakup jam is very rough with and often visually dramatic, with large blocks of ice jutting out of the surface. Very large and rapid rises are possible behind breakup jams, which pose the primary ice-induced flood threat in the MARFC area of responsibility.

(3) Shear walls - cliff-like remnants of an ice cover or jam still attached to the shore after breakup.

(4) Sheet ice - forms on slowly flowing reaches of a stream or lake; generally smooth and clear, but may appear dark blue or black. Crystals are columnar in shape which can produce a very strong ice cover.


Major Flooding

Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations.

Minor Flooding

Minimal or no property damage, but possibly some public threat.

Moderate Flooding

Some inundation of structures and roads near stream. Some evacuations of people and/or transfer of property to higher elevations.



The long-term average value of a meteorological paramater (i.e., temperature, humidity, etc.) for a certain area. For example, "temperatures are normal for this time of year" means that temperatures are at or near the average climatological value for the given date. Normals are usually taken from data averaged over a 30-year period (e.g., 1971-2000 average), and are concerned with the distribution of data within limits of common occurrence.


Point (location) Floods Rank

Provides an indication of how widespread a particular flood event was within the MARFC service area. Our records show over 1,000 flood events but numerous floods have the same number of locations (forecast points) flooding. Therefore ranking range from 1 to 636. For example, presently the highest-ranked flood event (#1) caused 138 locations to reach flood stage, while the lowest-ranked flood event (#636) caused only 1 location to reach flood stage. Dozens of flood events are tied for having a rank of 636 because only one location within the MARFC service area reached flood stage during those events. Generally, the lower the total number of forecast points during a flood the more numerous the rank is.



A section of river or stream between an upstream and downstream location, for which the stage or flow measured at a point somewhere along the section (e.g., gaging station or forecast point) is representative of conditions in that section of river or stream. The forecast points designated by the weather forecast offices are used by the river forecast centers.

River Gage Datum

The arbitrary zero datum elevation which all stage measurements are made from.


The methods of predicting the attenuation of a flood wave as it moves down the course of a river.


The part of precipitation that flows toward the streams on the surface of the ground or within the ground. Runoff is composed of baseflow and surface runoff.


Seasonal Floods

Seasonal floods are a 3 month total of a meteological season.

Winter = January 1 - March 31 Spring = April 1 - May 31
Summer = July 1 - September 30 Fall = October 1 - December 31



A structure over or through which excess or flood flows are discharged. If the flow is controlled by gates, it is a controlled spillway, if the elevation of the spillway crest is the only control, it is an uncontrolled spillway.


The level of the water surface of a river or stream above an established datum at a given location.


In hydologic terms, the rate at which water passes a given point. Streamflow is expressed in volume per time with units of cubic feet per second (CFS). Streamflow is often used interchangeably with discharge or flow.


Snow Water Equivalent (the amount of water content in a snowpack or snowfall).


Threshold Runoff

In hydrologic terms, the runoff in inches from a rain of specified duration that causes a small stream to slightly exceed bankfull. When available, flood stage is used instead of slightly over bankfull.



U.S. Geological Survey

The Federal Agency chartered in 1879 by congress to classify public lands, and to examine the geologic structure, mineral resources, and products of the national domain. As part of its mission, the USGS provides information and data on the Nation's rivers and streams that are useful for mitigation of hazards associated with floods and droughts.

Unit Hydrograph

The discharge hydrograph from one inch of surface runoff distributed uniformly over the entire basin for a given time period.


Water Year

The time period from October 1 through September 30.


Land area from which water drains toward a common watercourse in a natural basin.


An area that is regularly wet or flooded and has a water table that stands at or above the land surface for at least part of the year.


Year Rank by Floods

The number of flood events that occurred in the MARFC service area during a calendar year. Events are based on a period of time where one or more location (forecast points) crest at or above flood stage. Our records show several years having the same amount of events. For example, presently the highest-ranked year by events (#1) saw 24 different flood events, while the lowest-ranked flood event (#123) saw only 1 event. Generally, the lower the yearly flood events the more numerous the rank is.

Year Rank by Forecast Points

The accumulated number of locations (forecast points) that occurred in a calendar year within the MARFC service area. Our records show several years having the same amount of events. For example, presently the highest-ranked year by locations (forecast points) (#1) saw 335 locations flood, while the lowest-ranked flood event (#128) saw only 1 location flood. Generally, the lower the total number of forecast points during the year the more numerous the rank is.


Zero Datum

A reference "zero" elevation for a stream or river gage. This "zero" can be referenced (usually within ten feet of the bottom of the channel) to mean sea level, or to any other recognized datum.

Data provided by the U.S. Geological Survey

Additional support for this project was provided by the Susquehanna River Basin Commission, National Weather Service and Penn State University (PSU) Department of Meteorology. Special thanks to Michael Kozar (PSU student) for writing numerous weather summaries in the Floods by Events section.

Questions should be directed to Charles Chillag or Alaina MacFarlane.

NOAA's National Weather Service
Middle Atlantic River Forecast Center
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State College, PA 16803
Page Authors: Charles Chillag & Alaina MacFarlane
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Page last modified: December 31, 2012
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