Changes in River Forecasts on Illinois River Affecting Morgan and Scott Counties

Effective at 10 am on Tuesday, January 26, the National Weather Service will be making the following changes to flood stages and products for the Illinois River at Meredosia and Valley City, affecting Morgan and Scott Counties.

Flood stages for both locations are being changed as follows:

Value Old Stage New Stage
Flood Stage 432.0 feet 435.0 feet
Moderate Flood Stage 435.0 feet 440.0 feet
Major Flood Stage 438.0 feet 442.0 feet

Valley City
Value Old Stage New Stage
Flood Stage 11.0 feet 14.0 feet
Moderate Flood Stage 16.0 feet 21.0 feet
Major Flood Stage 21.0 feet 23.0 feet

Another change will take place at Meredosia.  Since its inception as a forecast point, stages have been given in vertical distance above a common vertical datum, currently the National Geodetic Vertical Datum of 1929 (NGVD29).  This is known as an "elevation based system".  However, the U.S. Geological Survey (USGS) and the U.S. Army Corps of Engineers (COE) also reference a vertical distance above a commonly accepted gage zero elevation, known as a "stage based system".  All agencies plan to change from the current NGVD29 to North American Vertical Datum 1988 (NAVD88) within the next year or two.  This should result in a change in reported elevations of more than a third of a foot.  To avoid changing flood stages and impact elevations again with the conversion of NAVD88, the NWS is planning to change to a stage based system at the same time as the flood stage change.  Emergency managers in Brown, Pike, and Morgan Counties all agree with these proposed changes.

The USGS gage zero elevation is 418.0 feet above NGVD29, so the final flood stage for the Illinois River at Meredosia will be 435.0 minus 418.0, or 17.0 feet.  The new moderate flood stage will be updated to 22.0 feet, and the new major flood stage will be 24.0 feet.

"Flood stage" is the river level at which minimal human impact from floodwaters begins, and the level the NWS uses as a threshold for issuing river flood warnings.  Moderate and major flood stages are levels at which human impact increases noticeably.  At moderate levels, numerous secondary roads are often inundated and some outbuildings may be flooded.  At major levels, primary roads and highways can become flooded, along with residences and businesses.

The NWS welcomes public feedback.  If you have any questions or commons about these changes, please contact:

Mark Fuchs, Service Hydrologist
National Weather Service
12 Missouri Research Park Dr.
St. Charles, MO  63304

Phone:  636-447-1876


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