USGS water specialist Dwight Sparks measures floodwaters from County Road 402 bridge over the Navasota River near Groesbeck, Texas on June 12, 2025.
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. See their National Water Prediction Service website which includes a selection for Long Range Flood Outlook in the right column.
The USGS maintains a network of streamflow-gaging stations throughout the country.
Related
What are the two types of floods? 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...
Does an increase in the 100-year flood estimate originate from climate or land-use change? 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: The 100-Year Flood The 100-Year Flood--It's All About Chance
How can a 1,000-year rainfall not result in a 1,000-year flood? 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 off into streams, but if the ground is dry, it will...
We had a "100-year flood" two years in a row. How can that be? 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 with the same 1-percent chance that a 100-year...
What is a 1,000-year flood? 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.
Where can I find flood maps? 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 National Flood Hazard Layer NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data: NWS - National Water Prediction Service: River forecasts and long range flood outlook Coastal Inundation Dashboard: Real-time and historical...
Why do the values for the 100-year flood seem to change with every flood? 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...
USGS water specialist measures floodwaters near Groesbeck, Texas
USGS water specialist Dwight Sparks measures floodwaters from County Road 402 bridge over the Navasota River near Groesbeck, Texas on June 12, 2025.
Abandoned cars on a flooded street in Brooklyn, NY.
Abandoned cars on a flooded street in the Gowanus neighborhood of Brooklyn, NY during a storm in 2023.
Abandoned cars on a flooded street in the Gowanus neighborhood of Brooklyn, NY during a storm in 2023.
Glacial outburst flooding from Suicide Basin submerges trails around the Mendenhall Glacier visitors center
linkA path near the Mendenhall Glacier Visitor Center in Juneau, Alaska is completely submerged by floodwaters from a glacial lake outburst flood.
Glacial outburst flooding from Suicide Basin submerges trails around the Mendenhall Glacier visitors center
linkA path near the Mendenhall Glacier Visitor Center in Juneau, Alaska is completely submerged by floodwaters from a glacial lake outburst flood.
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.
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.
Flooded Street in Denham Springs
Backwater flooding across Florida Blvd near the Amite River Bridge in Denham Springs, LA.
Backwater flooding across Florida Blvd near the Amite River Bridge in Denham Springs, LA.
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.
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.
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.
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.
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.
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.
What is a "500 year flood", and how do scientists determine if a flood is considered a "500 year flood"?
linkListen to hear the answer.
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.
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.
We had a "100-year flood" two years in a row. How can that be?
Listen to hear the answer.
Related
What are the two types of floods? 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...
Does an increase in the 100-year flood estimate originate from climate or land-use change? 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: The 100-Year Flood The 100-Year Flood--It's All About Chance
How can a 1,000-year rainfall not result in a 1,000-year flood? 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 off into streams, but if the ground is dry, it will...
We had a "100-year flood" two years in a row. How can that be? 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 with the same 1-percent chance that a 100-year...
What is a 1,000-year flood? 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.
Where can I find flood maps? 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 National Flood Hazard Layer NOAA is responsible for producing flood forecast maps that combine precipitation data with USGS streamflow data: NWS - National Water Prediction Service: River forecasts and long range flood outlook Coastal Inundation Dashboard: Real-time and historical...
Why do the values for the 100-year flood seem to change with every flood? 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...
USGS water specialist measures floodwaters near Groesbeck, Texas
USGS water specialist Dwight Sparks measures floodwaters from County Road 402 bridge over the Navasota River near Groesbeck, Texas on June 12, 2025.
USGS water specialist Dwight Sparks measures floodwaters from County Road 402 bridge over the Navasota River near Groesbeck, Texas on June 12, 2025.
Abandoned cars on a flooded street in Brooklyn, NY.
Abandoned cars on a flooded street in the Gowanus neighborhood of Brooklyn, NY during a storm in 2023.
Abandoned cars on a flooded street in the Gowanus neighborhood of Brooklyn, NY during a storm in 2023.
Glacial outburst flooding from Suicide Basin submerges trails around the Mendenhall Glacier visitors center
linkA path near the Mendenhall Glacier Visitor Center in Juneau, Alaska is completely submerged by floodwaters from a glacial lake outburst flood.
Glacial outburst flooding from Suicide Basin submerges trails around the Mendenhall Glacier visitors center
linkA path near the Mendenhall Glacier Visitor Center in Juneau, Alaska is completely submerged by floodwaters from a glacial lake outburst flood.
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.
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.
Flooded Street in Denham Springs
Backwater flooding across Florida Blvd near the Amite River Bridge in Denham Springs, LA.
Backwater flooding across Florida Blvd near the Amite River Bridge in Denham Springs, LA.
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.
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.
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.
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.
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.
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.
What is a "500 year flood", and how do scientists determine if a flood is considered a "500 year flood"?
linkListen to hear the answer.
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.
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.
We had a "100-year flood" two years in a row. How can that be?
Listen to hear the answer.
Updated Date: June 27, 2025