# 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 valuable for determining possible severity of the flooding.
• Knowledge about the characteristics of a river's drainage basin, such as soil-moisture conditions, ground temperature, snowpack, topography, vegetation cover, and impermeable land area, which can help to predict how extensive and damaging a flood might become.

The National Weather Service (an agency within NOAA) collects and interprets rainfall data throughout the United States and issues flood watches and warnings as appropriate. They use statistical models that incorporate USGS streamflow data to try to predict the results of expected storms.

The USGS maintains a network of streamflow-gaging stations throughout the country.

## Related Content

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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 rainstorm 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...

### 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...

### 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: 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: August 22, 2016

### Fighting the Floods

The USGS response to the Louisiana floods is part of the larger USGS flood science mission...

Date published: January 14, 2011

August 8, 2011

### 2011: The Year of the Flood

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.

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.

June 26, 2008

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.

June 4, 2008