We had a "100-year flood" two years in a row. How can that be?

The term "100-year flood" is used to describe the recurrence interval of floods. 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.

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What is a 1,000-year flood?

The term “1,000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1,000 chance of occurring in any given year. In terms of probability, the 1,000-year flood has a 0.1% chance of happening in any given year. These statistical values are based on observed data.

Why do the values for the 100-year flood seem to change with every flood?

The amount of water corresponding to a 100-year flood, a 500-year flood, or a 1,000-year flood is known as a "flood quantile". For instance, on a given river, the flood quantile corresponding to the 50-year flood might be 10,000 cubic feet per second (cfs) and the flood quantile corresponding to the 100-year flood might be 15,000 cfs. The...

Does an increase in the 100-year flood estimate originate from climate or land-use change?

Climate variability (dry cycles to wet cycles) and land-use change play a significant role, but there is a large amount of uncertainty around the flood quantile estimates (the value of discharge corresponding to the 100-year flood), particularly if there isn’t a long record of observed data at a stream location. Learn more: Flood recurrence...

How can a 1,000-year rainfall not result in a 1,000-year flood?

It comes down to a number of factors, including the pattern of movement of the rain storm in each particular watershed, the conditions of the soil and plant matter in the watershed, and the timing of the rainstorm in one watershed versus other watersheds. For example, if the ground is already saturated before a rainstorm, much of the rain will run...

Where can I find flood maps?

FEMA is the official public source for flood maps for insurance purposes: FEMA’s Flood Map Service Center NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data: National Flood Forecasts Interactive Flood Information Map USGS flood map products include: USGS Flood Information : Maps and...

How are floods predicted?

Flood predictions require several types of data: The amount of rainfall occurring on a realtime basis. The rate of change in river stage on a realtime basis, which can help indicate the severity and immediacy of the threat. Knowledge about the type of storm producing the moisture, such as duration, intensity and areal extent, which can be valuable...

What are the two types of floods?

There are two basic types of floods: flash floods and the more widespread river floods. Flash floods generally cause greater loss of life and river floods generally cause greater loss of property. A flash flood occurs when runoff from excessive rainfall causes a rapid rise in the water height (stage) of a stream or normally-dry channel. Flash...
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Date published: September 18, 2018

USGS Science – Leading the Way for Preparedness

Learn About USGS Hazards Science and More About National Preparedness Month: The very nature of natural hazards means that they have the potential to impact a majority of Americans every year.  USGS science provides part of the foundation for emergency preparedness whenever and wherever disaster strikes.

Date published: August 29, 2017

USGS Crews Measure Record Flooding in South-Central Texas

Reporters: Do you want to interview USGS scientists as they measure flooding? Please contact Jennifer LaVista or Lynne Fahlquist. 

U.S. Geological Survey field crews are measuring record flooding in parts of south-central Texas following intense rainfall from Tropical Storm Harvey.

Date published: August 10, 2017

Study Links Major Floods in North America and Europe to Multi-Decade Ocean Patterns

The number of major floods in natural rivers across Europe and North America has not increased overall during the past 80 years, a recent study has concluded. Instead researchers found that the occurrence of major flooding in North America and Europe often varies with North Atlantic Ocean temperature patterns.

Date published: February 22, 2017

Stormy weather: How the USGS goes to work monitoring its effects

Atmospheric rivers are a global weather phenomenon that can bring large amounts of rain or snow to the U.S. West Coast each year. These rivers of wet air form over the Pacific Ocean near Hawaiʻi and pick up large amounts of moisture from the tropics and on their way to the West Coast. This moisture is carried in narrow bands across the Pacific Ocean to California, Oregon, Washington and Nevada.

Attribution: Natural Hazards, Pacific
Date published: October 12, 2016

Severe flooding in NC breaks more than a dozen USGS peak records

Reporters: Want more information? Please contact Jeanne Robbins, jrobbins@usgs.gov, 919-571-4017 

Date published: September 28, 2016

Fragmented Patterns Seen in the Recent History of U.S. Floods

Some regional trends; no widespread national pattern

Attribution: Water Resources
Date published: August 18, 2016

USGS Records Historic Flooding in South Louisiana

Six streamgages Set peaks of record and 50 stations were overtopped by floodwaters.

Date published: April 22, 1997

Flow of Red River Sets Official 100-Year Record

The flow of the Red River officially broke the 100-year-old record on Thurs. April 17, 1997, according to the U.S. Geological Survey.

Date published: November 30, 1995

WASHINGTON FLOOD EXCEEDS 100-YEAR RECURRENCE LEVELS

Flood waters are peaking and beginning to recede in the Seattle-Tacoma, Washington, area according to streamflow specialists of the U.S. Geological Survey (USGS), a science agency of the Department of the Interior.

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man in yellow slicker standing in rushing river up to his thighs, holding a stick-like instrument in the water
December 31, 2017

Measuring streamflow in fast moving floodwater.

USGS hydrographer measuring streamflow using a handheld Acoustic Doppler Velocimeter in fast moving floodwater Cajon Creek near Keenbrook, California. 

video thumbnail: The Anatomy of Floods: The Causes and Development of 2011's Epic Flood Events
July 31, 2012

The Anatomy of Floods: The Causes and Development of 2011's Epic Flood Events

Flooding costs the United States more than $7 billion per year and claims more than 90 lives annually. During the Spring and Summer of 2011, the central U.S. experienced epic flooding, while Hurricane Irene followed by Tropical Storm Lee caused severe flooding in the east and northeastern U.S, setting numerous flood records at USGS streamgages. Dr. Robert Holmes discusses

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flood sign
January 11, 2012

Flood Sign in Tualatin River, OR

Peak flood height of 1996 flood, as observed in the Tualatin River, OR.

Car submerged in floodwaters
September 1, 2011

Car submerged in Floodwaters

Car Submerged in Floodwaters in Middlesex, New Jersey during September 2011 flood event

USGS Streamgage Destroyed by Flood Flows
July 1, 2011

Flood Waters Take USGS Streamgage Wind River near Crowheart, WY

Gagehouse at 06225500 Wind River near Crowheart WY right before it washed away.

Jul 01 2011; 13,900 ft3/s

October 14, 2009

Streamgages: The Silent Superhero

Whether you drink water from your tap, use electricity or canoe down your local river, chances are you benefit from USGS streamgage information. So what is a streamgage and what does it do for you? This CoreCast episode gives you the inside scoop on your silent superhero.

Transcript and captions available soon.

USGS scientists working during the epic September 2009 flood in Georgia. Power Springs Creek (02336870)
September 21, 2009

USGS scientists measuring Sept. 2009 flooding, Powder Springs Cr, GA

Epic Flooding in Georgia, 2009

Metropolitan Atlanta—September 2009 Floods

  • The epic floods experienced in the Atlanta area in September 2009 were extremely rare. Eighteen streamgages in the Metropolitan Atlanta area had flood magnitudes much greater than the estimated 0.2-percent (500-year) annual exceedance probability.
  • The
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video thumbnail: Interview: 2009 Fargo, ND Flooding
March 25, 2009

Interview: 2009 Fargo, ND Flooding

Interview with USGS crews regarding the 2009 flooding events in Fargo, ND.

Attribution:
USGS
June 18, 2008

Two 500-Year Floods Within 15 Years?

We talk to Bob Holmes about some of the recent flooding events occuring in the Midwest, how does a 500-year flood occur twice in 15 years, and what do the recent events have in store for folks downriver.

USGS
February 5, 2008

We had a "100-year flood" two years in a row. How can that be?

Listen to hear the answer.