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Hazard Information

This page provides easy access to useful information regarding the hazards that Illinois faces. While the Illinois Natural Hazard Mitigation Plan goes into greater detail about all of Illinois' hazards, this page looks at those that pose the biggest threat.


Except for fire, the most common hazard in the United States is flooding, with thousands occurring each year from oceans, rivers, lakes, small streams, gullies, creeks, culverts, dry streambeds or low-lying ground. The standard definition of a flood is: "A general and temporary condition of partial or complete inundation of normally dry land areas from: (1) the overflow of inland or tidal waters, (2) the unusual and rapid accumulation or runoff of surface waters from any source, or (3) mudflows or the sudden collapse of shoreline land." A simpler definition is: "too much water in the wrong place."    

Since 1981, 99 of Illinois' 102 counties have been declared by the President as major disaster areas due to flooding. Ten counties were declared in both the 1993 and 1995 floods. Calhoun County, less than 10 miles wide and approximately 42 miles from north to south and located between the Mississippi and Illinois rivers, has had 10 major flood declarations since 1981.

If you would like to know how many Federal Declarations your county has had, click here.     

For more information on Illinois' disaster history, please visit:

Useful Information Related to Floods:

Flood Guages

Flood Insurance (NFIP)

Flood Maps

Levee Information

Preparedness Information

Severe Storms

All counties are susceptible to severe storms. The past 49 years of data include more than 11,000 reports of severe thunderstorm damage (approximately 7,000 wind and 4,000 hail reports) in the state of Illinois. In an analysis of thunderstorm-caused catastrophes, Illinois ranked 4th in the United States in total thunderstorm catastrophes between 1949 and 1998. Illinois has never seen a year go by without having a severe storm.

Illinois is also situated on the northeast edge of "tornado alley," the tornado-prone area that extends approximately 400 miles on either side of a line from Fort Worth, Texas, to Detroit, Michigan. This area is the battleground of warm, moist air from the Gulf of Mexico and cold air from Canada, resulting in the world's largest breeding ground for twisters. The greatest frequency of tornadoes in Illinois occurs in a wide band from Madison and St. Clair counties northeastward to Lake and Cook counties. Officially, there have been 2,103 tornadoes in Illinois from 1950 through August 30, 2007.  

Although they do not receive as much recognition as tornado events, thunderstorm winds cause more damage year-to-year than tornadoes. In 1993 alone, 38 events of thunderstorm winds caused a minimum of $5 million in damage, while 34 tornadoes caused just over $1.5 million in damage.  

In Illinois, thunderstorms occur when there is a collision of moist, warm air moving north from the Gulf of Mexico with colder fronts moving east from the Rocky Mountains. This results in cold air overriding a layer of warm air causing the warm air to rise rapidly. Thunderstorms may occur singly, in clusters, or in lines. In the course of a few hours, it is possible for several thunderstorms to affect one location or a single thunderstorm to affect one location for an extended time. Thunderstorms typically are 15 miles in diameter and produce heavy rain anywhere from 30 minutes to an hour.    

With that in mind, homeowners and businesses alike can take part in mitigation techniques to reduce their chances of falling victim to wind related property damage. For examples, click here.    

For more information about severe storms, click here.    

Taking Shelter from the Storm: Building a Safe Room for Your Home or Small Business (FEMA P-30, Fourth Edition/Dec. 2014)

Preparedness Information


Over 250 small to moderate earthquakes are known to have occurred in Illinois during the past two centuries. Of these, 31 caused at least some damage. Earthquakes occur when rocks forming the earth's crust slip past each other along a fault. This slippage occurs when the buildup of stresses gets to the point that they are greater than the strength of the locked up section of rocks along the fault plane. When faulting takes place, the sudden release of energy produces vibrations or seismic (shock) waves that radiate from the main fault movements. These waves cause the shaking or quaking that lasts tens of seconds to a few minutes, depending on the magnitude of the event (energy released), what kinds of rocks they travel through, and the stiffness or lack of stiffness of the soils at a site.  

