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Field Book/Vol. VI, Events/FB-EVT-102

River flood

A river flood is a river risen over its flood stage, the height on its gauge at which the water begins to reach roads, homes and farms. It comes slowly, from days of rain or melting snow over a large basin, and it is warned at points along the river, each with its own gauge, its own flood stage and a forecast of its crest: minor, moderate or major.

Plate FB-EVT-102Water, flood, coast and marineRevision 1, Status draftAlso called flood warning, river flooding, forecast point, flood stage, flood categories, crest, minor flooding, moderate flooding, major flooding, record flooding, hydrograph

Section across a river valley at a forecast point, its stage to scale and its width not: the gauge house on the bluff with its well down to the river; the gauge's zero at the bed of the channel; bankfull, where the river leaves its banks below flood stage; the categories of the manual's example gauge, flood stage at 8 feet (2.4 metres), minor flooding from 8 to 15 feet, moderate from 15 to 24 feet and major from 24 feet (7.3 metres); a road on the floodplain at 9 feet, under water in minor flooding; houses near the river at 15 feet, reached in moderate flooding; the town on its terrace at 24 feet, reached in major flooding; and the river at the crest of Fig. B, 19 feet (5.8 metres), over the road and into the houses.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-EVT-102-ASECTION ACROSS A RIVER VALLEY AT A FORECAST POINT, THE MANUAL'S EXAMPLE GAUGE, THE GROUND DRAWN FOR THE PLATEFIG. A A RIVER AT ITS GAUGECREST 19 ft (5.8 m)0 ft (0 m)5 ft (1.5 m)10 ft (3.0 m)15 ft (4.6 m)20 ft (6.1 m)25 ft (7.6 m)30 ft (9.1 m)STAGEMINOR FLOODING8 to 15 ft (2.4 to 4.6 m)MODERATE FLOODING15 to 24 ft (4.6 to 7.3 m)MAJOR FLOODING24 ft (7.3 m) and aboveFLOOD STAGE8 ft (2.4 m)BELOW FLOOD STAGEKEYThe river at its crestGround, cut12345678TITLERiver flood, a river at its gaugeVOL. VI EVENTS · WATER, FLOOD, COAST AND MARINETYPESECTIONSCALESTAGE TO SCALE, WIDTH NTSREVREV 1 DRAFT SHEET 1 of 3DATE2026-10-17IDFB-EVT-102-ADRAWN AS linework on paperSOURCES NWS
Fig. A A river at its gauge. Section across a river valley at a forecast point, the manual's example gauge, the ground drawn for the plate STAGE TO SCALE, WIDTH NTSMaximizeThe sheet, SVG, 11 by 17
  1. The gauge house and its well
  2. The gauge's zero, at the bed of the channel
  3. Bankfull, where the river leaves its banks
  4. Flood stage, 8 ft (2.4 m)
  5. Minor: the road under water, 9 ft (2.7 m)
  6. Moderate: houses near the river, 15 ft (4.6 m)
  7. Major: the town on its terrace, 24 ft (7.3 m)
  8. The river at its crest, 19 ft (5.8 m)

What it is

A river flood is a river risen over its flood stage. Every point on a river that the Weather Service forecasts has a gauge, and every gauge has a flood stage: the height on it at which the rising water begins to reach roads, homes, farms and businesses. Above it the flooding is named in three categories, set for that point from what the water reaches there. Minor flooding brings little or no damage, though roads go under. Moderate flooding reaches buildings and roads near the river, and people leave or move their belongings to higher ground. Major flooding covers buildings and roads widely, and many people leave. A flood at or above the highest stage ever measured at the gauge is record flooding, whatever its category.

A river flood comes slowly. Rain falls for a day or several over a basin of hundreds or thousands of square miles, or snow melts across it, and the water gathers into the river over days. The crest, the highest stage of the flood, then travels down the river, reaching each point a little later than the one above it. A river can stay over its flood stage for days, and the largest for weeks.

