USGS Fact Sheet 024-00
Significant Floods in the United States During the 20th Century - USGS Measures a Century
-Charles A. Perry
Table of Contents
During the 20th century, floods were the number-one natural disaster in the United States
in terms of the number of lives lost and property damage. For more than 110 years the U.S.
Geological Survey (USGS) has measured floods for the Nation's benefit while supplying
additional streamflow data with its extensive stream-gaging network. Thirty-two of the most
significant floods (in terms of number of lives lost and (or) property damage) in the United
States during the 20th century are listed according to the various types of floods. Internet
sites for acquiring near-real-time streamflow data and other pertinent flood information are
The USGS currently (2000) maintains more than 7,000 stream-gaging stations throughout the
United States, Puerto Rico, and the Virgin Islands that monitor streamflow and provide data to
various Federal, State, and local cooperating agencies as well as the general public. Some of
these stream-gaging stations have been in operation since before 1900, providing more than a
century of water informaton for the Nation. In addition to providing critical information on
flood heights and discharges, these stations provide data used in the effective management of
water-supply and water-quality needs, protection of aquatic habitat, recreation, and
The basic building block for a stream-flow data network is the stage-discharge relation that
is developed at each gaging-station location. Measurements of the flow (discharge) are related
graphically to the respective water depths (stage), which then enables discharge to be
determined from stage data.
Discharge measurements can either be direct, using a current meter, or indirect, using
mathematical flow equations. Both methods require that an elevation of the floodwater surface
be determined by a water-depth gage or by a detailed survey of high-water marks. If time
allows and conditions are safe, a direct measurement by USGS hydrographers is preferred.
However, during major floods, direct measurements are often impossible or extremely dangerous,
and indirect methods must be used.
Accurate identification and measurement of high-water marks from floods are very important in
the accurate mapping of inundated areas as well as in the analysis of water-surface profiles
for indirect discharge measurements. These elevations, in combination with flood-frequency
analysis using many years of annual flood maximums, are used by the Federal Emergency
Management Agency (FEMA) to determine flood-insurance rates.
During the 20th century, floods were the number-one natural disaster in the United States in
terms of number of lives lost and property damage. They can occur at any time of the year, in
any part of the country, and at any time of the day or night. Most lives are lost when people
are swept away by flood currents, whereas most property damage results from inundation by
sediment-laden water. Flood currents also possess tremendous destructive power, as lateral
forces can demolish buildings and erosion can undermine bridge foundations and footings
leading to the collapse of structures. The accompanying map and table locate and describe 32
of the most significant floods of the 20th century.
Floods are the result of a multitude of naturally occurring and human-induced factors, but
they all can be defined as the accumulation of too much water in too little time in a specific
area. Types of floods include regional floods, flash floods, ice-jam floods, storm-surge
floods, dam- and levee-failure floods, and debris, landslide, and mudflow floods.
Some regional floods occur seasonally when winter or spring rains coupled with melting snow
fill river basins with too much water too quickly. The ground may be frozen, reducing
infiltration into the soil and thereby increasing runoff. Such was the case for the New
England flood of March 1936 in which more than 150 lives were lost and property damage totaled
Extended wet periods during any part of the year can create saturated soil conditions, after
which any additional rain runs off into streams and rivers, until river capacities are
exceeded. Regional floods are many times associated with slow-moving, low-pressure or frontal
storm systems including decaying hurricanes or tropical storms. Persistent wet meteorological
patterns are usually responsible for very large regional floods such as the Mississippi River
Basin flood of 1993 wherein damages were $20 billion.
Flash floods can occur within several seconds to several hours, with little warning. Flash
floods can be deadly because they produce rapid rises in water levels and have devastating
Several factors can contribute to flash flooding. Among these are rainfall intensity, rainfall
duration, surface conditions, and topography and slope of the receiving basin. Urban areas are
susceptible to flash floods because a high percentage of the surface area is composed of
impervious streets, roofs, and parking lots where runoff occurs very rapidly. Mountainous
areas also are susceptible to flash floods, as steep topography may funnel runoff into a
narrow canyon. Floodwaters accelerated by steep stream slopes can cause the floodwave to move
downstream too fast to allow escape, resulting in many deaths. A flash flood caused by 15
inches of rain in 5 hours from slow-moving thunderstorms killed 237 people in Rapid City,
South Dakota, in 1972.