Did you know? Although 80 percent of Illinois' previous earthquake activity has occurred in the southern third of the state, one of the largest earthquakes in Illinois occurred in northern Illinois on May 26, 1909. The exact location of the magnitude 5.1 (estimated) earthquake isn't known, but the greatest intensities occurred in and near Aurora, where many chimneys fell, a stove overturned, gas lines broke, and a fire started. Although considerable excitement ensued, the Aurora fire was quickly extinguished and soon forgotten. It was felt over 500,000 square miles; buildings swayed in Chicago where there was fear that the walls would collapse; houses were jostled out of plumb in Beloit, Wis.; and brick walls cracked as far away as Bloomington, Ill.  

If you would like to learn more about earthquakes and earthquake mitigation, please click here or follow some of the links below.

Preparedness Information


Technological hazards are those that are caused by tools, machines and substances that we use in our everyday lives. The major technological hazards that Illinois faces are hazardous materials. The term "hazardous materials" refers generally to hazardous substances, petroleum, natural gas, synthetic gas, and acutely toxic chemicals. The term Extremely Hazardous Substance (EHS) is used in Title III of the Superfund Amendments and Reauthorization Act of 1986 to refer to those chemicals that could cause serious health effects following short-term exposure from accidental releases. Illinois has more than 7,000 fixed facility locations that report the presence of an EHS in federally mandated threshold amounts.

Another major technological hazard in Illinois is radiation. Nuclear power generating facilities have the greatest concentration of radioactive materials of any private source. Illinois has six functioning nuclear power plants: Braidwood (Kankakee County), Byron (Ogle County), Clinton (DeWitt County), Dresden (Will County), LaSalle (LaSalle County) and Quad Cities (Rock Island County).

Preparedness Information

Winter Storms

One hundred percent of the population is at risk from a severe winter storm in the state of Illinois. An Illinois winter does not pass without a severe winter storm somewhere in the state. On average, five severe storms strike each year. As few as two, and as many as 18 have occurred each year in Illinois history.

When you hear the term "severe winter storm warning," freezing temperatures, heavy snowfall or freezing rain comes to mind. There are actually three categories of winter storms:

  • Blizzard: This is the most dangerous of all winter storms. A blizzard combines low temperatures, heavy snowfall and winds of at least 35 miles per hour, reducing visibility to only a few yards.
  • Heavy Snow Storm: Will produce six inches or more of snow in 48 hours or less.
  • Ice Storm: Occurs when moisture falls and freezes immediately upon impact.

One of the worst winter storms to impact the State was on January 26-27, 1967, when 23 inches of snow fell in Moline (Rock Island County) and the Chicago area, paralyzing O'Hare International Airport. Travel throughout Northern Illinois was curtailed, and areas to the south experienced a glaze of ice which made travel virtually impossible until January 29, 1967. Fifty deaths were directly attributed to this storm.

More information about Illinois' winter storm threats can be accessed by clicking here.

Preparedness Information


A drought originates from a deficiency of precipitation over an extended period of time, usually a season or more. This deficiency results in a water shortage for some activity, group or environmental sector. Operational definitions help people identify the beginning, end and degree of severity of a drought.

There are four commonly used operational definitions:

  • Meteorological Drought: A period of well-below-average precipitation that spans from a few months to a few years.
  • Agricultural Drought: A period when soil moisture is inadequate to meet the demands for crops to initiate and sustain plant growth.
  • Hydrological Drought: A period of below-average stream flow and/or depleted reservoir storage (i.e., stream flow, reservoir and lake levels, ground water).
  • Economic Drought: This definition deals with the supply and demand of water. Some years there is an ample supply of water and in other years there is not enough to meet human and environmental needs.

Both the timing and amount of precipitation are responsible for the occurrence of a drought. The mean annual precipitation in Illinois varies from 34 inches in northern Illinois to 46 inches in southern Illinois. Annual amounts fluctuate primarily within a 10-inch range of the median. The most severe drought in recent years was 1988, when rainfall was 88 percent of normal. The timing or distribution was also abnormal because 1988 saw less than 50 percent of the April through August normal rainfall. Droughts of this magnitude occur about once every 21 years.

Scientists use the Palmer Drought Severity Index (PDSI) to measure droughts. The PDSI compares weekly temperature and precipitation readings over a defined climatic region in order to identify periods of abnormally dry (or wet) weather. To view, click here.