Fig. A draws the example gauge of the Weather Service's own manual: flood stage at 8 ft (2.4 m), minor flooding from 8 to 15 ft (2.4 to 4.6 m), moderate from 15 to 24 ft (4.6 to 7.3 m) and major from 24 ft (7.3 m). The numbers are the manual's; every forecast point has its own.

What it is not

It is not a flash flood. Water that rises fast, generally within 6 hours of its cause, in a small steep basin, a city street or below a failed dam, is a flash flood and is warned by a Flash Flood Warning, Flash flood FB-EVT-100. A river flood rises over days, and its warning usually comes before the water does.

It is not an areal flood. A Flood Warning can also be issued for an area: streams, fields and low ground flooding over hours where no gauge stands to measure them. That is Areal flood FB-EVT-101. This plate is a flood at a point on a river, with a gauge, a stage and a forecast crest.

It is not a coastal flood. Water pushed onto the shore by the tide and the wind is Coastal flood FB-EVT-105. Near the mouth of a river the two can come together, and the office says so in its warning.

Lookalikes

Flash flood
Water that rises fast, generally within 6 hours of its cause, in a small steep basin or a city. Its warning is a Flash Flood Warning, Flash flood FB-EVT-100. A river flood rises over days.
Areal flood
A Flood Warning can also cover an area: streams and low ground flooding over hours, with no forecast point to measure them. That is Areal flood FB-EVT-101. This plate is a flood at a point on a river, with its gauge, its stage and its crest.
Coastal flood
Water from the sea, driven by the tide and the wind, Coastal flood FB-EVT-105. Near the mouth of a river both can come together, and an office says so in its warning.

Machine

A basin gathers its water

Every drop of rain that falls on a basin and is not taken in by the ground, the plants or the air runs downhill toward the same river. The larger the basin, the longer the water takes to arrive, and the more of the rain from a long storm it gathers. Ground already soaked, or frozen, takes in little, so the same rain floods a river in spring that would not in late summer. Snowmelt adds water for days without any rain at all, and rain on melting snow adds both. The whole cycle of the water is drawn on The hydrologic cycle FB-WAT-030, and the melt on Melt and runoff FB-WAT-032.

The crest travels

Fig. B draws one flood at the gauge of Fig. A. The rain falls on the first day. The river rises slowly at first, then faster as the water from the far parts of the basin arrives, crosses flood stage and reaches its crest at four days, 19 ft (5.8 m), moderate flooding. It falls more slowly than it rose, because the water spread over the floodplain drains back to the channel, and it is back below flood stage almost three days after the crest. A point downstream sees the same flood later and, where tributaries join, larger.

Deeper water moves faster

The equation below, Manning's, gives the speed of water flowing in an open channel. In a wide river 10 ft (3.0 m) deep, with the slope and roughness of a large river, it moves 4.42 ft s⁻¹ (3.01 mph); at 20 ft (6.1 m) deep, 7.01 ft s⁻¹ (4.78 mph). Twice the depth carries 3.17 times the water. This is why the last few feet of a rise carry so much more than the first, and why water moving over a road a foot deep, 12 in (30 cm), can carry away a small car.

The floodplain

Once the river is over its banks, the water spreads across the floodplain, the flat ground beside the channel that past floods built. The water there is shallower and slower, slowed by fields, fences, trees and buildings, and the river's rise slows because each foot of stage now covers far more ground. Levees hold the water off the land behind them until they are overtopped or fail, and then that land can fill quickly. An ice jam does the opposite: it dams the river, and the water rises behind it within hours.

