|Ice jam on the Kankakee River near I-55 bridge. Jan 27, 2008||
I-55 bridge over the Kankakee River. Jan 25, 2008.
What is an ice jam?
An ice jam is a stationary accumulation of ice that restricts flow. Ice jams can cause considerable increases in upstream water levels, while at the same time downstream water levels may drop, exposing water intakes for power plants or municipal water supplies. Types of ice jams include freezeup jams, made primarily of frazil ice; breakup jams, made primarily of fragmented ice pieces; and combinations of both.
Why are we experiencing so much ice jam flooding at this time?
In early January, northern Illinois and northwest Indiana experienced warm temperatures that melted the existing snow in combination with heavy rainfall that resulted in near record flooding on some streams. Although water levels have receded since the near record levels, they remain above normal for this time of year. These higher than normal flows are resulting in more substantial flooding when an ice jam forms. The recent sub zero temperatures have increased the ice thickness.
What causes ice to break up once a jam is in place?
Ice cover can begin to breakup as a result of thermal effects--direct sunlight melting the cover. This causes ice cover to melt in place and normally does not result in flood problems. However, this normally will only occur on a thin ice cover. More extensive jams require increased flows in the river from either snowmelt or rain or a combination of both. The increased flows begin to lift and break the ice and transport it downstream. This type of breakup can result in flash flooding if a new jam forms at a sharp meander in the river, bridge, or where there is a change in the slope of the river bed.
Can you predict ice jams?
The National Weather Service monitors river levels at select gages on area streams, maintained and operated by the U.S. Geological Survey. Current river levels are available from the NWS Advanced Hydrologic Prediction Service page. However, ice jams can be very localized and the gage reading may not always reflect what is happening a few miles upstream or downstream from the gage. In addition, jams can form and break with little or no warning making ice jams very difficult to predict.
If you live along streams prone to ice jams you should continue to closely monitor river levels and listen for possible flood statements or warnings. When a jam forms, water can rise many feet in minutes versus hours and days in a normal river flood, and you may have little time to take action. Report ice jams and ice jam flooding to local authorities for relay to the National Weather Service.
Do you have photos from the recent ice jam flooding that you would be willing to share?
We would be interested in any ice jam flooding photos to help document this event. Please follow the instructions here to submit a photo.