Climate Change Indicators in the United States: River Flooding
www.epa.gov/climate-indicators - August 2016

River Flooding

This indicator examines changes in the size and frequency of inland river flood events in the United
States.

Background

Rivers and streams experience flooding as a natural result of large rain storms or spring snowmelt that
quickly drains into streams and rivers. Although the risk for flooding varies across the United States,
most areas are susceptible to floods, even in dry and mountainous regions. The size, or magnitude, of
flood events is influenced by how much water enters the waterway upstream—and how quickly. Flood
frequency largely depends on the frequency of weather events.

Large flood events can damage homes, roads, bridges, and other infrastructure; wipe out farmers' crops;
and harm or displace people. Although regular flooding helps to maintain the nutrient balance of soils in
the flood plain, larger or more frequent floods could disrupt ecosystems by displacing aquatic life,
impairing water quality, and increasing soil erosion. By inundating water treatment systems with
sediment and contaminants, and promoting the growth of harmful microbes, floods can directly affect
the water supplies that communities depend on.

Climate change may cause river floods to become larger or more frequent than they used to be in some
places, yet become smaller and less frequent in other places. As warmer temperatures cause more
water to evaporate from the land and oceans, changes in the size and frequency of heavy precipitation
events may in turn affect the size and frequency of river flooding (see the Heavy Precipitation
indicator).1 Changes in streamflow, the timing of snowmelt (see the Streamflow indicator), and the
amount of snowpack that accumulates in the winter (see the Snowpack indicator) can also affect flood
patterns.

About the Indicator

The U.S. Geological Survey maintains thousands of stream gauges across the United States. Each gauge
measures water level and discharge—the amount of water flowing past the gauge. This indicator uses
total daily discharge data from about 500 long-term stream gauge stations where trends are not
substantially influenced by dams, reservoir management, wastewater treatment facilities, or land-use
change.

One way to determine whether the magnitude of flooding has changed is by studying the largest flood
event from each year. This indicator examines the maximum discharge from every year at every station
to identify whether peak flows have generally increased or decreased. This indicator also analyzes
whether large flood events have become more or less frequent over time, based on daily discharge
records.

This indicator starts in 1965 because flood data have been available for a large number of sites to
support a national-level analysis since then.

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Climate Change Indicators in the United States: River Flooding
www.epa.gov/climate-indicators - August 2016

Key Points

•	Floods have generally become larger in rivers and streams across large parts of the Northeast
and Midwest. Flood magnitude has generally decreased in the West, southern Appalachia, and
northern Michigan (see Figure 1).

•	Large floods have become more frequent across the Northeast, Pacific Northwest, and northern
Great Plains. Flood frequency has decreased in some other parts of the country, especially the
Southwest and the Rockies (see Figure 2).

•	Increases and decreases in frequency and magnitude of river flood events generally coincide
with increases and decreases in the frequency of heavy rainfall events.2,3

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Climate Change Indicators in the United States: River Flooding

www.epa.gov/climate-indicators - August 2016

Figure 1. Change in the Magnitude of River Flooding in the United States, 1965-2015

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T	V	A	A

Significant	Insignificant	Insignificant	Significant

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This figure shows changes in the size of flooding events in rivers and streams in the United States
between 1965 and 2015. Blue upward-pointing symbols show locations where floods have become
larger; brown downward-pointing symbols show locations where floods have become smaller. The
larger, solid-color symbols represent stations where the change was statistically significant.

Data source: Slater and Villarini, 20164

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Climate Change Indicators in the United States: River Flooding

www.epa.gov/climate-indicators - August 2016

Figure 2. Change in the Frequency of River Flooding in the United States, 1965-2015

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\ m ^ L









£

~	V	A	A

Significant	Insignificant	Insignificant	Significant

decrease	decrease	increase	increase

This figure shows changes in the frequency offloading events in rivers and streams in the United States
between 1965 and 2015. Blue upward-pointing symbols show locations where floods have become more
frequent; brown downward-pointing symbols show locations where floods have become less frequent.
The larger, solid-color symbols represent stations where the change was statistically significant.

Data source: Slater and Viilarini, 2016s

Indicator Notes

This indicator is based on U.S. stream gauges that have recorded data consistently since 1965. Besides
climate change, many other types of human influences could affect the frequency and magnitude of
floods—for example, dams, floodwater management activities, agricultural practices, and changes in
land use. To remove these influences, this indicator focuses on a set of sites that are not heavily
influenced by human activities, in watersheds that do not have a large proportion of impervious surfaces
such as concrete and asphalt. Increased flooding does not necessarily result in an increased risk to
people or property if an area has protective infrastructure, such as levees or floodwalls.

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Climate Change Indicators in the United States: River Flooding
www.epa.gov/climate-indicators - August 2016

Data Sources

Daily stream gauge data were collected by the U.S. Geological Survey. These data came from a set of
gauges in watersheds with minimal human impacts, which have been classified as reference gauges.6
Daily discharge data are stored in the National Water Information System and are publicly available at:
http://waterdata.usgs.gov/nwis.

1	Mallakpour, I., G. Villarini. 2015. The changing nature of flooding across the central United States. Nature
Climate Change 5:250-254.

2	Mallakpour, I., G. Villarini. 2015. The changing nature of flooding across the central United States. Nature
Climate Change 5:250-254.

3	Melillo, J.M., T.C. Richmond, and G.W. Yohe (eds.). 2014. Climate change impacts in the United States: The third
National Climate Assessment. U.S. Global Change Research Program, http://nca2014.globalchange.gov.

4	Slater, L., and G. Villarini. 2016 update and expansion to data originally published in: Mallakpour, I., G. Villarini.
2015. The changing nature of flooding across the central United States. Nature Climate Change 5:250-254.

5	Slater, L., and G. Villarini. 2016 update and expansion to data originally published in: Mallakpour, I., G. Villarini.
2015. The changing nature of flooding across the central United States. Nature Climate Change 5:250-254.

6	Lins, H.F. 2012. USGS Hydro-Climatic Data Network 2009 (HCDN-2009). U.S. Geological Survey Fact Sheet 2012-
3047.

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