What is a 1,000-year flood?

The term “1000-year flood” means that, statistically speaking, a flood of that magnitude (or greater) has a 1 in 1000 chance of occurring in any given year.  In terms of probability, the 1000-year flood has a 0.1% chance of happening in any given year.  

These statistical values are based on observed data.  

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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

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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 large 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.  

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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

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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.

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Where can I find flood maps?

FEMA’s Flood Map Service Center is the official public source for flood hazard information for insurance purposes.

NOAA (National Oceanic and Atmospheric Administration) has flood forecast maps:

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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
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What are the two types of floods?

There are two basic kinds 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 stage (water height) of a stream or

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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 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: September 28, 2016

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

Some regional trends; no widespread national pattern

Attribution: Water Resources
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man in yellow slicker standing in rushing river up to his thighs, holding a stick-like instrument in the water
2017 (approx.)

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

Image shows aerial imagery of flooding in Louisiana, courtesy of the National Geodetic Survey
August 18, 2016

Aerial imagery of flooding in Louisiana. Image created by Jason Burton, USGS, using aerial imagery taken by NOAA aviators on behalf of the National Geodetic Survey.

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

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 cause and effect of flooding, including a look at aspects of the 2011 epic flooding, and how USGS science assists in the overall flood mitigation efforts of the United States.

USGS Streamgage Destroyed by Flood Flows
July 1, 2011

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

Jul 01 2011; 13,900 ft3/s

October 14, 2009

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.

video thumbnail: Interview: 2009 Fargo, ND Flooding
March 25, 2009

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

Image: Flooding in Finchfield, IA
June 10, 2008

Flooding of a house

St. John River threatens to swamp a steel bridge.
April 29, 2008

Understanding the forces that influence major floods can help inform the design of more resilient infrastructure. Image shows a major flood on the St. John River on the border of Maine, United States and New Brunswick, Canada, April 29, 2008. This site was part of the study. USGS Public Domain.

A USGS crew prepares to measure Hurricane Harvey floodwaters off of a bridge.

A USGS crew prepares to measure Hurricane Harvey floodwaters off of a bridge. Photo by Cassi Crow, USGS.