Time strip of one typical flood at the gauge of Fig. A, drawn for the plate, by days from the start of the rain: rain over the basin upstream for the first day; a Flood Watch as it begins; a Flood Warning at one day, for minor flooding more than 80 percent likely; the river over flood stage, 8 feet (2.4 metres), 15 hours later; a Flood Warning again at two days, the forecast raised from minor to moderate; Flood Statements every half day with each new reading and forecast; the crest at four days, 19 feet (5.8 metres), moderate flooding; a statement at five and a half days as the flooding falls to minor; the river back below flood stage at six days and 17 hours, after 5 days above it; and the warning's end at seven days, once a reading confirms it.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-EVT-102-BA TYPICAL FLOOD AT THE GAUGE OF FIG. A, THE STAGE EVERY HALF DAY FOR SEVEN AND A HALF DAYS, DRAWN FOR THE PLATE AND NOT A MEASURED FLOODFIG. B ONE FLOOD AT THE GAUGEFLOOD STAGE, 8 ft (2.4 m)MINORMODERATEMAJORRAIN OVER THE BASIN UPSTREAMCREST 19 ft (5.8 m) AT 4 DAYS, MODERATEABOVE FLOOD STAGE FOR 5 DAYSFLOOD WATCHflooding 50 to 80 percent likelyFLOOD WARNINGminor flooding, more than 80 percent likelyFLOOD WARNING AGAINforecast raised from minor to moderateFLOOD STATEMENTthe flooding observed down to minorTHE WARNING ENDSa reading confirms it below flood stageFLOOD STATEMENTS, EACH WITH THE NEW READING AND FORECAST051015202530ft02468(m)01234567 DAYSDAYS FROM THE START OF THE RAIN; A TYPICAL FLOOD AT THE GAUGE OF FIG. A, DRAWN FOR THE PLATE12345678TITLERiver flood, one flood at the gaugeVOL. VI EVENTS · WATER, FLOOD, COAST AND MARINETYPETIME STRIPSCALETIME TO SCALE, STAGE TO SCALEREVREV 1 DRAFT SHEET 2 of 3DATE2026-10-17IDFB-EVT-102-BDRAWN AS linework on paperSOURCES NWS
Fig. B One flood at the gauge. A typical flood at the gauge of Fig. A, the stage every half day for seven and a half days, drawn for the plate and not a measured flood TIME TO SCALE, STAGE TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. The rain over the basin upstream
  2. The stage at the gauge
  3. Flood stage, 8 ft (2.4 m)
  4. The crest, 19 ft (5.8 m), moderate flooding
  5. The Flood Watch
  6. The Flood Warning, raised to moderate
  7. The Flood Statements
  8. The warning's end
Classification of the products for a point on a river, by how likely the flooding is and where the stage stands: at or above flood stage, about 50 to 80 percent likely within 48 hours, a Flood Watch, which also covers a dam or levee that may fail; at or above flood stage, more than 80 percent likely or reported, a Flood Warning, issued again when the flooding rises to a higher category; below flood stage, elevated flow more than 80 percent likely or occurring, a Flood Advisory; a Flood Statement following each warning; and flooding that rises fast, generally within 6 hours of its cause, a flash flood with its own warning.1122334455667788AABBCCDDEE© 2026 STORM STATION 247THE FIELD BOOKPLATE FB-EVT-102-CTHE PRODUCTS FOR A FORECAST POINT, BY THE CHANCE OF FLOODING AND WHERE THE STAGE STANDS AGAINST FLOOD STAGE, AFTER NWSI 10-922FIG. C WHICH PRODUCT A RIVER GETSABOUT 50 TO 80 PERCENT LIKELYMORE THAN 80 PERCENT, OR OCCURRINGAT OR ABOVEFLOOD STAGEfloodingBELOWFLOOD STAGEelevated flowFLOOD WATCHflooding at the forecast point,about 50 to 80 percent likelywithin 48 hours; or a dam or leveethat may fail, not yet imminentFLOOD WARNINGflooding more than 80 percent likely,or reported; issued again when theflooding forecast or seen risesto a higher category-FLOOD ADVISORYelevated flow, more than 80 percentlikely or occurring; never for ariver above flood stage orforecast to rise above itFLOOD STATEMENTfollows the warning: each new reading andforecast, a lower category, and its endFLOODING THAT RISES FASTgenerally within 6 hours of its cause, is a flashflood, with its own warning: plate FB-EVT-100THE CHANCE OF FLOODING, AS THE OFFICE JUDGES IT FROM THE RIVER FORECAST12345TITLERiver flood, which product a river getsVOL. VI EVENTS · WATER, FLOOD, COAST AND MARINETYPECLASSIFICATIONSCALENOT TO SCALEREVREV 1 DRAFT SHEET 3 of 3DATE2026-10-17IDFB-EVT-102-CDRAWN AS linework on paperSOURCES NWS
Fig. C Which product a river gets. The products for a forecast point, by the chance of flooding and where the stage stands against flood stage, after NWSI 10-922 NOT TO SCALEMaximizeThe sheet, SVG, 11 by 17
  1. Flood Watch
  2. Flood Warning
  3. Flood Advisory
  4. Flood Statement
  5. A flash flood, warned on its own

