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 estimates of the flood quantiles are calculated using actual data collected at a site. 

For a particular river, the USGS collects data over time, determines the largest flood in each year, and then calculate statistical data for that river. The more years of data available, the more accurate the estimates for the various flood quantiles. 

As more years of data become available, the estimates become more refined, which can result in the quantiles changing.

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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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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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Picture of tow truck driver assisting car stuck in flood waters.
August 1, 2016

A tow truck driver wades through waist-deep water to assist motorists as Stewart Creek overtops Morehead Street. (From FS 036-98; Photograph from The Charlotte Observer/Kent D. Johnson)

August 31, 2012

Devastation from the 1913 flood is shown primarily through photographs taken during the March 1913 flood. In the aftermath of the 1913 flood, State and Federal funds were allocated for the installation of a streamgage network to monitor the water level and flow of Ohio's rivers and streams. The modern streamgaging network and uses of streamflow data are described.

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.

flood sign
January 11, 2012

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

video thumbnail: 2011: The Year of the Flood
August 8, 2011

Devastating floods across much of the U.S. were severe and unrelenting during the spring and summer of 2011. When floods happen, USGS crews are among the first-responders. Often working in dangerous conditions, USGS scientists measure streamflow and river levels, repair and install streamgages, measure water quality and changes in sediment flow, and assess river changes. From flood forecasts to difficult decisions about flood management, science is critical for flood preparations and response.

Image: USGS personnel measure flood waters on Wind River, WY
July 1, 2011

USGS hydrographer Jerrod Wheeler (in cablecar) measures flood flows right before the gagehouse washes away.

06225500 Wind River near Crowheart, WY: Jul 01 2011; 13,900 ft3/s

Image: Missouri River flood of 2011
May 27, 2011

Releases of 80,000 cubic feet per second from Oahe Dam near Pierre, SD, on May 27, 2011, during 2011 Missouri River flood.

Image: Missouri River flood of 2011
May 27, 2011

Releases of 80,000 cubic feet per second from Oahe Dam near Pierre, SD, on May 27, 2011, during 2011 Missouri River flood.

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 Waverly, IA
June 10, 2008

Flooding of a house