River Flooding FAQ

Where can I get detailed river information including current stages and forecast stages?
Detailed river information is available from our Advanced Hydrologic Prediction Service page.

I have seen the river forecasts change occasionally...Why the change?
Hydrologists use computer models to produce a river forecast. One important input to the models is rainfall. Each new model run incorporates the latest new rainfall in the last 24 hours. If additional heavy rain has fallen since the previous forecast, the new river forecast will likely change to reflect the new data. In addition to actual rainfall, river forecasts also use the amount of future or forecast rainfall expected during the next 24 hours. This can change greatly and can have a major impact on the river forecast. In addition, the forecasts may be adjusted based on actual flow measurements taken during the event by U.S. Geological Survey (USGS) hydrologic technicians.

How do you measure the current water levels?
The National Weather Service relies heavily on a network of river gages operated by the U.S. Geological Survey (USGS). Data is received from these river gages either by phone telemetry or satellite. In addition, during times of flooding, USGS technicians are out taking flow measurements and relaying that back to the NWS and other agencies which provides vital information for our river flood warning and forecast program. The NWS also uses gages on the Illinois  Waterway operated by the Rock Island US Army Corps of Engineers.

USGS taking flow measurement
USGS Hydrologist David Schrader makes a wading road overflow measurement at the USGS gage 
on the East Branch of the Dupage River in  Bolingbrook IL on 9/14/2008

What is a river stage?
A river stage represents the level of the water surface of a river or stream above an established datum at a given location.

What is meant by the term flood stage?
Flood stage is an established gage height for a given location above which a rise in water surface level begins to create a hazard to lives, property, or commerce. The issuance of flood warnings for river locations is linked to flood stage. Flood stage is not necessarily the same as bankfull stage.

What is the meaning of a "100 year" flood?
The 100-year flood  or x-year flood refers to the probability of those events occurring. That is, for a 100-year flood, there is a 1% chance in any given year of having a flood of that magnitude. For a 500-year flood, there is a 0.2% chance of having a flood of that magnitude occurring.

It should be stressed that the 100-year and 500-year events are independent events, from the perspective of probability. That means that if one of those event occurs, it has no effect on future events occurring. In other words, if a 100-year flood event occurs, that does NOT mean that people are “safe” for 99 years. The risk of having the flood in any given year is the same, regardless of if it occurred recently.

Where can I find information about flood insurance?
Floodsmart.gov is the official site of the National Flood Insurance Program.

What about flood safety?
More than half of all flood related fatalities are a result of driving into hazardous water covered roadways.  If you encounter a flooded roadway follow this simple advice: Turn Around, Don’t Drown! Learn more about flood safety and the Turn Around, Don’t Drown campaign here.


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