Ingredients

  • Rain over days on a large basin, or heavy rain on ground already wet or frozen
  • Snow melting, alone or with rain falling on it
  • A river already high, so the next rain brings it over flood stage
  • Or an ice jam that dams the river, or a dam or levee that fails

Scales

time
days to weeks, from the rain to the crest and back below flood stage
horizontal
a basin of hundreds to tens of thousands of square miles, its crest travelling hundreds of miles down the river
vertical
a stage from a few feet to tens of feet above flood stage
orlanski
meso-beta to meso-alpha

Equations

Manning's equation, in U.S. units

V=1.49n R2/3 S1/2V = \frac{1.49}{n}\,R^{2/3}\,S^{1/2}
VV
the mean speed of the water, ft s⁻¹
nn
the roughness of the channel, Manning's n
RR
the hydraulic radius, the area of the flow divided by the wetted edge of the channel, ft; in a wide river nearly its depth
SS
the slope of the water surface, ft per ft

Assumes Steady flow in a channel so wide that its hydraulic radius is its depth, with one roughness at every depth. The factor 1.49 is the cube root of 3.28 ft per metre; in metres the equation is the same with 1 in its place. A real river spills onto a floodplain rougher than its channel, so a gauge's rating, its stage against its flow, is measured, not computed.

Working form In a wide channel with n = 0.035 and a slope of 2.64 ft a mile (0.0005), water 10 ft (3.0 m) deep moves 4.42 ft s⁻¹ (3.01 mph, 1.35 m s⁻¹) and carries 44.2 ft³ s⁻¹ for each foot of width (4.10 m³ s⁻¹ a metre); 20 ft (6.1 m) deep, 7.01 ft s⁻¹ (4.78 mph, 2.14 m s⁻¹) and 140.3 ft³ s⁻¹ (13.03 m³ s⁻¹ a metre). Twice the depth is 1.59 times as fast and 3.17 times the flow.

Signatures

surface
a river gauge rising day after day; its forecast crest above flood stage; rain gauges upstream with a day's or several days' rain; snow melting in the basin
radar
rain over the basin for a day or more; the totals the radar estimates large over the whole of it
satellite
a slow storm or a stalled front over the basin; later; the river and its floodplain wide with water
sounding
deep moisture; the air holding much more water than usual; over days

