|Ice jam on the Kankakee River near Warner Rd bridge. 2.23.14
Ice jam on Rock Creek in Kankakee River State Park. 2.23.14
|Iroquois River ice photo courtesy James Myers, NWS ice spotter||Illinois River ice near LaSalle, IL. Photo courtesy LaSalle County Emergency Management Agency|
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?
This winter has been unusually cold with many subzero temperatures recorded. The bitter cold temperatures have allowed an extensive ice cover to develop on area streams. The heavy rain, snowmelt, and brief warm-up event on Feb 20 initiated runoff that increased flows on area rivers. Increasing river levels beneath the ice cover began to lift and break the ice and transport it downstream. When the ice encounters an obstruction, it jams and result in rapid rises in water levels.
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.
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|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. A network of NWS Chicago River Ice Spotters provides weekly reports on the amount of ice at select locations on rivers across northern Illinois and northwest Indiana.
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 several feet in just minutes compared to hours or 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.
Get Text or Email Alerts of Water Levels from the USGS
At USGS river gage locations, you can sign up for alerts when water levels rise to a threshold that's important to you. Learn more about the USGS WaterAlert progam here.