Floodwaves more than 30-feet high have occurred many miles from the rainfall area, catching
people unaware. Even desert arroyos are not immune to flash floods, as distant thunderstorms
can produce rapid rises in water levels in otherwise dry channels. Early-warning gages
upstream save lives by providing advanced notice of potential deadly floodwaves.
Ice-jam floods occur on rivers that are totally or partially frozen. A rise in stream stage
will break up a totally frozen river and create ice flows that can pile up on channel
obstructions such as shallow riffles, log jams, or bridge piers. The jammed ice creates a dam
across the channel over which the water and ice mixture continues to flow, allowing for more
jamming to occur. Backwater upstream from the ice dam can rise rapidly and overflow the
channel banks. Flooding moves downstream when the ice dam fails, and the water stored behind
the dam is released. At this time the flood takes on the characteristics of a flash flood,
with the added danger of ice flows that, when driven by the energy of the floodwave, can
inflict serious damage on structures. An added danger of being caught in an ice-jam flood is
hypothermia, which can quickly kill. Ice jams on the Yukon River in Alaska contributed to
severe flooding during the spring breakup of 1992.
Storm-surge flooding is water that is pushed up onto otherwise dry land by onshore winds.
Friction between the water and the moving air creates drag that, depending upon the distance
of water (fetch) and the velocity of the wind, can pile water up to depths greater than 20
feet. Intense, low-pressure systems and hurricanes can create storm-surge flooding. The storm
surge is unquestionably the most dangerous part of a hurricane as pounding waves create very
hazardous flood currents.
Nine out of 10 hurricane fatalities are caused by the storm surge. Worst-case scenarios occur
when the storm surge occurs concurrently with high tide. Stream flooding is much worse inland
during the storm surge because of backwater effects. In September 1900, the hurricane and
storm surge at Galveston, Texas, killed more than 6,000 people, making it the worst natural
disaster in the Nation's history.
Dams and levees are built for flood protection. They usually are engineered to withstand a
flood with a computed risk of occurrence. For example, a dam or levee may be designed to
contain a flood at a location on a stream that has a certain probability of occurring in any 1
year. If a larger flood occurs, then that structure will be overtopped. If during the
overtopping the dam or levee fails or is washed out, the water behind it is released to become
a flash flood. Failed dams or levees can create floods that are catastrophic to life and
property because of the tremendous energy of the released water. Warnings of the Teton Dam
failure in Idaho in June 1976 reduced the loss of life to 11 people.
Debris or landslide floods are created by the accumulation of debris, mud, rocks, and (or)
logs in a channel, which form a temporary dam. Flooding occurs upstream as water becomes
stored behind the temporary dam and then becomes a flash flood as the dam is breached and
rapidly washes away. Landslides can create large waves on lakes or embayments and can be
deadly. Mudflow floods can occur when volcanic activity rapidly melts mountain snow and
glaciers, and the water mixed with mud and debris moves rapidly downslope. These mudflow
events are also called lahars and, after the eruption of Mt. St. Helens in 1980, caused
significant damage downstream along the Toutle and Cowlitz Rivers in southwest Washington.
Streamflow information can be accessed through the Internet at several adresses, including:
Click here for an example of the streamflow information on the
Flood Insurance-http: //www.fema.gov/library/frnfip.htm
Regional Flood Safety-http://www.nws.noaa.gov/om/nh-flood.htm
River Forecast Centers-http://info.abrfc.noaa.gov/rfc_wfo.html
U.S. Army Corps of Engineers regional sites and districts-
USGS Streamflow Data
- Most flood-related deaths are due to flash floods.
- Fifty percent of all flash-flood fatalities are vehicle related.
- Ninety percent of those who die in hurricanes drown.
- Most homeowners insurance policies do not cover floodwater damage.
- Individuals and business owners can protect themselves from property losses by
purchasing flood insurance through FEMA's National Flood Insurance Program.
Some information in this fact sheet was obtained from U.S. Army Corps of Engineers, Federal
Emergency Management Agency, and National Weather Service.
For additional information contact:
Charles A. Perry
U.S. Geological Survey
4821 Quail Crest Place
Telephone: (703) 648-5305
Fax: (785) 832-3500