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 off into streams, but if the ground is dry, it will soak up more of the rain and the runoff will be less significant.

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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...

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...

Where can I find flood maps?

FEMA is the official public source for flood maps for insurance purposes: FEMA’s Flood Map Service Center FEMA’s Flood Hazard Map FAQs NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data: National Flood Forecasts Interactive Flood Information Map Coastal Inundation Dashboard : Real-time...

How are floods predicted?

Flood predictions require several types of data: The amount of rainfall occurring on a real-time basis. The rate of change in river stage on a real-time 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...

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 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.

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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Photos show change in water clarity in agricultural runoff before and after a grassed waterway was installed
June 23, 2016

Edge-of-field: runoff before and after conservation practice install

These photos, taken before and after a grassed-waterway conservation practice was installed, show a change in the water clarity of agricultural runoff at an edge-of-field surface monitoring site in Wisconsin.

Image: Stormwater Runoff in Rapid City, SD
May 21, 2008

Stormwater Runoff in Rapid City, SD

Stormwater runoff following a May 2008 storm event in the Arrowhead drainage basin in Rapid City, SD. Runoff from this drainage discharges into Rapid Creek. Stormwater runoff from urbanized lands is known to harm surface-water resources by increasing stream velocities, destroying natural habitat, and increasing pollutant loads in the receiving waters. The City of Rapid

Atlanta, Georgia: Impervious areas affect rainfall runoff, flooding, and water quality
December 31, 2005

Impervious areas affect rainfall runoff, flooding, and water quality.

Impervious areas can affect precipitation runoff and flooding, and water quality

If you are not familiar with the term "impervious surface," this picture of a typical landscape in suburban Atlanta, Georgia, USA, will help explain it. As cities grow and more development occurs, the natural landscape is replaced by roads, buildings, housing