Numbers

QuantityValue, and the kind of number it is
Flood stageThe gage height at which a rise begins to impact lives, property or commerce; not necessarily bankfull stageStandard, Manual 10-950
Bankfull stageThe gage height above which the river overflows its lowest natural bank somewhere in its reachStandard, Manual 10-950
Action stageThe gage height at which action is taken to prepare for possible significant hydrologic activityStandard, Manual 10-950
Minor floodingMinimal or no property damage, but possibly some public threat, such as roads under waterStandard, Manual 10-950
Moderate floodingSome structures and roads near the stream under water; people evacuated or property moved to higher groundStandard, Manual 10-950
Major floodingExtensive inundation of structures and roads; significant evacuations of people or property moved to higher groundStandard, Manual 10-950
Record floodingAt or above the highest stage or flow observed at the site in its period of record, in any category, or below them all where the record is shortStandard, Manual 10-950
The manual's example gaugeFlood stage 8 ft (2.4 m); minor 8 to 15 ft (2.4 to 4.6 m); moderate 15 to 24 ft (4.6 to 7.3 m); major 24 ft (7.3 m) and above; each forecast point has its ownStandard, Manual 10-950
Flood WarningFlooding at a forecast point more than 80 percent likely, or reported; issued again when the flooding forecast or observed rises to a higher categoryStandard, Directive 10-922
Flood WatchFlooding about 50 to 80 percent likely within 48 hours, or a dam or levee that may fail with the threat not yet imminentStandard, Directive 10-922
Flood AdvisoryElevated streamflow occurring or more than 80 percent likely; never for a river above flood stage or forecast to rise above itStandard, Directive 10-922
Flood StatementFollows a Flood Warning with each new reading and forecast, a fall to a lower category, and its endStandard, Directive 10-922
Manning's nAbout 0.03 for a clean, straight channel and 0.035 for a large river; more for a floodplain of brush or treesTypical
Moving water that carries away a small car12 in (30 cm)Standard, Turn Around Don't Drown
The site's gaugesThe stage as each gauge reports it, as often as every 15 minutes: the height of the water above the gauge's own zero, not its depthThis site

How the station sees it

A river gauge measures the stage: the height of the water's surface above the gauge's own zero, which is usually set near the bed of the channel, so the stage is not the depth of the water. Some gauges set their zero at a datum such as sea level instead, and read hundreds of feet. Where a gauge reports its flow, in cubic feet a second, the flow is read from the stage by the gauge's rating, a curve made from many measurements of the flow at different stages. The river and lake gauges on this site report their stage as often as every 15 minutes, with its change over a day and its last 30 days.

The Weather Service's River Forecast Centers forecast the stage at each forecast point days ahead, from the rain that has fallen, the rain expected, the snow and the state of the ground, and the office near the river issues the products from those forecasts. The site's record of severe weather keeps every river point an office warns will flood, with its category, its forecast crest, the highest stage reported and its flood stage, as the office gave them.

How it is warned

A Flood Watch may be issued when flooding at a forecast point is about 50 to 80 percent likely within 48 hours, or when a dam or levee may fail and the threat is not yet imminent. A Flood Warning is issued when the flooding is more than 80 percent likely or is reported, and it is issued again, under a new headline, when the flooding forecast or observed rises to a higher category. Flood Statements follow the warning with each new reading and forecast, tell when the flooding falls to a lower category and end the warning once a reading confirms the river is back below flood stage. Some offices issue a Flood Advisory for a river running high but forecast to stay below flood stage. Fig. C sets them side by side.

Each warning names its point on the river, its flood stage, the latest stage and the forecast crest, with what that stage floods there. Its code line carries the gauge's identifier, the category, what caused the flood and the times it is forecast to begin, crest and end.

When a Flood Warning is issued for a river near you, move belongings and vehicles to higher ground before the water comes, and leave when told to. Never drive or walk into flood water: turn around, don't drown. Water over a road can hide a road that is already gone.

VTECPhenomenonThe alerts that carry it
FLflood (point)Flood Advisory; Flood Statement; Flood Warning; Flood Watch

Weather radio (SAME) codes: FLA FLW FLS

Every alert in force now

See also

Sources

  1. National Weather Service. Manual 10-950, Definitions and General Terminology.
  2. National Weather Service. Directive 10-922, Weather Forecast Office Hydrologic Products Specification.
  3. National Weather Service. Directive 10-1703, Valid Time Event Code (VTEC).
  4. National Weather Service. Flood Related Hazards.
  5. National Weather Service. Turn Around Don't Drown.
  6. Chow, V. T., D. R. Maidment and L. W. Mays. Applied Hydrology (1988).

Definition after the Manual 10-950, Definitions and General Terminology. Plate FB-EVT-102, revision 1, 2026-10-17. The number is permanent; cite it.