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 floods are more common in areas with a dry climate and rocky terrain because lack of soil or vegetation allows torrential rains to flow overland rather than infiltrate into the ground.

River flooding is generally more common for larger rivers in areas with a wetter climate, when excessive runoff from longer-lasting rainstorms and sometimes from melting snow causes a slower water-level rise over a larger area. Floods also can be caused by ice jams on a river or high tides, but most floods can be linked to a storm of some kind.

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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...

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 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 and historic coastal flooding...

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...
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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: May 19, 2011

Bayou Landscape Changing Due to Floods

The opening of the Morganza Spillway to alleviate flooding on the Mississippi River is diverting water into the Morganza floodway and downstream into the Atchafalaya River, potentially impacting as much as 800,000 acres of wetlands, navigational waterways, and recreational and fishery waters in Louisiana.

Date published: June 20, 2008

Two 500-Year Floods Within 15 Years: What are the Odds?

The term "500-year flood" has been used to describe the recent flooding in the Midwest.

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June 30, 2018

Understanding Floods | Long-term Streamflow Data Collection

The USGS is developing methods to improve data collection during floods to gain new insight into the rise and fall of flood waters. In the past, the only data left behind after a flood was how high the water got, or the peak of the flood. This video presents the methodology that hydrologists are using to set up a Continuous Slope-area Reach in remote areas that are

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Image:  In Louisiana fields and forests, USGS scientists find floodwaters' marks
March 17, 2016

In Louisiana fields and forests, USGS scientists find floodwaters' marks

USGS hydrographer Aaron Pugh of Little Rock, Arkansas documents a high water mark left by flooding in Shreveport, Louisiana near Cross Lake.

Image: Ice Jams
March 14, 2014

Ice Jams

Ice Jams in Redwater River near Vida, MT.

Image: Ice Jams
March 8, 2014

Ice Jams

Ice Jams in Powder River at Arvada, WY, downstream of the bridge.

Image: A Flooded Field in East Boulder, Colo.
September 13, 2013

A Flooded Field in East Boulder, Colo.

Numerous rivers flooded during a significant September 2013 rain event along Colorado's Front Range, damaging or destroying several USGS streamgages. In response, USGS field crews measured flood flows, made gage repairs, and assessed sites to replace those gages destroyed. 

Attribution: Natural Hazards
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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video thumbnail: 2011: The Year of the Flood
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.

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March 21, 2010

Explaining Flood Measurements

Explaning why USGS streamgagers make discharge measurements.

Image: Flooded Street Sign in Moorhead MN
March 18, 2010

Flooded Street Sign in Moorhead MN

Traffic sign surrounded by floodwater from Red River of the North at Moorhead, MN. Photo taken at 2nd Ave and 3rd St, Moorhead, MN.

Attribution: Natural Hazards