Changes to Flood Levels Des Plaines River near Des Plaines, IL

Effective November 14, 2014...the gage zero datum at the U.S. Geological survey(USGS) streamgage on the Des Plaines River near Des Plaines IL will be changed by 10.0 feet. Gage zero datum is a horizontal surface used as the zero point for measurement of river stage, also referred to as gage height.

Background
The physical location of the USGS streamgage near Des Plaines IL has changed. The gage is now located at the Euclid Avenue/West Lake Avenue bridge, approximately 0.4 mile upstream from the previous location. The move was necessitated by the removal of dam #2 by the Illinois Department of Natural Resources beginning on May 28 2014. At the current datum, low flows are resulting in the reporting of negative stages. The USGS will be adjusting the zero datum used to reference gage heights, flood stage, and impact levels. The zero datum will be changed from 626.31 to 616.31 feet to avoid negative readings.

Impacts to users
All existing flood and impact stages will be increased by 10 feet. New flood thresholds will be as follows:

	Old	New
Flood Stage	5.0	15.0
Moderate flood	8.0	18.0
Major flood	9.0	19.0

Gage datum change graphic

In addition, it was determined that some minor impacts begin at levels slightly below the existing action stage. Thus, the new action stage will be adjusted to 13.5 feet.

Frequently Asked Questions (FAQ)

Will river stages I see on the AHPS hydrograph page for Des Plaines change?
Yes, after the change, current river stage observations will be 10.0 feet higher.

Will the impact statements that list flood impacts at various water levels change?
Yes, all existing flood impacts will be increased by 10.0 feet after the change on 11/14/14.

After the change, will a river stage of 12.0 feet at the Des Plaines USGS river gage mean that the water is 12.0 feet deep?
No, a river stage or gage height of 12.0 feet only indicates that the gage height is 12 feet above the horizontal reference point known as gage datum. This gage datum will be lowered by 10 feet on 11/14/14. See the graphic above.

How can I compare crests after the change with previous historical crests referencing the old gage datum?
The historical crests will be revised using the new datum after the change.

I use the x ft level for a flood threshold at my location. How will this change?
After the datum change, simply add 10.0 feet to your previous value to determine your new threshold. For example, if 8.0 ft was a critical stage before the change, that threshold will be 18.0 feet after the datum change.

These planned changes have been coordinated with other agencies including the U.S. Geological Survey and the Illinois Dept. of Natural Resources.

River stage and forecast data can be found on the Advanced Hydrologic Prediction Service page. River stage data is also available from the USGS here. Learn more about the USGS Streamgage program here.

If you have any questions or comments about this change...please contact:

William Morris
Service Hydrologist
National Weather Service
333 W University Dr
Romeoville, IL 60446
815.834.0600 x493
william.morris@noaa.gov

Jon Hortness
Hydrologist
U.S. Geological Survey
Dekalb, IL

815.752.2036
hortness@usgs.gov